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-   -   Are We Living In A Void? (http://www.physicsforums.com/showthread.php?t=246466)

81+ Jul23-08 08:21 AM

Are We Living In A Void?
 
I find Timothy Clifton's paper (arXIV: 0807.1443) "Living In A Void: Testing The Copernican Principle With Distant Supernovae" very interesting (at least where I think I understand what's being proposed). The abstract is:

[A fundamental presupposition of modern cosmology is the Copernican Principle; that we are not in a central, or otherwise special region of the Universe. Studies of Type Ia supernovae, together with the Copernican Principle, have led to the inference that the Universe is accelerating in its expansion. The usual explanation for this is that there must exist a ‘Dark Energy’, to drive the acceleration. Alternatively, it could be the case that the Copernican Principle is invalid, and that the data has been interpreted within an inappropriate theoretical frame-work. If we were to live in a special place in the Universe, near the centre of a void where the local matter density is low, then the supernovae observations could be accounted for without the addition of dark energy. We show that the local redshift dependence of the luminosity distance can be used as a clear discriminant between these two paradigms. Future surveys of Type Ia supernovae that focus on a redshift range of  0.1 − 0.4 will be ideally suited to test this hypothesis, and hence to observationally determine the validity of the Copernican Principle on new scales, as well as probing the degree to which dark energy must be considered a necessary ingredient in the Universe].

Has he missed something critical?

Frank

marcus Jul23-08 09:57 AM

Re: Are We Living In A Void?
 
I haven't read the paper yet but I will gamble a response to the answer to your question "Is he missing something." Then I'll have a look and see how well my response holds up.

The gist is: It doesn't look to me as if they are arguing that we actually live in a void, or that the Copernican assumption is wrong.

Sounds to me as if they are proposing a nice way to further test the Copernican notion---by raising a question which can be answered by a new bunch of Supernova observations.

The Copernican assumption (that we live in a fairly typical neighborhood) has been tested in the past and has been repeatedly confirmed. In a way, you do a scientific principle a favor by explicitly putting it at risk, because only by being tested can it be confirmed. The more times something is tested (and passes) the more confidence people tend to have in it.

So I would hope that they are not missing anything in their analysis and that the continuing SN IA observations ARE a good test.

But wouldn't it be exciting if it could be determined that we DO live in an unusual neighborhood! :biggrin:

Here's background on Pedro Ferreira. He has 88 papers on arxiv.
http://arxiv.org/find/astro-ph/1/au:.../0/1/0/all/0/1
He has published a lot, peer-review. He has co-authored with prominent people.

Here's some background on Tim Clifton. He's young evidently. He already has 18 papers on arxiv.
http://arxiv.org/find/grp_physics/1/.../0/1/0/all/0/1
He has co-authored with prominent people (John Barrow, Steve Shenker, Joel Weinberg)
He has a good publication record for someone at his stage.

Here is the full link to the preprint you brought up, for the convenience of anyone who wants to take a look. I shall, in a moment.

http://arxiv.org/abs/0807.1443

George Jones Jul23-08 10:12 AM

Re: Are We Living In A Void?
 
Quote:

Quote by marcus (Post 1811011)
I haven't read the paper yet but I will gamble a response to the answer to your question "Is he missing something." Then I'll have a look and see how well my response holds up.

The gist is: It doesn't look to me as if he is arguing that we actually live in a void, or that the Copernican assumption is wrong.

Sounds to me as if he is proposing a nice way to further test the Copernican notion---by raising a question which can be answered by a new bunch of Supernova observations.

The Copernican assumption (that we live in a fairly typical neighborhood) has been tested in the past and has been repeatedly confirmed. In a way, you do a scientific principle a favor by explicitly putting it at risk, because only by being tested can it be confirmed. The more times something is tested (and passes) the more confidence people tend to have in it.

So I would hope that he is not missing anything in his analysis and that the continuing SN IA observations ARE a good test.

But wouldn't it be exciting if it could be determined that we DO live in an unusual neighborhood! :biggrin:

Note: it is "they", not "he". I'm fairly certain that Kate Land is a "she". :biggrin:

Yes, I agree. According to their analysis, observation will soon determine which gives better agreement between theory and experiment, dark energy or a void. This is far different than saying that we definitely live in a void.

