Here you go.... again.
How do we know the Big Bang happened?
Astrophysicists have uncovered a great deal of compelling evidence over the past hundred years to support the Big Bang theory. Among this evidence is the observation that the universe is expanding. By looking at light emitted by distant galaxies, scientists have found that these galaxies are rapidly moving away from our galaxy, the Milky Way. An explosion like the Big Bang, which sent matter flying outward from a point, explains this observation.
Did you know that the static on your television is caused by radiation left over from the Big Bang?
Another critical discovery was the observation of low levels of microwaves throughout space. Astronomers believe these microwaves, whose temperature is about -270 degrees Celsius, are the remnants of the extremely high-temperature radiation produced by the Big Bang.
Interestingly, astronomers can get an idea of how hot the universe used to be by looking at very distant clouds of gas through high-power telescopes. Because light from these clouds can take billions of years to reach our telescopes, we see such bodies as they appeared eons ago. Lo and behold, these ancient clouds of gas seem to be hotter than younger clouds.
Scientists have also been able to uphold the Big Bang theory by measuring the relative amounts of different elements in the universe. They've found that the universe contains about 74 percent hydrogen and 26 percent helium by mass, the two lightest elements. All the other heavier elements -- including elements common on earth, such as carbon and oxygen -- make up just a tiny trace of all matter.
So how does this prove anything about the Big Bang? Scientists have shown, using theoretical calculations, that these abundances could only have been made in a universe that began in a very hot, dense state, and then quickly cooled and expanded. This is exactly the kind of universe that the Big Bang theory predicts.
And for the umpteeth time....
Red shift, cosmic background radiation, Friedmann's solutions to Einstein's field equations and the Second Law of Thermodynamics tells us that space and time did have a beginning. If the universe is expanding then it must have a beginning. If you follow it backwards in time, then any object must come to a boundary of space time. You cannot continue that history indefinitely. This is still true even if a universe has periods of contraction. It still has to have a beginning if expansion over weights the contraction. Physicists have been uncomfortable with the idea of a beginning since the work of Friedman which showed that the solutions of Einstein's equation showed that the universe had a beginning. The problem with a cyclical universe is with the 2nd Law of Thermodynamics. For every matter to energy or energy to matter exchange there is a loss of usable energy. So while the total energy of the universe does not decrease, the usable energy of the universe does decrease. If it is a periodic or cyclical universe then the entropy will increase with each cycle. The 2nd Law of Thermodynamics is a fundamental law of nature which tells us that entropy can only increase or stay the same. Entropy can never decrease. Which means that in a finite amount of time, a finite system will reach a maximum state of disorder which is called thermal equilibrium and then it will stay in that state. A cyclical universe cannot avoid this problem. Since we do not see thermal equilibrium (good thing too because there would be no life) we know that the universe did have a beginning.
There. I have explained it to you and provided links to my sources.