# The Big Bang

The equations of General Relativity allow us to deduct the evolution of the universe. These universes are non-static (in expansion). This fact seemed inconceivable for Einstein, for primarily philosophical reasons. He thus introduced a supplementary variable into his equations, called the cosmological constant, which aim is to counterbalance the gravity due to the matter contained in this universe.

But, a few years later, Hubble, by observing distant galaxies, realizes that they move away from us, and that their recessional velocity increases in proportion to their distance.
The Hubble law, which connects the recessional velocity and the distance is expressed by a linear relationship : Vc = H * D, where Vc represents the velocity, D the distance. H is called the Hubble constant.

Hubble understands that the universe is expanding : what other explanation could we find to explain this escape of the galaxies, proportional to their distance ?

## The expansion of the universe

You need to understand that this expansion is a property of space-time itself. Galaxies don't drift away in a solidified space, they are motionless in an expanding "web".
Actually , we measure the redshift of their spectrum. But the wavelengths of the emitted radiation grow with the expansion, like all the lengths. The "apparent escape" of the galaxies is thus only the translation of a Doppler effect created by this expansion of the universe.

One usual image used to describe this effect is a cooking raisin cake : the cake inflates and the raisins move away the ones from the others. The further they are, the faster they move.

Each raisin can see its neighbours moving away from it, all the more quickly as they are far, but their own size doesn't change.

This expansion has no center. Each raisin can see the others moving away, whatever their initial position. It is the same for the galaxies in the universe.
Let us note that, if the distances between the galaxies increase with time, it's not the same for distances inside the galaxies : the gravitational force sets again this expansion and, hence, allows the full galaxy to stay binded.

## The past of the universe

Since the universe is expanding, it was thus smaller in the past than nowadays.
But, if it was smaller, it was denser, hence hotter. We can calculate that the temperature of the universe is divided by two each time the distances double.

Let us extrapolate by moving back in the past : there was a time where the universe was as small as we can imagine, and at a very high temperature. The "birth" of the universe happened from this tiny ball of energy which inflated.
So, we see that the universe had a beginning, and is expanding : the old idea of a static and overlasting universe just collapsed.

The Belgian Lemaitre was the first, in 1925, to propose this theory that he had called " the primitive atom".

By derision, the astrophysicist Fred Hoyle in 1950, who supported a different theory, gave to this one the name of "Big Bang". This name is always in use nowadays.

Once again, take care not to see the Big Bang as an explosion of matter inside an empty universe : the universe itself is expanding.

To take again the analogy with the raisin cake, you must imagine that nothing exists apart from the cake itself, and at the very beginning, all the mixture is concentrated is a volume as small as you can imagine, and which inflates with time.
Space and time are born with the Big Bang : before and elsewhere are not meaningful words, because nothing can exists outside the universe and before its creation.

In the same manner that a heated piece of metal keeps its heat for a long time and radiates, then, if the universe has been hot enough in its past, it must continue to radiate like a black body.
But this radiation must now be found in the field of microwaves, corresponding to a very low temperature : in 1948, George Gamow estimates it at 6 K ; and in 1965, Penzias and Wilson indeed observe a radiation equivalent to a temperature of approximately 3 K.

Since, COBE and WMAP satellites allowed finer measurements, and the temperature of cosmological microwave background is 2.736 K. We can say that it is the current temperature of the universe.

Fluctuations of the fossil radiation, measured by the COBE satellite in 1990. Influences of the motions of Earth and the Galaxy in space have been taken into account.

This radiation is exceedingly uniform : red areas are hotter than the blue ones for only 1 in 100,000 parts.

Source NASA / COBE.

The same fluctuations, measured by WMAP satellite in 2003.
Always the same fluctuations, measured by Planck satellite in 2013. Notice the advance of the resolution satellite after satellite.

The tiny variations that you can observe are the seeds from which the matter grow, forming later stars and galaxies.

This radiation, whose spectrum is a nearly perfect black body one, is the first really certain evidence of the theory of Big Bang.
Very recently the temperature of a gaz cloud situated in front of a 12 billion light years apart quasar has been measured by the VLT.
This cloud is warmed up by the cosmological radiation, and the measured temperature of 14 K is in agreement with the estimated temperature of the cosmological radiation at this time.

The expansion of the universe is sometimes presented as an evidence for the Big Bang, but it is based on redshift measurements, and some people think that this redshifts may not be cosmological.

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