tag:blogger.com,1999:blog-22973357.post7049320763201219086..comments2023-09-27T07:44:19.769-04:00Comments on Sabine Hossenfelder: Backreaction: Dear Dr B: Where does dark energy come from and what’s it made of?Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger87125tag:blogger.com,1999:blog-22973357.post-62706951354458914912019-02-26T11:35:40.318-05:002019-02-26T11:35:40.318-05:00Thanks for the reference. Seeing as you are also w...Thanks for the reference. Seeing as you are also working on modified gravitation in addition to dark matter/energy, I suppose you need to write a book on this too, especially the apparent connection between modified gravity and dark matter/energy.Usmanhttps://www.blogger.com/profile/09759123857246721561noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-36868748337143944872019-02-24T05:24:56.459-05:002019-02-24T05:24:56.459-05:00Hi Usman,
Maybe this will help. There are also ma...Hi Usman,<br /><br /><a href="https://arxiv.org/abs/astro-ph/0603114" rel="nofollow">Maybe this will help</a>. There are also many good popular science books on cosmology, eg <a href="http://backreaction.blogspot.com/2015/12/book-review-beyond-galaxy-by-ethan.html" rel="nofollow">Ethan Siegel's</a>.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-71327568061188616102019-02-24T04:14:49.616-05:002019-02-24T04:14:49.616-05:00Thank you Prof.Hossenfelder. I have recently start...Thank you Prof.Hossenfelder. I have recently started reading your blog and I must say there is knowledge and wisdom here not on many other physics blogs. Can you recommend a review article(s) that encompasses most of which you wrote and the discussion in the comments below?Usmanhttps://www.blogger.com/profile/09759123857246721561noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-25979478359205350592017-08-08T12:01:18.655-04:002017-08-08T12:01:18.655-04:00As a non-scientist I thought I was on to something...As a non-scientist I thought I was on to something novel, some years back, when it occurred to me that a dual-universe emerging from the big-bang, one having positive energy density, the other negative energy density could explain the 120 order discrepancy of the Cosmological Constant, between its predicted and measured values. The idea being that the other universe's negative energy David Schroederhttps://www.blogger.com/profile/18048116250413347228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-25506337411410186852016-11-03T05:18:21.106-04:002016-11-03T05:18:21.106-04:00Dear Sabine
Much appreciated the explanation. All...Dear Sabine<br /><br />Much appreciated the explanation. All things being equal is what its appears to all be about.Alanhttps://www.blogger.com/profile/00375734596064810061noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-81757979307845774942016-10-31T09:12:34.491-04:002016-10-31T09:12:34.491-04:00Phillip H, for a 'mainstream' analysis in ...Phillip H, for a 'mainstream' analysis in this regard, see the paper by Smoot et al:<br />https://arxiv.org/abs/1002.4278<br />I have a similar shorter and slightly more heuristic analysis giving the same result. It is also supplemented by another heuristic analysis using the Principle of Maximm Force/Power advanced by Gary Gibbons and Christoph Schiller that, in addition, indicates that Anonymoushttps://www.blogger.com/profile/14318195499972536515noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-71693706402568498222016-10-31T07:12:52.202-04:002016-10-31T07:12:52.202-04:00"Using the Holographic Principle and recogniz...<i>"Using the Holographic Principle and recognizing that the Cosmological Horizon has the Gibbons-Hawking Temperature, Dark Energy can be viewed as the result of thermodynamic/entropic effects - and one finds that its energy density is 2/3 the Friedman Critical Total Density that makes space 'flat' (as observed!)."</i><br /><br />Reference?<br />Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-73281385273435991832016-10-31T05:40:42.124-04:002016-10-31T05:40:42.124-04:00David Thornton, would the holographic principle pr...David Thornton, would the holographic principle predict that the energy density as a fraction of critical density has always been 2/3? That would be different from the cosmological constant which predicts that the ratio has increased from near zero in the early universe. PeteHHhttps://www.blogger.com/profile/08545699636937390967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-33396351856464261942016-10-29T18:34:40.966-04:002016-10-29T18:34:40.966-04:00Using the Holographic Principle and recognizing th...Using the Holographic Principle and recognizing that the