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started something. For the moment really just a glorified pointer to Buchert et al. 15 and putting Scharf 13 into perspective
expanded a bit more and introduced subsections.
found a useful conclusion of the arguments exchanged in the “backreaction debate”, in Ostrowski-Roukema 15, and added it as a quote (here)
added pointer to Coley 18, p. 28, where it is confirmed that it remains open whether cosmic inhomogeneity may have substantial backreaction on cosmic evolution (in view of the debate about a claim by Green-Wald to the contrary)
added pointer to
This is remarkable, as they claim an analytic solution exhibiting the effect:
Our analysis is based on the discovery of a closed ansatz for perturbations of the SM during the p$= 0$ epoch of the Big Bang which triggers instabilities that create unexpectedly large regions of accelerated uniform expansion within Einstein’s original theory without the cosmological constant. We prove that these accelerated regions introduce precisely the same range of corrections to redshift vs luminosity as are produced by the cosmological constant in the theory of Dark Energy.
Cool!
Should this hold water, and dark energy shown to be just an artifact of an inappropriate model, this would be the most dramatic triumph of rigour over handwaving in physics. Cosmology/HEP physics has entered an era of storytelling since the last decades, and this might well be what brings mathematical physics back to the forefront.
added pointer to today’s
added pointer to
added this pointer:
added also this recent reference
abd this quote (from its p. 3):
Cosmic backreaction is particularly interesting because it in principle has the potential to explain the apparent accelerated expansion of the Universe without introducing any exotic dark energy component as well as possibly being able to mimic dark matter.
Less ambitiously, cosmic backreaction might solve the $H_0$-problem through the emergence of curvature (Bolejko 17), or a small backreaction may bias the values obtained from analyses of data based on FLRW models and must therefore be identified and taken into account in an era of precision cosmology. Yet another option is that cosmic backreaction is entirely negligible in the real universe.
Whichever is the case, a theoretical quantification of cosmic backreaction is necessary for getting the foundations of cosmology onto solid ground; the mathematics clearly shows that in principle backreaction terms affect the overall dynamics of the Universe. It is therefore an important goal of cosmologists to obtain a theoretical understanding of the size of cosmic backreaction in the real universe similarly to e.g. the desire to theoretically understand the value of the vacuum energy density.
added pointer to this:
added pointer to and quotes from this webpage:
added pointer to today’s
I have added pointer to this exposition, which appeared today (and which in turn references this nLab entry):
as well as this classical article, which it cites,
and finally this quote, from the former:
The Fitting Problem in cosmology was written in 1987. In the context of this work and the significant theoretical difficulties involved in inferring fundamental physics from the real Universe, any claims of having measured a cosmological constant from directionally skewed, sparse samples of intrinsically scattered observations should have been taken with a grain of salt. By honouring this claim with a Nobel Prize, the Swedish Academy may have induced runaway prestige bias in favour of some of the least principled analyses in science, strengthening the confirmation bias that seems prevalent in cosmology.
also added pointer to
and a quote of their conclusion:
some proposals suggest some local/environmental factors ($z \leq 0.03$) can bias the local determinations. This would mean the locally measured $H_0$ cannot be interpreted as the global Hubble constant of the homogeneous universe. An example is a local underdense region (Lombriser 2019; Shanks et al. 2019). Recent studies have shown observational evidence supporting a small-scale local underdense region (Boehringer et al. 2019; Pustilnik et al. 2019), though it has been argued that the likelihood for a local void to substantially affect the local measurement may be low (Kenworthyet al. 2019). These local factors do not pose a problem tothe standard ΛCDM model at large scales, but instead point to the need for a more detailed description of our local environment to account for such a systematic effect that can shift all local measurements in the same way. If all local measurements produce high values of $H_0$, it would favor such a local/environmental-factor explanation over systematic effects that may be unique to each observation.
Updated the following to include doi link and full journal reference
J. Colin, R. Mohayaee, Mohamed Rameez, Subir Sarkar, Evidence for anisotropy of cosmic acceleration, Astronomy & Astrophysics 631 L13 (2019) doi:10.1051/0004-6361/201936373, (arXiv:1808.04597)
Thanks.
