tag:blogger.com,1999:blog-22973357.post8581367098341541028..comments2023-09-27T07:44:19.769-04:00Comments on Sabine Hossenfelder: Backreaction: The Comeback of Massive GravitySabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger48125tag:blogger.com,1999:blog-22973357.post-67124325562586114082020-01-25T19:13:28.434-05:002020-01-25T19:13:28.434-05:00"universal negative energy parameter""universal negative energy parameter"Emmette Davidsonhttps://www.blogger.com/profile/15566381168125492666noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-20718689357082200852020-01-25T13:18:13.826-05:002020-01-25T13:18:13.826-05:00Phillip Helbig wrote: "I really hate the term...Phillip Helbig wrote: "I really hate the term "dark energy"."<br /><br />What term do you like better? It's good to have a term referring to the observed phenomena; "cosmological constant" doesn't work for that, because it refers to a specific theoretical attempt to explain these phenomena - with the built-in assumption that something is constant.<br /><br /John Baezhttps://www.blogger.com/profile/11573268162105600948noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10111655127666565432014-02-19T09:36:52.559-05:002014-02-19T09:36:52.559-05:00There is a slight inaccuracy in crediting the auth...There is a slight inaccuracy in crediting the authors. The key element to avoid the BD ghost in massive gravity was introduced by de Rham-Gabadadze-Tolley, ie, the theory was constructed by dRGT, hence the prevalence use of the term "the dRGT model" in the active community. <br /><br />Hassan and Rosen proved that the dRGT model is free of the BD ghost. They also showed that promoting stevenhttps://www.blogger.com/profile/09309927779069160886noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-68230376564709189862013-12-19T10:40:53.740-05:002013-12-19T10:40:53.740-05:00"There are in my counting 3 different cosmolo...<i>"There are in my counting 3 different cosmological constant problems, which one are you referring to?"</i><br /><br />Let's see:<br /><br />1. What <i>is</i> dark energy?<br /><br />2. Why is its energy density roughly comparable to that of matter (coincidence problem)?<br /><br />3. Why is its energy density much smaller than QFT estimates on the back of the proverbial envelope Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-36283549446423577672013-12-19T09:12:46.878-05:002013-12-19T09:12:46.878-05:00Phillip,
Yes, one of the motivations to look into...Phillip,<br /><br />Yes, one of the motivations to look into this is to have an alternative to LambdaCDM. You want there to be modifications on supergalactic scales, but not screw up solar system physics. That roughly tells you the ballpark for the graviton mass. There are in my counting 3 different cosmological constant problems, which one are you referring to? Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-74612686009901446222013-12-19T09:08:29.558-05:002013-12-19T09:08:29.558-05:00Shantanu,
It's an interesting question, but n...Shantanu,<br /><br />It's an interesting question, but not one there is an easy answer to. To begin with, in the bi-metric gravity setting there are two propagating fields, one massless and one massive, but the metrics are (if I recall correctly) actually superpositions of these (ie not mass eigenstates). So I don't think the constraints can be applied just by looking at the numbers. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-8459225074199312882013-12-19T08:59:28.108-05:002013-12-19T08:59:28.108-05:00"I thought one of the motivations of these mo...<i>"I thought one of the motivations of these models<br />is an alternative to standard Lambda CDM?"</i><br /><br />Maybe. That's why I want to know where the idea came from.<br /><br />Of course, not all alternatives are worth pursuing. Sometimes the cure is worth than the disease. Actually, I am among those who think that there is no "problem" at all. See, for Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-7525542373528157262013-12-19T07:17:01.284-05:002013-12-19T07:17:01.284-05:00Philip,
I thought one of the motivations of these ...Philip,<br />I thought one of the motivations of these models<br />is an alternative to standard Lambda CDM?<br />Shantanuhttps://www.blogger.com/profile/16322812456382858228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-71527951639090066912013-12-19T06:53:05.813-05:002013-12-19T06:53:05.813-05:00"I presume the required graviton mass needed ...<i>"I presume the required graviton mass needed to<br />explain dark energy is much smaller than this bound?"</i><br /><br />Where did you get the idea that a massive graviton has anything to do with dark energy?<br />Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-43663156144265885842013-12-19T06:34:20.214-05:002013-12-19T06:34:20.214-05:00Bee,
In these massive graviton theories, do gravi...Bee,<br />In these massive graviton theories, do gravitational waves travel slower than speed of light? If so, is it<br />consistent with the lower limit on speed of gravity<br />calculated by Caves(1980) and Moore-Nelson(2001)<br />http://adsabs.harvard.edu/abs/1980AnPhy.125...35C<br />and http://arxiv.org/abs/hep-ph/0106220 ?<br /><br />Or is it that the above argument cannot be applied?<br />Shantanuhttps://www.blogger.com/profile/16322812456382858228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-31981267899993074522013-12-18T05:01:33.111-05:002013-12-18T05:01:33.111-05:00Giotis,
Sorry for being vague, I was trying to gau...Giotis,<br />Sorry for being vague, I was trying to gauge (excuse the choice of word :p) how much you had read of the papers rather than guessing the reason for your conviction. Best,<br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-24330357938032371892013-12-17T14:58:26.616-05:002013-12-17T14:58:26.616-05:00My third sentence doesn’t make sense.
