# FeaturedA Erik Verlinde's new view on dark matter

1. May 25, 2014

### MTd2

UPDATE: New Paper discussion starts here: https://www.physicsforums.com/threads/erik-verlindes-new-view-on-dark-matter.755235/#post-5615947

This is all I could gather from abstracts of talks and colloquiums he gave this year. Each of them have a different bit of information

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String Theory and the Entropic Origin of Gravity, Dark Energy and Dark Matter
II Tuscan Meeting on Theoretical Physics. Feb 25th

Erik Verlinde (Amsterdam U.)
Abstract:
Recent developments indicate that quantum entanglement and its associated entropy play a central role in explaining the connectivity of space time and the origin of gravity. In string theory a concrete mechanism for the quantum entanglement of space time is provided by the so-called long string phenomenon. In a cosmological setting these entangled long strings form a dynamical system whose energy can be identified with the dark energy. Using an accurate analogy with entanglement network of polymers, we calculate the effects due to slow relaxation processes and find excellent quantitative agreement with the phenomena attributed to dark matter in galaxies, clusters of galaxies, and the cosmos.

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In Arizona State University Department of Physics, January 30th

Recent developments indicate that quantum entanglement and its associated entropy play a central role in explaining the connectivity of space time and the origin of gravity. String theory provides a concrete mechanism of the quantum entanglements of the underlying microscopic degrees of freedom. In a cosmological setting these degrees of freedom form a dynamical system whose energy can be identified with the dark energy of our universe. The usual laws of gravity, which are derived from the first law of thermodynamics, are valid in an adiabatic approximation. Using an accurate analogy with entanglement network of polymers, I show that the slow relaxation processes are responsible of the phenomena attributed to dark matter. These considerations lead to a universal formula for the dark matter distribution in galaxies, clusters of galaxies, and the cosmos, which agrees extremely well with observations.

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The Entropic Origin of Gravity, Dark Energy and Dark Matter

UserErik Verlinde (Amsterdam)
ClockWednesday 21 May 2014, 14:15-15:15
HouseMR2, Centre for Mathematical Sciences.

Recent insights from black hole physics, string theory and the AdS/CFT correspondence suggest that gravity is an emergent phenomenon. The microscopic counting of black hole states together with the recently found connection between gravity and quantum entanglement give important clues towards the underlying mechanism. Motivated by these ideas I propose that space-time can be viewed as an entanglement network of “long strings”. Anti-de Sitter and de Sitter space correspond to the distinct situations in which these “long strings” are all in their ground state or all in an excited state. By using an analogy with entanglement networks of polymers, I show that this description leads to memory effects that quantitatively describe the observed phenomena attributed to dark matter.

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String Theory and the Entropic Origin of Gravity, Dark Energy, and Dark Matter

Department Colloquia
Spring 2014
Erik Verlinde
Affiliation: University of Amsterdam
Seminar Date: Thu, 01/30/2014 - 3:15pm - 4:15pm
Location: PSF 101
Host:
Maulik Parikh
Abstract

Recent developments indicate that quantum entanglement and its associated entropy play a central role in explaining the connectivity of space time and the origin of gravity. String theory provides a concrete mechanism for the quantum entanglement of the underlying microscopic degrees of freedom. In a cosmological setting these degrees of freedom form a dynamical system whose energy can be identified with the dark energy of our universe. The usual laws of gravity, which are derived from the first law of thermodynamics, are valid in an adiabatic approximation. Using an accurate analogy with entanglement network of polymers, I show that the slow relaxation processes are responsible for the phenomena attributed to dark matter. These considerations lead to a universal formula for the dark matter distribution in galaxies, clusters of galaxies, and the cosmos, which agrees extremely well with observations.

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It seems that the slow relaxation, with an analogy of a network of entangled polymers, of the phenomenon of "long strings", which lead to a formula of dark matter distribution, which agrees extremely well with observations.

Last edited by a moderator: Nov 15, 2016
2. May 26, 2014

### craigi

In string theory, gravity is a fundamental force and not an entropic force, so how can he use string theory with an argument that gravity is entropic in nature?

