Table of Contents

  

 

The Wilkinson Microwave Anisotropy Probe (WMAP)

    Introduction

    What WMAP Records

    Surface of last Scatter

    Microwave Background Fluctuation Spectrum

    The Sonic Horizon

    Measuring the Geometry of the Universe

    Counting the Electrons in the Universe

    Measuring the Matter Content of the Universe

    Microwave Background Polarization

    WMAP 5-year Results Released - March 7, 2008  

    Content of the Universe

    Time Line of the Universe  

    Temperature Fluctuation by Angular Size  

    Conclusion

 

Big Bang

    Introduction

Timeline of the Big Bang  

    The very early universe

    Augustinian era - Before the Big Bang

    The Planck epoch: Up to 10–43 seconds after the Big Bang

    The grand unification epoch: Between 10–43 seconds and 10–36 seconds after the Big Bang

    The electroweak epoch: Between 10–36 seconds and 10–12 seconds after the Big Bang

    The inflationary epoch: Between 10–36 seconds and 10–32 seconds after the Big Bang

    Reheating

    Baryogenesis

    The early universe  

    Supersymmetry breaking

    The quark epoch: Between 10–12 seconds and 10–6 seconds after the Big Bang

    The hadron epoch: Between 10–6 seconds and 1 second after the Big Bang

    The lepton epoch: Between 1 second and 3 minutes after the Big Bang

    The photon epoch: Between 3 minutes and 380,000 years after the Big Bang

    Nucleosynthesis: Between 3 minutes and 20 minutes after the Big Bang

    Matter domination: 70,000 years

    Recombination: 240,000–310,000 years

    Dark ages

    Structure formation

    Reionization: 150 million to 1 billion years

    Formation of stars

    Formation of galaxies

    Formation of groups, clusters and superclusters

    Formation of our solar system: 8 billion years

    Today: 13.7 billion years

Graphical timeline of the Big Bang

Ultimate fate of the universe

    The future according to the Big Bang theory

    Big freeze: 1014 years and beyond

    Big crunch: 100+ billion years

    Big rip: 200+ billion years

    Vacuum metastability event

 

Big bang theory assumptions  

    Friedmann-Lemaître-Robertson-Walker metric

    Horizons

    Observational evidence

    Hubble's law and the expansion of space

    Cosmic microwave background radiation

    Abundance of primordial elements

    Galactic evolution and distribution

    Other lines of evidence

 

Features, issues and problems

    Horizon problem

    Flatness/oldness problem

    Magnetic monopoles

    Baryon asymmetry

    Globular cluster age

    Dark matter

    Dark energy

 

Speculative physics beyond the Big Bang 

 

 

   

The Wilkinson Microwave Anisotropy Probe (WMAP)

       

Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big Bang.

 

 

Introduction

"We are living in an extraordinary time," said Gary Hinshaw of NASA's Goddard Space Flight Center in Greenbelt, MD. "Ours is the first generation in human history to make such detailed and far-reaching measurements of our universe."  

 

WMAP measures a remnant of the early universe - its oldest light. The conditions of the early times are imprinted on this light.  It is the result of what happened earlier, and a backlight for the later development of the universe.  This light lost energy as the universe expanded over 13.7 billion years, so WMAP now sees the light as microwaves.  By making accurate measurements of microwave patterns, WMAP has answered many longstanding questions about the universe's age, composition and development.  

 

The cosmic microwave background (CMB) radiation is the radiant heat left over from the Big Bang. It was first observed in 1965 by Arno Penzias and Robert Wilson at the Bell Telephone Laboratories in Murray Hill, New Jersey. The properties of the radiation contain a wealth of information about physical conditions in the early universe and a great deal of effort has gone into measuring those properties since its discovery. This radiation (and by extension, the early universe) is remarkably featureless; it has virtually the same temperature in all directions in the sky.  

      

In 1992, NASA's Cosmic Background Explorer (COBE) satellite detected tiny fluctuations, or anisotropy, in the cosmic microwave background. It found, for example, one part of the sky has a temperature of 2.7251 Kelvin (degrees above absolute zero), while another part of the sky has a temperature of 2.7249 Kelvin. These fluctuations are related to fluctuations in the density of matter in the early universe and thus carry information about the initial conditions for the formation of cosmic structures such as galaxies, clusters, and voids. COBE had an angular resolution of 7 degrees across the sky, 14 times larger than the Moon's apparent size. This made COBE sensitive only to broad fluctuations of large size.  

      

The Wilkinson Microwave Anisotropy Probe (WMAP) was launched in June of 2001 and has made a map of the temperature fluctuations of the CMB radiation with much higher resolution, sensitivity, and accuracy than COBE. The new information contained in these finer fluctuations sheds light on several key questions in cosmology. By answering many of the current open questions, it will likely point astrophysicists towards newer and deeper questions about the nature of our universe.

The Big Bang theory of the universe allows plenty of room for variations in the details (parameters) of the actual structure and behavior of our universe. These "free parameters" are important, but must be determined by observations, not theory. The parameters effect very basic aspects of our universe:

  • Will the universe expand forever, or will it collapse?

  • Is the universe dominated by exotic dark matter?

  • What is the shape of the universe?

  • How and when did the first galaxies form?

  • Is the expansion of the universe accelerating rather than decelerating?

  • And more...

Since the CMB radiation was emitted so long ago (and far away), it carries a great deal of information about the properties of our universe which can be measured in no other way. This early radiation was effected everywhere by the physics of matter within the bounds set by the parameters. So we have a large statistical sample of microwave radiation (across the whole sky) to help us determine these parameters. Because WMAP can measure the CMB patterns of radiation with tremendous accuracy, it accurately determines most of the basic parameters of cosmology.

In order to understand how WMAP determines cosmological parameters, we need to give a little background on the physical evolution of the early universe and understand how cosmologists describe the statistical properties of the microwave background radiation.  

 

What WMAP Records

   

The Big Bang theory implies that the early universe was a hot, dense fluid. During the first three hundred and eighty thousand years of the Big Bang, the temperature of the universe exceeded 2967 Kelvin (4880 F), so most of the hydrogen in the universe was ionized: the electron of the hydrogen atom was so energized it could not stay bound to the proton. Thus, the early universe was a hot sea of energetic protons and electrons. This hot gas constantly emits, scatters and reabsorbs particles of light called photons and is ultimately the source of the cosmic microwave background radiation. As long as the gas remains ionized, these strong particle interactions tie the photons, electrons, and protons together so they behave like a single fluid. As discussed below, this single fluid behavior has implications for how sound waves move through the gas. We can learn a great deal about the properties of the super heated gas of the early universe in which the sound waves moved and thus learn a great deal about the physical properties of the early universe. The CMB Radiation is a fossil record of those sound waves!  

