I have mentioned recently a little about what the CMBR is and why scientists think its so important to understanding the formation of our universe. it's easy to wonder though, just because you can see a relatively uniform (at least in the space between galaxies) wash of microwaves, how does that indicate some kind of formative explosion?
To undertsand that we have to understand something called redshift.
Ever stand near a moving train or car while some sort of horn is blowing and noticed that it was a little higher pitch before it passed you, and lower afterwards? Or perhaps you saw a jet plane pass over you before you heard it, and when you did it was really loud? The reason for this occurence is known as the Doppler effect (I just like to call it Doppler Shift). To undertstand what that is first you have to know a little about how sound works.
Now, we call those ripples waves, and they only stay circular like that as long as nothing is moving. In the case of a moving car or train, or even if you are talking to someone while walking these sound waves can "bend" a little. Right in front of the car or train or whatever is making noise these waves can "bunch up" and get compressed, because of this you hear it at a higher pitch than normal. And behind the source of this noise the waves get "stretched out" and you hear them at a lower pitch. This happens more and more as you approach the speed of whatever it is we are talking about (in this case the speed of sound).
Anyway, thats the idea behind doppler shift, in case you were wondering where I was going with this, many different things are expressed as these waves not just sound. Earthquakes, waves in water, and most importantly light are all different kinds of waves. All that matters is what they move through (sound through air, waves in water, earthquakes in land) however light is special, as of the current day nobody understand exactly what it is that light moves through, or if it has to move through anything at all since it can exist in a perfect vaccuum.
Everything I just mentioned applies to light as well as sound, however light is much less noticable, and it only really matters when you start approaching the speed of light (so very incredibly fast that nothing we have created even comes close, breaking the speed of sound was hard enough). Being able to notice these small distortions in light is the basis of why physicists think the universe was created in a large explosion, or "big bang".
To relate, just like the sound waves bunch up in the direction the car is moving, if you were to take a lightbulb and move it really fast the light waves would bunch up too, and you would see the light as "blueshifted", contrary to that behind the lightbulb you would see light that has been stretched out or "redshifted".
So here is the deal, and kinda the point of this whole thing. When an astronomer analyzes light coming from a star thats really far away, it looks as if it is redshifted, that is the star he is looking at is moving away from us at some speed based on just how red it is. The thing is, no matter what star you are looking at the further away it is the more redshifted it looks. This leads scientists to believe that the universe is expanding in every direction at all times, thus that a large "explosion" was where the universe began.
To summarize stars that are very distant are "redshifted" or moving away quickly. This combined with the CMBR are the basis for the "big bang theory", and its why scientsts tell you they can figure out just how long the universe has been around, because if we know how fast everything is moving away we can figure out how long its been moving like that.
Next time ill wrap up Cosmological Recycling and explain why these scientists may have been mistaken, as always the facts dont neccecarily add up to the conclusions we want to believe.