Cosmological Horizon has the Gibbons-Hawking Temperature, Dark Energy can be viewed as the result of thermodynamic/entropic effects - and one finds that its energy density is 2/3 the Friedman Critical Total Density that makes space 'flat' (as observed!).Anonymoushttps://www.blogger.com/profile/14318195499972536515noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-26930660256124242412016-10-29T04:42:34.657-04:002016-10-29T04:42:34.657-04:00Although 10 dimensions is sufficient to embed a 4 ...Although 10 dimensions is sufficient to embed a 4 dimensional manifold locally (i.e. in a sufficiently small neighbourhood), it is not sufficient globally. For that a higher dimensional space is required see https://en.wikipedia.org/wiki/Nash_embedding_theorem PeteHHhttps://www.blogger.com/profile/08545699636937390967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-91577009029350464732016-10-28T12:29:18.892-04:002016-10-28T12:29:18.892-04:00just zis,
I don't know if there are any pitfa...just zis,<br /><br />I don't know if there are any pitfalls, but you know how it goes with interpretations, once one becomes widely spread, it takes over. It's like with the Copenhagen interpretation. Could have been some other one, but it just wasn't. The way it went historically is that Einstein's whole motivation was to come up with an internal measure for curvature, so Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-3892179144720301452016-10-28T12:06:02.075-04:002016-10-28T12:06:02.075-04:00Dear Sabine,
You wrote: "Now you can take an...Dear Sabine,<br /><br />You wrote: "Now you can take any 4-dimensional space and embed it into a 10 dimensional space, and then you could say it expands 'into' that larger space. But that's an interpretation nobody uses, at least not that I know of."<br /><br />What are the pitfalls of that interpretation? Are the efforts of AdS/CFT trying to move in that direction (but withjust zis guy, you know?https://www.blogger.com/profile/13420165390513200266noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-76641358979974994362016-10-27T16:06:12.989-04:002016-10-27T16:06:12.989-04:00Bee is not simply slowing down the debate. She is ...Bee is not simply slowing down the debate. She is granting her mercy to a dead horse.TheBigHenryhttps://www.blogger.com/profile/04917973198063733316noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-64206506808238701762016-10-27T02:53:04.428-04:002016-10-27T02:53:04.428-04:00I dont blame Bee for moderating, just saying it sl...I dont blame Bee for moderating, just saying it slows down the debate. I'm glad she has been willing to allow my very direct comments.<br /><br />MTW is one of the textbooks that initiated the myth that there is no local formulation of energy conservation. In earlier work Wheeler even claimed that the energy conservation law was a trivial result because it gave a value of zero for a closed PeteHHhttps://www.blogger.com/profile/08545699636937390967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-55911146116736061732016-10-27T02:16:29.835-04:002016-10-27T02:16:29.835-04:00Giotis,
I am sincerely sorry about the moderation...Giotis,<br /><br />I am sincerely sorry about the moderation delay, but dark matter/energy is a topic that attracts a ridiculous amount of crank theories and, trust me, if I'd post them all, that wouldn't benefit this comment section. It's easier to approve the reasonable comments than to later delete the useless ones. In any case, thanks for your effort with PhilG.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-8248252164057325402016-10-26T17:59:08.036-04:002016-10-26T17:59:08.036-04:00Yes, the stubbornness of the moderator to keep on ...Yes, the stubbornness of the moderator to keep on this comment policy is unexplainable.<br /><br />No one else does that, it kills any kind of sensible discussion.<br /><br />Not even juntas do this kind of censorshipGiotishttps://www.blogger.com/profile/01538762241438887298noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-52216322095689491712016-10-26T17:41:07.550-04:002016-10-26T17:41:07.550-04:00It's not just Wald, there isn't a single t...It's not just Wald, there isn't a single textbook that defends your position.<br /><br />Check Misner et al beautiful analysis. <br /><br />page 466 under Chapter with title:<br /><br />20.4. WHY THE ENERGY OF THE GRAVITATIONAL FIELD CANNOT BE LOCALIZED<br /><br />It can only be understood globally as total energy and only then you can speak about energy conversation in a meaningful way<Giotishttps://www.blogger.com/profile/01538762241438887298noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-1006520882948363722016-10-26T15:46:31.803-04:002016-10-26T15:46:31.803-04:00PhilG,
