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Old May 13th 2014, 02:24 PM   #1
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Spacetime

Hi,

I'm wondering about something. First, accordingly to phycisicits space is expanding. So spacetime is expanding. Would it be possible to think that space near massive objects expands less quickly? I mean, because of general relativity, mass effects spacetime. Therefore spacetime contracts because of the mass.

Hope someone could answer this hypothesis.
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Old May 13th 2014, 03:32 PM   #2
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Originally Posted by Dylss View Post
Hi,

I'm wondering about something. First, accordingly to phycisicits space is expanding. So spacetime is expanding. Would it be possible to think that space near massive objects expands less quickly? I mean, because of general relativity, mass effects spacetime. Therefore spacetime contracts because of the mass.

Hope someone could answer this hypothesis.
This is not really a good question for absolutes. Yes the Universe is expanding and yes space-time is distorted near massive objects. But bear in mind that space-time is "flexible" so the massive object an still expand like everything else even though locally things are being contracted.

There is a large scale version of this...the Universe as a whole is expanding, but the Andromeda and Milky Way galaxies are on a collision course and will collide some large number of millions of years.

I'm not sure that I've really answered your question. If I haven't just let us know.

-Dan
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Old May 14th 2014, 08:04 AM   #3
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accordingly to physicists space is expanding.

Yes.

So spacetime is expanding.

No! Think of spacetime as "space at all times". You could depict it as in inverted cone. Or as a "bell" shape on its side like the NASA picture on Wikipedia. But it's space that's expanding, not spacetime.

Would it be possible to think that space near massive objects expands less quickly?

Yes. See the raisin-cake analogy. Space expands between the galaxies but not within. It can't expand if its gravitationally bound. Or electromagnetically bound.

I mean, because of general relativity, mass effects spacetime. Therefore spacetime contracts because of the mass.

Ummm. This is getting tricky. Gravity is the result of a concentration of energy, not just mass. And curved spacetime is not curved space. See the bottom of this Baez article where he says this:

"Similarly, in general relativity gravity is not really a 'force', but just a manifestation of the curvature of spacetime. Note: not the curvature of space, but of spacetime. The distinction is crucial".

But see this depiction of a gravitational field? It's back to front. It features tension pulling space inwards. It ought to feature pressure, like the Earth is pushing out on the lattice. See the stress-energy-momentum tensor. Look at the diagonal. It's an energy-pressure diagonal. And there's a radial length contraction. So in a way, mass does make space contract. But not much.
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Old May 14th 2014, 03:00 PM   #4
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Thanks for clearing things up.

It could be i didn't use the right words for space and spacetime. For me they seem strongly connected. If space curves, so will spacetime. Because of they rely on each other.

But i'm still not completely clear about this isn't true. Try seeing it from the other side. If dark energy wouldn't exist, gravity would be a bigger influence? Maybe the answer of this question makes it easier for me to understand the fundementals of expanding space.

Thanks!
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Old May 15th 2014, 02:35 AM   #5
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Originally Posted by Dylss
If space curves, so will spacetime. Because of they rely on each other.
Curved spacetime isn't curved space. Look at the bottom of this Baez article:

"Similarly, in general relativity gravity is not really a 'force', but just a manifestation of the curvature of spacetime. Note: not the curvature of space, but of spacetime. The distinction is crucial".

Perhaps I can explain this distinction via an analogy. Imagine you're standing on a headland, looking out over the sea. It's flat calm, glassy like a mirror. On your left is an estuary, on your right is the open ocean. So there's a salinity gradient from right to left. After a while you notice something out to sea, coming towards you. You realise that it's a wave, just one, all on its own. You plot its course, and you come to appreciate that its path is slightly curved. It's curving slightly towards the right because of the salinity gradient. This wave is standing in for a photon. Its path is curved because the water density isn't uniform. The water is standing in for space. The path of the wave through space over time is curved, so we say spacetime is curved. Now look back to that wave moving across the ocean. Look closely at the surface of the sea where that wave is. It's curved.

Originally Posted by Dylss
But i'm still not completely clear about this isn't true. Try seeing it from the other side. If dark energy wouldn't exist, gravity would be a bigger influence? Maybe the answer of this question makes it easier for me to understand the fundementals of expanding space.
I can't give you an authoritative answer on this, just a personal take: see the stress-energy tensor on Wikipedia? Look at the picture on the right. See the energy-pressure diagonal? Think of a gravitational field as a "pressure gradient in space". Now look at dark energy and note the reference to "negative pressure". Negative pressure is tension. Like in the skin of a balloon. See the balloon analogy used in cosmology to represent the expanding universe. A balloon is the size it is because the pressure inside is balanced by the tension in its skin. And there are two ways to make that balloon bigger. You can increase the pressure or you can reduce the tension. Think bubblegum. If the skin gets thinner it expands, so the skin gets thinner so it expands some more and so on. Space of course is three dimensional, so the balloon analogy only takes you so far, and nobody knows the answer for sure. For myself I play around with stress balls and silly putty and plumbers' expanding foam to try to get an understanding of what might be going on. I'd say ask around and get as much input on this as you can.
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Old May 18th 2014, 03:00 AM   #6
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Originally Posted by topsquark View Post

There is a large scale version of this...the Universe as a whole is expanding, but the Andromeda and Milky Way galaxies are on a collision course and will collide some large number of millions of years.
Now that you raise the question ... we can measure the radial velocity component of M31 (the great galaxy in Andromeda) with respect to the Galaxy more or less directly, but how do we know/measure the velocity component of M31 in the plane of the sky?

.

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Old May 18th 2014, 03:26 AM   #7
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Originally Posted by zzephod View Post
Now that you raise the question ... we can measure the velocity component of M31 (the great galaxy in Andromeda) with respect to the Galaxy more or less directly, but how do we know/measure the velocity component of M31 in the plane of the sky?

.
We can measure the lateral speed of the galaxy by noting the displacement angle and direction. I presume that is what "velocity component in the plane of the sky" means. To get the speed perpendicular to this plane we can measure its redshift.

-Dan
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Old May 18th 2014, 12:45 PM   #8
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Originally Posted by topsquark View Post
We can measure the lateral speed of the galaxy by noting the displacement angle and direction. I presume that is what "velocity component in the plane of the sky" means. To get the speed perpendicular to this plane we can measure its redshift.

-Dan
But at some billions of years (~6 Gyr), and the measurements of the relative velocity of M31 wrt the Galaxy is difficult, requiring the measurement of a very small proper motion and the subtraction of a number of velocities which are somewhat similar to remove the Sun's velocity about the Galactic centre from the estimate of the motion of M31 wrt the Sun.

The direct measurements of M31's proper motion is recent dating from ~2012.

One reference is here and that should have references to the other key papers.

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Last edited by zzephod; May 18th 2014 at 12:52 PM.
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