     

 Surface of last Scatter


As the universe expanded, the temperature of the cosmic soup dropped below 2967 Kelvin. The electrons and protons were then able to combined to make neutral hydrogen. Hydrogen is almost completely transparent to the cosmic background radiation, thus after the universe reached this temperature cosmic radiation could propagate freely through the universe. The appearance of the sky on cloudy days is a good analogy to the appearance of the microwave background radiation. Water droplets scatter optical light, much like the free electrons scatter the photons of cosmic background radiation. But water vapor is nearly transparent to optical light, just as neutral hydrogen is nearly transparent to the cosmic background radiation. On a cloudy day, we can look out through the air and water vapor to the water droplets in the clouds which obscure our view beyond them.

Person looking up through
the air to view the underside of a cloud compared to looking through the space to the CMB Surface of Last Scatter

 

When WMAP observes the microwave background sky it looks back to when there were free electrons that could readily scatter cosmic background radiation. This cosmic background "cloud surface" is called the "surface of last scatter". If there were any "features" imprinted in this surface of last scatter (i.e.., regions that were brighter or dimmer than average) they will remain imprinted to this day because emitted light travels across the universe largely unimpeded. Thus, observations of the cosmic background radiation directly probe the physical conditions at this epoch, only 380,000 years after the Big Bang. 

   

Microwave Background Fluctuation Spectrum


 

 

When WMAP measures the temperature of the CMB radiation over the full sky, it sees a nearly, but not exactly, uniform glow - there will be variations, or fluctuations, across the sky, like ripples on a pond. There are a number of different effects that can produce temperature variations in the microwave background: variations in the density or velocity of the gas at the surface of last scatter (i.e., sound waves), variations in the gravitational potential of the universe at the surface of last scatter as well as variations in the gravitational potential along the photon path. Different physical phenomena are responsible for each of these effects. By studying the microwave background fluctuations we can infer a wealth of information about the early universe.  

                

By measuring the CMB temperature in progressively smaller and smaller patches of the sky , variations are measured in terms of an "angular fluctuation spectrum", or the amplitude or strength of temperature fluctuations as a function of angular size. Cosmologists can very easily represent this complex idea with a simple wiggly line across a graph. The graph produced by the WMAP science team is shown below. This plot shows a plateau at large angular or length scales, then a series of coherent peaks at progressively smaller scales. These features are generated by the various physical processes that produce different amounts of energy (temperature) at the different angular scales. This graph line will change as one varies the details of the physical processes; and the physical processes are controlled by fundamental cosmological parameters.  

Comparison of angular power in the CMB and theoretical power evenly distributed

This graph shows the sky's temperature when measured at different angular scales across the sky. The wiggly line is the actual sky measurement. The straight line is the same measurement of sky energy if there were a true uniform distribution of temperature. The sky images above three separate portions of the graph show their appearance under these two scenarios.  

      

On the largest length scales (left side of the graph) the main source of temperature fluctuations are due to variations in the strength of gravity at the time of last scatter. Photons that must climb out of a gravitational potential well will lose some energy and as a result, the radiation reaching us from that direction will appear slightly dimmer or cooler than average, and vice-versa. Since the universe appears to be dominated by some form of dark matter that has yet to be directly observed the variations in the gravitational potential are largely determined by variations in the density of the dark matter. It is unclear what mechanism established these variations to begin with (remember that the early universe is almost featureless) but many cosmologists believe that a period of inflation may have produced the dark matter/gravitational variations a small fraction of a second after the Big Bang. The COBE satellite first detected fluctuations on these angular scales in 1992.  

      

As one moves to smaller angular scales (to the right on the graph) one starts to see the imprint of sound waves moving through the ionized hydrogen gas. Recall from above that the photons, electrons, and protons behave much like a single fluid before the epoch of last scatter so any regions where the density of protons and electrons are higher than average (a region of compression in a sound wave) will also be a region where the photon density is higher, and vice-versa. When the radiation decouples from the hydrogen gas at the time of last scatter the waves will "freeze out" because the gas has lost its ability to support waves (the photons were providing an important source of pressure or "stiffness"). Thus a region in compression at this time will appear to us as a region that is brighter or hotter than average and vice-versa.  

 

The Sonic Horizon

Most cosmologists believe that sound waves were induced in the hydrogen gas in response to the gravitational fluctuations set up shortly after the Big Bang (perhaps by the episode of inflation). Since these waves have only 380,000 years to live, they can only move for a limited distance (or oscillate a certain number of times) before freezing out. This distance is called the "sonic horizon" and sets a fundamental length scale in the early universe. The first and largest peak in the above spectrum corresponds to sound waves that were just starting their first period of compression when the freeze out occurred. (These are very low frequency sound waves!) The successive peaks correspond to higher frequency waves alternately caught in periods of rarefaction and compression at the time of last scatter. The relative heights and locations of these peaks contains signatures of the properties of the gas at the time of last scatter.  

   

Measuring the Geometry of the Universe

The dominant angular scale for microwave background fluctuations is the angle subtended (linked directly) by the sonic horizon at the surface of last scatter. This distance serves as a ruler for measuring the geometry of the universe. In a flat universe, where light will move in a straight line, this scale is roughly one degree. In our temperature fluctuation spectrum, this corresponds to l = 180 on our graph.  

 

The Angle of view or the sonic horizon ia observered to be different depending on the content of the universe

If the universe is open, without enough gravitational matter to stop expansion, photons move on more rapidly diverging paths in a negatively curved space as shown on the right. This effect would cause our cosmic ruler to appear to have a smaller angular size. Thus if the universe were open, the location of the first peak in the fluctuation spectrum would appear at smaller angular scales, as indicated by the grey line.  

If the universe were closed, with enough gravitational matter to reverse expansion, the angle would appear larger than one degree, represented by a first peak shifting to the left on our graph. A flat universe, where the force of gravitational matter is not forcing a collapse or expansion, leaves our ruler image undistorted and remains at roughly 1 degree in size (red line). 

 

Counting the Electrons in the Universe

The behavior of the early universe fluid will also depend upon the relative number densities of electrons and photons. The more electrons (and protons), the stronger their response to the gravitational potential wells. This leads to an enhancement of the first and third acoustic peaks, which are produced by the fluid falling into the potential wells and a suppression of the second acoustic peak, which is produced by the fluid moving outward. Thus, by measuring the relative heights of the peaks in the fluctuation spectrum, we should be able to determine the relative number density of protons and electrons.  

 

 

The relative hight of fluctuation peaks on the graph determine the relative number density of protons and electrons in the universe.  

            

Increasing the ratio of electrons to photons also has the effect of decreasing the sound speed of the fluid. Since the fluid moves more slowly, the secondary oscillations occur at larger angular scales. This effect shifts the location of the latter peaks in the fluctuation spectrum. 

