Dark matter versus modified gravity

The scenario consistent with a wealth of observations for the missing mass problem is that of weakly interacting dark matter particles. However, arguments or proposals for a Newtonian or relativistic modified gravity scenario continue to be made.

A distinguishing characteristic between the two scenarios is that dark matter particles can produce a gravitational effect, in principle, without the need of baryons while this is not the case for the modified gravity scenario where such an effect must be correlated with the amount of baryonic matter.

This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACSJ0717.5+3745, one of the most massive galaxy clusters known and also the largest known gravitational lens. Credit: NASA, ESA and the HST Frontier Fields team (STScI)

In a recent paper (Lin & Ishak 2016) the authors consider ultra-faint dwarf (UFD) galaxies as a promising arena to test the two scenarios based on the above assertion. They compare the correlation of the luminosity with the velocity dispersion between samples of UFD and non-UFD galaxies, finding a trend of loss of correlation for the UFD galaxies.

For example, they find for 28 non-UFD galaxies a strong correlation coefficient of -0.688 which drops to -0.077 for the 23 UFD galaxies. Incoming and future data will determine whether the observed stochasticity for UFD galaxies is physical or due to systematics in the data.

Such a loss of correlation (if it is to persist) is possible and consistent with the dark matter scenario for UFD galaxies but would constitute a new challenge for the modified gravity scenario.

Lin & Ishak 2016 (preprint) - Ultra faint dwarf galaxies: an arena for testing dark matter versus modified gravity (arXiv)

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