3 ms·
One important caveat to all this is that the gravitational redshift for a galaxy or galaxy cluster is an incredibly tiny effect. For example, the mean gravitati
by Keysh 5y ago
One important caveat to all this is that the gravitational redshift for a galaxy or galaxy cluster is an incredibly tiny effect. For example, the mean gravitational redshift for massive galaxy clusters has been measured (e.g., link below), and has a value of ~ 10 or 15 km/s. This can be compared with the typical motions of galaxies within a cluster (dispersions of ~ 1000 km/s) and the cosmological redshift of the clusters (for the sample in the paper I linked to, it's equivalent to an average Doppler shift of ~ 50,000 km/s).
In order to make that measurement, the authors had to combine redshift measurements for ~ 20,000 galaxies from ~ 2,500 different clusters; as they note, it's impossible to detect the effect for just a single cluster: "... even assuming that every galaxy in a cluster could be spectroscopically measured, there are simply too few galaxies to allow the statistical detection of a non-zero gravitational redshift."
So while it's technically true that adding dark matter to a galaxy would increase its gravitational redshift and thus its total observed redshift, this effect is immeasurably small.
Mpetha et al. (2021), "Gravitational redshifting of galaxies in the SPIDERS cluster catalogue"
https://arxiv.org/abs/2102.11156 https://arxiv.org/abs/2102.11156
- raattgift 5y agoI can only reply to your "impossible to detect" with respect to clusters with the "technically challenging but apparently real and prospects for better measurements and extension to larger scales is promising" at the end of Kaiser's 2018 slide deck https://chalonge-devega.fr/Kaiser_29mars2018.pdf https://chalonge-devega.fr/Kaiser_29mars2018.pdf which I do not think is contradicted by the (interesting, thank you!) Mpetha et al (2021) paper, which cites at least three of Kaiser's earlier publications. Broadhurst & Scannapieco (2000) https://iopscience.iop.org/article/10.1086/312630 https://iopscience.iop.org/article/10.1086/312630 is also relevant here, as it distinguishes between X-Ray studies and optical ones (SPIDERS being an example of the latter) pointedly at §4.2. I do not disagree that ~ 10-20 km/s is small, but it's the little things that cause "tension" headaches, isn't it?
- Keysh 5y agoKaiser's presentation doesn't suggest you can do the measurement for individual clusters; the example analysis is from Wojtak et al. (2011), which is similar to Mpetha et al. in using thousands of galaxies and thousands of clusters. The Broadhurst & Scannapieco discussion about X-ray studies is interesting and potentially promising, but no one seems to have tried doing for real.