New Theory of Gravity as an Emergent Phenomena in the Cosmos –“First Tests Looks Interesting”

 

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Emergent gravity, a new theory developed by theoretical physicist Erik Verlinde at University of Amsterdam, predicts the exact same deviation of motions that is usually explained by invoking dark matter. According to Verlinde, gravity is not a fundamental force of nature, but an emergent phenomenon. In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime.


Now, a team led by astronomer Margot Brouwer at the Leiden Observatory, The Netherlands, has tested Verlind’e new theory for the first time through the lensing effect of gravity.

Brouwer and her team measured the distribution of gravity around more than 33,000 galaxies to put Verlinde’s prediction to the test. She concludes that Verlinde’s theory agrees well with the measured gravity distribution. The results have been accepted for publication in the British journal Monthly Notices of the Royal Astronomical Society.

Astronomers have measured, however, that at distances up to a hundred times the radius of the galaxy, the force of gravity is much stronger than Einstein’s theory of gravity predicts. The existing theory only works when invisible particles, the so-called dark matter, are added.

Brouwer’s conclusion is that his prediction agrees well with the observed gravity distribution, but she emphasizes that dark matter could also explain the extra gravitational force. However, the mass of the dark matter is a free parameter, which must be adjusted to the observation. Verlinde’s theory provides a direct prediction, without free parameters.

The new theory is currently only applicable to isolated, spherical and static systems, while the universe is dynamic and complex. Many observations cannot yet be explained by the new theory, so dark matter is still in the race.

“The question now is how the theory develops, and how it can be further tested,” says Brouwer. “But the result of this first test definitely looks interesting.”

In his 2010 article (On the origin of gravity and the laws of Newton), Verlinde showed how Newton’s famous second law, which describes how apples fall from trees and satellites stay in orbit, can be derived from these underlying microscopic building blocks.

Extending his previous work and work done by others, Verlinde showed last month how to understand the curious behavior of stars in galaxies without adding the puzzling dark matter.

In the image below, the gravity of galaxies bends space, such that the light traveling through this space is bent. This bending of light allows astronomers to measure the distribution of gravity around galaxies, even up to distances a hundred times larger than the galaxy itself. (APS/Alan Stonebraker; galaxy images from STScI/AURA, NASA, ESA, and the Hubble Heritage Team).

 

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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.

According to Erik Verlinde, there is no need to add a mysterious dark matter particle to the theory.

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.

“We have evidence that this new view of gravity actually agrees with the observations, ” says Verlinde. “At large scales, it seems, gravity just doesn’t behave the way Einstein’s theory predicts.”

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.

Investigating the effects of this additional information on ordinary matter, Verlinde comes to a stunning conclusion. Whereas ordinary gravity can be encoded using the information on the imaginary sphere around the universe, as he showed in his 2010 work, the result of the additional information in the bulk of space is a force that nicely matches that attributed to dark matter.

Gravity is in dire need of new approaches like the one by Verlinde, since it doesn’t combine well with quantum physics. Both theories, crown jewels of 20th century physics, cannot be true at the same time. The problems arise in extreme conditions: near black holes, or during the Big Bang.

“Many theoretical physicists like me are working on a revision of the theory, and some major advancements have been made,” says Verlinde. “We might be standing on the brink of a new scientific revolution that will radically change our views on the very nature of space, time and gravity.”

More information: Margot M. Brouwer et al. First test of Verlinde’s theory of Emergent Gravity using Weak Gravitational Lensing measurements, Monthly Notices of the Royal Astronomical Society (2016). DOI: 10.1093/mnras/stw3192 , On Arxiv: https://arxiv.org/abs/1612.03034

Journal reference: Monthly Notices of the Royal Astronomical Society arXiv

The Daily Galaxy via Netherlands Research School for Astronomy and Delta Institute for Theoretical Physics

Image credit: NASA/Hubble Space Telescope

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