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Fraser
2007-Oct-29, 03:00 PM
It's about time for a question show again, so we'll have one last interruption to our planetary tour, to deal with the questions that arose from our <a href="http://www.astronomycast.com/cosmology/episode-58-inflation/">inflation show</a>.

So if you still don’t understand inflation, take a listen to this week's show and as always, send us your questions.

<strong><a href="http://media.libsyn.com/media/astronomycast/AstroCast-071029.mp3">Episode 60: Questions on Inflation (19.6MB)</a></strong><br />&nbsp;<br />

Read the full blog entry (http://www.astronomycast.com/cosmology/episode-60-questions-on-inflation/)

EvilEye
2007-Oct-29, 06:45 PM
Pamela's beetle analogy can further be simplified.

At themeparks, and airports, they have moving walk-ways (like a flat escalator) that go about 2 mph also.

As you walk, you are going about 2 mph.

When you walk onto the mover, you are still walking at 2 mph relative to it (space), but to a person that is not on the mover, you are moving at 4 mph... but without speeding up your motion. You've doubled your apparent speed relative to everything outside the mover, but to you, you haven't done anything different.

Steve Limpus
2007-Oct-30, 03:13 AM
When we look at distant galaxies receding due to the expansion of space, everywhere, in all directions, aren't we, as the observer, the underlying frame of reference? The universe is expanding relative to us, to our position; and the same thing applies to any observer anywhere in the universe. We're all at the very centre of our own expanding observable universes. Mine is different to yours by a couple thousand miles or whatever distance separates us.

But we observers don't violate relativity; relativity always refers to our velocity through the space around us, wherever we are. The expansion of space does not add to my velocity through space because the local expansion of the space around me is tiny (it only becomes significant over vast distances when the expansion in between adds upon expansion, upon expansion... and the forces of my own particles, and gravity overcome any local expansion anyways) so I never violate relativity, and neither does anyone else, anywhere in the universe. Those distant galaxies don't move through their own local space faster than light, even though they appear to recede from us faster than light.

Did I get that right? :think:

Here's another thought: is 'expansion' always the right term? If I understand it right, the planck length/volume/time is not expanding--so then neither is spacetime really? (Even if the planck length was expanding, we couldn't tell anyway, because our 'measuring stick' would be expanding too?) Wouldn't it be more accurate to say spacetime is 'multiplying?' In other words: planck spaces aren't expanding as such, there are just *more* of them between us and distant galaxies? If that's right--is there any way to observe it?

When you stretch a rubber band, the number of units of rubber band (rubber molecules?) doesn't increase, rather the units themselves become longer. As space expands, the number of units of space does increase, but the units themselves don't become larger. I guess its the same difference between the anaolgy of raisin bread, and the anology of adding blocks of pavement, that Pamela has used before. No raisin bread is added. The pavement doesn't get bigger. Which anology is most correct?

If space truly was 'expanding' rather than 'multiplying' wouldn't it cause the constants of the universe to change--or the properties of spacetime to be 'diluted' in some way?

Does the force of gravity (and the other fundamental forces) hold back the expansion of spacetime--or does it somehow interrupt the mechanism by which spacetime multiplies? :confused:

Last thought: Let's say I take a line of sight to a distant galaxy, and there are a certain number of galaxy clusters along that line of sight, each constricting the expansion of the universe in each local region of space. Then, I take a line of sight to a distant galaxy in a completely different direction, where twice as many galaxy clusters are constricting the expansion of the universe along the way.

Would I observe any difference? Or would each line of sight appear the same to me today?

What about in the past: does double the galaxy clusters mean half the expansion, one line of sight compared to the other? Should I perceive my observable, expanding universe to be perfectly spherical; or is it lumpy over time?

Burt
2007-Oct-30, 12:18 PM
Very nice discussions/descriptions, but I've got 2 concerns.

Pamela said that Dark energy is trying to 'push' the Andromeda galaxy away from us at 58 odd km/s, while gravity 'pulls' us together at about 359 km/s, giving a resultant of some 301 km/s. I believe it will be the same if there was no Dark energy. The pure kinematical expansion rate (the 'momentum like' expansion) that slows down over time would do exactly the same thing without Dark energy.