    

Measuring the Matter Content of the Universe

The cosmic background radiation propagating from the surface of last scatter to our experiment is also affected by gravitational fluctuations along its path. Photons falling into the gravitational potential wells of clusters gain energy. They lose this energy when they climb out of the potential wells. If the universe is flat and dominated by dark matter (as opposed to dark energy), then these two effects cancel and the matter along the photon path has no net gravitational effect.  

If there is dark energy, then the depth of gravitational potential wells decay with time. Thus, a photon which falls into a deep potential well gets to climb out of a slightly shallower well. The net effect leads to a slight increase in photon energy along this path. Another photon which travels through a low density region (which produces a potential "hill") will lose energy as it gets to descend down a shallower hill than it climbed up. Because of this effect, a model with a cosmological constant will have additional fluctuations on large angular scales. Large angular scale measurements are most sensitive to variations in the gravitational potential at low red shift.  

 

Additional graph fluctuations on a large scale indicate a cosmological constant.  

      

There is a similar effect operating at high red shift (z ~ 500 - 1300): during this epoch, photons and neutrinos make a significant contribution to the total energy density of the universe. Because of this contribution, gravitational potential fluctuations also decay during period. This leads to enhanced fluctuations on angular scales smaller than approximately two degrees. This is the angular scale subtended by the horizon at z ~ 500. The larger the ratio of photon energy density to matter density, the more important role radiation plays during this epoch and the higher amplitude of temperature fluctuations. Since the FIRAS measurements of the spectrum have already determined the photon energy density, we can use the measurement of the amplitude of fluctuations on this angular scale to determine the matter energy density.  

    

Parameters of Cosmology: Microwave Background Polarization

 

top image: Picket fence as a polarizer filter, Middle: Water as a polatizing filter, Bottom: WMAP measures strenght of polarized light.  

  

WMAP measures not only temperature fluctuations in the microwave background, but also the polarization of the microwave background. Theoretical models predict that it should exist at an amplitude detectable with WMAP's sensitivities. Multiple passes of the WMAP detectors across the sky have finally detected a faint microwave background polarization signal.  

                

Scattered light, whether it is sunlight reflected off of haze, or microwave background photons that are reflected off of free electrons in the early universe is often polarized. This effect occurs because the electron-photon scattering cross-section depends upon the polarization of the incoming photon (which corresponds to the direction of the photon electric field). Because of this effect, we can probe the properties of the electrons that the photon encounters as it propagates from the surface of last scatter to our experiment.  

                

At this point in the discussion, the astute reader might object, "but wait, I thought that there were no free electrons left once the universe cooled below 3000 K". Fortunately, this is not true. Once stars start forming their radiation will ionize hydrogen, thus liberating free electrons. These free electrons will produce polarization fluctuations in the microwave background that are detectable.  

   

WMAP 5-year Results Released - March 7, 2008


Credit: NASA/WMAP Science Team
 

The cosmic microwave temperature fluctuations from the 5-year WMAP data seen over the full sky. The average temperature is 2.725 Kelvin (degrees above absolute zero; equivalent to -270 C or -455 F), and the colors represent the tiny temperature fluctuations, as in a weather map. Red regions are warmer and blue regions are colder by about 0.0002 degrees. 

 

Content of the Universe

 

WMAP measures the composition of the universe. The top chart shows a pie chart of the relative constituents today. A similar chart (bottom) shows the composition at 380,000 years old (13.7 billion years ago) when the light WMAP observes emanated. The composition varies as the universe expands: the dark matter and atoms become less dense as the universe expands, like an ordinary gas, but the photon and neutrino particles also lose energy as the universe expands, so their energy density decreases faster than the matter. They formed a larger fraction of the universe 13.7 billion years ago. It appears that the dark energy density does not decrease at all, so it now dominates the universe even though it was a tiny contributor 13.7 billion years ago.  

 

Time Line of the Universe  

 

 
A representation of the evolution of the universe over 13.7 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 380,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe.  

 

Temperature Fluctuation by Angular Size  

       

 

This graph illustrates how much the temperature fluctuates on different angular sizes in the map. Very large angles are on the left, and smaller angles are on the right. Note that there is a large first peak, illustrating a preferred spot size in the map. This means that there is a preferred length for the sound waves in the early universe, just as a guitar string length produces a specific note. The second and third peaks are the harmonic overtones of the first peak. The third overtone is now clearly captured in the new 5-year WMAP data. It helps provide evidence for neutrinos.  

 

Conclusion

The WMAP observations were a huge success for the standard model of cosmology, which describes a flat, homogenous universe dominated by dark matter (unidentified gravitating but non-luminous matter) and dark energy (a mysterious entity speeding up the expansion of the universe).  

 

Among the tens of other cosmological parameters that have been tightened by the new WMAP results are the age of the universe (13.73 ± 0.12 billion years)/ WMAP has found evidence for this so-called "cosmic neutrino background" from the early universe. Neutrinos made up a much larger part of the early universe than they do today. Microwave light seen by WMAP from when the universe was only 380,000 years old, shows that, at the time, neutrinos made up 10% of the universe, atoms 12%, dark matter 63%, photons 15%, and dark energy was negligible.  In contrast, the current universe consists of 4.6% percent atoms, 23% dark matter, 72% dark energy and less than 1 percent neutrinos. Cosmic neutrinos existed in such huge numbers they affected the universe’s early development. That, in turn, influenced the microwaves that WMAP observes. WMAP data suggest, with greater than 99.5% confidence, the existence of the cosmic neutrino background - the first time this evidence has been gleaned from the cosmic microwaves.

 

The cosmic microwave background provides an independent estimate of the number of neutrino “families” in nature: 4.4 ± 1.5. Despite having been inferred from a totally different cosmological epoch, this value agrees with constraints from Big-Bang nucleosynthesis, the first few minutes of the universe during which light nuclei were manufactured, and with precision measurements at particle accelerators which fix the number of families at three. The WMAP data also constrain the combined mass of all types of neutrino to be less than 0.61 eV.

 

The hot and dense young universe was a nuclear reactor that produced helium.  Theories based on the amount of helium seen today predict a sea of neutrinos should have been present when helium was made.  The new WMAP data agree with that prediction, along with precise measurements of neutrino properties made by Earth-bound particle colliders.

 

A second big result for the five-year data concerns the origin of stars and galaxies. Because the polarization of the cosmic background photons is affected by the presence of ionizing material, WMAP provides new insights into the end of the “cosmic dark ages” when the first generation of stars began to shine. The breakthrough derived from WMAP data is clear evidence the first stars took more than a half-billion years to create a cosmic fog. The data provide crucial new insights into the end of the "dark ages," when the first generation of stars began to shine. The glow from these stars created a thin fog of electrons in the surrounding gas that scatters microwaves, in much the same way fog scatters the beams from a car’s headlights. "We now have evidence that the creation of this fog was a drawn-out process, starting when the universe was about 400 million years old and lasting for half a billion years," said WMAP team member Joanna Dunkley of Princeton University in Princeton, N.J. “We now know that the first stars needed to come earlier than the first billion years in order to give a large enough polarization signal in the CMB, but since quasars at later times indicate that the universe was still partly neutral at around 1 billion years, the lighting up process was likely quite extended.

