Emergent Gravity?

[bystander]

New Theory of Gravity Might Explain Dark Matter
Delta Institute for Theoretical Physics | 2016 Nov 08

A new theory of gravity might explain the curious motions of stars in galaxies. Emergent gravity, as the new theory is called, predicts the exact same deviation of motions that is usually explained by inserting dark matter in the theory. Prof. Erik Verlinde, renowned expert in string theory at the University of Amsterdam and the Delta Institute for Theoretical Physics, published a new research paper today in which he expands his groundbreaking views on the nature of gravity.

In 2010, Erik Verlinde surprised the world with a completely new theory of gravity. 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. ...

Emergent Gravity and the Dark Universe - Erik P. Verlinde

• arXiv.org > hep-th > arXiv:1611.02269 > 07 Nov 2016 (v1), 08 Nov 2016 (v2)

On the Origin of Gravity and the Laws of Newton - Erik P. Verlinde

• Journal of High Energy Physics 2011(4):029 (Apr 2011) DOI: 10.1007/JHEP04(2011)029
  arXiv.org > hep-th > arXiv:1001.0785 > 06 Jan 2010

http://asterisk.apod.com/viewtopic.php?t=18539#p122239

[bystander]

Verlinde's new theory of gravity passes first test
Netherlands Research School for Astronomy (NOVA) | 2016 Dec 12

A team led by astronomer Margot Brouwer (Leiden Observatory, The Netherlands) has tested the new theory of theoretical physicist Erik Verlinde (University of Amsterdam) 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. ...

[img3=" 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. Credit: APS/Alan Stonebraker; galaxy images from NASA, ESA, and the Hubble Heritage Team (STScI/AURA)"]http://www.astronomie.nl/media/medialib ... e_Team.png[/img3][hr][/hr]

The gravity of galaxies bends space, such that the light traveling through this space is bent, as through a lens. Background galaxies that are situated far behind a foreground galaxy (the lens), thereby seem slightly distorted. This effect can be measured in order to determine the distribution of gravity around a foreground-galaxy. 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.

Verlinde now claims that he not only explains the mechanism behind gravity with his alternative to Einstein's theory, but also the origin of the mysterious extra gravity, which astronomers currently attribute to dark matter. Verlinde’s new theory predicts how much gravity there must be, based only on the mass of the visible matter.

Brouwer calculated Verlinde's prediction for the gravity of 33,613 galaxies, based only on their visible mass. She compared this prediction to the distribution of gravity measured by gravitational lensing, in order to test Verlinde’s theory. Her 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. ...

First test of Verlinde's theory of Emergent Gravity using Weak Gravitational Lensing measurements - Margot M. Brouwer et al

• Monthly Notices of the RAS (online 2016 Dec 11) DOI: 10.1093/mnras/stw3192
  arXiv.org > astro-ph > arXiv:1612.03034 > 09 Dec 2016

[Ann]

Interesting. Chris, and other smart people here (Rob? alter-ego? Art? Case? Geck? Others?), what do you think about this?

I found this particularly interesting:

Her 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.

Suppose that emergent gravity receives more observational support. What would happen to the Lambda Cold Dark Matter model of the universe? Would it have to be scrapped? Could dark energy also be emergent, or maybe even an illusion?

What about Einstein's theory of general relativity? That one seems extremely robust so far, doesn't it?

Ann

[Chris Peterson]

Interesting. Chris, and other smart people here (Rob? alter-ego? Art? Case? Geck? Others?), what do you think about this?

I have no opinion. I don't understand it well enough. It doesn't seem to offer any advantages over current theory (that is, there aren't things better explained), so in that sense it isn't really needed. I'll start paying attention when people who are experts on gravity start taking it seriously, start thinking it offers a valuable way of looking at things.

[Markus Schwarz]

Interesting. Chris, and other smart people here (Rob? alter-ego? Art? Case? Geck? Others?), what do you think about this?

There are good reasons to believe that general relativity is not the final theory of gravity because it leads to singularities in some cases (e.g. big bang and center of black holes). However, in these scenarios matter is compressed to such small distances that quantum mechanics cannot be ignored. Unfortunately, a theory of quantum gravity is not yet known, with several contenders on the market. Emergent gravity (EG) is one of those, but I don't know much about it.

In general, I advice to be very sceptic about what is written in press releases. Treat them the same way you tread the advertisement of a used-car salesman. Fortunately, bystander also provided the original puplications I quickly browsed through the arXiv paper of the group that did the measurement. Already the abstract reads less sensational:

We find that the prediction from EG, despite requiring no free parameters, is in good agreement with the observed galaxy-galaxy lensing profiles in four differ- ent stellar mass bins. Although this performance is remarkable, this study is only a first step. Further advancements on both the theoretical framework and observational tests of EG are needed before it can be considered a fully developed and solidly tested theory.

One problem seems to be that, so far, EG does not say anything about the dynamical evolution of the universe and, currently, applies only to the present day situation. Another one seems to be that the lensing equation only applies to spherically symmetric objects. The group then selected only galaxies that matched that criterium. That seems strange to me, but I am no expert on this. On the plus side, EG does seem to make predictions that can be tested, which is a rare feature of quantum gravity theories.

Personally, I agree with the last sentence of the abstract: "Further advancements on both the theoretical framework and observational tests of EG are needed before it can be considered a fully developed and solidly tested theory."

[Ann]

Thanks, Markus! I should have mentioned you among the smart people here.

Ann

[geckzilla]

I don't know enough about the topic, either, but I do follow at least one person whose well-informed opinions make for interesting reads. Here are a couple of links to Sabine Hossenfelder's blog posts on the matter.
http://backreaction.blogspot.com/2016/1 ... avity.html
http://backreaction.blogspot.com/2016/1 ... merge.html

[rstevenson]

Thanks, Markus! I should have mentioned you among the smart people here.

Ann

And you should have left me out! But thanks anyway.

Rob

[Ann]

I don't know enough about the topic, either, but I do follow at least one person whose well-informed opinions make for interesting reads. Here are a couple of links to Sabine Hossenfelder's blog posts on the matter.
http://backreaction.blogspot.com/2016/1 ... avity.html
http://backreaction.blogspot.com/2016/1 ... merge.html

Thanks, Geck. I only read the the first link, but I got the impression that gravity, for all we know, may be emergent from an unknown source (the question is what). Or else it's not emergent. But it might be.

Interesting!

Ann

[neufer]

[b][color=#0000FF][size=175]The ultra-diffuse galaxy Dragonfly 44[/size] Image credit: Pieter van Dokkum / Roberto Abraham / Jean Brodie.[/color][/b]

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<<Dragonfly 44 is an ultra diffuse galaxy in the Coma Cluster. Observations of the rotational speed suggest a mass of about one trillion solar masses, about the same as the mass of the Milky Way. This is also consistent with about 90 globular clusters observed around Dragonfly 44. However, the galaxy emits only 1% of the light emitted by the Milky Way. The galaxy was discovered with the Dragonfly Telephoto Array. In August 2016, astronomers reported that this galaxy might be made almost entirely of dark matter.>>