tag:blogger.com,1999:blog-22973357.post1897341046856633647..comments2020-05-27T12:17:44.312-04:00Comments on Sabine Hossenfelder: Backreaction: I totally mean it: Inflation never solved the flatness problem.Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger87125tag:blogger.com,1999:blog-22973357.post-80956643531989693482017-11-08T00:46:07.602-05:002017-11-08T00:46:07.602-05:00Alejandro,
I feel really bad for never having rea...Alejandro,<br /><br />I feel really bad for never having read Penrose's book. They look so scary... But one day, I promise, I'll give them a read.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-23967194320627051642017-11-07T16:18:16.928-05:002017-11-07T16:18:16.928-05:00Sabine,
I believe you are not alone, by any means,...Sabine,<br />I believe you are not alone, by any means, in your healthy skepticism of the motivations for Inflation. At least in writing, one can find R. Penrose suggesting similar arguments in his 2006 book "Road to Reality", section 28.5 "Are the motivations for inflation valid?". I suppose that he makes a longer argument for that in his most recent book. I believe there areAlejandro Corichihttps://www.blogger.com/profile/07408718574331884796noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-4964931207767834672017-10-29T00:45:08.171-04:002017-10-29T00:45:08.171-04:00Johannes,
I think you should do a dimensional ana...Johannes,<br /><br />I think you should do a dimensional analysis. You seem to be speaking of the curvature term k/a^2, which is dimensionful, rather than of the curvature density, which is dimensionless. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-81372394419292222592017-10-28T16:54:11.347-04:002017-10-28T16:54:11.347-04:00OK, let's round it up. My point is that your s...OK, let's round it up. My point is that your statement...<br /><br />"This has the consequence that if you start with something of order 1 and add inflation, the result today is compatible with observation. But of course if you start with some very large value, say 10^60, then the result will still be incompatible with data. That is, you really need the assumption that the initial valuesJohanneshttps://www.blogger.com/profile/05371418313799513738noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-47216166153387048412017-10-28T12:57:29.050-04:002017-10-28T12:57:29.050-04:00Johannes,
Well, you began your comment referring ...Johannes,<br /><br />Well, you began your comment referring to an infinite initial value. I merely told you that's nonsense. If that's not what you mean, then maybe explain what you mean. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-65123796633471681742017-10-28T12:35:38.851-04:002017-10-28T12:35:38.851-04:00OK, but the closed lambdavacuum model does not hav...OK, but the closed lambdavacuum model does not have a singularity at t=0! Note that the value of the scale factor at t=0 is a0 = sqrt(3 / L), i.e. nonzero and finite.<br /><br />The reason why both density parameters, for curvature and for lambda, become infinite at t=0 in that model is just because H(0) = 0, i.e. because the universe starts at rest. Let's recall that the density parameter Johanneshttps://www.blogger.com/profile/05371418313799513738noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-92177077055571907042017-10-28T00:57:44.040-04:002017-10-28T00:57:44.040-04:00Initial condition are chosen at an initial time, n...Initial condition are chosen at an initial time, not necessarily t=0. You never put initial conditions on a singularity. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-2531232347002637762017-10-27T14:21:47.496-04:002017-10-27T14:21:47.496-04:00Sorry, but since initial conditions are just the c...Sorry, but since initial conditions are just the conditions at t=0, I don't get your last point.<br /><br />My point was that the geometry of the universe is just another initial condition, which a priori can be closed or open, and that, if it was closed, then the initial absolute value of the curvature density parameter was infinite. Thus, your statement "that there are still initial Johanneshttps://www.blogger.com/profile/05371418313799513738noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-16034466641171136832017-10-27T02:05:13.962-04:002017-10-27T02:05:13.962-04:00Johannes,
