In 1929 Edwin Hubble compared the galaxies' distances to their redshifts, and found a linear relationship. He interpreted the redshifts as being caused by the receding velocities of the galaxies. Although the explanation is widely accepted by astronomers today, at the time Hubble proposed it there were critics of this interpretation and it was they who coined the name big bang. By running time backwards, an effect called the big crunch, we can infer that the Universe used to be much smaller and extremely dense.
The high level of confidence that the theory enjoys is mostly built on 4 pillars of observation:
A generally accepted timeline of the history of the Universe:
The Universe experiences a cataclysm that generates all of space and time, as well as all the matter and energy the Universe will ever hold. For an incomprehensibly small fraction of a second, the Universe is an infinitely dense, hot fireball. The prevailing theory describes a peculiar form of energy (dark energy) that pushes out the fabric of space. A runaway process called "Inflation" causes a vast expansion of space filled with this energy. The inflationary expansion is slowed only when this energy is transformed into matter and energy as we know it.
After inflation, one millionth of a second after the Big Bang, the Universe continues to expand but not nearly so quickly. As it expands, it becomes less dense and cools. The most basic forces in nature become distinct: first gravity, then the strong force, which holds nuclei of atoms together, followed by the weak and electromagnetic forces. By the first second, the Universe is made up of fundamental particles and energy: quarks, electrons, photons, neutrinos and less familiar types. These particles smash together to form protons and neutrons.
Protons and neutrons come together to form the nuclei of simple elements: hydrogen, helium and lithium. It will take another 300,000 years for electrons to be captured into orbits around these nuclei to form stable atoms.
The first major era in the history of the Universe is one in which most of the energy is in the form of radiation -- different wavelengths of light, X rays, radio waves and ultraviolet rays. This energy is the remnant of the primordial fireball, and as the Universe expands, the waves of radiation are stretched and diluted until today, they make up the faint glow of microwaves which bathe the entire Universe.
At this moment, the energy in matter and the energy in radiation are equal. But as the relentless expansion continues, the waves of light are stretched to lower and lower energy, while the matter travels onward largely unaffected. At about this time, neutral atoms are formed as electrons link up with hydrogen and helium nuclei. The microwave background radiation hails from this moment, and thus gives us a direct picture of how matter was distributed at this early time.
300 million years
Gravity amplifies slight irregularities in the density of the primordial gas. Even as the Universe continues to expand rapidly, pockets of gas become more and more dense. Stars ignite within these pockets, and groups of stars become the earliest galaxies. This point is still perhaps 12 to 15 billion years before the present. The Hubble Space Telescope recently captured some of the earliest galaxies ever viewed. They appear as tiny blue dots in the Hubble Deep Field images.
15 billion years
The Sun is born.
Despite the self-consistency and remarkable success of the Big Bang model in describing the evolution of the Universe back to only a fraction of a second, a number of unanswered questions remain regarding the initial state of the Universe.
The flatness problem
Why is the matter density of the Universe so close to the unstable critical value between perpetual expansion and recollapse into a Big Crunch?
The horizon problem
Why does the Universe look the same in all directions when it arises out of causally disconnected regions? This problem is most acute for the very smooth cosmic microwave background radiation.
The density fluctuation problem
The perturbations which gravitationally collapsed to form galaxies must have been primordial in origin; from whence did they arise?
The dark matter problem
Of what stuff is the Universe predominantly made? Nucleosynthesis calculations suggest that the dark matter of the Universe probably does not consist of ordinary matter - neutrons and protons.
The exotic relics problem
Phase transitions in the early Universe inevitably give rise to topological defects, such as monopoles, and exotic particles. Why don't we see them today?
The thermal state problem
Why should the Universe begin in thermal equilibrium when there is no mechanism by which it can be maintained at very high temperatures.
The cosmological constant problem
Why is the cosmological constant 120 orders of magnitude smaller than naively expected from quantum gravity?
The singularity problem
The cosmological singularity at t=0 is an infinite energy density state, so general relativity predicts its own breakdown.
The timescale problem
Are independent measurements of the age of the Universe consistent using Hubble's constant and stellar lifetimes?
Cosmology is a fast changing area of research and solutions to these problems are currently being studied and proposed. While some mysteries are solved, others endure, and new mysteries are uncovered.