How Big Was the Universe When It Was First Born?

StartsWithABang writes: Looking out at the distant stars, galaxies and radiation in the Universe today, we've been able to determine not only what it's made out of, but how long it's been since the Big Bang: 13.8 billion years. Put all that information together, and you can also figure out how large the observable part of that Universe is today. From our point of view, it appears to extend for 46.1 billion light years in all directions. So what if you extrapolate backwards, to the very end of inflation and the start of the hot, dense state we identify with the Big Bang, and ask how large that 46.1 billion light year "size" was back then? How big would it be? Depending on the particulars of when inflation came to an end, the answer is somewhere between the size of a soccer ball and the size of a city block, no smaller and no larger.

Comments

Re:Refers to Observable Universe Only

[Anonymous Coward]

You've managed to argue that a definitely finite 2-d surface is infinite. The infinite surface would be mapped onto a flat plane.

The relevant 3-d surfaces are hyperspheres, flat surfaces, and hyperboloids (ie saddles). The first is definitely finite, the other two ae infinite pending any further topological assumptions. However, a flat surface can still lead to a finite universe for a given topology, the simplest and most common example being a torus; this looks like Pacmanland, given that if you walk in any direction you get wrap round.

It's worth noticing that as a local theory, general relativity (and indeed all metric theories of gravity) says nothing about global topology.

It is not ratioanal to believe in the big bang!

[anwyn]

The early Universe postulated by the big bang was low in entropy. Another way to say low in entropy is low probability! The probability that the matter and energy of the Universe organized itself into the form postulated is an extremely low number. I mean really really really really small! Take the smallest number you can think of and make it a gazillion time smaller than that!

Now consider the probability that human beings made a fatal error in constructing the big bang theory. This number may be extremely small, but it is a gazillion times bigger than the former number!

The more likely possibility is that human beings made an error. Thus it is more rational to believe in the possibility with the greater probability, namely that human beings made a fatal error in constructing the big bang theory.

Welcome to David Hume's argument against miracles! It is valid.