All you learn from that is that you bet...Johannes,<br /><br />All you learn from that is that you better not use initial conditions at t=0.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-51932266894380401382017-10-27T01:49:25.895-04:002017-10-27T01:49:25.895-04:00"But of course if you start with some very la..."But of course if you start with some very large value, say 10^60, then the result will still be incompatible with data. That is, you really need the assumption that the initial values are likely to be of order 1."<br /><br />Not in a universe of closed geometry, in which the initial absolute value of the curvature density parameter is infinite. In a simplified model of exponential Johanneshttps://www.blogger.com/profile/05371418313799513738noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-7438934683040931482017-10-24T04:15:00.185-04:002017-10-24T04:15:00.185-04:00Phillip-
The reason a ~30eV neutrino was raised wa...Phillip-<br />The reason a ~30eV neutrino was raised was because it would explain the Dark Matter component of the universe. From hindsight it was untenable.<br /><br />Why is the Earth like it is? From a chemistry viewpoint, the Earth is vastly reducing. The atmosphere is highly oxidising. As a consequence of life it has been transformed and maintained in a far from equilibrium thermodynamic Graham Dungworthhttps://www.blogger.com/profile/18355209024912564624noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-4103482762876711922017-10-23T03:47:33.032-04:002017-10-23T03:47:33.032-04:00"The reason the critical density and the Hubb...<i>"The reason the critical density and the Hubble parameter at time t are derived from Birkhoff's geometric proof etc is that they vastly simplify the calculations. The galaxies, groups,clusters and superclusters we observe and their structures appear similar in all directions of the sky, the cosmological Principle."</i><br /><br />The question is <i>why</i> does the cosmological Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-44701359657578435632017-10-23T03:44:27.088-04:002017-10-23T03:44:27.088-04:00"A 30eV neutrino(mass eigenstates for 3 gener...<i>"A 30eV neutrino(mass eigenstates for 3 generations) would close the universe."</i><br /><br />But a 30ev neutrino is ruled out by other observations.<br />Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58155754169410184682017-10-23T01:29:32.198-04:002017-10-23T01:29:32.198-04:00Sabine,
I'm glad that you referred to your ear...Sabine,<br />I'm glad that you referred to your earlier post about the "widely-spread and yet utterly wrong idea that the Bullet-cluster rules out modified gravity". I recently noted another article relevant to that discussion. <br />You cited two articles estimating tiny probabilities for observing such an extremely high energy collision of galaxy clusters. I noted a 2015 <a hrefAnonymoushttps://www.blogger.com/profile/15729194602104030005noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-80391699494001157352017-10-21T13:03:13.411-04:002017-10-21T13:03:13.411-04:00This whole business with some initial conditions b...This whole business with some initial conditions being "difficult" or "fine tuned" or whatever starts being unnervingly incomprehensible. It'd be much more understandable if someone was to talk of their "computability' - "compressibility" and such. Then again, there is this terible problem with what allows passage to dissipative chaotic structures where Theophanes Raptishttps://www.blogger.com/profile/01269614280130174555noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-70079232461230564722017-10-21T10:36:10.664-04:002017-10-21T10:36:10.664-04:00Haelfix,
Indeed it seems like we're talking p...Haelfix,<br /><br />Indeed it seems like we're talking past each other. Please re-read my first reply to you. You are conflating simplicity with finetuning. It's *not* the same thing. Difficult initial conditions are quantifiably problematic. Finetuning isn't. It's an aesthetic problem. GR simplifies initial conditions over Newtonian gravity. Inflation doesn't make the initialSabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-66304180694136179562017-10-21T09:53:20.030-04:002017-10-21T09:53:20.030-04:00Bee,
The fact that it would be a 'mess' a...Bee,<br /><br />The fact that it would be a 'mess' as you put it, is the point. It tells you very much that the theory is sick. Again, you can arrange your initial data so that every terrestrial observation ever made just so happens to be a classical cosmic ray hitting a detector in a sort of wonderful conspiracy such that it doesn't matter what you think you see (this is right Haelfixhttps://www.blogger.com/profile/16980241518510365230noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-86018953229368349172017-10-20T17:45:40.302-04:002017-10-20T17:45:40.302-04:00The initial low-entropy of the universe seems to a...The initial low-entropy of the universe seems to a similar problem. It seems like the same logic applies.<br />Anonymoushttps://www.blogger.com/profile/14727554887260418977noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-18638190954656812972017-10-20T15:32:24.779-04:002017-10-20T15:32:24.779-04:00Phillip-
