Ted Bunn on expansion, redshifts and the dangers of 'rubber sheets'

[Wallace]

Ted Bunn (a cosmologist) has written an interesting blog post following up on papers he and others have written. You can find it http://blog.richmond.edu/physicsbunn/2009/12/02/interpreting-the-redshift/". While this is probably not a good place to start if you want to learn about the expanding Universe from scratch, I do think there are some very important points made that anyone wanting to go the next step, and get to grips with cosmology beyond the 'party supplies' level, should understand.

The basic idea is that while the 'balloon analogy/expanding space' type analogies have their use, they also have siginificant drawbacks, in terms of engendering confusion about how to interpret recession velocities and other issues. Bunn certainly doesn't argue that this analogy has no use, but points out that insisting on using this alone, or insisting on physical truths derived from it must hold are dangerous.

I'd stress, as does Bunn, that this is a question of interpretation only, but none the less an important question.

 

[Chronos]

The balloon analogy is a 2D model. It is fairly certain the 'real' universe is at least 3D+1. I think Ted Bunn is trying to raise awareness above the balloon 'trap'. My sources suggest you are a Bunn synpathizer, Wallace - XD.

 

[Wallace]

I don't think the fact that the balloon model is 2D is a problem, it's a potential pitfall if someone mistunderstands that part I guess, but I think that's a peripheral issue.

 

[Ich]

Thanks Wallace,

that article expresses exactly what I think.
A core part, IMHO:

In its most extreme form (found in a lot of textbooks and pop-science books), the rubber-sheet metaphor says that distant galaxies are not moving with respect to us — that is, not that their velocities with respect to us are undefined, but that they’re zero!
In effect, the rubber sheet acts in people’s minds like a sort of aether, i.e., a preferred frame to use in defining all motions. This is precisely the opposite of how you want to understand space in relativity.

So I'll keep pushing that point.

Chodorowski's approach seems a bit strange to me, as he identifies lines of constant cosmological time with geodesics, which they generally aren't. There is not one, but many natural choices of paths, and that's exactly why cautious people claim that there is no unambiguous way to decompose large redshifts.

Along the same lines, did you spot this in Philip Helbig's comment:

Note that v = HD is exact for all redshifts and all cosmological models. It doesn’t apply to “directly observable” quantities, but does apply if D is the conventional proper distance in relativity and and v is its derivative with respect to cosmic time as measured now (H is of course the Hubble constant). Thus, it is something people can imagine

The very same misconception that plagues Davis's papers. If D were the "conventional proper distance in relativity", this universe would be a strange place indeed.

 

[oldman]

that ...expresses exactly what I think.
A core part, IMHO:

So I'll keep pushing ...Ted Bunn's statement that:

... In effect, the rubber sheet acts in people’s minds like a sort of aether, i.e., a preferred frame to use in defining all motions. This is precisely the opposite of how you want to understand space in relativity.

Would you keep pushing if one replaced "rubber sheet" with "local CMB frame" ? If not, why not?

 

[Ich]

Would you keep pushing if one replaced "rubber sheet" with "local CMB frame" ? If not, why not?

As long as you take the time to spell out explicitly "at rest with respect to the local CMB frame", I don't take issue. The point is: all motion is relative, and the same laws apply to all states of motion. So: peculiar motion is in no absolute sense different from "motion through expansion", there is no way to tell from observations whether a redshift is cosmological or doppler.
And if you're not moving along with the CMB, the universe doesn't care and won't start dragging you with it - as you are lead to expect. In fact, it's this very notion of "dragging" that contradicts relativity and makes people ask how fast the solar system expands (just look at the comments in http://blogs.discovermagazine.com/cosmicvariance/2008/10/06/does-space-expand/" ).

 

[Wallace]

Quite right Ich. I've often seen posts around here encourage a distinction between 'motion through space' as opposed to expansion motion or some other phrase. It's a moot, non-existent distinction. As you say, all motion is relative so there are infinite ways to describe it.

We shouldn't throw the baby out with the bathwater; the balloon, or raisins in bread type analogies are very useful ways of conveying the first and most important point about expansion, that is the neither we are anywhere else is the 'centre' of some original explosion. That's a very important point to get across and the idea that balloon type approaches best address.

That being said, that is about as far as one would want to take that analogy. It's only when it gets pushed further than that that it causes problems.

 

[Chronos]

Personally, I prefer motion in space. Motion through space infers an absolute reference frame. I'm not comfortable with that.