The Big Bang Theory is a cosmological model of the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from an initial state of very high density and high temperature and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, large-scale structure of the universe and the Hubble’s law.
Extrapolation of the expansion of the universe backward in time using general relativity yields an infinite density and temperature at a finite time in the past. This irregular behavior is known as gravitational singularity.
Initial Phases of the Big Bang
The earliest phases of the Big Bang are subject to much speculation since astronomical data about them are not available. In the most common models, the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures and was very rapidly expanding and cooling.
The Big Bang
When we work backward to the conditions in the early universe, we find that there must have been an era in which the material was very hot and dense. This hot dense era is called the big bang. The debris of the big bang is all around us. Just as the early universe was filled with hot matter, it was also filled with hot radiation. The radiation and matter stayed in contact as long as the radiation could scatter off the charged particles.
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Cosmic Microwave Background Radiation
When the temperature of the universe fell below 3000 K, the electrons and protons combined to form neutral atoms, and the universe became transparent. The radiation and matter decoupled. The radiation is still everywhere in the universe traveling in all directions. It is called cosmic background radiation.
As the universe expanded, the cosmological redshift increased the wavelengths of all of the photons, and the effect on the background radiation was to produce ever cooler blackbodies. The current temperature is about 2.7 K. Following its accidental discovery, the background radiation has been studied extensively. Studies of the spectrum were made difficult by the Earth’s atmosphere.
Radio observations from the ground, some balloon and rocket observations, and some indirect measurements involving interstellar CN were all employed to study the radiation until the Cosmic Background Explorer Satellite was launched. COBE provided definitive evidence that the spectrum is indeed that of a blackbody. These studies also showed that the radiation is very isotropic.
There is a dipole anisotropy, due to the Earth’s motion. COBE revealed very low-level fluctuations in intensity all over the sky. It is believed that these fluctuations are a snapshot of the small density enhancements in the early universe that gave rise to the clusters of galaxies that we see today. Apart from these fluctuations, the radiation is actually too smooth.
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The Causality Problem
The degree of smoothness that we see would suggest that points that were farther than 1,00,000 light-years apart would have had to communicate with each other in the first 1,00,000 years. This is called the causality problem. One of the great successes of big-bang cosmologies is that they can explain the abundance of light elements.
For approximately the first three minutes the universe was hot enough for nuclear reactions to take place. Those reactions produced essentially all the helium and deuterium that we see in the universe today (though deuterium is destroyed and helium is produced in stars). The abundance of deuterium is particularly sensitive to the density of the material in nuclear reactions.
The denser the material, the less deuterium there is. The density of material in the first three minutes can be related to the current density and can tell us if nuclear reacting material (protons and neutrons) can close the universe. The best estimate is that this material is only about 5% of what is needed to reach the critical density.
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When the universe was less than one second old, the temperature was so high that matter was stripped down into its fundamental components, the elementary particles.
Applying particle physics to our study of the early universe has helped solve some cosmological problems. The excess of matter over antimatter, about one part in ten billion, arose from a slight asymmetry in certain weak interactions.
This also explains why there are approximately ten billion photons for every baryon in the universe. The GUTs also suggest that, in the first fraction of a second, the universe went through a rapid expansion, or inflation, in which the scale factor increased by about 1035 almost instantaneously. This might explain why the universe appears to be flat and so isotropic. It might even explain how galaxy formation started.
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The Hubble’s Law
The earliest and most direct observational evidence of the validity of the theory is the expansion of the universe according to Hubble’s law (as indicated by the redshifts of galaxies), the discovery, and measurement of the cosmic microwave background, and the relative abundances of light elements produced by BBN. More recent evidence includes observations of galaxy formation and evolution and the distribution of cosmic structures. These are sometimes called the “four pillars” of the Big Bang theory.
Observations of distant galaxies and quasars show that these objects are redshifted: the light emitted from them has been shifted to longer wavelengths. v = H0D where v is the recessional velocity of the galaxy or other distant object, D is the comoving distance to the object, and H0 is Hubble’s constant, measured to be 70.4+1.3-1.4 km/s/Mpc by the WMAP.
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This universal expansion was predicted from general relativity by Friedmann in 1922and 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.
In 1964, Arno Penzias and Robert Wilson discovered cosmic background radiation, an omnidirectional signal in the microwave band. Their discovery provided substantial confirmation of the big-bang predictions by Alpher, Herman, and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a blackbody spectrum in all directions.
This spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded the 1978 Nobel Prize in Physics.
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Abundances of the 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. The relative abundances depend on a single parameter, the ratio of photons to baryons. This value 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 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.
Detailed observations of the morphology and distribution of galaxies and quasars are in agreement with the current state of the Big Bang theory.
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