What are the grounds for believing in Big Bang Cosmology? I have listed below, seven powerful pieces of evidence for the big bang theory. By the way, I am very open to the idea that the big bang is only one of an infinite number of big bangs, with new universes being created all the time ad infinitum. Also I would like to say there that I find it quite amusing that some religious people say they don’t believe in the big bang because it denies the existence of God, while other equally religious people claim that the big bang is the best scientific evidence we have for the existence of God. The big bang theory, like all scientific theories, gives us no evidence either way concerning the existence of God.

                          (1) The Universe is expanding.      

          In the 1920s astronomers discovered that the observable galaxies were each moving away from each other. It is not as though they are rushing through empty space, but the space between galaxies cluster is actually stretching. Astronomers arrived at this conclusion by an examination of the light spectra of galaxies. As with sound waves, light waves change depending on whether an object is moving away or moving toward an observer. Sound waves shift to a lower frequency as objects move away from an observer, and they shift to a higher frequency as objects move toward an observer. This is known as the “Doppler” effect. (The sound of a siren rushing by an observer provides a good example of this). Something similar is observed in the case of light waves. In the 1920s Edwin Hubble discovered that light from distant galaxies is shifted to longer wavelengths, toward the red end of the light spectrum, whereas light from some nearby stars is shifted to shorter wavelengths, toward the blue end of the light spectrum. Distant galaxies exhibit what is called a cosmological red shift, and Hubble claimed that the red shift of a galaxy was proportional to its distance. He calculated the distance of galaxies by measuring the brightness of their stars, especially a type of star known as a Cepheid variable, which is a type of star whose light output varies in a particular manner, and whose period of light output variations is proportional to its peak absolute brightness, thus making it perfect as a distance indicator to whatever galaxy it resides in. All you need to do is calibrate the method by measuring a nearby Cepheid variable in our own galaxy with an independent means, such as triangulation. Although Hubble’s observations originally applied only to fairly nearby galaxies that he observed, subsequent observations have extended Hubble’s observation to galaxies and galactic clusters to billions of light years away from us.

     The actual situation turns out to be that it is not individual galaxies that are moving away from each other. It is galaxy clusters that are all moving away from each other. The crucial point to note here is that the data imply that galaxy clusters are moving away from us. More precisely, every cluster is moving away from all of the other clusters – much like the movements of dots on a balloon that is being filled with air, but you have to visual that in three dimensions. Big Bang cosmology of course predicts that the Universe is and has been expanding from its inception. So expansion is important evidence that confirms the Hot Big Bang theory.

     Technical note: Light arriving at earth from distant galaxies provides information about objects at the time the light left them. Owing to the distances between galactic objects, we see them literally as they were in the past. Modern telescopes can see galaxies as far as 10 billion light years away. Thus, these telescopes give us a glimpse of the Universe 10 billion years ago and it shows that galaxies billions of light years away are in an earlier state of evolution than nearby galaxies. They are rather primordial in their appearance, with rapid star formation taking place. This provides one reason among many for being quite sure that expansion is not merely a local phenomenon. It can observationally be traced back 10 billion years. And if the Universe is not expanding, we must suppose that 10 billion years ago these primordial galaxies all mysteriously “turned on” in their present positions.

     Moreover, general relativity implies that the universe must either be expanding or contracting, so expansion must be viewed as a feature of the universe from the inception of its existence. Let me elaborate a bit on that last statement. Einstein faced the same problem Newton faced when formulating a theory of gravity. If every object in the universe attracts all other objects in the universe through the force of gravity, then it would contract, unless of some unknown force opposed gravity. Newton solved this problem by postulating that the universe is infinite. According to equations in Einstein’s General Relativity, the Universe must either be expanding or collapsing, or some unknown force must be preventing its collapse. When Einstein discovered this consequence of his theory, he was profoundly disturbed, as there was at that time no experimental evidence suggesting that the Universe is expanding. Einstein added the idea of the so-called cosmological constant as a force that would keep the Universe from contracting. The cosmological constant as Einstein envisioned it, was a force that worked against gravity on large scales. Edwin Hubble’s observations of the cosmos later confirmed that the Universe is indeed expanding, and Einstein quickly retracted his idea of a cosmological constant, saying that it was the biggest mistake of his career. Einstein missed his chance to be the fist person to propose an expanding universe.

                          (2) Uniform Background Microwave Radiation    

        The entire visible cosmos is bathed in a sea of background microwave radiation. This was predicted by big bang cosmologists before its discovery by Arnold Penzias and Robert Wilson in the Bell Telephone Laboratories in 1964-65, and the insights of Robert Dicke. It was confirmed to a high degree of precision in 1990 by COBE, the Cosmic Microwave Background Explorer, a satellite designed to measure the microwave background radiation. The data from COBE and more recent experiments shows that microwave radiation is uniformly distributed throughout the visible cosmos, present in all directions at the same temperature, about 2.7 degrees Kelvin, and peaks at approximately 200GHz. This is precisely what was predicted by the cosmologists. The spectrum of background microwave radiation which comes from all directions is an exact match to the spectrum that was predicted. This is profound because it is the best evidence for the big bang that you could possibly ask for. There is absolutely no other explanation for this background radiation.

