tag:blogger.com,1999:blog-1513704378254120283.post7063325023832529916..comments2024-01-23T13:58:48.688-08:00Comments on The Trenches of Discovery: Cosmological BackreactionShaun Hotchkisshttp://www.blogger.com/profile/04832423210563130467noreply@blogger.comBlogger7125tag:blogger.com,1999:blog-1513704378254120283.post-30949954801378983732016-02-08T12:55:08.973-08:002016-02-08T12:55:08.973-08:00Thanks for the comment David. I think your words b...Thanks for the comment David. I think your words below summarise the situation to me quite well. I'm confused as to why they're still pushing this results as so general.<br /><br />"To be clear we have not claimed that they make an error of logic, but rather that their framework is too narrow to capture the essential physics of the averaging and backreaction problems."Shaun Hotchkisshttps://www.blogger.com/profile/04832423210563130467noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-19044604931478576272016-02-06T17:49:05.542-08:002016-02-06T17:49:05.542-08:00Shaun: you are perfectly correctly that Green and ...Shaun: you are perfectly correctly that Green and Wald are using a different definition of backreaction, and in <a href="http://arxiv.org/abs/1506.06452" rel="nofollow">arxiv:1506.06452</a> they state that their formalism was "never intended or claimed to apply" to various approaches to backreaction that constitute much of the literature.<br /><br />Unfortunately, time-challenged non-experts only read titles and abstracts, rather than studying the fine print of the content of papers peripheral to their own research. In fact, Green and Wald's mathematical results are inapplicable to much of the literature, because the averaged cosmic variables in many backreaction schemes are not an exact solution of Einstein's equations, but rather of equations with additional backreaction terms on any coarse-grained scale, and not only in an ultra-local limit.<br /><br />In their past work, Green and Wald have strongly criticized other approaches while arguing that their own framework should provide an accurate description of the actual Universe, leaving it unclear to what extent their mathematical results are limited by restrictive assumptions which may not apply to the actual physical problem.<br /><br />In Green and Wald's framework backreaction can only contribute a radiation-like trace-free term to the effective stress-energy tensor in the average equations. We show that such a result is not a feature of general averaging schemes. Furthermore, even if an average background FLRW metric is assumed, Green and Wald's proof of the trace-free property relies on specific technical assumptions when taking weak-limits, one of them implying that the scheme does not actually provide an averaging operation as it is generally understood. If one takes a more general mathematical framework in dealing with such limits, specifically by emphasizing mathematical and physical control on the properties of curvature and matter fluctuations, then their result does not follow.<br /><br />In a new paper, <a href="http://arxiv.org/abs/1601.06789" rel="nofollow">arxiv:1601.06789</a>, they refer to their non-peer reviewed note <a href="http://arxiv.org/abs/1506.06452" rel="nofollow">arxiv:1506.06452</a> as giving a "refutation" of our arguments. Our referees, one of whom effectively also wrote a report on arxiv:1506.06452 disagreed. To be clear we have not claimed that they make an error of logic, but rather that their framework is too narrow to capture the essential physics of the averaging and backreaction problems.<br /><br />We see no point in continuously going over these arguments. We wish to move things on, as there is much exciting work to be done, and the point of our article was to dispell a few myths so people are not discouraged to work in this field. Thus when invited to write a piece for CQG+, we have focused on the challenges of the future:<br /><br /><a href="http://cqgplus.com/2016/01/20/the-universe-is-inhomogeneous-does-it-matter/" rel="nofollow">http://cqgplus.com/2016/01/20/the-universe-is-inhomogeneous-does-it-matter/</a>David Wiltshirehttp://www2.phys.canterbury.ac.nz/~dlw24/noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-50730066742291986262015-12-09T10:09:23.834-08:002015-12-09T10:09:23.834-08:00My guess is that, 10 years from now, a popular equ...My guess is that, 10 years from now, a popular equation will be: dark-matter-particles = phlogiston. My guess is that Gravity Probe B discovered the Fernández-Rañada-Milgrom effect — can anyone point out a previous case in which 4 (out of 4) scientific instruments malfunctioned in a remarkably predictable manner? I say that that MOND has been outrageously ignored — for example:<br /><a href="http://arxiv.org/abs/1504.05774" rel="nofollow">"Spacetime, Spin and Gravity Probe B" by James M. Overduin, 2015</a><br />According to the research of Milgrom, McGaugh, Kroupa, Pawlowski, and several others, there are 2 possibilities:<br />(1) Newtonian-Einsteinian gravitational theory is 100% correct but appears to be significantly wrong for some unknown reason.