The concluding paragraph of the paper:
Quote:

Quote by Clifton, Ferreira, and Land
Two very different paradigms have been invoked to explain the current observation of an apparently accelerating Universe, depending on whether we invoke or reject the Copernican Principle. We have shown that in the coming years it will be possible to experimentally distinguish between these two scenarios, allowing us to experimentally test the Copernican Principle [26, 27, 28], as well as determine the extent to which Dark Energy must be considered a necessary ingredient in the Universe.


marcus Jul23-08 10:55 AM

Re: Are We Living In A Void?
 
Hi Eightyone-plus, I think that's a good paper!
On technical questions concerning it I would defer to the more expert folks (George Jones, Cristo, etc etc) if any of them takes a look. But on first glance I don't see any major flaw.
There are some minor things.
They have a reference [8] to something I believe may be fishy, but it is a side-issue.
Their paper does not depend on it. There are a number of really imaginative inflation scenarios floating around and I don't buy a lot of what Andrei Linde makes up about what might be based on this or that idea of inflation. So I pass on their side remark that "it can be argued" we are more likely than not to live in the midst of a void. Sure "it can be argued" :wink:

What matters is not what this or that theorist thinks is the a priori likelihood. Clearly what matters is whether or not we actually DO live at the center of a large void. And that is something that observationalists like Pedro Ferreira and Kate Land can find out for us by looking.

On the whole I think Ferreira et al are on the level except for one thing. Unless I missed it, they are always saying DARK ENERGY instead of positive cosmological constant. There is a rhetorical feel to that overly consistent choice of language. To offset that, note there are various ways a positive cosmological constant could enter the picture. We don't necessarily need to picture it as an exotic fluid or particle with paradoxical properties. Perhaps nobody does.

Certain quantum geometry approaches which bid to replace classic 1915 General Relativity seem to REQUIRE a positive cosmological constant in order to compute, and not break down. It could be tht if we understood space and time a little better---what the fundamental thing is from which they arise---we would have an explanation for the positive constant.

And it might not be an exotic matter-field (if anyone really thinks of it that way) but simply an inherent feature of the fundamental picture. It is too early to say.

There are, moreover, other places that the cosmological constant is invoked, other needs it fills, so proving a void might not get rid of it entirely----might just change the estimate of how big the constant is, or of how much dark energy there is (if you prefer that figure of speech).

Aside from that faint rhetorical note (and the dubious inflationary side business) I didn't see anything to have reservations about.

George Jones Jul23-08 11:34 AM

Re: Are We Living In A Void?
 
Quote:

Quote by marcus (Post 1811068)
On the whole I think Ferreira et al are on the level except for one thing. Unless I missed it, they are always saying DARK ENERGY instead of positive cosmological constant. There is a rhetorical feel to that overly consistent choice of language. To offset that, note there are various ways a positive cosmological constant could enter the picture. We don't necessarily need to picture it as an exotic fluid or particle with paradoxical properties. Perhaps nobody does.

Even though there are some complaints, I think they are just using the same term that most of their community uses.

I talked about what I think are some of the possibilities in the post

http://www.physicsforums.com/showthread.php?p=1327486

I think that sometimes all of these possibilities get lumped together under the generic umbrella "dark energy."

I was at at a talk given by Roy Maartens 14 months ago, and he took pains to talk about all theses possibilities and more, so I don't think the community has lost sight of the different possibilities.

marcus Jul23-08 12:05 PM

Re: Are We Living In A Void?
 
Quote:

Quote by George Jones (Post 1811120)

I was at at a talk given by Roy Maartens 14 months ago, and he took pains to talk about all theses possibilities and more, so I don't think the community has lost sight of the different possibilities.

I believe it! What I read most often is where an author will say positive cosmological constant or some alternative term part of the time (not always say dark energy) and it serves among other things as a reminder that it isn't settled-----in line with the Roy Maartens talk you heard.

So the Ferreira paper overly consistent language had a slightly off-key sound to me. But it doesn't really matter.


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