For the reader resolved to eschew theory and admit only definite observational facts, all astronomical books are banned. There are no purely observational facts about the heavenly bodies. Astronomical measurements are, without exception, measurements of phenomena occuring in a terrestrial observatory or station; it is only by theory that they are translated into knowledge of a universe outside. ~ Arthur Eddington, The Expanding Universe
In modern scientific cosmology, there are an abundance of models that are used to describe our own Universe. Many models have been developed in the past, and many are currently being developed. But only one model can be right, and we must depend on our observations to rule out models that are wrong.
Observations of galactic redshifts at various distances by Edwin Hubble led to the discovery of the expansion of our Universe, ruling out all the other static and contracting models proposed by Einstein and other cosmologists. Of course, this does not mean that our universe will never arrive at a stage where it is static or contracting, or that it has never done so in the past.
One of the greatest triumphs of observational cosmology has to be the discovery of the cosmic microwave background (CMB) radiation, left over from the early stages of the Big Bang. Before its discovery, cosmologists were divided into two camps. One camp held on to the Steady State model of the Universe proposed by Fred Hoyle, Hermann Bondi and Thomas Gold. In this model, the Universe is infinite, eternal and unchanging; but to account for its expansion, new matter has to be spontaneously and continuously created from nothing so that the density of the Universe remains constant. This model was favoured by the heavyweight cosmologists of the time, and for awhile remained the dominant theory. The other camp, which was a minority among cosmologists, preferred the ‘primeval atom’ (later called the ‘Big Bang’ by Fred Hoyle as a form of derision) model of the Universe, initially proposed by Georges Lemaitre. In this model, the Universe expanded from an initial state of very high density, and continues to expand today. Although the model was based on a solution of the Friedmann equations (the main equation that describes the Universe in cosmology), there were reservations about the theory because Lemaitre was a Catholic priest, and many regarded his work as a mixture of science and theology. Even the Pope acknowledged that the ‘primeval atom’ theory agreed with Catholic theology of the creation of the Universe.
The following work is concerned with this aspect of the matter and arose from a discussion with Mr. T. Gold who remarked that through continuous creation of matter it might be possible to obtain an expanding universe in which the proper density of matter remained constant. This possibility seemed attractive, especially when taken in conjuction with aesthetic objections to the creation of the universe in the remote past. For it is against the spirit of scientific enquiry to regard observable effects as arising from ’causes unknown to science,’ and this in principle is what creation-in-the-past implies. ~ Fred Hoyle, A New Model for the Expanding Universe
But the serendipitous discovery of the CMB by Arno Penzias and Robert Wilson in the 1960s put the debate to rest by confirming that the Universe began in a hot, dense state. Today, steady state models remain only as historical artifacts. Since then, the discovery of large numbers of quasars and active galaxies at high redshifts have provided further evidence for an evolving universe. Further measurements of the CMB led to the conclusion that the early Universe was very smooth and homogenous, but very small inhomogeneities detected by the COBE satellite are considered to be the seeds for the formation of the structure and galaxies that we see today. Measuring these inhomogeneities also tell us that the Universe is flat. Coupled with the measurements of the abundances of lithium, dueterium and helium in the Universe, and the dynamics of galaxies and their clusters, we now know that the Universe consists mostly of matter that we have yet to discover (the so called dark matter). Studying supernovae allowed scientists to determine that the expansion of the Universe was accelerating, rather than slowing down as previously thought.
Beginning as an almost entirely speculative field, cosmology has turned into a full-blown observational science.
Now, in the physical universe of today, even a simple metal coin testifies to the existence of stars that died before the formation of the Solar System, and hence the universe is billions of years old, spans billions of light years in space, and contains billions of galaxies. ~ Edward Harrison, Cosmology: The Science of the Universe
Astronomers have an advantage over geologists in that they can directly observe the past. ~ Martin Rees, Perspectives in Astrophysical Cosmology