General Relativity not only provides a nice history of the universe, but it also points out viable measurements which can support its validity. The most important is the so-called Microwave Background Radiation.
When the universe began the density and temperature of the initial fireball was so high that all matter dissociated into its primary components. Note also that in this initial setting the force of gravity was enormous. As the expansion progressed the universe cooled and the initial fundamental constituents formed increasingly more complicated objects. This is so because when the temperature is very high everything is jiggling very fast and anything that can be dissociated will; as the temperature drops so does the jiggling and, eventually, composite structures can form and survive. Thus, if we had been able to film the contents of the universe as it cooled, and then run the film backwards we would first see atoms which are then broken apart into nuclei and electrons by the intense heat, then we would see the nuclei themselves decomposing into protons and neutrons, then the protons and neutrons decomposing into quarks . The microwave background radiation is a messenger from this primordial soup.
To understand how why is this microwave radiation present and how it was generated I need to talk a bit about the way charged bodies interact with light. Remember now that light is described by the same equations that describe the physics of electric charges (Maxwell's equations), this suggests (and it is true) that light will interact with charged objects. In fact this is how your skin gets hot when exposed to the sun: your skin is composed of molecules which are made of atoms. Atoms in their turn are composed of a small heavy nucleus (with positive charge) surrounded by a cloud of negatively charged light particles, the electrons. When light shines on your skin it is absorbed by the electrons which get agitated, and it is this agitation which you perceive as heat. This is not as efficient as it might be because the electrons are not free, they are inside atoms, so that on average the atoms are neutral. Much more light would be absorbed by a set of free electrons. This also works in the reverse: if you jiggle electrons sufficiently rapidly they will give off light, this is how a light-bulb works.
Suppose now that you have a box with perfectly reflecting walls and which are kept very hot. Into that box we introduce a bunch of electrons and nuclei and also light. Assume that the system is so hot that the electrons are not bound to the nuclei: as soon as they come close they are wrenched apart by the intense heat of the environment. So, on average, what you see is a bunch of charged particles and light running amok. In this case light is constantly being absorbed and emitted by the electrons and nuclei.
Now imagine that you cool the box by making it larger. Eventually things will get cold enough for the electrons to stay attached to the nuclei, the heat is not sufficiently high for them to be wrenched apart. At this point the rate at which light is absorbed and emitted drops rather suddenly for now the particles in the box are neutral (on average). From this point on light will just stream forth unimpeded (until it is reflected by a wall).
This is precisely what happens in the universe. After the big bang there came a point where electrons and nuclei were formed. They were immersed in intense electromagnetic radiation (light, X-rays, gamma rays, etc.). As time progressed and the expansion of the universe continued, the system became cooler (much as for the box when we increased its size). Eventually a point was reached where the universe was cool enough for atoms to form and from this moment on most of the radiation just streamed forth unimpeded. This happened when the universe was a mere 300,000 years old.
So, can we see this relic of the ancient universe? The answer is yes! But before we look for it one thing must be kept in mind. The universe has been getting bigger and bigger and less and less dense. This implies that the average gravitational force is getting smaller with time. So the radiation, from the moment it no longer interacted with the newly formed atoms has been shifting from an environment where gravity's force is large to that where gravity is small and, using (again!) General Relativity, it must be red-shifted. In fact the prediction of General Relativity is that this radiation should be seen mostly as microwaves...and it has been seen. This prediction is not only of the existence of this relic radiation, but also how this radiation depends frequency. These predictions have been confirmed to great accuray (see Fig. 8.17). This ubiquitous sea of radiation that permeates the cosmos is called the microwave background radiation.
The microwave background radiation was created in approximately the same environment everywhere (remember that it came from an epoch in which everything was a very homogeneous hot mixture of nuclei and electrons) and because of this we expect it to look the same in every direction. This is precisely what happens, but, as it turns out, it is too much of a good thing: the microwave background radiation is the same everywhere to a precision of 0.1%, and understanding this presents problems, see Sect. 8.5.3.
But one can go even farther. Even though the microwave background radiation is very homogeneous, there are small deviations. These represent inhomogeneities in the universe at the time radiation and atoms stopped interacting strongly. These inhomogeneities provide a picture of the universe in its most tender infancy, see Fig. 8.18. As the universe expands and cools atoms will conglomerate into stars and stars into galaxies; the initial seeds for this process to start are these inhomogeneities. They correspond to regions where the matter was slightly mode dense than the average, and will, in the eons that follow, attract other matter to form the structures we see today.
It is very hard to explain the microwave background radiation by any theory other than the Big Bang. It represent one of its biggest successes.