In 1922-24 Alexander Friedmann from Russia and later in 1927 Abbe' Lemaitre from Belgium, a Jesuit priest but with interest in cosmology, had worked out mathematical models of the expanding universe i.e., the concept in which galaxies are moving away from one another. If you look at the universe from any one of the galaxies as the vantage point you will find the other galaxies moving away from you. Models with this symmetry were discussed independently by Friedmann and Lemaitre. A galaxy may be 100 light years across and may contain as many as a hundred billion stars. In a vast universe even these galaxies become like points. This was the mathematical idealisation which Lemaitre and Friedmann had proposed. At that time nobody took them seriously because there was no evidence that the universe is indeed expanding. But shortly thereafter, in 1929, Edwin Hubble, who had been analyzing the spectra of galaxies had found that the absorption lines in these spectra were shifted towards the red end. This shifting towards the red end meant that the wavelengths of these lines were increased. The increase in wavelength was attributed by Hubble to the fact that the galaxies were moving away from us. This is the very picture which I described just now in the context of mathematical models of Friedmann and Lemaitre.
Moreover, Hubble found that the velocity of a galaxy was in proportion to its distance from us. The farther a galaxy was the faster would be it recession. This effect was very well described by the Friedmann-Lemaitre models. This was the reason why these became accepted as the starting point for any discussion of cosmology or the large scale structure of the universe. There is a simple relation that emerges in this model, for example. If the universe expands by a certain factor while light leaves a source and travels to the receiver, then the wavelength of light also increases by the same factor.
So 1930 onwards theoreticians began to think in terms of expanding universe models rather than models in which all galaxies were static, that is staying where they were rather than moving. There were some physicists who considered the possibility that in the early stages of the universe the conditions were very different from what they are today. If a system is expanding the density of matter in it becomes thinner and thinner. It is like a gas ball whose density becomes lower and lower when it is expanding. So, if you look back into the past this gas ball was smaller and smaller and so denser and denser. The universe was like this gas ball and was denser and denser as you probed it further and further back into the past. If you compress a gas it heats up. So the argument was that the densities and temperatures of matter and radiation were both increasing as you went toward the past epochs of the universe.
Ultimately how far back can you probe the past? The answer is : you can go upto a stage when the entire universe was so small that you could imagine it to have been compressed to a point. So the convention is to regard this instant when the entire universe was compressed to a point as the beginning of the universe and say that the universe came into existence very hot as well as very dense. It exploded into existence because the velocities of created matter were all very high. One of the scientists who never liked this particular idea was Fred Hoyle and he called it the ``Big Bang" in a derisory sense. He so named it for the fun of it but everybody liked that phrase so much that it has become popularly known as the Big Bang Model. So in a big bang universe you start off with infinite density, infinite temperature but as time proceeds the temperature and the density become less and less. If you ask the mathematicians who work out the consequences of such a model, they will tell you that the density of matter drops as the third inverse power of the scale of expansion. So, if the universe expands in all directions by a factor of ten its volume increases by a factor thousand. And so matter density would fall to a value 1/1000 of what it was. However, if you look at the radiation density, it would fall faster, as the inverse fourth power of the scale factor. So, in this particular example it would fall to 1/10000 of its original value. What this implies is that today you find the matter density in the universe is at least a thousand times more than radiation density, that is the universe is matter dominated. But if you went back into the past epochs, you will find that radiation was more important and the universe was very hot and dominated by radiation. The physicist George Gamow was the first to explore those early epochs.
