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How Big is the Universe?

As I continue to struggle through the task of listening to over 100 episodes of the AstronomyCast podcast (about 30 minutes each) I find more new and interesting ideas being presented. The podcast is a good one – the general level is not overly complex but it isn’t dumbed down either. I have noticed a few errors and misleading statements but its easy to be overly critical – even of a professional astronomer – because she is discussing ideas outside of her specialist area.

Minor errors aside, I do recommend this podcast if you are interested in astronomy because it is useful for both beginners and more experienced amateurs. It has been years since I taught astronomy myself and its amazing how much has changed in that time. I first discussed this in a blog entry titled “Things Have Changed” from 2009-02-16 and have mentioned it a few times since.

The latest thing which I have noticed has changed is our understanding of the size of the Universe. I can remember that the standard way to teach this in the 80s was to say the Universe had a radius of less than 14 billion light years because that was the maximum distance it could have expanded at less than the speed of light in the time available (its age at the time was estimated at 14 billion years).

Well things have changed since then. The age was quite accurate – we now say it is 13.7 billion years – but the rest is very different!

Here’s one current interpretation of the size of the Universe: the observable Universe is 78 billion light years in radius and that part is only about 1% of the total. To make things worse, the Universe we can see is only composed of 5% of stuff we understand quite well – the rest is dark matter and dark energy which is not well understood currently.

If you want to be more speculative still you can hypothesise the existence of a multiverse, as I discussed in “Is There a Multiverse?” from 2009-03-03, but even without that its obvious that the Universe is a lot bigger that we used to think (the multiverse theories usually say it is infinite).

But to get back to the commonly accepted values above. How can the radius be more than the age? Doesn’t that mean the Universe expanded faster than light? Well, yes, it does actually. Space is allowed to expand at any speed. Its only physical objects which can’t exceed the speed of light. So an object which emitted light 13 billion years ago could now be much more than 13 billion light years away because the space between it and us has expanded so much.

And the 99% of the Universe being invisible is a result of the ultra-fast expansion during the inflationary period. Star light (or anything else) from that part of the Universe will never reach us because the space between it and us is expanding too fast.

As I said in previous comments on the subject, the Universe just keeps getting bigger and more mysterious and our part in it seems to get less and less significant. But that’s just what we would expect based on history.

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