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0 pointsraattgift9y ago0 comments

It's not complete for other reasons, several of which are detailed in Verlinde's paper itself; however, it wholly reproduces GR in the EFT limit by design. Indeed, Verlinde starts with standard dS as his unremovable background on which he puts strings. What in GR we would consider perturbations of dS and IR corrections of GR (at ~ galactic scales) emerge from the behaviour of the strings and how they form long-distance entanglements.

However, it's not outrageously wrong, and it is noteworthy that an accomplished string theorist has decided to tackle gravity in a universe like ours rather than leaning on AdS/CFT arguments.

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acqq9y ago

Thanks for the correction and clarification.

For others who, like me, aren't in the field, the "EFT" mentioned is explained, for example, here:

https://arxiv.org/abs/gr-qc/0311082

"The problems of quantizing gravity within the experimentally accessible situations are similar to those which arise in a host of other non-gravitational applications throughout physics. As such, the size of quantum corrections can be safely estimated and are extremely small. The theoretical framework which allows this quantification is the formalism of effective field theories."

dS is "de Sitter space": https://en.wikipedia.org/wiki/De_Sitter_space

I'd also appreciate a link to the explanation of "What in GR we would consider perturbations of dS and IR corrections."

raattgiftOP9y ago

> I'd also appreciate a link to the explanation of "What in GR we would consider perturbations of dS and IR corrections."

Here's a quick overview of perturbation theory then an example of how one might apply it in GR.

https://www.wikiwand.com/en/Perturbation_theory

http://theory.physics.helsinki.fi/~inf/Lectures/Lecture2.pdf

IR is "infrared", low-energy physics, as opposed to UV, "ultraviolet", high-energy physics. In this context, UV means strong gravity where quantum effects are expected to be important. Galaxy-galaxy gravitational lensing (which is the subject of the Brouwer et al. paper that's the subject of the newscientist article linked at the top) is purely a weak gravity problem[1], so is in the IR limit of General Relativity. If you want different results from standard General Relativity you can add corrections by hand in a variety of ways; in the galaxy-galaxy lensing case they would be in the infrared.

[1] strong gravity is hidden behind the event horizons of the black holes of the galaxies and, arguendo, if we could see the strong gravity near the black hole centres, it would not be relevant by virtue of being drowned out by all the stars, gas, dust and other matter.

ETA: This is nice and terse, too: https://www.wikiwand.com/en/Non-exact_solutions_in_general_r...

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