Emergent Gravity theory may explain galaxy rotation without dark matter
The outer regions of galaxies, like our own Milky Way, rotate much faster around the center than can be accounted for by the quantity of ordinary matter like stars, planets and interstellar gasses. Something else has to produce the required amount of gravitational force, so physicists proposed the existence of dark matter. Dark matter seems to dominate our universe, comprising more than 80 percent of all matter. Hitherto, the alleged dark matter particles have never been observed, despite many efforts to detect them.
In his new paper, Verlinde showed how his theory of gravity accurately predicts the velocities by which the stars rotate around the center of the Milky Way, as well as the motion of stars inside other galaxies. At first glance, Verlinde’s theory presents features similar to modified theories of gravity like MOND (modified Newtonian Dynamics, Mordehai Milgrom). However, where MOND tunes the theory to match the observations, Verlinde’s theory starts from first principles. “A totally different starting point,” according to Verlinde.
One of the ingredients in Verlinde’s theory is an adaptation of the holographic principle, introduced by his tutor Gerard ‘t Hooft (Nobel Prize 1999, Utrecht University) and Leonard Susskind (Stanford University).According to the holographic principle, all the information in the entire universe can be described on a giant imaginary sphere around it. Verlinde now shows that this idea is not quite correct—part of the information in our universe is contained in space itself.
This extra information is required to describe that other dark component of the universe: Dark energy, which is believed to be responsible for the accelerated expansion of the universe.
This experiment is merely the tip of a twenty year long iceberg of research on alternative theories of gravity. These theories first appeared around 1995, in reaction to some of the inconsistencies in the theory of black holes (Hawking). Those theories introduced the idea that black holes had entropy. In trying to quantify the entropy, people (t’Hooft, Susskind) came up with the idea that reality lives on the 2D surface of the black hole, not the 3D interior – it was a way to avoid the issue with the singularity at the center of the black hole. People then took this explanation and used it outside of black holes, by drawing non-existent spherical boundaries in space and asking what the entropy of the points of matter enclosed might be.
The new theory makes an analogy to the well-known (to physicists) ability of statistical mechanics (a theory at the atomic level) to explain the laws of thermodynamics (a theory at the macroscopic level). In this theory, gravity is something like temperature – an emergent phenomenon. That is, you can’t really talk about the temperature of one atom. Temperature is not only a bulk (macroscopic) property. To have a temperature, a collection of atoms must be in thermal equilibrium with each other. When that equilibrium is reached, the property of temperature “emerges”.
The new theory says that gravity emerges from behind these fake spherical boundaries, that it is property of bulk matter, not indivdual atoms. It says that gravitational force is an entropic phenomenon: in trying to reach a gravitational equilibrium (in order to maximize their entropy) gravity emerges. The idea resembles osmotic pressure.
As I mentioned, there is a twenty year literature on this stuff. It is mainstream science, not some kooky nonsense. I was surprised to hear about this stuff, because dark matter has gotten all the attention. But this explanation is so elegant, so cool. Did I mention that they have no free parameters? The holographic theory has generated a whole bunch of analogies that are being explored under the rubric of “conformal field theories”. Although this first result is non-relativistic (they selected galaxies with minimal red shift), the theory has been extended to cover General Relativity.
You might notice that I am blown away, not by this experiment, but by the underlying 20 years of mainstream physics that I managed to miss. I would love to hear from other physics-oriented JPR folk.HIP56948, like this
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