Comparing Emergent Gravity to Dark Matter in Dwarf Spheroidal Galaxies

Emergent Gravity (EG) is a theory of gravity that reproduces the effects of dark matter and dark energy without the need for new particles or exotic energy densities. EG operates via the interaction between baryonic matter’s quantum entanglement entropy and the cosmological horizon. Because EG works through the stress-energy tensor, it reproduces Modified Newtonian Dynamics’ (MOND) results while overcoming MOND’s struggles with gravitational lensing. We test EG in the extreme low-density environment of the dwarf spheroidal galaxies (dSphs) that surround the Milky Way. We employ a novel method that combines large-scale photometric survey stellar position data with spectroscopic line-of sight velocity measurements. We find that EG can reproduce the observed velocity dispersion profiles of most of our sample of dSphs; however, EG fails in the cases of Draco and Fornax, underpredicting the velocity dispersion at large radii. We find that the EG model is not favored in a Bayes Factor comparison to a dark matter NFW profile in any galaxy, and is only favored in a small minority of cases when compared to a NFW model that also allows a central core as well as tidal truncation. Furthermore, we find that EG predicts unphysically low stellar mass to light ratios (<0.5 solar masses per solar luminosity) in the galaxies Crater 2 and Antlia 2. At the same time, EG predicts unphysically high stellar mass to light ratios (>15 solar masses per solar luminosity) in Ursa Major I and Tucana. Finally, we discuss whether any modifications to EG, or additional effects due to the presence of the Milky Way, can rescue EG’s poor fit to dSph galaxies.