The four key observational successes of the standard Hot Big Bang model are the following:
The Big Bang model makes accurate and scientifically testable hypotheses in each of these areas and the remarkable agreement with the observational data gives us considerable confidence in the model.
The Universe began about ten billion years ago in a violent explosion; every particle started rushing apart from every other particle in an early super-dense phase. The fact that galaxies are receding from us in all directions is a consequence of this initial explosion and was first discovered observationally by Hubble. There is now excellent evidence for Hubble's law which states that the recessional velocity v of a galaxy is proportional to its distance d from us, that is, v=Hd where H is Hubble's constant. Projecting galaxy trajectories backwards in time means that they converge to a high density state - the initial fireball.
The Copernican or cosmological principle states that the Universe appears the same in every direction from every point in space. It amounts to asserting that our position in the Universe - with respect to the very largest scales - is in no sense preferred. There is considerable observational evidence for this assertion, including the measured distributions of galaxies and faint radio sources, though the best evidence comes from the near-perfect uniformity of the relic cosmic microwave background radiation. This means that any observer anywhere in the Universe will enjoy much the same view as we do, including the observation that galaxies are moving away from them.
The fact that the Universe is expanding - about every point in space - can be a difficult concept to grasp. The analogy of an expanding balloon may be helpful: Imagine residing in a curved flatland on the surface of a balloon. As the balloon is blown up, the distance between all neighbouring points grows; the two-dimensional universe grows but there is no preferred centre.
About 100,000 years after the Big Bang, the temperature of the Universe had dropped sufficiently for electrons and protoons to cobine into hydrogen atoms, p + e --> H. From this time onwards, radiation was effectively unable to interact with the background gas; it has propagated freely ever since, while constantly losing energy because its wavelength is stretched by the expansion of the Universe. Originally, the radiation temperature was about 3000 degrees Kelvin, whereas today it has fallen to only 3K.
Observers detecting this radiation today are able to see the Universe at a very early stage on what is known as the `surface of last scattering'. Photons in the cosmic microwave background have been travelling towards us for over ten billion years, and have covered a distance of about a million billion billion miles.
Prior to about one second after the Big Bang, matter - in the form of free neutrons and protons - was very hot and dense. As the Universe expanded, the temperature fell and some of these nucleons were synthesised into the light elements: deuterium (D), helium-3, and helium-4. Theoretical calculations for these nuclear processes predict, for example, that about a quarter of the Universe consists of helium-4, a result which is in good agreement with current stellar observations.
The heavier elements, of which we are partly made, were created later in the interiors of stars and spread widely in supernova explosions.
The standard Hot Big Bang model also provides a framework in which to understand the collapse of matter to form galaxies and other large-scale structures observed in the Universe today. At about 10,000 years after the Big Bang, the temperature had fallen to such an extent that the energy density of the Universe began to be dominated by massive particles, rather than the light and other radiation which had predominated earlier. This change in the form of the main matter density meant that the gravitational forces between the massive particles could begin to take effects, so that any small perturbations in their density would grow. Ten billion years later we see the results of this collapse.
The standard cosmology, then, provides a framework for understanding galaxy formation, but it does not tell us about the origin of the primordial fluctuations required at 10,000 years. We must seek answers to questions like these from earlier epochs in the history of the Universe.
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