 

A third major finding arising from the new WMAP data places tight constraints on the astonishing burst of growth in the first trillionth of a second of the universe, called inflation, when ripples in the very fabric of space may have been created. WMAP team had measured the polarization of the background photons. This provided rare if not rigorous constraints on models of inflation — a period of enormous expansion thought to have taken place 10–35 seconds after the Big Bang, and a key component of the standard cosmological model. Some versions of the inflation theory now are eliminated. Others have picked up new support.

By measuring the incredibly weak polarization signal of the photons, the WMAP3 data were able to tighten the limits on the “spectral index” of the fluctuations, ns. This is a central parameter in inflationary models which describes the slope of the angular power spectrum once its oscillatory features have been removed: the WMAP3 data favored a “tilted” spectrum (ns < 1), which is a natural feature of simple inflation models. WMAP5 strengthens this picture: ns = 0.960 ± 0.014. Another hallmark of inflation is gravitational waves, which would have been produced by motion on the quantum scale and then blown up during inflation. “The five-year results put tighter limits on the gravitational wave amplitude,” says Gary Hinshaw of NASA’s Goddard Flight Center in Maryland, who heads the data analysis for the WMAP science team. “Now gravity waves can contribute no more than 20% to the total temperature anisotropy [corresponding to a “tensor–scalar ratio”, r = 0.2], as opposed to r = 0.3 for the three-year result.” The new combined limits on spectral index and gravitational waves rule out a swathe of inflation models.  

"The new WMAP data rule out many mainstream ideas that seek to describe the growth burst in the early universe," said WMAP principal investigator, Charles Bennett, of The Johns Hopkins University in Baltimore, MD. "It is astonishing that bold predictions of events in the first moments of the universe now can be confronted with solid measurements."  

 

 

Big Bang

 

Introduction

The Big Bang is the cosmological model of the universe that is best supported by all lines of scientific evidence and observation. As used by scientists, the term Big Bang generally refers to the idea that the universe has expanded from a primordial hot and dense initial condition at some finite time in the past, and continues to expand to this day. 

          

After Edwin Hubble discovered in 1929 that the distances to far away galaxies were generally proportional to their red shifts, this observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point. The farther away, the higher the apparent velocity. If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures, and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of the theory, but these accelerators have limited capabilities to probe into such high energy regimes. Without any evidence associated with the earliest instant of the expansion, the Big Bang theory cannot and does not provide any explanation for such an initial condition; rather, it describes and explains the general evolution of the universe since that instant. The observed abundances of the light elements throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.  

       

Fred Hoyle is credited with coining the phrase 'Big Bang' during a 1949 radio broadcast, as a derisive reference to a theory he did not subscribe to. Hoyle later helped considerably in the effort to figure out the nuclear pathway for building certain heavier elements from lighter ones. After the discovery of the cosmic microwave background radiation in 1964, and especially when its collective frequencies sketched out a blackbody curve, most scientists were fairly convinced by the evidence that some Big Bang scenario must have occurred.  

       

The Big Bang theory developed from observations of the structure of the universe and from theoretical considerations. In 1912 Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way. Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einstein's equations of general relativity, showing that the universe might be expanding in contrast to the static universe model advocated by Einstein. In 1924, Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, predicted that the recession of the nebulae was due to the expansion of the universe.  

       

In 1931 Lemaître went further and suggested that the evident expansion in forward time required that the universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the universe into a single point, a "primeval atom", at a point in time before which time and space did not exist. As such, at this point, the fabric of time and space had not yet come into existence. This perhaps echoed previous speculations about the cosmic egg origin of the universe.  

       

Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2,500 mm) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose red shifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recession velocity now known as Hubble's law. Lemaître had already shown that this was expected, given the Cosmological Principle.    

       

During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model, the oscillatory universe (originally suggested by Friedmann, but advocated by Einstein and Richard Tolman) and Fritz Zwicky's tired light hypothesis.  

       

After World War II, two distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the universe seemed to expand. In this model, the universe is roughly the same at any point in time. The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background radiation. Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it derisively as "this big bang idea" during a BBC Radio broadcast in March 1949. For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor the latter. The discovery and confirmation of the cosmic microwave background radiation in 1964 secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the universe at earlier and earlier times, and reconciling observations with the basic theory.  

       

Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances in telescope technology as well as the analysis of copious data from satellites such as COBE, the Hubble Space Telescope and WMAP. Cosmologists now have fairly precise measurement of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.   

   

     

Timeline of the Big Bang  

   

     

This timeline of the Big Bang describes the events according to the scientific theory of the Big Bang, using the cosmological time parameter of comoving coordinates. Observations suggest that the universe as we know it began around 13.7 billion years ago. Since then, the evolution of the universe has passed through three phases. The very early universe, which is still poorly understood, was the split second in which the universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth. Therefore, while the basic features of this epoch have been worked out in the big bang theory, the details are largely based on educated guesses. Following this period, in the early universe, the evolution of the universe proceeded according to known high energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted. Then matter started to aggregate into the first stars and ultimately galaxies, quasars, clusters of galaxies and superclusters formed. There are several alternative theories about the ultimate fate of the universe.  

             

The very early universe

All ideas concerning the very early universe (cosmogony) are speculative. As of today no accelerator experiments probe energies of sufficient magnitude to provide any insight into the period. All proposed scenarios differ radically, some examples being: the Hartle-Hawking initial state, string landscape, brane inflation, string gas cosmology, and the ekpyrotic universe. Some of these are mutually compatible, while others are not.  

       

Augustinian era - Before the Big Bang

In 1952, George Gamow, one of the founding fathers of Big Bang cosmology, proposed that the period before the Big Bang be called the Augustinian era, after the philosopher Saint Augustine, who believed time was solely a property of the God-created Universe, so that there was no time prior to the creation of the universe. The phrase "Augustinian Era" is meant to convey the idea that the known laws of physics break down in a gravitational singularity of infinite density at the time zero of the Big Bang, so that according to Albert Einstein's general theory of relativity there were no times prior to that point. However, physicists believe that general relativity becomes incompatible with quantum mechanics at the Planck scale, so that the predictions of general relativity cannot be trusted before the Planck era when energies and temperatures reached the Planck scale, and that we need a theory of quantum gravitation before we can say anything about times before the Planck era  

       

The Planck epoch - Up to 10–43 seconds after the Big Bang

If supersymmetry is correct, then during this time the four fundamental forces — electromagnetism, weak nuclear force, strong nuclear force and gravitation — all have the same strength, so they are possibly unified into one fundamental force. Little is known about this epoch, although different theories propose different scenarios. General relativity proposes a gravitational singularity before this time, but under these conditions the theory is expected to break down due to quantum effects. Physicists hope that proposed theories of quantum gravitation, such as string theory and loop quantum gravity, will eventually lead to a better understanding of this epoch.  

