Expansion of the Universe

  • Thread starter WendyE
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  • #1
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I was wondering how much we know these days about how the universe is expanding. Has space actually been stretching since the big bang? Or is new space being created somewhere? Or maybe the matter and energy of the universe is just expanding into space that already exists... Or is the matter and energy creating new space itself as it pushes outward?
 

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  • #2
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I believe I've read that space is stretching, but this somehow doesn't make sense to me. We can't percieve the stretching of space ourselves, can we? If everything is expanding, then we are too, but relative to everything else, we wouldn't notice any difference. So, how could we actually observe things getting farther away?
 
  • #3
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WendyE said:
I believe I've read that space is stretching, but this somehow doesn't make sense to me. We can't percieve the stretching of space ourselves, can we? If everything is expanding, then we are too, but relative to everything else, we wouldn't notice any difference. So, how could we actually observe things getting farther away?

Consider coins glued just in one point over a balloon; if you inflate the balloon what you see is the bi-dimensional view of how the universe is behaving. At least @ my understanding.

gijeqkeij

Universe it's so simple that it's almost impossible for us to understand it
 
  • #4
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Hmm... but what if instead of coins, you make little marker dots all in one area on the surface. As you blow up the balloon, the marker dots expand too. I guess the space between the dots will expand also, but would a stretched dot be able to percieve the difference? It would look porportionally the same, it seems like, wouldn't it?
 
  • #5
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WendyE said:
Hmm... but what if instead of coins, you make little marker dots all in one area on the surface. As you blow up the balloon, the marker dots expand too. I guess the space between the dots will expand also, but would a stretched dot be able to percieve the difference? It would look porportionally the same, it seems like, wouldn't it?

yes, but the universe seems not to follow this way. Don't ask me why...

gijeqkeij

Universe it's so simple that it's almost impossible for us to understand it
 
  • #6
EL
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Rulers (as well as other material stuff) do not co-expand with the universe. How would we, as you mensioned, otherwise be able to measure the expansion?
The expansion only takes place on large scales where our universe can be considered homogenous and isotropic. For example the planet orbit radii in our solar system do not co-expand, and neither do our galaxy or even our local group of galaxies.
 
  • #7
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Ok. Maybe space is expanding, but where there's matter, it isn't expanding as rapidly because of gravity. Matter stitches space together. Then expansion would cause space to be stretched more near matter, and increase gravity (or maybe space stretching outside of matter is responsible for gravity?). Did I read somewhere that gravitational constant may not be constant over time? Or was that debunked?

This is too much speculation based on not enough knowledge, I know. I'll do some reading. Sorry...
 
  • #8
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Ah... thanks for your response EL. I wrote my last one before I read it...
So, the universe isn't expanding uniformly. I see..
 
  • #9
EL
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The thing is that the expansion comes from a solution of the equations of General Relativity for a homogenous and isotropic matter distribution. And our universe is only homogenous and isotropic on very large scales. On smaller scales (like the solar system) where we see "clumpiness" in the matter distribution this expanding solution does not hold anymore. The solution is instead static and no expansion takes place.
But on large enough scales, the expansion is uniform!
 
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  • #10
DaveC426913
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WendyE said:
...where there's matter, it isn't expanding as rapidly because of gravity. Matter stitches space together. Then expansion would cause space to be stretched more near matter...
It is not as complex as that. There is not a mystical connection between the amount of mass in a location and how much the space is expanding there.

The balloon-with-pennies analogy is correct.

The expansion of space is extremely weak, much weaker than any forces between atoms, molecules and even weaker than gravity. It acts everywhere, it just doesn't have an effect everywhere.

The analogy to the balloon is that the glue holding the pennies onto the balloon is much weaker than the metal of the pennies. While the balloon does pull on the pennies (by way of the glue), the force has an inconsequential effect on them.
 
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  • #11
pervect
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Several suggestions:

First of all, try Ned Wright's cosmology FAQ. I've cut and pasted it here so you can get an idea of what the whole thing is, but the original has links for additional reading.


http://www.astro.ucla.edu/~wright/cosmology_faq.html#MX

Are galaxies really moving away from us or is space just expanding?

This depends on how you measure things, or your choice of coordinates. In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift. General relativity explains how to transform from one view to the other, and the observable effects like the redshift are the same in both views. Part 3 of the tutorial shows space-time diagrams for the Universe drawn in both ways.

In the absence of the cosmological constant, an object released at rest with respect to us does not then fly away from us to join the Hubble flow. Instead, it falls toward us, and then joins the Hubble flow on the other side of the sky, as discussed by Davis, Lineweaver & Webb (2003, AJP, 71, 358). In what are arguably the most reasonable coordinates, the cosmic time t and the distance D(t) measured entirely at the cosmic time t, the acceleration is given by g = -GM(r<D)/D2 where M(r<D) is the mass contained within radius D. This gives g = -(4*pi/3)*G*(rho(t)+3P(t)/c2)*D(t). The 3P/c2 term is a general relativistic correction to the otherwise Newtonian dynamics. Galaxies all move under the influence of this acceleration and their initial position and velocity. In other words, F = ma and gravity provides the force. Nothing extra or weird is needed.

Also see the Relativity FAQ answer to this question.


Why doesn't the Solar System expand if the whole Universe is expanding?

This question is best answered in the coordinate system where the galaxies change their positions. The galaxies are receding from us because they started out receding from us, and the force of gravity just causes an acceleration that causes them to slow down, or speed up in the case of an accelerating expansion. Planets are going around the Sun in fixed size orbits because they are bound to the Sun. Everything is just moving under the influence of Newton's laws (with very slight modifications due to relativity). [Illustration] For the technically minded, Cooperstock et al. computes that the influence of the cosmological expansion on the Earth's orbit around the Sun amounts to a growth by only one part in a septillion over the age of the Solar System. This effect is caused by the cosmological background density within the Solar System going down as the Universe expands, which may or may not happen depending on the nature of the dark matter. The mass loss of the Sun due to its luminosity and the Solar wind leads to a much larger [but still tiny] growth of the Earth's orbit which has nothing to do with the expansion of the Universe. Even on the much larger (million light year) scale of clusters of galaxies, the effect of the expansion of the Universe is 10 million times smaller than the gravitational binding of the cluster.

Also see the Relativity FAQ answer to this question.

The short version - think about observable differences (if any) between space expanding and the objects moving away from us. If you can't find any, a difference that makes no difference is no difference.

Note also that the solar system is NOT expanding, no matter how you look at it.

A final note: you might also want to try the cosmology forum.
 
  • #12
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Awesome! Thanks for the references.

Yes... I should have posted in the Cosmology forum. Actually, when I first posted, I was thinking I was going to ask a question about how light travels through space, but then realized I had questions about space first.
And I now realize that I don't really know enough to be asking good questions yet! Thanks all...
 
  • #13
DaveC426913
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WendyE said:
And I now realize that I don't really know enough to be asking good questions yet! Thanks all...
No, you're doing fine!

The thing about asking is that, even if you don't get to the answer right away, people are happy to guide you there.
 

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