<br />(2) Newtonian-Einsteinian gravitational theory really is slightly wrong.David Brownhttps://www.blogger.com/profile/10537922851243581921noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-80231447233336015582015-12-07T06:14:50.299-08:002015-12-07T06:14:50.299-08:00Just because Kroupa (no, he is not the new Kepler)...Just because Kroupa (no, he is not the new Kepler) says so doesn't make it so, and neither does your posting similar comments wherever a blog post touches this topic (at least they are not grossly off-topic).<br /><br />Why no dark matter haloes in our own solar system? Is this a serious question? Check out the size of even a small halo.<br /><br />I'm sure Stacy has learned some things since 1998. Yes, MOND seems to work better in some regimes, which, probably not coincidentally, are regimes where simulations are very hard, because small-scale "gastrophyics" and what not are involved. However, there are other regimes where LCDM works better. Touting one and ignoring the other doesn't help the debate. Even if his LSB galaxies work better with MOND, one has to come up with some explanation what the alternative to LCDM is where it does work, and MOND doesn't.<br /><br />The same comments apply to Kroupa, though perhaps he hasn't learned as much since 1998. Debate is good, and Kroupa even debated Simon White. But the it seems that MOND, while intriguing, is not the last word, and LCDM won't just be thrown out the window. Probably, a solution which explains why MOND works, whether or not such a solution involves some sort of MOND, would probably leave LCDM untouched in other areas.<br />Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-41395338791873245412015-12-07T03:25:02.356-08:002015-12-07T03:25:02.356-08:00"This model is the FLRW metric with cold dark..."This model is the FLRW metric with cold dark matter and a cosmological constant." The cosmological constant seems very good on the evidence, but Kroupa says that the Lambda-CDM model has been ruled out and that galactic dynamics is Milgromian. It seems to me that Kroupa is definitely correct. If cold dark matter particles exist, then why are there no dark matter haloes in our own solar system?<br />“I came to the subject a True Believer in dark matter, but it was MOND that nailed the predictions for the LSB galaxies that I was studying (McGaugh & de Blok, 1998), not any flavor of dark matter. So what I am supposed to conclude? …” — McGaugh<br />“The currently (2010) widely accepted/believed description of the birth and evolution of the universe and of its contents is "Lambda Cold Dark Matter Concordance Cosmological Model" (LCDM CCM) … My own research was very much confined to the early version of the LCDM CCM (mid-1990's) when I began performing numerical experiments on the satellite galaxies of the Milky Way. I was quite happy with the CCM, as everyone else, and did not bother with the fundamental issues raised by some. With time, however, it became apparent that the LCDM CCM accounts poorly for the properties of the satellite galaxies and their distribution about the Milky Way. Warm dark matter models fared no better.” — KroupaDavid Brownhttps://www.blogger.com/profile/10537922851243581921noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-30971905348209457532015-08-26T11:59:00.256-07:002015-08-26T11:59:00.256-07:00"There is still some speculation that dark en...<i>"There is still some speculation that dark energy might just be backreaction but that particular possibility seems very unlikely, at least to me, in 2015."</i><br /><br />I agree. Back when the observers said "supernovae are fainter than they should be in the Einstein-de Sitter universe" then, yes, it could have been backreaction, or grey dust, or calibration errors, or any number of things. Now the data are much better. Not only can they be fit with 1920s cosmological models, but the allowed region of parameter space is consistent with that from completely independent cosmological tests. This would require a very finely tuned conspiracy for all tests to give the "wrong" answer in precisely the same way.Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-61788018555882978922015-07-01T10:40:06.237-07:002015-07-01T10:40:06.237-07:00Because I didn't get around to it in the bulk ...Because I didn't get around to it in the bulk of the post, I'll clarify my thoughts on the disagreement. Green and Wald, who claim to have proven that backreaction isn't important explicitly only consider scenarios where the true metric is close to some background metric, everywhere, for all time. Although they do allow the metric's derivatives to be large they explicitly require the metric itself to be close to the background. In most scenarios where backreaction (at least how I would define it) is important, my best understanding is that this won't be true. Green and Wald explicitly acknowledge this in all their recent papers and claim that they don't consider such a scenario to be suitably described as "backreaction".Shaun Hotchkisshttps://www.blogger.com/profile/04832423210563130467noreply@blogger.com