3. May 26, 2014

### mitchell porter

In his talks, Verlinde sometimes appeals to open-closed string duality, which sort of refers to all the string-theory consequences of the fact that a cylinder is a circle times a line segment. In general, a graviton is a closed string (circle) and a gauge boson is an open string (line segment). So a cylinder can be seen as a closed string evolving along a time interval (the line segment), or as an open string evolving around a time loop (the circle). This is apparently the geometric basis of a number of known relations between gauge field theories and gravity. Verlinde wants to use it too but I can't tell you how.

Something I'd like to know, is whether it ever makes sense to have an entropic force in which the entropy is a von Neumann entropy, the definition of entropy for a quantum state. Back when I was trying to make sense of Verlinde's talks (they have not been turned into a paper yet), my intuition said that maybe he needed a "quantum entropic force", if there is such a thing. But I never reasoned it out myself.

4. May 26, 2014

### MTd2

Mitchel, these are new views. I am not sure how they represent his old views. For example, he didn't talk about dark matter before.

5. May 26, 2014

### craigi

I was just wondering myself, if the degeneracy pressure from the Pauli exclusion principle, would be well represented as an entropic force.

6. May 27, 2014

### Blackforest

7. May 27, 2014

### Chronos

I would call this a 'weak string' view of gravity. It seems to make sense.

8. Nov 8, 2016

### kodama

Emergent Gravity and the Dark Universe
Erik P. Verlinde
(Submitted on 7 Nov 2016 (v1), last revised 8 Nov 2016 (this version, v2))
Recent theoretical progress indicates that spacetime and gravity emerge together from the entanglement structure of an underlying microscopic theory. These ideas are best understood in Anti-de Sitter space, where they rely on the area law for entanglement entropy. The extension to de Sitter space requires taking into account the entropy and temperature associated with the cosmological horizon. Using insights from string theory, black hole physics and quantum information theory we argue that the positive dark energy leads to a thermal volume law contribution to the entropy that overtakes the area law precisely at the cosmological horizon. Due to the competition between area and volume law entanglement the microscopic de Sitter states do not thermalise at sub-Hubble scales: they exhibit memory effects in the form of an entropy displacement caused by matter. The emergent laws of gravity contain an additional dark' gravitational force describing the elastic' response due to the entropy displacement. We derive an estimate of the strength of this extra force in terms of the baryonic mass, Newton's constant and the Hubble acceleration scale a_0 =cH_0, and provide evidence for the fact that this additional `dark gravity~force' explains the observed phenomena in galaxies and clusters currently attributed to dark matter.
Subjects: High Energy Physics - Theory (hep-th)
Cite as: arXiv:1611.02269 [hep-th]

9. Nov 9, 2016

### Staff: Mentor

Back in 2010, when Verlinde published an earlier paper along the same lines, neutron interferometry experiments were cited as evidence against an entropic model of gravity, as explained for example here:

http://motls.blogspot.nl/2010/01/erik-verlinde-why-gravity-cant-be.html

I don't know if these criticisms are still relevant to this latest paper, but it looks to me like they should be.

10. Nov 9, 2016

### Demystifier

Gravitational force is reversible. (Think of elliptic planet trajectories that, in the absence of other forces, can last forever.)

Entropic force is not reversible. (You need an external force if you want to separate objects that were attracted by the entropic force.)

Last edited: Nov 9, 2016
11. Nov 9, 2016

### RockyMarciano

I think they are since the idea of entropic gravity and entanglement entropy is at the root of his new paper too. But it is debatable if the criticism based on the the lack of the loss of the interference pattern at a distance in presence of gravity is convincing. It seems like a question of degree in Verlinde's view and the influence of entropy in the neutron experiments would be too small, while in the link interference patterns is treated in simple absence/presence terms.

The classical Newtonian view of gravitational force is certainly as a reversible force. But already GR's treatment of gravity is not exactly reversible, for instance using your own example of the elliptic orbits, the Einsteinian correction makes them open, the ellipses cannot close due to curvature, and that is not exact reversibility in the Newtonian sense. Actually the very idea of variable curvature to explain gravity departs from reversibility.