It’s an intriguing argument. My instinctive preference, for what little it’s worth, is for inhomogeneity over dark energy, as it’s much more parsimonious a solution. And it seems more generic that there is an inhomogeneity rather than a mysteriously homogeneous distribution of the universe. But, of course, one needs to follow the experimental data on this one.
(It would be even more revolutionary if there were a similarly credible (I guess this is credible, given that it’s published and getting attention) alternative to dark matter. But that’s off-topic for this thread)
added pointer to today’s
presenting a model of inhomogeneous cosmology that is claimed to be (p. 14):
a natural and consistent explanation of
(i) dark energy,
(ii) the coincidence problem (here conceptually,not quantitatively),
(iii) positive initial curvature,
(iv) the small matter density cosmological parameterfound in local probes of the matter density,
(v) the large angular diameter distance tothe CMB consistent with JLA supernova sample parameter constraints,
and (vi) the local expansion rate measurements (removal of the ’Hubble tension’).
We believe that this model architecture needs convincing arguments to be rejected as a physically viable show-case, on the basis of which the model ingredients can be improved in order to build a physical cosmology in the future.
Added these two pointers
Kevin J. Ludwick, Examining the Viability of Phantom Dark Energy, Phys. Rev. D 92, 063019 (2015) (arXiv:1507.06492)
Kevin J. Ludwick, The Viability of Phantom Dark Energy as a Quantum Field in 1st-Order FLRW Space, Phys. Rev. D 98, 043519 (2018) (arXiv:1804.02987)
on inhomogeneity mimicking phantom dark matter
added this pointer:
added pointer to today’s
added pointer to
added pointer to
added pointer to today’s
added pointer to
together with a quote on “void models”
added pointer to:
added pointer to today’s survey:
added pointer to today’s update
Together with this line quoted from the Conclusions:
The joint significance of this rejection of the cosmological principle is 5.2σ.
Well if the authors of the new article are right and lambda CDM and FLRW are proven to be wrong, then this might as well save string theory from its current sorry state, as the main conflict of string theory vs the cosmological constant would have been resolved against the cosmological constant in favor of string theory, and everything would be reset to the mid 1990s.
Yes, that’s one motivation for keeping an eye on this development. The entry briefly comments on this in the section Impact.
added pointer to today’s review of observed violations of common cosmological assumptions:
Interesting with this article also the implicit showcasing of the underlying sociodynamics: The author (who holds immense reputation in the field) is torn between his intrigue for the reported facts and the fear to be perceived (“misunderstood”) as a contrarian or rebel (p. 1: “To reduce the chance of misunderstanding I emphasize that…”; p. 22: “To prevent misunderstandings I repeat…”)
By the way, the author also comments on the reference SHRMS22 from #34 above:
Some authors conclude that the anomalously large dipole anisotropy of distant quasars and radio galaxies and the large quasar groups, if physically real, violate the cosmological principle (SHRMS22). This depends on the definition of this principleL of course, it does not violate the definition explained in section (1.1.1). But it likely violates the assumption of Gaussian near scale-invariant and adiabatic initial conditions that has served so well for many other cosmological test.
added pointer to:
with this quote:
it is possible that data has reached a requisite precision that the cosmological principle has already become obsolete
That’s a nice quote. I was seeing independent discussion in blog comments recently about how the assumption of cosmological homogeneity may need to be dropped. I recall where though, sorry!
pointer to yesterday’s paper
Anonymouse
new paper:
John Ochs
Thanks for making updates!
That arXiv:2308.10738 is curious, I have cross-linked the reference item with topological data analysis and persistent homology.
I am always looking for genuine applications of TDA in the form of persistent homology (since upon inspection of the large literature on TDA it turns out to be hard to pinpoint specific achievements in applications).
I have only skimmed Buchert et al.’s latest article now and I don’t really understand yet what exactly is going on. They seem to mention persistence diagrams only in passing? I’d have to have a closer look.
But from reading the intro and the conclusions, the impression I get is that whatever aspect of persistent homology they compute, they don’t claim to see signatures of inhomogeneity right in the TDA data itself, rather what they do is compare the persistent homology of experimental data with the theory data and look for discrepancies.
Surprisingly, they find such discrepancies for the northern but decidedly not for the southern hemisphere?! Isn’t that a red flag warning of some big systematic error hiding somewhere?
Anyways, thanks for keeping us updated on these matters. Interesting to follow this development.
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