Thursday, February 16, 2017

Verlinde's Emergent Gravity Doesn't Work

Verlinde sought to reproduce the successes of other modified gravity theories in a variant of his entropy based gravity theories called "emergent gravity" which arises from quantum entanglement.

The entanglement based "emergent gravity" theory itself is discussed in two recent posts:

* Emergent Gravity (November 30, 2016)
* Quick Physics Hits (November 14, 2016)

Verlinde's entropic gravity theories that were predecessors to emergent gravity were discussed in two 2012 posts:

* A Review of Fundamental Physics in 2012 (December 24, 2012)
* Two Provocative Papers On Gravity (December 4, 2012)

Unfortunately, "emergent gravity" doesn't reproduce the observed data.
Verlinde (2016) has recently proposed that spacetime and gravity may emerge from an underlying microscopic theory. In a de Sitter spacetime, such emergent gravity (EG) contains an additional gravitational force due to dark energy, which may explain the mass discrepancies observed in galactic systems without the need of dark matter. For a point mass, EG is equivalent to Modified Newtonian Dynamics (MOND). We show that this equivalence does not hold for finite-size galaxies: there are significant differences between EG and MOND in the inner regions of galaxies. We confront theoretical predictions with the empirical Radial Acceleration Relation (RAR). We find that (i) EG is consistent with the observed RAR only if we substantially decrease the fiducial stellar mass-to-light ratios; the resulting values are in tension with other astronomical estimates; (ii) EG predicts that the residuals around the RAR should correlate with radius; such residual correlation is not observed.
Federico Lelli, Stacy S. McGaugh, and James M. Schombert "Testing Verlinde's Emergent Gravity with the Radial Acceleration Relation"(February 14, 2017).

Another study looking at a different set of data with different investigators reaches basically the same conclusion.
It was recently proposed that the effects usually attributed to particle dark matter on galaxy scales are due to the displacement of dark energy by baryonic matter, a paradigm known as emergent gravity. This formalism leads to predictions similar to Modified Newtonian Dynamics (MOND) in spherical symmetry, but not quite identical. In particular, it leads to a well defined transition between the Newtonian and the modified gravitational regimes, a transition depending on both the Newtonian acceleration and its first derivative with respect to radius. Under the hypothesis of the applicability of this transition to aspherical systems, we investigate whether it can reproduce observed galaxy rotation curves. We conclude that the formula leads to marginally acceptable fits with strikingly low best-fit distances, low stellar mass-to-light ratios, and a low Hubble constant. In particular, some unobserved wiggles are produced in rotation curves because of the dependence of the transition on the derivative of the Newtonian acceleration, leading, even in the most favorable case, to systematically less good fits than MOND. Then, applying the predicted transition from emergent gravity in a regime where it should be fully applicable, i.e. in spherical symmetry and outside of the bulk of matter, we show that the predictions for the secular advances of Solar System planets' perihelia are discrepant with the data by seven orders of magnitude, ruling out the present emergent gravity formalism with high confidence.
Aurelien Hees, Benoit Famaey, and Gianfraco Bertone, Emergent gravity in galaxies and in the Solar System (February 14, 2017)

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