     This observation further implies that the Universe is, in its large-scale features, uniform and smooth. Big Bang cosmology leads us to expect all this. The primordial fireball would leave a permanent, uniform microwave signature as it cooled. But of course it could not have been perfectly uniform or stars and galaxies would not have ever formed. Quantum theory predicted extremely small quantum fluctuations in the density of the universe in the first moments of the big bang. These density fluctuations would have left slight temperature fluctuations in the microwave background radiation. These variations would be only one part in 100,000 and thus would be very hard to detect. But the COBE satellite had extremely sensitive instruments on it, and those minute temperature fluctuations showed up just as predicted.

     In short, the background microwave radiation of the universe, as well as its spectral curve, and its minute temperature fluctuations, is exactly what was predicted by the big bang theory and there is simply no other way to explain it.

                         (3) Uniform Abundance of Hydrogen and Helium 

       In our cosmos there is a universal abundance of hydrogen and helium. Stars run on hydrogen, and although stars produce helium, they cannot be the source of all the helium in the Universe, because most of the helium in a star either gets processed into heavier elements via nuclear reactions, or remains in the star as it finally runs out of fuel and dies. There is too much helium in the universe for it to have come only from stars, and it’s too uniformly distributed throughout the cosmos to be accounted for through mere stellar processes. The abundance of hydrogen and helium, as well as its uniform distribution, and the relative proportion of hydrogen to helium, is precisely what we would expect given Big Bang cosmology, which postulates the emergence of the lighter elements out of the fires of the Big Bang, through a process called nucleosynthesis, a process that took place in the first few minutes of the Universe’s existence. Also the relative abundance of helium and deuterium (heavy hydrogen), as determined from the spectra of hydrogen clouds backlit by quasars, is exactly what was predicted by big bang cosmology.

                           (4) Second Law of Thermodynamics   

      Several independent facts and scientific laws preclude an infinitely old Universe. First, the second law of thermodynamics entails that all physical systems move increasingly toward greater states of disorder or entropy. If the Universe is infinitely old, then it would have reached thermodynamic equilibrium or maximal entropy a finite time ago, which clearly it has not. Among other things, we would not be here if it did.

                                    (5) Darkness of the sky   

        The night sky is largely dark. This would not be the case if the Universe had existed for infinite time. For in that case, the night sky would be filled with starlight, as infinite time would have allowed light from all points of the sky to reach us, no matter how distant its source.           

                                           (6) Age of Stars  

         No star is older than approximately 12 billion years old, a rather odd fact if the Universe is infinitely old. If the Universe is infinitely old there should be stars older much than that. I will not go into detail here on how they can estimate the ages of stars, but I am well-read on that subject and can elaborate at length if you’d like. Evidence against an infinite universe also lends support to Big Bang cosmology, since the latter postulates a temporally finite universe.

                          (7) Evolution of Galaxies    

 The farther away, (and thus farther back in time) one looks, using the Hubble and also powerful, Earth-based telescopes, the more different the universe looks. There is a higher proportion of spiral to elliptical galaxies as one looks more and more billions of light years away. And there are more galaxies farther away. The merging of spiral galaxies to form elliptical galaxies is something that is seen taking place in various stages in different parts of the universe. Galaxy mergers are very well confirmed to have been an important part of the evolution of the universe. Even our own Milky Way is swallowing up a couple of small galaxies right now as I type this. One small galaxy called Sagittarius A is being swallowed by our galaxy on the side opposite from us. The larger number of galaxies in clusters, the higher proportion of spirals to ellipticals, and the higher rate of star formation seen in far away galaxies, are strong evidence of the big bang.

     As a corollary to the above paragraph, if some intelligent creatures in a galaxy 10 billion light years away looked at our galaxy, they would see our galaxy as it looked 10 billion years in the past. They would see more small satellite galaxies around the Milky Way than we see, and they would see a much more active nucleus in our galaxy than what we see now.

     One line of evidence that has not been checked is the pattern of polarization of the Microwave background radiation. Gravitational waves in the hot, dense plasma of the early universe would have left a pattern of polarization in the background radiation. The polarization of the microwave photons would be different according to which direction they are coming from. COBE did not have a polarization sensor. We will have to wait for an instrument with an extremely sensitive polarization sensor to be put in orbit before this line of evidence can be tested. The necessary sensitivity is beyond current technology, but should be obtainable in the near future.