       

The grand unification epoch - Between 10–43 seconds and 10–36 seconds after the Big Bang

As the universe expands and cools from the Planck epoch, gravitation begins to separate from the fundamental gauge interactions: electromagnetism and the strong and weak nuclear forces. Physics at this scale may be described by a grand unified theory in which the gauge group of the Standard Model is embedded in a much larger group, which is broken to produce the observed forces of nature. Eventually, the grand unification is broken as the strong nuclear force separates from the electroweak force. This occurs as soon as inflation does. According to some theories, this should produce magnetic monopoles. Unification of the strong and electroweak forces, means that the only particle expected at this time is the Higgs boson [citation needed].  

       

The electroweak epoch - Between 10–36 seconds and 10–12 seconds after the Big Bang

The temperature of the universe is low enough (1028K) to separate the strong force from the electroweak force (the name for the unified forces of electromagnetism and the weak interaction). This phase transition triggers a period of exponential expansion known as cosmic inflation. After inflation ends, particle interactions are still energetic enough to create large numbers of exotic particles, including W and Z bosons and Higgs bosons.  

       

The inflationary epoch - Between 10–36 seconds and 10–32 seconds after the Big Bang

The temperature, and therefore the time, at which cosmic inflation occurs is not known for certain. During inflation, the universe is flattened (its spatial curvature is critical) and the universe enters a homogeneous and isotropic rapidly expanding phase in which the seeds of structure formation are laid down in the form of a primordial spectrum of nearly-scale-invariant fluctuations. Some energy from photons becomes virtual quarks and hyperons, but these particles decay quickly. One scenario suggests that prior to cosmic inflation, the universe was cold and empty, and the immense heat and energy associated with the early stages of the big bang was created through the phase change associated with the end of inflation.          

Reheating

During reheating, the exponential expansion that occurred during inflation ceases and the potential energy of the inflation field decays into a hot, relativistic plasma of particles. If grand unification is a feature of our universe, then cosmic inflation must occur during or after the grand unification symmetry is broken, otherwise magnetic monopoles would be seen in the visible universe. At this point, the universe is dominated by radiation; quarks, electrons and neutrinos form.  

       

Baryogenesis

No known physics can explain the fact that there are so many more baryons in the universe than antibaryons. In order for this to be explained, the Sakharov conditions must be met at some time after inflation. There are hints that this is possible in known physics and from studying grand unified theories, but the full picture is not known.  

       

The early universe  

After cosmic inflation ends, the universe is filled with a quark-gluon plasma. From this point onwards the physics of the early universe is better understood, and less speculative.  

       

Supersymmetry breaking

If supersymmetry is a property of our universe, then it must be broken at an energy as low as 1 TeV, the electroweak symmetry scale. The masses of particles and their superpartners would then no longer be equal, which could explain why no superpartners of known particles have ever been observed.  

       

The quark epoch - Between 10–12 seconds and 10–6 seconds after the Big Bang

In electroweak symmetry breaking, at the end of the electroweak epoch, all the fundamental particles are believed to acquire a mass via the Higgs mechanism in which the Higgs boson acquires a vacuum expectation value. The fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction have now taken their present forms, but the temperature of the universe is still too high to allow quarks to bind together to form hadrons.  

       

The hadron epoch - Between 10–6 seconds and 1 second after the Big Bang

The quark-gluon plasma which composes the universe cools until hadrons, including baryons such as protons and neutrons, can form. At approximately 1 second after the Big Bang neutrinos decouple and begin traveling freely through space. This cosmic neutrino background, while unlikely to ever be observed in detail, is analogous to the cosmic microwave background that was emitted much later. (See above regarding the quark-gluon plasma, under the String Theory epoch).  

       

The lepton epoch - Between 1 second and 3 minutes after the Big Bang

The majority of hadrons and anti-hadrons annihilate each other at the end of the hadron epoch, leaving leptons and anti-leptons dominating the mass of the universe. Approximately 3 seconds after the Big Bang the temperature of the universe falls to the point where new lepton/anti-lepton pairs are no longer created and most leptons and anti-leptons are eliminated in annihilation reactions, leaving a small residue of leptons.  

       

The photon epoch - Between 3 minutes and 380,000 years after the Big Bang

After most leptons and anti-leptons are annihilated at the end of the lepton epoch the energy of the universe is dominated by photons. These photons are still interacting frequently with charged protons, electrons and (eventually) nuclei, and continue to do so for the next 300,000 years.  

       

Nucleosynthesis - Between 3 minutes and 20 minutes after the Big Bang

During the photon epoch the temperature of the universe falls to the point where atomic nuclei can begin to form. Protons (hydrogen ions) and neutrons begin to combine into atomic nuclei in the process of nuclear fusion. However, nucleosynthesis only lasts for about seventeen minutes, after which time the temperature and density of the universe has fallen to the point where nuclear fusion cannot continue. At this time, there is about three times more hydrogen than helium-4 (by mass) and only trace quantities of other nuclei.  

       

Matter domination: 70,000 years

At this time, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) are equal. The Jeans length, which determines the smallest structures that can form (due to competition between gravitational attraction and pressure effects), begins to fall and perturbations, instead of being wiped out by radiation free-streaming, can begin to grow in amplitude.          

Recombination: 240,000–310,000 years

Cosmic microwave background WMAP data shows the microwave background radiation variations throughout the Universe from our perspective, though the actual variations are much smoother than the diagram suggests. Hydrogen and helium atoms begin to form and the density of the universe falls. This is thought to have occurred somewhere between 240,000 and 310,000 years after the Big Bang. Hydrogen and helium are at the beginning ionized, i.e. no electrons are bounded to the nuclei which are therefore electrically charged (+1 and +2 respectively). As the universe cools down, the electrons get captured by the ions making them neutral. This process is relatively fast (actually faster for the helium than for the hydrogen) and is known as recombination. At the end of recombination, most of the atoms in the universe are neutral, therefore the photons can now travel freely: the universe has become transparent. The photons emitted right after the recombination, that can therefore travel undisturbed, are those that we see in the cosmic microwave background (CMB) radiation. Therefore the CMB is a picture of the universe at the end of this epoch.  

       

Dark ages

Before decoupling occurs most of the photons in the universe are interacting with electrons and protons in the photon-baryon fluid. The universe is opaque or "foggy" as a result. There is light but not light we could observe through telescopes. The baryonic matter in the universe consisted of ionized plasma, and it only became neutral when it gained free electrons during "recombination," thereby releasing the photons creating the CMB. When the photons were released (or decoupled) the universe became transparent. At this point the only radiation emitted is the 21 cm spin line of neutral hydrogen. There is currently an observational effort underway to detect this faint radiation, as it is in principle an even more powerful tool than the cosmic microwave background for studying the early universe .  

