The Big Bang is the expansion of the Universe from an extremely hot and dense state that started about 13.7 billion ago.
Einstein and others
Albert Einstein had famously missed predicting it when he worked out a cosmology with his general-relativity equations in 1919. However, he thought that the Universe has to be static, neither expanding nor contracting, and to make that happen, he introduced a "cosmological constant". This may be interpreted as a material with a very high negative pressure:
P = - (mass density) * c2
In the 1920's and 1930's, Alexander Friedmann, Georges Lemaître, Howard Percy Robertson, and Arthur Geoffrey Walker worked out how the Universe's expansion fits in with Einstein's equations, and negative-pressure material has reappeared as "dark energy" and in the "inflationary phase". It also reappeared in Fred Hoyle's "steady state" alternative to the Big Bang, but that was eventually discredited.
Redshifts of Galaxies. This was discovered by Edwin Hubble in the 1920's; in 1929, he noticed that they increase linearly with galaxy distance, indicating that the Universe is uniformly expanding. At high redshifts, this relation gets modified by relativistic effects, but at low redshifts, it is a good approximation. Mathematically, it is
v = H*r
v = velocity, r = distance, H = Hubble constant. Redshift = v/c to lowest order.
Hubble himself found a value of H = 500 km/s/megaparsec, but revisions in extragalactic-distance calibration have forced it downward. Recent values cluster around 70 km/s/Mpc.
The Cosmic Microwave Background. The Universe cools as it expands, and at about 380,000 years, its baryonic material in it reached a temperature of about 3000 K. It was then too cold to keep electrons separate form the hydrogen and helium nuclei. The electrons then went into orbit, making atoms, and thus making the Universe much more transparent. Photons could then travel essentially unimpeded, though the later Universe expansion redshifted them to a temperature of about 2.7 K, into the microwave part of the electromagnetic spectrum.
Its existence was first proposed by Ralph Alpher and Robert Herman in 1948, with a temperature of 5 K. It was discovered by Arno Penzias and Robert Woodrow Wilson in 1965. Since then, it has been measured to around 1 part in a million. The largest part of its variation is due to the Earth's motion relative to it, about 371 km/s to the constellation Leo. But about 7 parts per million of variation are due to primordial density fluctuations that had gotten frozen into the Universe.
Primordial Nucleosynthesis. It was first worked out by Ralph Alpher, Hans Bethe, and George Gamow ("alpha beta gamma") in 1948. From 3 to 20 minutes in the Universe's expansion, protons and neutrons from earlier in that expansion combined to make light nuclei, mostly helium-4. Many protons were left over, and the Universe's baryonic mass became about 75% hydrogen-1 and 25% helium-4 by mass, with much smaller proportions of hydrogen-2, helium-3, and lithium.
Carbon and more massive nuclei could not have been produced in any significant quantity in the early Universe. However, they have been produced in the interiors of massive stars and then spewed out in stellar winds and explosions. There is abundant evidence for this stellar nucleosynthesis, including the oldest stars having the smallest quantities of heavy elements.
Primordial Fluctuations. These was at first a problem with the theory behind the Big Bang, because the Universe's expansion makes more and more of them visible. This means that it would not have been possible to communicate across the size of some fluctuation in the early Universe. The Universe's overall shape not only has this "horizon problem", but also the "flatness problem". If one goes back to the early Universe, the expansion velocity was just enough to balance the gravitational pull. If it was a teeny teeny tiny bit less, the Universe would have collapsed upon itself long ago, and if it was a teeny teeny tiny bit more, the Universe would have become hopelessly dilute.
In 1980, Alan Guth proposed inflationary cosmology, a theory that stated that around 10^(-35) seconds into the Universe's expansion, it went into an exponentially-expanding "inflationary" phase. This was due to its matter density being dominated by a cosmological-constant-like material, negative pressure and all. The material is likely some spin-0 field that has been named the "inflaton" (no i).
In this phase, distant objects depart from accessibility as the Universe expands. This effect freezes into place quantum fluctuations, though they later become visible again. This solves the "horizon problem". It also flattens out the Universe, solving the "flatness problem". Inflation ends with the inflation-field energy going into ordinary particles and expansion switching to decelerating.
The best observational test of inflation to date is the primordial-fluctuation spectrum, fluctuation amplitude as a function of fluctuation extent. It agrees reasonably well with inflation predictions.
Cosmological inflation resembles Fred Hoyle's steady-state cosmology in their overall Universe behavior, though the Universe is much more dense in inflation than in steady-state theory. There is an additional analogy. Steady-state cosmology included a mysterious and ad hoc "C-field" that created neutrons and the like, while inflationary cosmology creates primordial fluctuations with quantum mechanics and the nature of its expansion.
Composition of the Universe
|Substance||Mass Fraction at present|
The Cosmic Neutrino Background's neutrinos decoupled when their interactions with other particles became so weak that they could travel over much of the then-observable size of the Universe without interacting. That happened about 2 seconds in the Universe's expansion, when the Universe's temperature reached 2.5 MeV or 2.8*10^(10) K, about 5 times as much as the rest-mass energy of the electron. The neutrinos' temperature is now about 1.95 K.
Photons are mostly the Cosmic Microwave Background.
Baryonic matter has about 1 baryon per billion CMB photons. But when the Universe was hot enough to produce protons and neutrons by pair production, it produced them in around the abundance of the photons. So there was about 1 + 10^(-9) ordinary proton or neutron for each antiproton and antineutron. Strictly speaking, around then was where hadrons had condensed out of a gas of quarks and gluons, but the basic premises are still correct. There was about 1 + 10^(-9) ordinary quarks for each antiquark.
In 1967, Andrei Sakharov, Soviet nuclear-bomb designer and dissident, proposed a solution. He proposed that there was some process early in the Universe's history that satisfied these conditions:
- Violation of baryon number
- Violation of C and CP symmetries, which can produce matter-antimatter and time asymmetries
- Departure from thermal equilibrium
The first of them is a common consequence of Grand Unified Theories. The second one was recently observed in the weak interactions. The third one is a result of the Universe's expansion.
There are several proposed mechanisms that satisfy these conditions, operating at energies ranging from electroweak-unification energies to grand-unified-theory energies.
The nature of dark matter continues to be obscure, though a favorite hypothesis is that it consists of Weakly Interacting Massive Particles (WIMP's) that froze out early in the Universe's history. When the Universe became too cold to produce them, most of them disappeared by running into each other, but a few escaped that fate, surviving to the present.
The nature of dark energy is even more obscure, though it behaves in cosmological-constant fashion, negative pressure and all.