Verlinde's emergent gravity versus MOND and the case of dwarf spheroidals

TL;DR

Verlinde's emergent gravity offers an alternative explanation for dark matter phenomena, matching observations in dwarf spheroidal galaxies without additional particles, but diverges from MOND in galaxy centers.

Contribution

The paper tests Verlinde's emergent gravity against dwarf spheroidal galaxy data, showing it can fit velocity dispersions without extra dark matter, and compares it with MOND.

Findings

Verlinde's theory fits dwarf spheroidal velocity profiles without dark matter.

The theory departs from MOND at galaxy centers by a factor of 2.

Results are consistent with observational scatter in galaxy data.

Abstract

In a recent paper, Erik Verlinde has developed the interesting possibility that spacetime and gravity may emerge from the entangled structure of an underlying microscopic theory. In this picture, dark matter arises as a response to the standard model of particle physics from the delocalized degrees of freedom that build up the dark energy component of the Universe. Dark matter physics is then regulated by a characteristic acceleration scale $a_0$, identified with the radius of the (quasi)-de Sitter universe we inhabit. For a point particle matter source, or outside an extended spherically symmetric object, MOND's empirical fitting formula is recovered. However, Verlinde's theory critically departs from MOND when considering the inner structure of galaxies, differing by a factor of 2 at the centre of a regular massive body. For illustration, we use the eight classical dwarf spheroidal satellites of the Milky Way. These objects are perfect testbeds for the model given their approximate spherical symmetry, measured kinematics, and identified missing mass. We show that, without the assumption of a maximal deformation, Verlinde's theory can fit the velocity dispersion profile in dwarf spheroidals with no further need of an extra dark particle component. If a maximal deformation is considered, the theory leads to mass-to-light ratios that are marginally larger than expected from stellar population and formation history studies. We also compare our results with the recent phenomenological interpolating MOND function of McGaugh {\it et al}, and find a departure that, for these galaxies, is consistent with the scatter in current observations.