How did the universe begin? Or did it begin?
Sir Fred Hoyle
didn't think so. He derisively coined the term the Big Bang. Now Fred Hoyle is dead and the theory is still alive.

Fourteen billion years ago, give or take a couple billion years, the universe as we know it wasn't here. Today we believe we live in an expanding universe. The event which started it all is known as the Big Bang. Why do we say fourteen billion years?
Well that is an estimate based on twentieth century research.
In the early 20th Century, an astronomer called Edwin Hubble made the discovery that distant stars emitted the same light as close-up stars (and the Sun), except they appeared more red. What's more, the more distant the star, the redder the star appeared. He recalled that another man Vesta M. Slipher had found the same thing in 1912 while researching the spectra of Nebulae.
Hubble reasoned that since the wavelengths of red are longer something must be stretching them. In 1929 he came up his formula v=Hd to account for the
red shift. As things recede, their wavelengths become longer. As things approach their wavelengths are compressed. In sound this is called the The Doppler shift
. The train whistle of a receding train is lower in pitch because the wavelength of the sound is being stretched out.

Since space is expanding, there must have been a time in the past when the universe was small. Perhaps infinitely small.
Here is what some think happened back then.
There was a singularity ¹ . In this singularity was all the matter needed to make a universe. This singularity exploded.
Maybe because of natural causes, or maybe because of divine intercession.
This explosion created temperatures on the order of 10³³ degrees. According to one model *
at 10^-35 seconds the universe made a large increase in size as the temperature cooled. This lasted until about 10^-30. Here the rate of expansion slowed.By the end of the first second it was cool enough for the first elements to appear. The Universe continued to cool and to grow.
As it cooled there was a clumping effect which gave birth to galaxies. Within those galaxies stars began to form. Many say that stars are still being born today, billions of years later. Microwave background radiation from the original state lingered and was used as evidence of the Big Bang.
The horizon line continued to move outward as the universe expanded. Some wonder if it will continue expanding forever.

What does this mean for us? Will the universe just keep expanding? Well, there are at least two forces at work here. One of these is the momentum from the original explosion. It (obviously) makes the universe expand. The opposing force is gravity. It will pull the universe back to whence it came.

If the momentum and the gravitational effect are equal, then the universe will continue to expand but at a decreasing rate. If there are other forces, outweighing the gravitational attraction then the rate of expansion may actually increase. But if there's more gravity, then eventually the universe will collapse back into a singularity. Some call this the Big Crunch. This may be followed by another Big Bang. We don't know.

Scientists like to think that the forces are balanced. It creates a nice flat universe and makes their calculations easier. How can we know if we have enough gravity to continue growing and never stop? That is a question being researched by Cosmologists.
Gravity is proportional to the mass of an object.We have methods of estimating the mass of objects in deep space. So we look for all the stuff in the universe, and from that calculate its mass, and therefore its gravity. Unfortunately, it's hard to see objects in the universe... we can only see bright things like stars, and they only make up about 10 percent of the critical mass we need. The other 90 percent is either dark matter or dark energy,(of which little will be said here).

Another evidence for the Big Bang is the Cosmic Microwave Background. Two scientists, by names of Penzias and Wilson, picked up some microwave signals in their antenna receiver.This background gave a uniform temperature of 3 degrees K in all directions.
This is consistent with the Big Bang theory.

This brings us back to the late Fred Hoyle. He didn't like this theory. Instead, he proposed the Steady State Theory, which provided an alternate version of cosmology. The discovery of the Cosmic Background Radiation and the confirmation of large amounts of Helium in the Universe 1000 seconds after creation were said to" spell the death knell"of the Steady State Theory.²

__ There are heaps of questions still unsolved, such as, where did all the Big Bang material come from? What happens before the beginning of time if anything? Is time something that also contracted to a point at the big bang?
What happens when you get a universe of zero size and infinite density? Is it the same as a black hole? We don't know. Perhaps the efforts of Cosmologists will bring some answers in our lifefime.
Perhaps not. But when you are dealing in Billions of years what is man that thou art mindful of him? Perhaps the answers are too big to fit in our small minds. Was there a Big Bang. The author of this article thinks that there was; but evidence is still coming in , and researchers will continue to wrestle with this issue for many years to come.

Related BBC LINKS
Cosmology of Solar system
Big Bang on BBC
Horizon on Massive Black Holes

Other References
Twentieth Century Cosmology
Time Graph