On Isotropy and homogeneity
The reason th...Phillip-<br />On Isotropy and homogeneity<br />The reason the critical density and the Hubble parameter at time t are derived from Birkhoff's geometric proof etc is that they vastly simplify the calculations. The galaxies, groups,clusters and superclusters we observe and their structures appear similar in all directions of the sky, the cosmological Principle.<br /><br />https://Graham Dungworthhttps://www.blogger.com/profile/18355209024912564624noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-68233890479441828622017-10-20T11:51:01.848-04:002017-10-20T11:51:01.848-04:00naivetheorist,
I do not recall any such comment. ...naivetheorist,<br /><br />I do not recall any such comment. In any case please note that I sometimes do not approve off-topic comments or questions that are repetitions of earlier asked questions or comments that contain any kinds of links to webpages that look fishy. And that's leaving aside that some comments simply land in the junk folder and I never see them in the first place.<br /><br /Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-26533177553942401262017-10-20T11:48:50.699-04:002017-10-20T11:48:50.699-04:00Phillip,your quote "It's easy to understa...Phillip,your quote "It's easy to understand: look at the CMB in two different places on the sky. Unless they are really close together, the two areas have never been in causal contact...".<br />This is valid for points located no more than 2 degees across, ca.4 fold the diameter of the Moon, or for the Andromeda galaxy that subtends a similar angle, although our eye only sees the Graham Dungworthhttps://www.blogger.com/profile/18355209024912564624noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-55090716008538094712017-10-20T10:27:38.604-04:002017-10-20T10:27:38.604-04:00bee:
i previously asked you (but you didn't p...bee:<br /><br />i previously asked you (but you didn't post the question or answer it) what distinction do you make between a theory and a model. you seem to conflate them or use them interchangeably. Do they differ and if so, how does one determine if a theory or a model 'works' and what does it mean?<br /><br />richardnaivetheoristhttps://www.blogger.com/profile/00425164894020381981noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-61830213042775094922017-10-20T10:11:34.523-04:002017-10-20T10:11:34.523-04:00Haelfix,
No, what you say is not correct, as I al...Haelfix,<br /><br />No, what you say is not correct, as I already said above. There is nothing whatsoever wrong with "finely-tuned" conditions, which I put in scare-quotes because it's ill-defined without a probability distribution. You can also not rule out 't Hooft's approach with that, and discarding it baselessly as "nonsense" won't help you either. <br /><Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-43704254522002680632017-10-20T09:32:07.396-04:002017-10-20T09:32:07.396-04:00Hi Bee,
Distinguish between the simplicity of a t...Hi Bee,<br /><br />Distinguish between the simplicity of a theory, and the simplicity of a solution of a theory. QCD is a simple theory, but its solution(s) are a complicated mess of bound states. Likewise Newtonian physics is simple (indeed a special case of SR), but the solution necessary to describe the real world must be ugly or contrived (PPN cosmology etc).<br /><br />For our purposes it Haelfixhttps://www.blogger.com/profile/16980241518510365230noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-53087594708213593052017-10-19T12:08:33.187-04:002017-10-19T12:08:33.187-04:00Haelfix,
I have no idea why you think that Newton...Haelfix,<br /><br />I have no idea why you think that Newtonian gravity - by which I suppose you mean post-post-Newtonian or whatever is necessary to fit current measurement precision - is axiomatically simpler than GR. <br /><br />Besides, you are misunderstanding the use of simplicity. Simplicity is not an absolute criterion. It doesn't make any sense to require a description of nature to Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.com