Monday, October 31, 2016

The Latest In The Dark Matter v. Modified Gravity Debate

Sabine Hossenfelder's latest post at her blog Backreaction reports on some of the latest volleys in the particle dark matter v. modified gravity debate that I have been following for many years at this blog. She does a good job at summing up the competing perspectives involved:
In its simplest form the concordance model has sources which are collectively described as homogeneous throughout the universe – an approximation known as the cosmological principle. In this form, the concordance model doesn’t predict how galaxies rotate – it merely describes the dynamics on supergalactic scales.

To get galaxies right, physicists have to also take into account astrophysical processes within the galaxies: how stars form, which stars form, where do they form, how do they interact with the gas, how long do they live, when and how they go supernova, what magnetic fields permeate the galaxies, how the fields affect the intergalactic medium, and so on. It’s a mess, and it requires intricate numerical simulations to figure out just exactly how galaxies come to look how they look. 
And so, physicists today are divided in two camps. In the larger camp are those who think that the observed galactic regularities will eventually be accounted for by the concordance model. It’s just that it’s a complicated question that needs to be answered with numerical simulations, and the current simulations aren’t good enough. In the smaller camp are those who think there’s no way these regularities will be accounted for by the concordance model, and modified gravity is the way to go.
The simulations of particle dark matter can now reproduce MOND-like relationships that exist in the actual data, but only with considerable fine tuning of the models and many degrees of freedom. MOND-like models, in contrast, only add one or two parameters to the equations of gravity in most cases (which may be derivable from existing physical constants like the Hubble constant and the speed of light). 

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