Several lines of evidence lead to the standard big-bang cosmology. In this picture, the Universe is expanding, but it is space itself that does the expansion - ordinary matter is only going along for the ride. Thus, the answer to "where was the big bang?" is everywhere. The main lines of observational evidence leading to this model are
The expanding Universe (otherwise known as the Hubble expansion). There is a (locally linear) relationship between the redshift of galaxy spectra and their distance (see graph of data from Hubble key project). This redshift is due to the expansion of space between the time the light left the source and the time we receive it. Simply extrapolated backwards, this says that everything was once in the same place at the same time. For our best current estimate of the expansion rate (Hubble constant) and its history, this would have been about 14 billion years ago.
The cosmic microwave background radiation. This is a bath of photons with an exact thermal (blackbody) form, coming from all directions, now observed at a temperature 2.728 K (view graph). This implies that the Universe was at one time a dense, opaque, and very smooth gas, quite unlike today's view. Tiny fluctuations, a few parts per million, can be observed, as expected for the seeds of today's superclusters of galaxies. The images are all-sky maps from the COBE mission, first showing the overall uniformity of the radiation, then processed to show our motion relative to the cosmic average, and finally to show these protoclusters as tiny ripples. This radiation was released when the Universe became transparent, about 300,000 years along, when it cooled enough for hydrogen and helium to form neutral atoms.
This picture gives a consistent explanation of the relative amounts of the lightest elements (hydrogen, deuterium, helium, lithium). They would occur as the result of cooling from 3-15 minutes into cosmic history. The competition among particle collisions, the expansion, and decay of free neutrons into protons and electrons yields sets of these abundances which are in good agreement with what we see in the most pristine parts of the Universe. (View nucleosynthesis network). Heavier elements are due to later processes in stars.
So, starting at 300,000 years, we see an expanding gas of 77% hydrogen, 23% helium, with smatterings of deuterium and lithium. This gas is very smooth, with parts-per-million density fluctuations. The story has another player, the dark matter, which (as far as we can tell) interacts with ordinary matter only by gravitational, and outweighs it by something like 10:1. This plays a crucial role in starting the formation of galaxies and the large-scale structures they trace.
Next: Formation of large-scale structures
Back:The Cosmic Backdrop
Last changes: 8/2002