Sunday, March 07, 2010

Expanding universe

Everybody Knows that the universe is expanding. Cuz that's what the clever people say. But I'm wondering how they can say it. What is the universe expanding into?
If it's expanding into something, why isn't that something part of the universe? And if it's expanding into nothing, what do you measure it's size against?

DDD links to an attempt at answering.


Alexander Greene said...

Edwin Hubble, the astronomer, discovered that all of the galaxies he could see had a peculiar red shift to them - like the Doppler shifting of a police siren as the car screams past you and recedes from you.

So he thought that the red shift meant that everything in the universe is expanding away from us, and that the further away we are from these objects, the faster they are moving from us - as if the universe was a skin of an expanding balloon.

What they thought was that if we are expanding, it is because we have the presence of Something - matter, energy - and the space into which we are expanding has Nothing - no matter, no energy, nothing formed.

It could be even simpler than this. We're living in a holographic universe, where the "fabric of space time" has been shown to be a kind of a grainy quantum-level foam. It's not a smooth void, as Hubble might once have thought.

And the light from a distant corner of the universe has a long way to travel across that grainy foam. A very long distance, and a lot of foam.

So maybe the reason why we see a lot of red shifting is not because everything's moving away from us, but because we are looking at photons which have been travelling for a billion years across a not quite frictionless surface ... and those photons are very old, and very tired.

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Eolake Stobblehouse said...

Thanks, Alexander, those are interesting thoughts.

Ray said...

Another consideration in all that is that as light passes close to a star or our own sun, the light-rays get bent from their original path by the intense gravitational effect of that
star or sun. Across a galaxy, there
would be many instances of that between us and the original light source. Across a universe, it would be an even greater effect. So these red shifts or blue shifts may not be exactly what we first thought.

Kent McManigal said...

If this kind of thing interests you, I highly recommend you read some Michio Kaku books. You'd love them and your imagination will be tested and stretched.

DeltaCubed said...

What is the universe expanding into?

I am very confused about things my science book says about the expanding universe. Every book I have seen has defined the universe as "everything". If the universe is expanding what is it expanding into? It would have to expand into even more universe. I understand that the red spectra indicates that things are moving away from us but that is drifting not expanding, right? If you could help me to understand this, it would be appreciated. Thank you for your time.

This is a very good question which is not at all easy to give a satisfactory answer to! The first time I tried to write an answer to this, we got so many follow-up questions from people who were still confused that I decided to try to answer it again, this time much more comprehensively. The long explanation is below. However, if you just want a short answer, I'll say this: if the universe is infinitely big, then the answer is simply that it isn't expanding into anything; instead, what is happening is that every region of the universe, every distance between every pair of galaxies, is being "stretched", but the overall size of the universe was infinitely big to begin with and continues to remain infinitely big as time goes on, so the universe's size doesn't change, and therefore it doesn't expand into anything. If, on the other hand, the universe has a finite size, then it may be legitimate to claim that there is something "outside of the universe" that the universe is expanding into. However, because we are, by definition, stuck within the space that makes up our universe and have no way to observe anything outside of it, this ceases to be a question that can be answered scientifically. So the answer in that case is that we really don't know what, if anything, the universe is expanding into.

Now, for those of you who want a more comprehensive discussion: see

Ask an Astronomer is run by volunteers in the Astronomy Department at Cornell University.

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