Recent Claims Invalid: Emergent Gravity Might Deliver A Universe Without Dark Matter
https://www.forbes.com/.../02/28/is-dark-matter-about-to-be-killed-by-emergent-gravity/#2018cc775359
Gravity is one of the four fundamental forces of nature, which means it’s not derived from anything else – it just is. At least,
that's according to our presently accepted theories. But this may be about to change.
Physicists today describe the gravitational interaction through Einstein’s Theory of General Relativity, which dictates the effects of gravity are due to the curvature of space-time.
But it's already been 20 years since Ted Jacobson demonstrated that General Relativity resembles thermodynamics, which is a framework to describe how very large numbers of individual,
constituent particles behave. Since then, physicists have tried to figure out whether this similarity is a formal coincidence or hints at a deeper truth: that space-time is made of
small elements whose collective motion gives rise to the force we call gravity. In this case, gravity would not be a truly fundamental phenomenon, but an emergent one.
The problem is, if emergent gravity just reproduces General Relativity, there’s no way to test the idea. What we need instead is a prediction from emergent gravity that deviates from
General Relativity.
Such a prediction was made two months ago by Erik Verlinde in his new paper. Verlinde pointed out that emergent gravity in a universe with a positive cosmological constant – like the
one we live in – would only approximately reproduce General Relativity. The microscopic constituents of space-time, Verlinde claims, also react to the presence of matter in a way that
General Relativity does not capture: they push inwards on matter. This creates an effect similar to that ascribed to particle dark matter, which pulls normal matter in by its
gravitational attraction.
Verlinde’s idea is interesting and solves two problems that had plagued previous attempts at emergent gravity.
First, he conjectures that the deviations from General Relativity come about because the microscopic constituents of space-time have an additional type of entropy. In the thermodynamic
formulation of gravity, the entropy – that is the number of possible microscopic configurations – which a volume can maximally have is proportional to the surface area of that volume.
This is also often referred to as a “holographic” entropy because it demonstrates that all what happens inside the volume can entirely be encoded on its surface. The additional entropy
that Verlinde introduces instead grows with the volume itself.
The modification to General Relativity then comes about because matter – so the conjecture goes – reduces the new, volume-scaling entropy in its environment. The entropy decrease leads
to a decrease in volume which, in turn creates a force pushing inwards on the matter. This force, Verlinde shows, is similar to the force normally attributed to dark matter – which pulls
in normal matter from its additional gravitational mass.