       

Structure formation

 

The Hubble Ultra Deep Fields often showcase galaxies from an ancient era that tell us what the early Stelliferous Age was like.

The Hubble Ultra Deep Fields often showcase galaxies from an ancient era that tell us what the early Stelliferous Age was like.  

     

Another Hubble image shows an infant galaxy forming nearby, which means this happened very recently on the cosmological timescale.  This is evidence that the Universe is not quite finished with galaxy formation yet.

       

Another Hubble image shows an infant galaxy forming nearby, which means this happened very recently on the cosmological timescale. This is evidence that the Universe is not quite finished with galaxy formation yet.  

        

Structure formation in the big bang model proceeds hierarchically, with smaller structures forming before larger ones. The first structures to form are quasars, which are thought to be bright, early active galaxies, and population III stars. Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory: that is, all structures could be understood as small deviations from a perfect homogeneous universe. This is computationally relatively easy to study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, N-body simulations with billions of particles.  

       

Reionization: 150 million to 1 billion years

The first quasars form from gravitational collapse. The intense radiation they emit re-ionizes the surrounding universe. From this point on, most of the universe is composed of plasma.  

       

Formation of stars

The first stars, most likely Population III stars, form and start the process of turning the light elements that were formed in the Big Bang (hydrogen, helium and lithium) into heavier elements. However, as of yet there have been no observed Population III stars which leaves their formation a mystery.

       

Formation of galaxies

Large volumes of matter collapse to form a galaxy. Population II stars are formed early on in this process, with Population I stars formed later. Johannes Schedler's project has identified a quasar CFHQS 1641+3755 at 12.7 billion light-years away, when the Universe was just 7 percent of its present age. On July 11, 2007, using the 10 meter Keck II telescope on Mauna Kea, Richard Ellis of the California Institute of Technology at Pasadena and his team found six star forming galaxies about 13.2 billion light years away and therefore created when the universe was only 500 million years old . Only about 10 of these really early objects are currently known. The Hubble Ultra Deep Field shows a number of small galaxies merging to form larger ones, at 13 billion light years, when the Universe was only 5% its current age. Based upon the emerging science of nucleocosmochronology, the Galactic thin disk of the Milky Way is estimated to have been formed 8.3 ± 1.8 billion years ago.  

       

Formation of groups, clusters and superclusters

Gravitational attraction pulls galaxies towards each other to form groups, clusters and superclusters.  

       

Formation of our solar system: 8 billion years

Finally, objects on the scale of our solar system form. Our sun is a late-generation star, incorporating the debris from many generations of earlier stars, and formed roughly 5 billion years ago, or roughly 8 to 9 billion years after the big bang.  

       

Today: 13.7 billion years

The best current data estimate the age of the universe today as 13.7 billion years since the big bang. Since the expansion of the universe appears to be accelerating, superclusters are likely to be the largest structures that will ever form in the universe. The present accelerated expansion prevents any more inflationary structures entering the horizon and prevents new gravitationally bound structures from forming.   

   

       

Graphical timeline of the Big Bang  

    

     

This timeline of the Big Bang shows the sequence of events as predicted by the Big Bang theory, from the beginning of time to the end of the Dark Ages. It is a logarithmic scale that shows 10 * log10 second instead of second. For example, one microsecond is 10 * log100.000001 = 10 * ( − 6) = − 60. To convert -30 read on the scale to second calculate 10 − 30 / 10 = 10 − 3 = 0.001 second = one millisecond. On a logarithmic time scale a step lasts ten times longer than the previous step.

Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch. The early hot, dense phase is itself referred to as "the Big Bang", and is considered the "birth" of our universe. Based on measurements of the expansion using Type Ia supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a calculated age of 13.73 ± 0.12 billion years. The agreement of these three independent measurements strongly supports the CDM model that describes in detail the contents of the universe.  

       

The earliest phases of the Big Bang are subject to much speculation. In the most common models, the universe was filled homogeneously and isotropically with an incredibly high energy density, huge temperatures and pressures, and was very rapidly expanding and cooling. Approximately 1035 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially. After inflation stopped, the universe consisted of a quark-gluon plasma, as well as all other elementary particles. Temperatures were so high that the random motions of particles were at relativistic speeds, and particle-antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and anti-leptons—of the order of 1 part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.  

       

The universe continued to grow in size and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form. After about 1011 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. At about 106 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton-antiproton pairs (similarly for neutrons-antineutrons), so a mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons, and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos).  

       

A few minutes into the expansion, when the temperature was about a billion (one thousand million; 109; SI prefix giga) Kelvin and the density was about that of air, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation.

        

The Hubble Ultra Deep Field showcases galaxies from an ancient era when the universe was younger, denser, and warmer according to the Big Bang theory.

Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The three possible types of matter are known as cold dark matter, hot dark matter and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the universe is cold dark matter. The other two types of matter make up less than 18% of the matter in the universe.  

       

Independent lines of evidence from Type Ia supernovae and the CMB imply the universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 72% of the total energy density of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity had the upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.  

       

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the CDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity. As noted above, there is no well-supported model describing the action prior to 1015 seconds or so. Apparently a new unified theory of quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the universe is currently one of the greatest unsolved problems in physics. 

   

 

Ultimate fate of the universe

 

The future according to the Big Bang theory

As with interpretations of what happened in the very early universe, advances in fundamental physics are required before it will be possible to know the ultimate fate of the universe with any certainty. 

 

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state that was similar to that in which it started—a Big Crunch. Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as all the interstellar gas in each galaxy is consumed; stars would burn out leaving white dwarfs, neutron stars, and black holes. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would asymptotically approach absolute zero—a Big Freeze. Moreover, if the proton were unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.  

Modern observations of accelerated expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the universe expands and cools. Other explanations of dark energy—so-called phantom energy theories—suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei and matter itself will be torn apart [dubious – discuss] by the ever-increasing expansion in a so-called Big Rip.

 

Below are some of the main possibilities.  

       

Big freeze: 1014 years and beyond

This scenario is generally considered to be the most likely, as it occurs if the universe continues expanding as it has been. Over a time scale on the order of 1014 years or less, existing stars burn out, stars cease to be created, and the universe goes dark. Over a much longer time scale in the eras following this, the galaxy evaporates as the stellar remnants comprising it escape into space, and black holes evaporate via Hawking radiation. In some grand unified theories, proton decay will convert the remaining interstellar gas and stellar remnants into leptons (such as positrons and electrons) and photons. Some positrons and electrons will then recombine into photons. In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known however whether it eventually achieves thermodynamic equilibrium..

