Just How Big Is the Damn Universe Anyway?

September 19th, 2008

There’s a basic misconception floating around out there about the size of the universe.   Astronomers trying to be accurate are probably guilty for some of this because we always hedge our bets.

We know that the universe is big, really big, from Douglas Adams.

He’s right, but there’s a difference between infinitely large and really big, and the fact is that we don’t know for certain which term applies for sure to our universe.

People talk about the entire universe starting in a singularity, a single point, with infinite density.   This is WRONG.

The entire observable universe today, previous to a period of inflation, was “infinitely small” in some sense to the limits of our physical understanding (it is possible to put some quantitative numbers on this, but it doesn’t help this discussion too much).   The truth is that the big bang is consistent with being an infinitely large, infinitely dense space.   As time goes on we can see farther away and see a larger universe out there (at least until some point where an accelerating expansion puts the distant universe beyond the limits of our vision).

OK, I’m not sure I’m not being confusing here.   The issues ARE confusing.   It’s understandable to be confused.   I have been on these issues before, and probably will get confused again about them when I think too seriously about the issues.

Oh, and about the issue of size of our universe, this is hard to even define.   The observable universe is the part we can see, which light has had time to cross, but keep in mind that the universe has been expanding while that light has been coming to us and has gotten bigger since then.

There are also some artifacts that might be expected if we lived in a finite universe, which is not favored, and the fact that we don’t see these features in the microwave background radiation lets cosmologists put some limits on the size of the universe.

So, let me finally put some numbers on this stuff:

We can see back to some few hundred thousand years after the big bang, about 13.7 billion light-years away.   Double that for about 27 billion light-years across.

The universe has continued to expand over that time, and if we could put up a ruler “now” between us and those distant parts of space, it would measure something like three times the above answer for a simple, closed universe model.   That would make it 79 billion light-years across.

However, for the preferred benchmark model we think is likely, that includes dark energy, it’s more like six times and the answer is 156 billion light-years across.   And that’s just the part we can see and how big it is today some 14 billion years after the big bang.   There’s more universe out there as far as we can tell.

Big enough to justify the ‘damn’?


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