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• CommentRowNumber1.
• CommentAuthorUrs
• CommentTimeJul 13th 2018

started something. For the moment really just a glorified pointer to Buchert et al. 15 and putting Scharf 13 into perspective

• CommentRowNumber2.
• CommentAuthorUrs
• CommentTimeJul 13th 2018

typo fixed

• CommentRowNumber3.
• CommentAuthorUrs
• CommentTimeJul 14th 2018
• (edited Jul 14th 2018)

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)

• CommentRowNumber4.
• CommentAuthorUrs
• CommentTimeJul 16th 2018

further added various bit of this and that, notably some quotes on the qualitative understanding of how inhomogeneity may give rise to accelerated expansion (here). That was fun, but I should leave it at that now.

• CommentRowNumber5.
• CommentAuthorUrs
• CommentTimeJul 24th 2018

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)

• CommentRowNumber6.
• CommentAuthorUrs
• CommentTimeAug 2nd 2018
• (edited Aug 2nd 2018)

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.

• CommentRowNumber7.
• CommentAuthorDavidRoberts
• CommentTimeAug 3rd 2018

Cool!

• CommentRowNumber8.
• CommentAuthorUrs
• CommentTimeAug 3rd 2018
• (edited Aug 3rd 2018)

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.

• CommentRowNumber9.
• CommentAuthorUrs
• CommentTimeMay 8th 2019

• Zhiqi Huang, Han Gao, Haoting Xu, Revisiting Ryskin’s Model of Cosmic Acceleration (arXiv:1905.02441)
• CommentRowNumber10.
• CommentAuthorUrs
• CommentTimeMay 8th 2019

also added a brief quote from their article

at the end of the section “The backreaction debate” (here)

• CommentRowNumber11.
• CommentAuthorUrs
• CommentTimeSep 7th 2019

• CommentRowNumber12.
• CommentAuthorUrs
• CommentTimeOct 25th 2019

• Vincent Deledicque, Theoretical developments on the adequacy of the fitting of the FLRW metric on the universe’s real metric (arxiv:1907.01580)
• CommentRowNumber13.
• CommentAuthorUrs
• CommentTimeOct 25th 2019

• S. M. Koksbang, Towards statistically homogeneous and isotropic perfect fluid universes with cosmic backreaction, Class. Quantum Grav. 36 185004, 2019 (arxiv:1907.08681)

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.

• CommentRowNumber14.
• CommentAuthorUrs
• CommentTimeOct 26th 2019
• (edited Oct 26th 2019)

• CommentRowNumber15.
• CommentAuthorUrs
• CommentTimeOct 26th 2019

added pointer to and quotes from this webpage:

• CosmoBack 2018 , From inhomogeneous gravity to cosmological backreaction. Theoretical opportunity? Observational evidence?
• CommentRowNumber16.
• CommentAuthorUrs
• CommentTimeOct 28th 2019

added more pointers to talks from the CosmoBack18 meeting

• CommentRowNumber17.
• CommentAuthorUrs
• CommentTimeOct 30th 2019

• Hayley J. Macpherson, Inhomogeneous cosmology in an anisotropic Universe (arxiv:1910.13380)
• CommentRowNumber18.
• CommentAuthorUrs
• CommentTimeNov 15th 2019
• (edited Nov 15th 2019)

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.

• CommentRowNumber19.
• CommentAuthorUrs
• CommentTimeNov 15th 2019

• Weikang Lin, Katherine J. Mack, Liqiang Hou, Investigating the Hubble Constant Tension – Two Numbers in the Standard Cosmological Model (arXiv:1910.02978)

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.

• CommentRowNumber20.
• CommentAuthorDavidRoberts
• CommentTimeDec 10th 2019

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)

• CommentRowNumber21.
• CommentAuthorUrs
• CommentTimeDec 11th 2019
• (edited Dec 11th 2019)

Thanks.

• CommentRowNumber22.
• CommentAuthorDavidRoberts
• CommentTimeDec 11th 2019

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)

• CommentRowNumber23.
• CommentAuthorUrs
• CommentTimeFeb 26th 2020

• Asta Heinesen, Thomas Buchert, Solving the curvature and Hubble parameter inconsistencies through structure formation-induced curvature (arXiv:2002.10831)

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.

• CommentRowNumber24.
• CommentAuthorUrs
• CommentTimeMar 12th 2020

• 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

• CommentRowNumber25.
• CommentAuthorUrs
• CommentTimeMar 24th 2020

• CommentRowNumber26.
• CommentAuthorUrs
• CommentTimeMay 8th 2020

• Yonadav Barry Ginat, Multiple-Scales Approach for Addressing The Averaging Problem in Cosmology (arXiv:2005.03026)
• CommentRowNumber27.
• CommentAuthorUrs
• CommentTimeOct 26th 2020

• Léo Brunswic, Thomas Buchert, Gauss-Bonnet-Chern approach to the averaged Universe, Class. Quantum Grav. 37 (2020) 215022 (arXiv:2002.08336)
• CommentRowNumber28.
• CommentAuthorUrs
• CommentTimeOct 26th 2020

• Szymon Sikora, Krzysztof Głód, Construction of the cosmological model with periodically distributed inhomogeneities with growing amplitude (arXiv:2005.06385)
• CommentRowNumber29.
• CommentAuthorUrs
• CommentTimeNov 6th 2020

• Martin Bojowald, Ding Ding, Canonical description of cosmological backreaction (arXiv:2011.03018)
• CommentRowNumber30.
• CommentAuthorUrs
• CommentTime4 days ago

• Ryan C. Keenan, Amy J. Barger, Lennox L. Cowie, Evidence for a ~300 Mpc Scale Under-density in the Local Galaxy Distribution, 2013, ApJ, 775, 62 (arXiv:1304.2884)

together with a quote on “void models”

• CommentRowNumber31.
• CommentAuthorUrs
• CommentTime4 days ago