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posted by Fnord666 on Wednesday March 01 2017, @05:21PM   Printer-friendly
from the seemed-like-a-hack-anyway dept.

The dominant Lambda-CDM model is the standard model of physical cosmology, and it has proved reasonably successful. It does, however, have problems, such as dark matter, whose true nature remains elusive. Dutch physicist Erik Verlinde has, in a recent paper, proposed that gravity might not actually be a fundamental interaction at all, but rather an emergent property of spacetime itself, and as such, what current cosmological theory considers dark matter is really an emergent gravity phenomenon. Sabine Hossenfelder has an article about several recent tests of Verlinde's theory, which show that the idea might have promise.

Physicists today describe the gravitational interaction through Einstein's Theory of General Relativity, which dictates the effects of gravity are due to the curvature of space-time. But it's already been 20 years since Ted Jacobson demonstrated that General Relativity resembles thermodynamics, which is a framework to describe how very large numbers of individual, constituent particles behave. Since then, physicists have tried to figure out whether this similarity is a formal coincidence or hints at a deeper truth: that space-time is made of small elements whose collective motion gives rise to the force we call gravity. In this case, gravity would not be a truly fundamental phenomenon, but an emergent one.

[...] Verlinde pointed out that emergent gravity in a universe with a positive cosmological constant – like the one we live in – would only approximately reproduce General Relativity. The microscopic constituents of space-time, Verlinde claims, also react to the presence of matter in a way that General Relativity does not capture: they push inwards on matter. This creates an effect similar to that ascribed to particle dark matter, which pulls normal matter in by its gravitational attraction.

[...] So, it's a promising idea and it has recently been put to test in a number of papers.

[...] Another paper that appeared two weeks ago tested the predictions from Verlinde's model against the rotation curves of a sample of 152 galaxies. Emergent gravity gets away with being barely compatible with the data – it systematically results in too high an acceleration to explain the observations.

A trio of other papers show that Verlinde's model is broadly speaking compatible with the data, though it doesn't particularly excel at anything or explain anything novel.

[...] The real challenge for emergent gravity, I think, is not galactic rotation curves. That is the one domain where we already know that modified gravity – at last some variants thereof – work well. The real challenge is to also explain structure formation in the early universe, or any gravitational phenomena on larger (tens of millions of light years or more) scales.

Particle dark matter is essential to obtain the correct predictions for the temperature fluctuations in the cosmic microwave background. That's a remarkable achievement, and no alternative for dark matter can be taken seriously so long as it cannot do at least as well. Unfortunately, Verlinde's emergent gravity model does not allow the necessary analysis – at least not yet.

Previously:
Emergent Gravity and the Dark Universe


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  • (Score: 0) by Anonymous Coward on Wednesday March 01 2017, @06:28PM (5 children)

    by Anonymous Coward on Wednesday March 01 2017, @06:28PM (#473437)

    GPS doesn't use GR. These use a post-Newtonian approximation (ie calculate the Newtonian result, then add some correction factor). Without GR, people would just add some other correction factor that approximates the correct result. No one uses GR for anything practical because it is too difficult to deal with.

  • (Score: 0) by Anonymous Coward on Wednesday March 01 2017, @06:43PM (1 child)

    by Anonymous Coward on Wednesday March 01 2017, @06:43PM (#473440)

    "It isn't GR... it's an approximation of GR!"

    What?! Any student of Newtonian mechanics will tell you that half the solutions involve making "simplifying" approximations of the underlying problem; I guess nobody uses even Newtonian mechanics for anything practical. Bish, please.

    • (Score: 0) by Anonymous Coward on Wednesday March 01 2017, @06:55PM

      by Anonymous Coward on Wednesday March 01 2017, @06:55PM (#473448)

      making "simplifying" approximations of the underlying problem

      Usually this is stuff like "treat the earth like a sphere" though. Not changing the fundamental assumptions (speed of gravity doesn't change between instantaneous to the speed of light, etc). You will still see some form of F = G*m1*m2/r^2 because what is to simplify? Ok, they use GM* since that is closer to what can be measured, but that is not equivalent.

      *https://en.wikipedia.org/wiki/Standard_gravitational_parameter

  • (Score: 1) by khallow on Friday March 03 2017, @03:32PM (2 children)

    by khallow (3766) Subscriber Badge on Friday March 03 2017, @03:32PM (#474382) Journal

    These use a post-Newtonian approximation

    Which readily comes from GR. As you note, they could needed some other approximation, but they would need to justify the correction, if they did so. And that would mean coming up with a model that wasn't current GR.

    You see this with a variety of complex equations. Two common examples are the Navier-Stokes equations and the corresponding equations for magnetohydrodynamics (plasma fluid dynamics). The two equations are bulky (and fundamentally broken at the subatomic level, which sometimes can bite you at far larger spatial scales) so they routinely use approximations where various terms of the equations are set to zero rather than merely be very small.

    • (Score: 0) by Anonymous Coward on Friday March 03 2017, @04:59PM (1 child)

      by Anonymous Coward on Friday March 03 2017, @04:59PM (#474438)

      the Navier-Stokes equations and the corresponding equations for magnetohydrodynamics (plasma fluid dynamics)

      But was there a pre-existing extremely simple model that was 99.99% correct in those cases?

      • (Score: 1) by khallow on Friday March 03 2017, @07:48PM

        by khallow (3766) Subscriber Badge on Friday March 03 2017, @07:48PM (#474531) Journal

        But was there a pre-existing extremely simple model that was 99.99% correct in those cases?

        What does 99.99% correct mean?