As dark matter continues to vex astronomers, new solutions to the dark matter question are proposed. Most focus on pinning down the form of dark matter, while others propose modifying gravity to account for the effect. But a third proposal is simply to remove gravity from the equation. What if the effects of gravity aren’t due to some fundamental force, but are rather an emergent effect due to other fundamental interactions? A new paper proposes just that, and if correct it could also explain the effects of dark matter.
The idea of emergent gravity isn’t entirely new. The most popular variation was proposed in 2010, where Erik Verlinde argued that gravity is not a fundamental force, but rather an effect that arises from the entropy of the Universe. Entropy is a property of thermodynamics. It’s often described as the unusable part of a system (or the waste heat if you will) and while that’s sometimes a useful description, a better description involves the amount of information contained within a system. An ordered system (say, marbles evenly spaced in a grid) is easy to describe because the objects have simple relations to each other. On the other hand, a disordered system (marbles randomly scattered) take more information to describe, because there isn’t a simple pattern to them. Basically, the more information it takes to describe a system, the more entropy it has.
Verlinde’s model uses this connection between thermodynamics (heat, energy, and forces) and information through a mathematical method known as the holographic principle. Since the information contained within a region of space depends upon the arrangement of objects within that region, moving the objects can change the entropy within the region. Verlinde demonstrated that this produces an entropic force that acts like gravity. From the basic idea of information entropy, one can derive Einstein’s equations of general relativity exactly.
Entropic gravity is an interesting idea, and it would explain why gravity is so difficult to bring into the fold of quantum physics, but it’s not without its problems. For one, since entropic gravity predicts exactly the same gravitational behavior as general relativity, there’s no experimental way to distinguish it as a better theory. There are also theoretical problems with the model. For example, if you try to describe a gravitationally closed system of masses within the model it only matches experiment if you place weird constraints on the entropy of the system.
But despite its problems the idea is at least worth exploring, and this latest work adds a new twist by describing the effects of dark matter. In the original formulation, the model focused on standard gravity. Specifically, it excluded dark energy. This new paper notes that since the dark energy of a region of space requires additional information to describe, including it in the model changes the entropy of a region of space. The paper then goes on to show how this additional information creates an additional entropic force. One that might account for the effects of dark matter similar to other modified gravity models such as Modified Newtonian Dynamics (MoND). Thus gravity, dark matter, and dark energy might all be connected through entropy.
While this seems like an elegant solution to several cosmological problems, there are plenty of reasons to be skeptical. For one, this new variation of emergent gravity still has the same theoretical difficulties of the original. Then there’s the fact that modified gravity models fail to explain large scale effects such as the clustering of galaxies, which regular gravity and dark matter explains very well. This new work is still more of an idea and less of a robust theory.
But even if the model doesn’t work out in the end, it demonstrates how thermodynamics and gravity are deeply connected in ways that aren’t obvious at first glance.
Paper: E. P. Verlinde. Emergent Gravity and the Dark Universe. arXiv:1611.02269 (2016)
Paper: E. P. Verlinde. On the Origin of Gravity and the Laws of Newton. arXiv:1001.0785 (2010)