 

Big crunch: 100+ billion years

If the energy density of dark energy were negative or the universe were closed, then it would be possible that the expansion of the universe would reverse and the universe would contract towards a hot, dense state. This is often proposed as part of an oscillatory universe scenario, such as the cyclic model. Current observations suggest that this model of the universe is unlikely to be correct, and the expansion will continue.  

       

Big rip: 200+ billion years

This scenario is possible only if the energy density of dark energy actually increases without limit over time. Such dark energy is called phantom energy and is unlike any known kind of energy. In this case, the expansion rate of the universe will increase without limit. Gravitationally bound systems, such as clusters of galaxies, galaxies, and ultimately the solar system will be torn apart. Eventually the expansion will be so rapid as to overcome the electromagnetic forces holding molecules and atoms together. Finally even atomic nuclei will be torn apart and the universe as we know it will end in an unusual kind of gravitational singularity. In other words, the universe will expand so much that the electromagnetic force holding things together will fall to this expansion, making things fall apart.  

 

Vacuum metastability event

If our universe is in a very long-lived false vacuum, it is possible that the universe will tunnel into a lower energy state. If this happens, all structures will be destroyed instantaneously, without any forewarning.

 

Big bang theory assumptions  

     

     

The Big Bang theory depends on two major assumptions: the universality of physical laws, and the Cosmological Principle. The cosmological principle states that on large scales the universe is homogeneous and isotropic.  

       

These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the universe is of order 105. Also, General Relativity has passed stringent tests on the scale of the solar system and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.  

       

If the large-scale universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler Copernican Principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 105 via observations of the CMB. The universe has been measured to be homogeneous on the largest scales at the 10% level.

       

FLRW metric - Friedmann-Lemaître-Robertson-Walker metric and Metric expansion of space

General relativity describes space-time by a metric, which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, themselves are specified using a coordinate chart or "grid" that is laid down over all space-time. The cosmological principle implies that the metric should be homogeneous and isotropic on large scales, which uniquely singles out the Friedmann-Lemaître-Robertson-Walker metric (FLRW metric). This metric contains a scale factor, which describes how the size of the universe changes with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates. In this coordinate system, the grid expands along with the universe, and objects that are moving only due to the expansion of the universe remain at fixed points on the grid. While their coordinate distance (comoving distance) remains constant, the physical distance between two such comoving points expands proportionally with the scale factor of the universe.  

       

The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distance between two comoving points. Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our universe only on large scales—local concentrations of matter such as our galaxy are gravitationally bound and as such do not experience the large-scale expansion of space.

 

Horizons

An important feature of the Big Bang space-time is the presence of horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe. Our understanding of the universe back to very early times suggests that there was a past horizon, though in practice our view is limited by the opacity of the universe at early times. If the expansion of the universe continues to accelerate, there is a future horizon as well.

 

Observational evidence

The earliest and most direct kinds of observational evidence are the Hubble-type expansion seen in the redshifts of galaxies, the detailed measurements of the cosmic microwave background, and the abundance of light elements (see Big Bang nucleosynthesis). These are sometimes called the three pillars of the big bang theory. Many other lines of evidence now support the picture, notably various properties of the large-scale structure of the cosmos which are predicted to occur due to gravitational growth of structure in the standard Big Bang theory.  

        

Hubble's law and the expansion of space

Observations of distant galaxies and quasars show that these objects are redshifted—the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission lines or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed:  

      

 v = H_0 D \,

      

where v is the recessional velocity of the galaxy or other distant object, D is the comoving proper distance to the object and Ho is Hubble's constant, measured to be 70.1 ± 1.3 km/s/Mpc by the WMAP probe.  

       

Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable given the Copernican Principle—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Alexander Friedman in 1922 and Georges Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson and Walker.  

       

The theory requires the relation v = HD to hold at all times, where D is the proper distance, v = dD / dt, and v, H, and D all vary as the universe expands (hence we write H0 to denote the present-day Hubble "constant"). For distances much smaller than the size of the observable universe, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity v. However, the redshift is not a true Doppler shift, but rather the result of the expansion of the universe between the time the light was emitted and the time that it was detected. That space is undergoing metric expansion is shown by direct observational evidence of the Cosmological Principle and the Copernican Principle, which together with Hubble's law have no other explanation. Astronomical redshifts are extremely isotropic and homogenous, supporting the Cosmological Principle that the universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.  

       

Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican Principle, that the Earth is not in a central position, on a cosmological scale. Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.  

Cosmic microwave background radiation

During the first few days of the universe, the universe was in full thermal equilibrium, with photons being continually emitted and absorbed, giving the radiation a blackbody spectrum. As the universe expanded, it cooled to a temperature at which photons could no longer be created or destroyed. The temperature was still high enough for electrons and nuclei to remain unbound, however, and photons were constantly "reflected" from these free electrons through a process called Thomson scattering. Because of this repeated scattering, the early universe was opaque to light.  

       

When the temperature fell to a few thousand Kelvin, electrons and nuclei began to combine to form atoms, a process known as recombination. Since photons scatter infrequently from neutral atoms, radiation decoupled from matter when nearly all the electrons had recombined, at the epoch of last scattering, 379,000 years after the Big Bang. These photons make up the CMB that is observed today, and the observed pattern of fluctuations in the CMB is a direct picture of the universe at this early epoch. The energy of photons was subsequently redshifted by the expansion of the universe, which preserved the blackbody spectrum but caused its temperature to fall, meaning that the photons now fall into the microwave region of the electromagnetic spectrum. The radiation is thought to be observable at every point in the universe, and comes from all directions with (almost) the same intensity.  

       

In 1964, Arno Penzias and Robert Wilson accidentally discovered the cosmic background radiation while conducting diagnostic observations using a new microwave receiver owned by Bell Laboratories. Their discovery provided substantial confirmation of the general CMB predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K—and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded a Nobel Prize for their discovery.  

       

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the Big Bang's predictions regarding the CMB. COBE found a residual temperature of 2.726 K and in 1992 detected for the first time the fluctuations (anisotropies) in the CMB, at a level of about one part in 105. John C. Mather and George Smoot were awarded Nobels for their leadership in this work. During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably BOOMERanG, found the universe to be almost spatially flat by measuring the typical angular size (the size on the sky) of the anisotropies.  

       

In early 2003, the first results of the Wilkinson Microwave Anisotropy satellite (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. This satellite also disproved several specific cosmic inflation models, but the results were consistent with the inflation theory in general, it confirms too that a sea of cosmic neutrinos permeates the universe, a clear evidence that the first stars took more than a half-billion years to create a cosmic fog. Another satellite like it will be launched within the next few years, the Planck Surveyor, which will provide even more accurate measurements of the CMB anisotropies. Many other ground- and balloon-based experiments are also currently running; see Cosmic microwave background experiments.  

