A second question concerning the Big Bang model of the universe has become known as the Horizon Problem. Essentially, the very homogeneity of the universe, as measured from the cosmic background radiation (see previous post), requires some explanation.
The problem concerns the finite age of the universe versus the finite speed of light. When you do the math, it turns out that the furthest flung regions of the universe are further apart than light could have travelled in the age of the universe. A simple claculation shows that the furthest regions could never have been in thermal contact – yet they have the same temperature to 1 part in 100,000.
So we have a paradox: the homogeneity of the background radiation suggests that all of the observable universe was once in contact long enough to reach thermal equilibrium, while simple calculations based on rewinding the Hubble graph suggest that the universe is too big for this to have happened in the time available.
Artist’s impression of the horizon problem
What is the solution to the paradox? One interesting solution could be that the speed of light in the very early universe was different from what we measure today. A less drastic solution is that we have made an unjustified assumption – namely, by extrapolating the Hubble slope back to the very early universe, we have superimposed an expansion rate of one era on an earlier era we know nothing about.. more on this later.
Antimatter 
Hello, I’m a physicist working for Spanish Nuclear Regulatory Council and I don’t understand this “horizon problem” from the explanations I read (wikipedia, blogs, etc.)Maybe you can explain better.
Apparently, it is based on the fact that we see the same CMB temperature from both sides of the universe (lets say east and west) and we are looking at a distance of 14000 mill light-years on each side. Then people say that the distance from east to west side is the sum of both! And then light didn’t have time to communicate both sides in the age of the universe.
But this is absurd. When we look to the past 14000 mill years, the universe was not as big as it is now. You cannot simply sum the distances. In fact, so far in the past the universe should be very small and for me it is evident that the whole of it was in the light cone of the big bang point. And on top of it, prior to the universe becoming transparent to light, light should be travelling much slower because of the extreme density of the plasma then.
I’m puzzled. How can everybody think thad the universe was 46000 mill light years big 14000 mill years ago?
Enrique
It is indeed confusing. What helps is to remember that we don’t really know anything of the initial BB – all we can deal with is what we can see today: the Hubble parameter, the age of the universe and the speed of light. Instead of starting at time zero (unknown zone), all we can do is rewind the expansion we see today. When we do this we get a shock: the size is wrong for distant parts of the univ to have known each other (which they must have done, both frm temp measurememnts and from GR)
You mean you apply standard cosmo model (Robertson-Walker metric) from now back to the so called recombination time and the size you get for the whole plasma soup is too big?
Anyway, this would not mean what I frecuently read that the distance between east and west is double the scale factor… I suppose what you imply is that you still get a size too big but, of course, not 26000 mill lightyears.
And, excuse my curiosity, how are this calculations done? Do you take into account a variable Hubble constant? How it is estimated backwards in time?
Well, maybe you can forward me to a reference for this type of calculations so that I don’t get too much of your time.
Thanks very much for the answer!
Best
Enrique
Another solution of the horizon problem may be that the universe is infinite and, on a large scale, uniform. The cosmic background is therefore the same in all directions, and by implication, is ancient but not the product of one event.
Another question this solves is why the cosmic background does not exhibit an asymmetry due to our position relative to the epicentre of the original inflation. Not all observers can be at the midpoint of the universe. If its supposed 3-dimensional size has any reality it follows (in a Big Bang universe) that we are almost certain to be significantly nearer to the horizon in one direction than another and might expect to see a difference.
special theme?