What I mean...My third sentence doesn’t make sense.<br /><br />What I meant to write is:<br /> <br />“You could have just said that instead of asking why I think diff is broken”<br />Giotishttps://www.blogger.com/profile/03594944884584261018noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-48174345569023145042013-12-17T11:50:27.283-05:002013-12-17T11:50:27.283-05:00
OK I checked a bit...
diff invariance is broken ...<br />OK I checked a bit...<br /><br />diff invariance is broken of course but it seems they restore it using the Stukelberg trick.<br /><br />You could just say that instead of asking why I think diff is broken. <br /><br />Anyway it seems to me that the Stukelberg trick is just that, a trick, since this way you can restore the gauge symmetry in any theory.<br /><br />But indeed I have to read Giotishttps://www.blogger.com/profile/03594944884584261018noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58202046793137450282013-12-17T08:57:50.516-05:002013-12-17T08:57:50.516-05:00Giotis,
Maybe you should actually read the papers...Giotis,<br /><br />Maybe you should actually read the papers I was referring to in my post, because you clearly haven't. For the non-linear generalization you introduce the second metric. The full action can be found eg in <a href="http://arxiv.org/abs/1106.3344" rel="nofollow">this paper, Eq (3)</a>. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-44379529823461702042013-12-17T07:09:04.269-05:002013-12-17T07:09:04.269-05:00There is the standard way to go from the linearize...There is the standard way to go from the linearized action to the full non linear action with general covariance. This method works only with a maseless graviton.<br /><br />I will try to find references for this when I find the time.Giotishttps://www.blogger.com/profile/03594944884584261018noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-54220801063969205662013-12-17T06:00:09.761-05:002013-12-17T06:00:09.761-05:00How does it do that?
As to the KK case. If you ca...How does it do that?<br /><br />As to the KK case. If you can do a massive graviton in flat space, I don't know what prevents you from adding compactified dimensions that give you excitations over that, but not sure that's what you mean.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-12052482414911792392013-12-17T05:53:31.343-05:002013-12-17T05:53:31.343-05:00How is that possible? A massive graviton breaks di...How is that possible? A massive graviton breaks diff invariance.<br /><br />For the other question I was trying to figure out how you can get a massive graviton (at lowest order) from a maseless graviton in higher dimension via KK compactification/reduction.Giotishttps://www.blogger.com/profile/03594944884584261018noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-36732736342303854412013-12-17T05:29:35.992-05:002013-12-17T05:29:35.992-05:00Giotis,
It doesn't break diff invariance, not...Giotis,<br /><br />It doesn't break diff invariance, not sure why you think so. I also don't know what KK tower you're referring to. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-17087784750007792392013-12-17T02:41:22.267-05:002013-12-17T02:41:22.267-05:00It breaks diff invariance. GR people will not be h...It breaks diff invariance. GR people will not be happy. Their precious background independence is lost:-)<br /><br />Sacrilege!<br /><br />In a KK the zero mode is massless and the tower is massive. How you get rid of the zero mode? Giotishttps://www.blogger.com/profile/03594944884584261018noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58490655647717959432013-12-16T21:55:53.027-05:002013-12-16T21:55:53.027-05:00Cumbersome? Yes, a bit depending on how many tooth...<br />Cumbersome? Yes, a bit depending on how many tooth fairies one discusses in one sitting.<br /><br />"Infinitely cumbersome? - No.<br /><br />Scientists used to be a lot more careful in their discussions, carefully distinguishing the empirically-based from pure speculation.<br /><br />I think the problem has developed because we now deal with so many purely speculative concepts, models,Robert L. Oldershawhttps://www.blogger.com/profile/15396555790655312393noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-89230473627092133702013-12-16T04:27:14.535-05:002013-12-16T04:27:14.535-05:00Robert,
Please read the footnote of my post. Grav...Robert,<br /><br />Please read the footnote of my post. Gravitons are the quanta of the graviational field. You can define them in perturbatively quantized gravity, which is a theory that we know exists, irrespective of the question what its uv completion looks like. Whether they 'really' exist is a question of experiment, and you know that I am cautiously optimistic that we will Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10887255603536119092013-12-16T04:23:03.140-05:002013-12-16T04:23:03.140-05:00Leo,
I don't know but I am guessing there is....Leo,<br /><br />I don't know but I am guessing there is. They have two metrics, but only one manifold. As long as you have at least one direction on which both agree is timelike I'd think it should work. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-57786906310055096512013-12-15T14:16:28.786-05:002013-12-15T14:16:28.786-05:00The concept of massive gravity doesn't require...The concept of massive gravity doesn't require the graviton, just the mass-energy equivalence. The existence of graviton follows just from consequential application of this principle, which is probably impossible to achieve in analytical way. And not all gravity is mediated via quantized gravitons, with massive gravitons the more. The concept of massive graviton is implicit (what mediates theZephirhttps://www.blogger.com/profile/06010623752049244967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-79149672611396153112013-12-14T23:28:09.147-05:002013-12-14T23:28:09.147-05:00I wonder if it is a good idea to use the phrase &q...I wonder if it is a good idea to use the phrase "the graviton". It makes it sound like the hypothetical particle actually exists, whereas there is not a shred of evidence that it does.<br /><br />Using the phrase "the putative graviton" may be cumbersome (but not really that much considering the risks of encouraging false beliefs) but I think we would be better off if it was Robert L. Oldershawhttps://www.blogger.com/profile/15396555790655312393noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-5529751288220027482013-12-14T16:53:55.442-05:002013-12-14T16:53:55.442-05:00Is it known if there is a well-posed IVP (Cauchy p...Is it known if there is a well-posed IVP (Cauchy problem) in the bimetric version of drgt?Leo C. Steinhttps://www.blogger.com/profile/09227175321097879772noreply@blogger.com