"Just stating that something cannot w...PhilG,<br /><br />"Just stating that something cannot work does not make it true. He needs to give a real physical reason"<br /><br />It's because of the equivalence principle. The Misner-Thorne-Wheeler book, Chapter 20, sections 20.3 and 20.4, elaborates why and provides the physical reason. The punchline is in the second-to-last paragraph of 20.4:<br /><br />"At issue is not vmarkohttps://www.blogger.com/profile/12660488689205445871noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-15461296715935286222016-10-26T11:57:04.611-04:002016-10-26T11:57:04.611-04:00Giotis, Wald's statements about energy are com...Giotis, Wald's statements about energy are completely without merit. He just makes claims about how you can't divide the metric into background and dynamical parts and complains that formulations are not covariant and are therefore meaningless. He also worries about non-uniqueness of the energy formulism. Einstein, Dirac, Landau and Lifschitz and Weinberg understood that these methods PeteHHhttps://www.blogger.com/profile/08545699636937390967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-66040267106747902642016-10-26T10:57:18.186-04:002016-10-26T10:57:18.186-04:00Phil,
It is well justified in other places e.g. p...Phil,<br /><br />It is well justified in other places e.g. page 84<br /><br />Komar is for static asymptotically flat space-times. Check page 289.<br /><br />Weinberg is not full-fledged GR, not covariant, he splits the metric and asymptotically flat<br /><br />I really can't understand why you keep defending an indefensible position.Giotishttps://www.blogger.com/profile/01538762241438887298noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-15321040692499684642016-10-26T06:24:52.984-04:002016-10-26T06:24:52.984-04:00Giotis, I am aware of Wald's view but I disput...Giotis, I am aware of Wald's view but I dispute it. Notice that he gives no detailed justification for his claim and he also inserts the weasel word "meaningful". What does he mean by that word and why does he consider it necessary to insert it? Why is the formulation provided by Weinberg not meaningful? If he means that it is not covariant (and I disagree that this makes it not PeteHHhttps://www.blogger.com/profile/08545699636937390967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-70883350295823263202016-10-25T13:58:04.078-04:002016-10-25T13:58:04.078-04:00Found it... and as I was expecting my statements w...Found it... and as I was expecting my statements were correct.<br /><br />My friend Wald will save the day once again :)<br /><br />If you want textbook references, here it goes:<br /><br />Wald, page 70, note 6:<br /><br />"One might hope to recover an energy conversation law by including the stress-energy of the gravitational field as it is done in Newtonian theory. However in General Giotishttps://www.blogger.com/profile/01538762241438887298noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-91187413957785573062016-10-25T03:26:09.859-04:002016-10-25T03:26:09.859-04:00Giotis, Sorry I did not respond before but you are...Giotis, Sorry I did not respond before but you are saying the same as Bee and I have already responded at length to her. <br /><br />No I don't agree with your statement that "In GR there is no notion of total (matter + gravitational field) locally conserved energy." Not all textbooks cover this but Weinberg gives one such formulation which I have cited. Komar wrote a paper giving aPeteHHhttps://www.blogger.com/profile/08545699636937390967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-77342995095516508072016-10-24T13:07:24.024-04:002016-10-24T13:07:24.024-04:00Giotis,
Yes, indeed! And that's a very intere...Giotis,<br /><br />Yes, indeed! And that's a very interesting point about which I wrote a paper a long time ago... But maybe somewhat too much detail for the purposes of this post. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-61596779484344982842016-10-24T12:51:41.542-04:002016-10-24T12:51:41.542-04:00PhilG, I will interpret the fact that you didn'...PhilG, I will interpret the fact that you didn't respond to my comment as an implicit agreement. <br /><br />That being said I read the post and Bee's statement that:<br /><br />"But General Relativity has a more complicated type of symmetry to which Noether’s theorem can be applied. This gives rise to a local conservation of stress-momentum when coupled to gravity (the Giotishttps://www.blogger.com/profile/01538762241438887298noreply@blogger.com