I think Verlinde's point(to get into his new paper proper) is more defendible phenomenologically than conceptually from what he writes. He is basically saying GR is wrong and yet he is still borrowing concepts from Einstein's GR, like the irreversibility inherent to curvature, and the entropy from black holes, although to be fair the connection with black hole thermodynamics is limited to the analogy between galactic dynamics entropy in de Sitter cosmology and the formula for bh entropy as shown in the inequalities 1.3 and 1.4 of the linked paper. So it looks more like a justification of his new approach to cosmologic phenomenology than anything else.

12. Nov 9, 2016

### Demystifier

I don't understand that argument. Fine, in GR the trajectories are not ellipses. But if you reverse the velocity at some point, the trajectory will be the reverse of the original trajectory, which is reversible dynamics in the Newtonian sense. Moreover, the scalar curvature $R$ of GR does not change if you make the transformation $x^0\rightarrow -x^0$, so curvature is time-inversion invariant.

One might argue that at least black holes are irreversible in GR, because once you are in the hole you cannot escape out. But even that is, strictly speaking, wrong. If you reverse all velocities at some point, the black hole will turn into a white hole, which is a time-inverted black hole. Of course, you cannot reverse all velocities in practice, but this is not forbidden in principle by general principles of GR.

If you write GR in the canonical Hamiltonian form, the Hamiltonian is strictly quadratic in canonical momenta, which is another way to see that GR is time-inversion invariant, i.e. reversible.

Last edited: Nov 9, 2016
13. Nov 9, 2016

### haushofer

The paper states a modification of GR at certain cosmological lengthscales, so I guess that these extra terms introduce non-reversibility . but I could be wrong, still have to read it.

14. Nov 9, 2016

### martinbn

They are irreversible because of the area theorem.

Also a trapped surface can form through evolution. Initial data that doesn't contain an trapped surface can have a maximal development that does contain a trapped surface. Not true in the time reversed situation. If the initial data doesn't have an anti-trapped surface it will not develop one.

15. Nov 9, 2016

### Demystifier

I wouldn't say that those properties make GR irreversible. Otherwise, one could argue that even Newtonian mechanics of a free particle in 1 dimension is irreversible. (By free, I mean in the absence of forces.) Namely, if free particle moves to the left, then it can never start to move on the right. This is absolutely true for free Newtonian mechanics, yet we don't call it irreversibility.

So what is irreversibility? Reversibility is the property that f(t) is a solution if and only if f(-t) is also a solution. Irreversibility, of course, is the negation of reversibility.

In the case of GR, if a black hole is a solution, then the corresponding white hole is also a solution.

16. Nov 9, 2016

### martinbn

Yes, but some solutions are considered not physical so they are excluded.

Your classical mechanics example is not completely analogous to the black hole case. A particle moving to the left can be made to move to the right. If something collapses to a black hole you cannot make it not a black hole.

17. Nov 9, 2016

### Demystifier

You think like a physicist, while I think like a mathematician. That cannot be good.

Not in the absence of forces.

You can if you can violate positive-energy assumption, which you can by some quantum effects.

18. Nov 9, 2016

### RockyMarciano

It is easier than all that, we are dealing with GR in vacuum here, so no scalar curvature R, just Weyl curvature. It is hard IMO to argue that gravity in the relativistic Einsteinian local vacuum approximation around a source is not locally time-oriented(even if globally , being asymptotically Minkowskian is independent of time and hypersurface orthogonal) and therefore irreversible. Clearly a lightcone must be chosen locally if the Weyl curvature is to induce post-Minkowskian and post-newtonian corrections.

19. Nov 9, 2016

### martinbn

Well, not sure about that. The likes of Dirac don't exclude solutions and can give answers as -2 fish, where a mathematician wouldn't.

Yes, but presence of forces isn't against the laws of nature.

Hm, you can shrink it, but will it go away completely? I am not even sure of QM violations of the energy conditions is enough.

20. Nov 10, 2016

### Demystifier

By the way, negative number of fish (i.e. fish made of antiparticles) is OK for me, but when a physicist proposes negative probability, that is something I cannot digest. This is related to my general views on probability discussed in