tag:blogger.com,1999:blog-22973357.post3281337347442580678..comments2017-04-30T01:10:10.920-04:00Comments on Backreaction: Yes, a violation of energy conservation can explain the cosmological constantSabine Hossenfelderhttps://plus.google.com/111136225362929878171noreply@blogger.comBlogger12125tag:blogger.com,1999:blog-22973357.post-34724956906575695422017-03-09T00:48:05.584-05:002017-03-09T00:48:05.584-05:00And, yes, what Haelfix says above is correct, ther...And, yes, what Haelfix says above is correct, there is a long back and forth in the literature about whether or not quantizing unimodular gravity helps with the cosmological constant problem by taming vacuum fluctuations, but the calculations in the paper above doesn't depend on the quantization. Best,<br /><br />B.Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-88355160112074449152017-03-09T00:45:14.135-05:002017-03-09T00:45:14.135-05:00Shantanu,
As I said, unimodular gravity plus ener...Shantanu,<br /><br />As I said, unimodular gravity plus energy conservation reproduces GR so it has no observable consequences. And if you relax energy conservation you want to make really sure it doesn't have observable effects that spoil the fit to data. Not much is being said in the paper about this, more work is needed etc. Best,<br /><br />B.Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-86698133589232858252017-03-08T10:43:02.455-05:002017-03-08T10:43:02.455-05:00Sabine/others: Does unimodular gravity have differ...Sabine/others: Does unimodular gravity have different observational consequences in solar system and binary pulsars as compared to GR? Which of its PPN parameters differ from GR?Shantanuhttp://www.blogger.com/profile/16322812456382858228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-88971658779268116962017-03-07T10:18:36.736-05:002017-03-07T10:18:36.736-05:00In 1918 Hermann Weyl derived a restriction-free fo...In 1918 Hermann Weyl derived a restriction-free form of the traceless Einstein equations by simply assuming R^2 in the Lagrangian rather than R. Since the energy-momentum tensor for the electromagnetic field is already traceless, such a straightforward method of getting traceless equations seems to be the way to go. It would thus appear that Weyl had effectively discovered unimodular gravity longBillhttp://www.blogger.com/profile/13243006930165511059noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-5849872863510506902017-03-05T03:14:03.418-05:002017-03-05T03:14:03.418-05:00It's not entirely settled whether Unimodular g...It's not entirely settled whether Unimodular gravity differs from GR's prediction at the quantum level. This goes back and forth endlessly in the literature.<br /><br />At the very least, its not clear what you gain when trying to solve the cosmological constant problem. There is still a finetuning problem, the difference is -they say- that there is only one number to explain, and not Haelfixhttp://www.blogger.com/profile/16980241518510365230noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-72627762644370286232017-03-04T03:27:51.501-05:002017-03-04T03:27:51.501-05:00@Professor R: In section 14c, 'The Final Steps...@Professor R: In section 14c, 'The Final Steps', of the superb Einstein biography 'Subtle is the Lord', Abraham Pais writes:<br /><br />"The remaining flaw was, of course, Einstein's unnecessary restriction to uni-modular transformations. The reasons which led him to introduce this constraint were not deep, I believe. He simply noted that this restricted class of John Fredstedhttp://www.blogger.com/profile/14392519442398073571noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-32540946636256869432017-03-04T02:01:59.782-05:002017-03-04T02:01:59.782-05:00Professor,
I'm not much of a historian, so I ...Professor,<br /><br />I'm not much of a historian, so I can't answer your question, sorry - maybe some of our readers can answer your question. What I recall is that Einstein tried various equations and ways to arrive at these equations and he was missing a constraint (the Bianchi identities), so it's quite plausible he'd have tried this. <br /><br />Having said that, note that Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-20708608411649400702017-03-03T13:19:49.354-05:002017-03-03T13:19:49.354-05:00One thing that confuses me is that, in Einstein...One thing that confuses me is that, in Einstein's seminal 1916 'Grundlage' paper, the condition [root (-g) = 1] is imposed in quite a few instances. It looks like he is using uni-modular gravity, but I presume he's just using the condition as a way of simplifying things, is that right? <br />Regards, CormacProfessor Rhttp://www.blogger.com/profile/08227006868149583276noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-51661131712663840002017-03-03T00:08:50.852-05:002017-03-03T00:08:50.852-05:00Baryogenesis, non-classical gravitations, dark mat...Baryogenesis, non-classical gravitations, dark matter, and SUSY are empirical failures. Add dark energy. If physics must be fantastical, perhaps physics should be different, Aristotle versus Galileo.<br /><br />https://www.facebook.com/GreenBloodNews/posts/409237282490972<br />...Empress Eugénie's Circle is a suspension bridge with no anchorages.Uncle Alhttp://www.blogger.com/profile/05056804084187606211noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-44429017047182799492017-03-02T17:37:48.013-05:002017-03-02T17:37:48.013-05:00Sabine,
"It is also clear, however, that it ...Sabine,<br /><br />"It is also clear, however, that it will require much more work to convince anybody this doesn’t lead to conflicts with observation."<br /><br />It seems to me that "much more work" would be welcomed by GR theoretical physicists.TheBigHenryhttp://www.blogger.com/profile/04917973198063733316noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-20279584438702331722017-03-02T13:43:20.232-05:002017-03-02T13:43:20.232-05:00Hi Sabine,
many thanks, this post is interesting ...Hi Sabine,<br /><br />many thanks, this post is interesting and very refreshing (which is so rare in physics :). <br /><br />The paper is of particular interest to me because I think it has direct relation with the one I pointed to you a few weeks ago. The point is that writing the non-conservation equation for a start (and then maybe understanding what it means), you get straight to the akidbellehttp://www.blogger.com/profile/12292741599925116131noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-74429975759163597762017-03-02T13:03:45.678-05:002017-03-02T13:03:45.678-05:00Thanks Dr. H. Another good read. Doesn't QFT a...Thanks Dr. H. Another good read. Doesn't QFT allow for local and temporary violations of energy conservation? Perhaps as with the process resulting in black hole evaporation some of these "temporary" violations become locked in?Matthew Rapaporthttp://www.blogger.com/profile/03016608637645316849noreply@blogger.com