       

The background radiation is exceptionally smooth, which presented a problem in that conventional expansion would mean that photons coming from opposite directions in the sky were coming from regions that had never been in contact with each other. The leading explanation for this far reaching equilibrium is that the universe had a brief period of rapid exponential expansion, called inflation. This would have the effect of driving apart regions that had been in equilibrium, so that all the observable universe was from the same equilibrated region.

 

Abundance of primordial elements

Using the Big Bang model it is possible to calculate the concentration of helium-4, helium-3, deuterium and lithium-7 in the universe as ratios to the amount of ordinary hydrogen, H. All the abundances depend on a single parameter, the ratio of photons to baryons, which itself can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are  about 0.25 for 4He/H, about 10−3 for ²H/H, about 10−4 for ³He/H and about 10−9 for 7Li/H.  

       

The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for 4He, and a factor of two off for 7Li; in the latter two cases there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium. Indeed there is no obvious reason outside of the Big Bang that, for example, the young universe (i.e., before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than ³He, and in constant ratios, too.  

Galactic evolution and distribution

 

This panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The galaxies are color coded by redshift.

This panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The galaxies are color coded by redshift.  

       

Detailed observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as galaxy clusters and superclusters. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions and larger structures agree well with Big Bang simulations of the formation of structure in the universe and are helping to complete details of the theory.

 

Other lines of evidence

After some controversy, the age of universe as estimated from the Hubble expansion and the CMB is now in good agreement with (i.e., slightly larger than) the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars.  

The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of temperature-sensitive emission lines in gas clouds at high redshift. This prediction also implies that the amplitude of the Sunyaev-Zel'dovich effect in clusters of galaxies does not depend directly on redshift; this seems to be roughly true, but unfortunately the amplitude does depend on cluster properties which do change substantially over cosmic time, so a precise test is impossible.

 

Features, issues and problems 

  

     

While very few researchers now doubt the Big Bang occurred, the scientific community was once divided between supporters of the Big Bang and those of alternative cosmological models. Throughout the historical development of the subject, problems with the Big Bang theory were posed in the context of a scientific controversy regarding which model could best describe the cosmological observations (see the history section above). With the overwhelming consensus in the community today supporting the Big Bang model, many of these problems are remembered as being mainly of historical interest; the solutions to them have been obtained either through modifications to the theory or as the result of better observations. Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as it is anticipated that they can be solved through further refinements of the theory.  

      

The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, the formation of galaxies—are derived from many independent observations including abundance of light elements, the cosmic microwave background, large scale structure and Type Ia supernovae, and can hardly be doubted as important and real features of our universe. Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark energy and dark matter are considered the most secure, while inflation and baryogenesis remain speculative: they provide satisfying explanations for important features of the early universe, but could be replaced by alternative ideas without affecting the rest of the theory. Explanations for such phenomena remain at the frontiers of inquiry in physics.

 

Horizon problem

The horizon problem results from the premise that information cannot travel faster than light. In a universe of finite age, this sets a limit—the particle horizon—on the separation of any two regions of space that are in causal contact. The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause these regions to have the same temperature.  

A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.  

Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the universe. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB.

 

Flatness/oldness problem

The overall geometry of the universe is determined by whether the Omega cosmological parameter is less than, equal to or greater than 1. From top to bottom: a closed universe with positive curvature, a hyperbolic universe with negative curvature and a flat universe with zero curvature.

The overall geometry of the universe is determined by whether the Omega cosmological parameter is less than, equal to or greater than 1. From top to bottom: a closed universe with positive curvature, a hyperbolic universe with negative curvature and a flat universe with zero curvature.  

       

        

 

The flatness problem (also known as the oldness problem) is an observational problem associated with a Friedmann-Lemaître-Robertson-Walker metric. The universe may have positive, negative or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density, positive if greater, and zero at the critical density, in which case space is said to be flat. The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat. Given that a natural timescale for departure from flatness might be the Planck time, 10-43 seconds, the fact that the universe has reached neither a Heat Death nor a Big Crunch after billions of years requires some explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the universe must have been within one part in 1014 of the critical density, or it would not exist as it does today.  

A resolution to this problem is offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that its curvature would have been smoothed out. Thus, it is believed that inflation drove the universe to a very nearly spatially flat state, with almost exactly the critical density.

 

Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted topological defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that searches have never found any monopoles. This problem is also resolved by cosmic inflation, which removes all point defects from the observable universe in the same way that it drives the geometry to flatness.

A resolution to the horizon, flatness, and magnetic monopole problems alternative to cosmic inflation is offered by the Weyl curvature hypothesis.

 

Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter. It is generally assumed that when the universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and anti-baryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. An unknown process called "baryogenesis" created the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the universe depart from thermodynamic equilibrium. All these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry.

 

Globular cluster age

In the mid-1990s, observations of globular clusters appeared to be inconsistent with the Big Bang. Computer simulations that matched the observations of the stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13.7-billion-year age of the universe. This issue was generally resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. There still remain some questions as to how accurately the ages of the clusters are measured, but it is clear that these objects are some of the oldest in the universe.

 

Dark matter

A pie chart indicating the proportional composition of different energy-density components of the universe, according to the best ?CDM model fits. Roughly ninety-five percent is in the exotic forms of dark matter and dark energy

A pie chart indicating the proportional composition of different energy-density components of the universe, according to the best ΛCDM model fits. Roughly ninety-five percent is in the exotic forms of dark matter and dark energy  

       

During the 1970s and 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far lumpier and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now indicated by numerous observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters.  

The evidence for dark matter comes from its gravitational influence on other matter, and no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.

 

Dark energy

Measurements of the redshift–magnitude relation for type Ia supernovae have revealed that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy". Dark energy is indicated by several other lines of evidence. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. Since dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy is also required by two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses, and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.  

       

Negative pressure is a property of vacuum energy, but the exact nature of dark energy remains one of the great mysteries of the Big Bang. Possible candidates include a cosmological constant and quintessence. Results from the WMAP team in 2008, which combined data from the CMB and other sources, indicate that the universe today is 72% dark energy, 23% dark matter, 4.6% regular matter and less than 1% of neutrinos. The energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.  

       

In the ΛCDM, the best current model of the Big Bang, dark energy is explained by the presence of a cosmological constant in the general theory of relativity. However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about quantum gravity. Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research.

  

 

Speculative physics beyond the Big Bang 

     

    

While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the universe's history. The Penrose-Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time. However, these theorems assume that general relativity is correct, but general relativity must break down before the universe reaches the Planck temperature, and a correct treatment of quantum gravity may avoid the singularity.  

       

There may also be parts of the universe well beyond what can be observed in principle. If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.  

       

Some proposals, each of which entails untested hypotheses, are:

  • models including the Hartle-Hawking no-boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.

  • brane cosmology models in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically.

  • chaotic inflation, in which inflation events start here and there in a random quantum-gravity foam, each leading to a bubble universe expanding from its own big bang.

Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or multiverse, and not the literal beginning.

 

 

 

 

 

END