Is this MOND advance a big deal?

[Frabjous]

I read this article
https://physics.aps.org/articles/v14/143
about this paper
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302

How potentially significant is this?

 

[mathman]

It would need analyses by experts to come up with a definite conclusion.

 

[Vanadium 50]

How potentially significant is this?

Not very.

 

[strangerep]

[...]
about this paper
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302

Arxiv version here.

 

[ohwilleke]

It is important as a proof of concept. It demonstrates that a modified gravitational theory that can produce a broad range of phenomena often attributed to dark matter at the scale of galaxies can also produce the cosmic background radiation phenomena often attributed to dark matter. Dark matter is thus no longer the only game in town when it comes to explaining this cosmology scale phenomena.

It is also not the first modified gravity theory to do so. JW Moffat's MOG theory got there first. See, e.g., here and here.

Generally speaking, dark matter particle theories have struggled to be predictive at the galaxy scale, while excelling at the cosmology and large scale structure scale. Neither, to be honest, do a great job at predicting behavior in galactic clusters.

This result also dovetails nicely with the EDGES 21cm signals which are a good fit to a universe without dark matter. But EDGES is among the biggest cosmology scale contradictions of the LambdaCDM theory by observational evidence in the cosmology/early universe context. See, e.g. here.

Fig. 1. Evolution of T21 with three different minimum halo masses. The black line is the EDGES best-fit model. From here.

Even strong MOND proponents recognize that MOND itself is a phenomenological toy model, but a quite successful one, viewed as such. MOND captures a basic relationship that is observed which is fairly easy to reproduce with a variety of different kinds of equations (also known as the Radial Acceleration Relation or RAR). But everyone realizes that it needs a relativistic generalization, and that it has known defects that need to be fixed in a more broadly applicable modified gravity theory.

 

[mbond]

Are the EDGES results independently verified? They are from 2018, with a signal/noise smaller than 1/1000.

 

[ohwilleke]

Are the EDGES results independently verified? They are from 2018, with a signal/noise smaller than 1/1000.

So far as I know, no one has seriously doubted the veracity of the EDGES result that was based on about three years of observations (although there are some methodological issues over just how precise it really is). Everyone seems to agree that it strongly contradict the LambdaCDM prediction, although there is great diversity of opinion regarding its cause of the disparity.

I believe that EDGES 21cm results are the first of its kind. A 2017 PhD thesis reviewed the experiment and put it in context. See also a 2019 article reviewing it and its results, and this website from M.I.T. about the observation technology used. It wasn't very expensive (although it is quite location sensitive to minimize background noise) and there are more than a hundred radio telescopes out there, so I suspect that it will be replicated before long. The 2019 article explains that:

The EDGES experiment is a very small radio telescope, 2 meter long and 1 meter high, located in the radio quiet zone in western Australia. The equipment consists in three broad-band antennas that cover a range of frequencies from 50 to 200 MHz. The low-band antenna (operating from 50 to 100 MHz) has been designed to observe a spectral distortion in the 21-cm energy band at cosmological redshift of 20 due to the absorption of CMB photons by the IGM. However, the detection of the 21-cm signal is very challenge because of the very large foregrounds of galactic diffuse synchrotron emission. The full-sky maps of the diffuse synchrotron emission at 45 MHz and 408 MHz can be found, for example, in [2] and [3] respectively. Before subtracting the foregrounds to the data is important to stress that: i) the brightness temperature in the frequency window of EDGES is always above 100 K even in region far away from the galactic center; ii) the galactic synchrotron emission is spectrally smooth above 50 MHz but might need several terms to model it in a proper way as discussed in details in [4]; iii) the synchrotron emission features a large spatial gradient especially in the region close to the galactic center where the activity is much larger (see e.g. [2, 3]).

Fig. 1 of [1] shows the EDGES detection in terms of brightness temperature as a function of the frequency obtained by looking at high galactic latitudes. It is evident from panel a. that the galactic synchrotron emission dominates the observed sky noise, yielding to an almost perfect power-law profile that decreases from about 5000 K at 50 MHz to about 1000 K at 100 MHz. Fitting and removing the galactic synchrotron emission from the spectrum with a physically motivated 5-terms polynomial the collaboration gets the residual in panel b.. This residual is not flat and it has a root-mean-square of 87 mK. Repeating the same exercise by adding to the 5-terms polynomial a template of the signal like the one in panel d., the collaboration gets the residuals in panel c.. This new residual is now flat and the fit substantially ameliorates with a root-mean-square of only 25 mK. Adding the template to the residual the 21-cm signal is finally reported in panel d.. Fig. 2 of Ref. [1] summarizes the detected signal obtained by using different experimental configurations. As one can see this is a signal in absorption because the brightness temperature is negative. It extends from redshift 20 to redshift 15 and it has an amplitude of 500+200 −500 mK at 99% CL. The value of the plateau, centered at a frequency of 78 MHz, translating to a redshift of 17.2, is quite surprising because is 3.8σ away from the prediction of standard cosmology. As I am going to discuss in Sec. 3, the global 21-cm signal predicted from ΛCDM can not ever be below −230 mK. If the measured amplitude is correct, BSM physics is required.

Reference 1 is J. D. Bowman, A. E. E. Rogers, R. A. Monsalve, T. J. Mozdzen and N. Mahesh, Nature 555 (2018) no.7694, 67 doi:10.1038/nature25792 [arXiv:1810.05912 [astro-ph.CO]].