How Einstein Discovered Dark Energy · HackerPapers discussions

How Einstein Discovered Dark Energy — mentioned by raattgift

Fri, 04 Oct 2024 20:20:46 GMT

Couple minor additions: geodesics are physical in that (i) they are the paths taken by objects in free-fall and (ii) light (massless waves, etc etc) in free space is always in free-fall: all allowable null curves are null geodesics. Massive objects and light may be accelerated (the direction part of the velocity vector changes with scattering even if the magnitude part does not) and during acceleration will not be bound to a geodesic because they aren't in free-fall. Even though I agree with the way you put it, I'll say it's probably a bit too strong to write that stress-energy is the (sole)…

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How Einstein Discovered Dark Energy — mentioned by pdonis

Mon, 25 Jul 2022 03:11:32 GMT

> Relativity allows some physical distributions of stress-energy to pick out a meaningful set of coordinates. More precisely, with certain distributions of stress-energy, computations become much simpler in particular coordinates. But that does not mean the coordinates have any physical meaning. If there are any actual invariant properties of the spacetime that particular coordinates make more evident, those properties can always be expressed in coordinate-independent terms, for example in terms of Killing vector fields. > (as has been known in cosmology for many years. cf.…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Mon, 25 Jul 2022 00:28:46 GMT

I agree with all your points except: > QM strongly suggests that it isn't an argument over which is certainly out of scope here (however it did answer my question); and > The Big Bang is not a single point. It's a spacelike line which I think is wrong for the standard cosmology. See Davis & Lineweaver 2004 <https://ui.adsabs.harvard.edu/abs/2004PASA...21...97D/abstra...> notably fig. 1 and the expalanatory text scattered within. Why would (cosmological) t=0 be a line rather than a point, a 3-volume, or some even weirder non-Lorentzian structure? I can't think of why it would be a line other…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Mon, 20 Dec 2021 10:56:18 GMT

It depends on what exactly you mean with "... non-zero energy", but the best answer is probably "no". On the one hand, in vacuum, an accelerating expanding universe's geometry breaks time-translation invariance. Let's look at the vacuum geometry with a pair of test photons in the otherwise empty test universe. In the early universe we have the photons widely spacelike-separated but on trajectories where they will ultimately collide in the far future. The photons in the future will be redshifted compared to their wavelengths in the past, with the redshift proportional to the expansion history…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Fri, 17 Sep 2021 15:45:49 GMT

Sunny Vagnozzi can't really be said to be suddenly coming up with the idea - there are several obviously related preprints at https://arxiv.org/a/vagnozzi_s_1.html I'd like to give some broad background on the link at the top of this discussion. If one substitutes "dark energy" with "quintessence field driving the accelerated expansion" one gets closer to what the press release (and associated paper, found at https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.0... ) discusses. The "quint" in "quintessence" refers to a "fifth fundamental force", which I'll get to below. As early as the…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Sun, 05 Sep 2021 20:39:23 GMT

Wikipedia covers this sufficiently at https://en.wikipedia.org/wiki/Cosmological_constant#Sequence... The five-line tl;dr: non-expanding cosmologies with no big bang but lots of galaxies tend to collapse in finite time. One can avoid collapse with a positive cosmological constant. That approach predated the work of Hubble (expansion) and Lemaître (big bang). Expanding cosmologies with a sufficiently large initial big bang do not need a cosmological constant to keep expanding forever. Einstein, not being stupid, recognized that and carried on. Since 1998: an accelerating expanding universe is…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Tue, 05 Nov 2019 12:28:03 GMT

The CMB was (and is) observed a lot: https://en.wikipedia.org/wiki/List_of_cosmic_microwave_backg... The CMB is the evidence for a hot big bang (note, this is not the same as a singularity) although of course observational cosmologists want to trace back from the first starlight to the CMB and have various plans to do so. Surprises in that "dark age" are more than welcome because they are bound to reveal things about low-density momentum-energy. > assume dark matter and dark energy are real Nobody has put together a self-consistent physical cosmology model that reasonably matches actual…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Sat, 08 Dec 2018 03:24:58 GMT

Well, so you write down an initial values surface with that approach and let the matter evolve: either you get something physically plausible or you don't. We have some guides as to what is physically plausible, and of course evidence that any of them is wrong would be very interesting. More theoretically, the LHS doesn't care about how you arrange the RHS at all. More finely, the metric does not care how you compose the metric. If you do it perturbatively, the leading term could be Minkowski or Gödel for all it cares, as long as the sub-leading terms then present a useful background for…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Tue, 22 Aug 2017 19:29:40 GMT

Second thing first: > ... much the same way physicists dislike the standard model in all of its' ugliness - even though it works Personally I think the U(1)xSU(2)xSU(3) group theory of the Standard Model of Particle Physics is quite pretty. AFAIK the problems with the Standard Model are rarely put as issues of "ugliness" but rather that it's known to be incomplete -- in particular the gravitational sector is not dealt with at all -- and is probably slightly incorrect (depending, for example, on how you deal with neutrino masses). It has some practical problems too, namely that there are at…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Wed, 01 Mar 2017 14:13:27 GMT

> I've seen people create variants of MOND that try to address the Bullet Cluster problem, but you start having to have some sort of really weird gravity memory effect in space or something, where space itself "remembers" that it used to have a lot of matter in it and still has a small amount of gravitation in it despite no longer containing the corresponding mass Which variants are these? cf. section 8.3, https://arxiv.org/abs/1112.3960 Several of the relativstic MOND theories (see chapter 7 of the same review) are detailed, and I don't see how any of the 8.3 set matches the quote above.…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Wed, 01 Mar 2017 12:41:53 GMT

The problem is that your "lens" statement doesn't convey much information; if it goes deep, unfortunately it's not obvious. For example, it doesn't tell anyone whether you think that CDM is sparticles or a quantization of a function on the Ricci scalar or of a second metric or whatever. The latter two are awfully hard to distinguish from relativistic approaches to MOND; indeed you could quantize most of the approaches in Chapter 7 of https://arxiv.org/abs/1112.3960 . More generally, you can put a scalar or other degree of freedom on either side of the usual write-down of the Einstein Field…

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How Einstein Discovered Dark Energy — mentioned by raattgift

Sun, 12 Feb 2017 18:35:56 GMT

Incidentally, and mostly just for myself here is : https://arxiv.org/abs/1211.6338 in which Schrödinger (in 1918!) describes, "matter in the large essentially as a compressible fluid of constant density at rest under a constant spatially isotropic tension which ... must be equal to 1/3 of the rest density of energy". The notation is old-fashioned, but check out Schrödinger's paragraph at the bottom of Harvey's page 5 !

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