Talk:Cosmic variance

Define three meanings of Cosmic Variance?

It seems to me that there are three distinct meanings of cosmic variance in widespread use, but before modifying the article to reflect this, I'd like to discuss it here to see if others agree.

• First, many astronomers (incorrectly, I believe) use the term to mean sample variance - i.e. the poissonian shot noise. So, one will read statements that source counts from different surveys differ because of cosmic variance (meaning sample variance).
• Second, some cosmologists use it to mean that we can only observe one realization of all the possible universes. For example, we can only observe one CMB, so the measured positions of the peaks in the WMAP spectrum are limited by the fact that WMAP can only see the observable Universe. The Universe next door might have the peaks in slightly different places, while still being consistent with the same physical laws, inflation, etc.
• Third, (and probably the most widespread use), is to use it to mean that measurements are affected by large-scale structure, so a measurement in any region of space may differ from a a measurement in a different region of space by an amount greater than the sample variance.

I believe the current article confuses the second and third meanings, and it would be helpful to distinguish between them. Anybody disagree? RayNorris (talk) 09:17, 27 February 2012 (UTC)

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Dear Ray, thanks for adding this nice distinction between the three possible meanings. However, I am not sure that the third meaning (the large scale structure one) is the most widespread. I usually think of Cosmic Variance as the variance related to the realisation we live in rather than something generated by the non-linearity of gravity; most of the cosmologist I spoke with agree with my view. So, I would say that the second meaning is the most widespread. However, I admit working on the CMB, so I might be biased :-)

In general, I think that the third meaning of cosmic variance can be simply stated as "we can only observe stuff inside our particle horizon", while the second one is "we can only observe stuff in our own realisation". The non-linearity of gravitational collapse makes the horizon limitation more severe, because it increases the variance of the perturbations, but the problem would be solved if we did not have such horizon. On the other hand, the realisation variance would not be overcome even with an infinite horizon (unless we trust the ergodicity condition).

Moreover, it seems to me that the rest of the article refers to the second meaning, not the third one. If nobody objects, I will take out the part "The most widespread use, to which the rest of this article refers...". I will also clarify that the third meaning is related to having a finite causal horizon. Cheers, Guido Pettinari Coccoinomane (talk) 19:03, 7 July 2013 (UTC)

Older Comments

The speed with which other people read a new wikipedia page and make corrections in a way which shows they appreciate the work is truly amazing. It really makes writing something satisfying in a way unlike any other pedagogical writing, where you only hope that others will eventually find your effort useful. Thanks to adrian for the typo correction :). Boud 14:01 2 Jun 2003 (UTC)

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My problem is that it might only take me a couple of minutes to proofread but will now take several days to understand (or try to)!! :^) Adrian

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:) Boud 14:14 2 Jun 2003 (UTC)

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Hi, I was unhappy with this statement "Since cosmic variance is an attempt to make a scientific statement about an unrepeatable scientific experiment (over the lifetime of any 21st century scientist remaining on Earth), this lies at the borderline between physcial cosmology and philosophical cosmology." and so I removed it. What lies at the borderline? Cosmic variance is a notion that cosmologists have to deal with in understanding how to deal with measurements that have a very small statistical sample, but it doesn't mean that these measurements are philosophy. The real philisophical problem cosmologists face is dealing with the anthropic principle. --Joke137 00:52, 12 Mar 2005 (UTC)

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Seems irrelevant to include the reference to the "Cosmic Variance" blog here. It's a great blog, but there's no way anyone interested in the cosmological concept necessarily has any interest in a coincidentally named entity. Should be listed elsewhere.

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Isn't "cosmic variance" just a pretentious new name for what's always been understood as "realization noise"?

Hugh Hudson (talk) 15:35, 28 October 2009 (UTC)

____ — Preceding unsigned comment added by Thebravelittlemuon (talk • contribs) 22:15, 25 March 2011 (UTC)

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