le MOND day

par Constantinos Skordis (FZU Prague), David Spergel (Princeton U. and Flatiron)

mardi 9 févr. 2021, 16:00 → 18:00 Europe/Paris

Part 1 by David Spergel: What is the Price of Abandoning Dark Matter? Cosmological Constraints on Alternative Gravity Theories. 

Any successful alternative gravity theory that obviates the need for dark matter must fit our cosmological observations. Measurements of microwave background polarization trace the large-scale baryon velocity field at recombination and show very strong O (1) baryon acoustic oscillations and anti-correlations between temperature and polarization on large angular scales. This constraint rules out most modified gravity theories and the modified theories that attempt to fit the data are far more complex and have more parameters than cold dark matter models.  Observations of weak lensing of the cosmic microwave background are well fit by lambda-CDM and deviate from predictions of MOND-like theories.  Measurements of the kSZ effect shows a correlation between galaxies and large-scale flows consistent with gravity falling as 1/r^2 rather than following a MOND-like law on large-scales.

 

Part 2 by Costas Skordis: New gravitational degrees of freedom as a solution to the dark matter problem

 

Cosmological and astronomical observations point to a gross mismatch between the observed dynamics of visible matter with its gravitational influence, suggesting the existence of dark matter. As at present a firm detection of a dark matter particle is lacking, a less explored interpretation is that the underlying theory of gravity may not be General Relativity. A hint that this may be the case is Milgrom’s observation that discrepancies concerning galaxies are controlled by a single, seemingly universal, acceleration scale. I will discuss this possibility and focus on a particular relativistic realization constructed to reproduce Milgrom’s Modified Newtonian Dynamics law at the scale of galaxies. I will how this proposal leads to (i) correct gravitational lensing on galactic scales, (ii) tensor modes propagating at the speed of light, and (iii) cosmological evolution in line with observations of the Cosmic Microwave Background anisotropies and the large-scale structure power spectrum on linear scales.

David Spergel: slides

David Spergel: video