What Gamow conjectured was that the nuclei of most chemical elements which we see in the universe today were synthesized by fusion in a very early hot stage. That hot stage lasted from around one to 200 seconds or approximately three minutes after the big bang. Subsequently, people found that what Gamow expected to achieve was only partially achieved. Gamow wanted to combine neutrons and protons as the building blocks of elements to form bigger and bigger nuclei of atoms. If you just take the proton by itself it forms the nucleus of the hydrogen atom which is the smallest atom. If you take two protons and two neutrons, and make them together into one element you get the next stable light element which is helium and so on. You can go on building bigger atoms like carbon, oxygen, nitrogen etc. The early universe would act as a hot thermonuclear reactor. It is a kind of furnace in which you put nuclei and they become fused together by thermonuclear fusion, the same kind of reaction that produces the hydrogen bomb. That same reaction goes on and on producing bigger and bigger nuclei. This is what Gamow was arguing for. But the real calculation showed subsequently that you can go only as far as making helium but you cannot go beyond to make bigger nuclei. This is because if you start combining two helium nuclei together you get something which does not last very long, it breaks apart. Nor can you make a stable nucleus of 5 particles by the fusion of hydrogen and helium. So you cannot get all the elements that Gamow was hoping for. Nevertheless the helium that is made agrees very well with the amount of helium found in the universe today. One can therefore say that there is some evidence for the very early hot universe coming from the calculation of synthesis of nuclei.
We are interested
in knowing where did the heavier elements come from. If they were not
made in the early universe where were they made? A brief answer to this
question is that they are believed to have been made inside stars, stars like the sun and
other stars which are more advanced in evolution than our sun. This was
shown by a team of scientists later on: a team commonly referred to as B
FH, which included Geoffrey and Margaret Burbidge, William Fowler and Fred Hoyle.
But there was another evidence for hot universe which
came from the work of Gamow's two younger colleagues, Ralph Alpher
and Robert Herman. They published a paper in 1948 in which they
argued that this early hot universe was dominated by radiation, and they conjectured
that the radiation should be seen today as a cool background spread all over the universe. They
estimated the temperature of that radiation as 
(absolute) which is
. Later in 1965 a radiation background in microwaves was discovered with a
temperature of 
,
reasonably close to what Alpher and Herman had predicted.
This prima facie evidence suggested that the universe might have started in a big bang, and was very hot, very dense in the early epochs. Gamow's work had related to the epochs when the universe was one second old. What about going further back in time when the universe was even denser and younger? What particles were present at that time when the universe was very hot? What was their energy? These questions do not belong entirely to cosmology but take you to particle physics where you are looking at the basic constituents of matter.
Cosmologists also wanted to know another aspect which I will explain to you later but I briefly mention it now. This was the following: when you look back into time, that is when you look at more and more distant past what do you see? You see galaxies, quasars and various parts of the universe but you don't see them as they are today. You see them as they were in the past, because light has taken some time to travel to you. When you see a star which is 10 light years away it means light has taken 10 years to travel to you. So the image that you are seeing is not the image of the star as it is today but its image 10 years ago. With the help of better and better instruments you can therefore see further into the past of the universe. To give you another example, if you can imagine somebody who has a very good telescope looking at the Earth today from a distance of 300 light years away that person is seeing the events on Earth not as they are happening today but as they were happening 300 years ago. So that observer would be able to see what was happening here during the Mughal era. You can see that it is possible to `view' history in this strange way. However, here we are interested in the history of the universe. Can we have a very good enough telescope which can let us see the universe as it came into existence at the big bang? The answer is alas no! There is a difficulty which intervenes.
It comes from this radiation which Alpher and Herman had predicted should exist and has been found today. What happens is as you start looking at the universe further and further back in time you also find that this radiation was present at that time and was denser and denser. When the radiation was very dense the universe was of course also dense in matter terms. Then you find that light was not able to travel in a straight line through that very dense universe. Suppose I send a ray of light from here. It will go straight upto the wall and once it hits the wall it will not go further because it gets absorbed. The dense distribution of matter in the wall does not allow light to penetrate. The same thing happens if you start looking at the universe very far back in time. Beyond a certain stage in the past it erected a `wall' so dense that light could not penetrate it. This problem is called the opacity of radiation background. So nothing could be observed prior to an epoch of redshift of around 1000. This means that beyond the epoch when the universe was a 1000 times smaller in linear size you cannot see anything. This is the limitation on observation. Nevertheless cosmologists wanted to know what the universe was like very early on. So they asked particle physicists to tell them what kind of physics might have existed in the very early stages.