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But apparently we don’t live in a void, we live in a big glob of higher density galaxies, the Laniakea supercluster. https://arxiv.org/pdf/2311.00215.pdf https

But apparently we don’t live in a void, we live in a big glob of higher density galaxies, the Laniakea supercluster. https://arxiv.org/pdf/2311.00215.pdf https://arxiv.org/pdf/2311.00215.pdf MOND hasn’t been having the rosiest of time lately. https://academic.oup.com/mnras/article/527/3/4573/7342478?login=false https://academic.oup.com/mnras/article/527/3/4573/7342478?lo...

due-rr 3y ago
Dr. Becky has a nice explainer video of the first paper. https://youtu.be/-kTe0xRAU1w https://youtu.be/-kTe0xRAU1w
OnACoffeeBreak 3y ago
Doesn't sound like the paper's (the one you linked) authors don't necessarily agree with your summary. From the discussion on page 16: These results seemingly worsen the recently established tension between the inferred value of H0 from early and late times Universe probes, which has been argued to potentially be the sign of new cosmological physics (see for example Refs. [78–83]). This might appear to be in contradiction with the possibility, explored for example in Refs. [74, 84– 93], that local gravitational physics could alleviate the Hubble tension. Amongst these, a class of models achieve a lowering of H0 under the assumption that we live in an underdense region, whose inner expansion rate is on average larger than the background one. Some results in the literature, see for example refs. [94–96], seem to corroborate the latter assumption finding evidence of local voids which averaged on spheres of r ≳ 100Mpc have density contrasts of δ ≤ −0.1, unexpected within the ΛCDM model. Computing the average density contrast of a sphere centered in Laniakea with radius r ≈ 110 Mpc (i.e. the average distance of the boundary of the ellipsoid from the center) using the CF4 reconstruction we found δ ∼ −0.06, within the prediction of the concordance model (see for example Fig. 6 of Ref. [55]). However, this sphere is not centered in the Milky Way, which might explain why the result differs from the aforementioned ones. Indeed, overdensities such as Laniakea are surrounded by voids (from which they have collected matter), and therefore any sufficiently spherical average will include these under-dense regions. On the other hand, Refs. [55, 79, 97] also found no evidence of any large void or overdensity, thus disfavoring a local resolution of the Hubble tension. Our analysis corroborates these results, suggesting instead that the tension is likely to be (slightly) worsened by Laniakea’s backreaction. An important caveat, however, is that our analysis does not exclude the possibility that large voids in the annular region between 110 − 400 Mpc outside Laniakea could balance and overcome the backreaction from Laniakea, like a rather picturesque Matryoshka doll. Alternative modelling choices accounting for the impact of these voids are therefore required to fully understand the impact our cosmic environment’s gravitational backreaction, which will be the focus of forthcoming studies.

due-rr 3y ago

Dr. Becky has a nice explainer video of the first paper. https://youtu.be/-kTe0xRAU1w https://youtu.be/-kTe0xRAU1w

OnACoffeeBreak 3y ago

Doesn't sound like the paper's (the one you linked) authors don't necessarily agree with your summary. From the discussion on page 16: These results seemingly worsen the recently established tension between the inferred value of H0 from early and late times Universe probes, which has been argued to potentially be the sign of new cosmological physics (see for example Refs. [78–83]). This might appear to be in contradiction with the possibility, explored for example in Refs. [74, 84– 93], that local gravitational physics could alleviate the Hubble tension. Amongst these, a class of models achieve a lowering of H0 under the assumption that we live in an underdense region, whose inner expansion rate is on average larger than the background one. Some results in the literature, see for example refs. [94–96], seem to corroborate the latter assumption finding evidence of local voids which averaged on spheres of r ≳ 100Mpc have density contrasts of δ ≤ −0.1, unexpected within the ΛCDM model. Computing the average density contrast of a sphere centered in Laniakea with radius r ≈ 110 Mpc (i.e. the average distance of the boundary of the ellipsoid from the center) using the CF4 reconstruction we found δ ∼ −0.06, within the prediction of the concordance model (see for example Fig. 6 of Ref. [55]). However, this sphere is not centered in the Milky Way, which might explain why the result differs from the aforementioned ones. Indeed, overdensities such as Laniakea are surrounded by voids (from which they have collected matter), and therefore any sufficiently spherical average will include these under-dense regions. On the other hand, Refs. [55, 79, 97] also found no evidence of any large void or overdensity, thus disfavoring a local resolution of the Hubble tension. Our analysis corroborates these results, suggesting instead that the tension is likely to be (slightly) worsened by Laniakea’s backreaction. An important caveat, however, is that our analysis does not exclude the possibility that large voids in the annular region between 110 − 400 Mpc outside Laniakea could balance and overcome the backreaction from Laniakea, like a rather picturesque Matryoshka doll. Alternative modelling choices accounting for the impact of these voids are therefore required to fully understand the impact our cosmic environment’s gravitational backreaction, which will be the focus of forthcoming studies.