On light orbiting a black hole, Pamela said that light can orbit at half the Schwarzschild radius. Isn't that rather at 1.5 times the Schwarzschild radius (r = 1.5Rs = 3GM/c^2)?

Burt
2007-Oct-30, 02:16 PM
... Those distant galaxies don't move through their own local space faster than light, even though they appear to recede from us faster than light.

Did I get that right? :think:

I think that's exactly what Pamela said.

Here's another thought: is 'expansion' always the right term? If I understand it right, the Planck length/volume/time is not expanding--so then neither is spacetime really? (Even if the Planck length was expanding, we couldn't tell anyway, because our 'measuring stick' would be expanding too?) Wouldn't it be more accurate to say spacetime is 'multiplying?' In other words: Planck spaces aren't expanding as such, there are just *more* of them between us and distant galaxies? If that's right--is there any way to observe it?
...
Does the force of gravity (and the other fundamental forces) hold back the expansion of spacetime--or does it somehow interrupt the mechanism by which spacetime multiplies? :confused:

Interesting thoughts, Steve. I think the jury is still out on the issue of "is space expanding" or is there somehow just more space "being generated between galaxies". Gravity does hold back the expansion of space and if it was not for dark energy, the expansion rate would have decreased forever, even possibly reversing one day.

What about in the past: does double the galaxy clusters mean half the expansion, one line of sight compared to the other? Should I perceive my observable, expanding universe to be perfectly spherical; or is it lumpy over time?

Astronomers observe the expansion to be uniform over very large scales, but 'clumpy' on smaller scales. The reason is that on the large scale there are roughly an equal number of galaxies in all directions and it is the large scale that determines the overall expansion rate. The observable universe is more or less spherical, of course, with very tiny 'lumps'.

NHR+
2007-Oct-30, 05:43 PM
On light orbiting a black hole, Pamela said that light can orbit at half the Schwarzschild radius. Isn't that rather at 1.5 times the Schwarzschild radius (r = 1.5Rs = 3GM/c^2)?

No, she definetly said half the Rs, though that does mean inside the horizon and is thus somewhat speculative, and she said that quite clearly, too.

If you put v = c in the circular orbit eq. and solve for r, you'll get
r = GM/c^2 = 0.5Rs.

I'm not sure about more elliptical orbits, but I don't think they can lie outside the horizon either.

Steve had some interesting points there above, but I don't think I wanna dislocate my brain just now...:)

Burt
2007-Oct-30, 06:16 PM
If you put v = c in the circular orbit eq. and solve for r, you'll get
r = GM/c^2 = 0.5Rs.

This is a strictly Newtonian view. The relativistic circular orbit equation includes a factor [1-2GM/(rc^2)]^(-.5) and it yields an orbital radius of 1.5Rs for light around a non-rotating black hole. For a popular (fun) account, see Prof. Kip Thorne's book "Black holes and time warps", 1994, Prologue, page 39. More technical info is available in many sources, e.g. Misner, Thorne and Wheeler's "Gravitation".

I'm actually quite surprised that Dr. Gay made this apparent mistake. Or am I missing something?

NHR+
2007-Oct-31, 12:08 AM
This is a strictly Newtonian view. The relativistic circular orbit equation includes a factor [1-2GM/(rc^2)]^(-.5) and it yields an orbital radius of 1.5Rs for light around a non-rotating black hole. For a popular (fun) account, see Prof. Kip Thorne's book "Black holes and time warps", 1994, Prologue, page 39.

Damn, I've like just finished reading that book by Thorne, but I still get it all wrong here! :doh: Yeah, my bad, sorry...

Steve Limpus
2007-Oct-31, 08:06 AM
Astronomers observe the expansion to be uniform over very large scales, but 'clumpy' on smaller scales. The reason is that on the large scale there are roughly an equal number of galaxies in all directions and it is the large scale that determines the overall expansion rate.

Thanks Burt--that makes sense. :)