Formation of the Universe...

 Before Hubble, most astronomers thought that the universe didn’t change. But if the universe is expanding, what does that say about where it was in the past? If the universe is expanding, the next logical thought is that in the past it had to have been smaller.

How Did the Universe Form?

The Big Bang theory - (the theory that the universe originated sometime between 10 billion and 20 billion years ago from the cataclysmic explosion of a small volume of matter at extremely high density and temperature) is the most widely accepted cosmological explanation of how the universe formed. If we start at the present and go back into the past, the universe is contracting -- getting smaller and smaller. 

What is the end result of a contracting universe?

According to the Big Bang theory, the universe began about 13 to 14billion years ago. Everything that is now in the universe was squeezed into a very small volume. Imagine the entire known universe compressed into a single, hot, chaotic mass. An explosive expansion caused the universe to start growing rapidly. All the matter and energy in the universe, and even space itself, resulted from this expansion.

What came before the Big Bang? There is no way for scientists to know since there is no remaining evidence.

After the Big Bang

In the first few moments after the Big Bang, the universe was unimaginably hot and dense. As the universe expanded, it became less dense and began to cool. After only a millionth of a second, protons (hydrogen nuclei) and neutrons could form. After a few minutes some of these subatomic particles came together to create helium nuclei. 

However it was not cool enough for electrons (which formed soon after protons and neutrons) to join with protons (hydrogen nuclei) or helium to make the first neutral atoms until about 380,000 years later. Matter in the early universe was smoothly distributed across space. However some of the hydrogen and helium were drawn together by gravity into clumps. These clumps were the seeds that eventually became countless trillions of stars, billions of galaxies, and other structures that now form most of the visible mass of the universe.

These stars provide us with another piece of evidence that the universe is aging. The hydrogen in stars is being changed by fusion into helium and bigger elements. As time goes on the hydrogen continues to be used up and turned into helium. When the first elements formed after the Big Bang about 92% were hydrogen, with the rest being 8% helium and traces of lithium. Today we have about 74% hydrogen and 24% helium as well as the 90 other naturally occurring elements. 

By measuring how much hydrogen the universe started with, and the rate that stars use it, we can determine when stars began fusing. We can also use this rate to estimate how much longer fusion will continue. Right now you live in the middle of the universe’s life. Hundreds of billions of stars in hundreds of billions of galaxies are using up hydrogen and turning it into heavier elements as the universe continues to age.

After the publication of the Big Bang hypothesis, many astronomers still thought the universe was static (or not changing). However, the Big Bang Theory made a prediction that was different from what was expected in a static universe. It predicted that there should be some heat energy left over from the Big Bang. With the continued expansion of the universe, the temperature should be very low today, only a few K (Kelvin) above absolute zero (Kelvin is an absolute temperature scale). 

In 1964, two researchers at Bell Laboratories built a microwave receiver for telecommunications research. Using it, they discovered microwave radiation in all parts of the sky. The radiation had a temperature of 3 K (Figure below). This is evidence in favor of the Big Bang and is called the cosmic background radiation.