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2019-01-28

89 Emergent Gravity(Entropic Gravity) and the Dark Universe(Dark Matter and Dark Energy)  #gravity #darkmatter #darkenergy

89 Emergent Gravity(Entropic Gravity) and the Dark Universe(Dark Matter and Dark Energy)  #gravity #darkmatter #darkenergy

○Erik P. Verlinde published 51 papers on-line November 7, 2016. In this paper, entropic gravity is said to be able to explain " Galaxy rotation curve problem". [1]
※A significant discrepancy exists between the experimental galaxy rotation curves observed, and a curve derived from theory. The theory of dark matter was postulated to account for the variance. [2]

○And in this paper he provides evidence for the fact that the observed dark energy and the phenomena currently attributed to dark matter have a common origin and are connected to the emergent nature of spacetime and gravity. [1]

○Discussion result
1)The positive dark energy and accelerated expansion are caused by the slow thermalization of the emergent spacetime. [1]
2) The observed phenomena that are currently attributed to dark matter are the conesquence of the emergent nature of gravity and are caused by an elastic response due to the volume law contribution to the entanglement entropy in our universe. [1]
3) Dark matter is not a collection of invisible particles but a result of the interaction of ordinary matter and dark energy. [1]

○When explaining the gravitational distribution around galaxies by dark matter, it is necessary to determine the mass of the dark matter so as to match the data obtained by the actual observation. In other words, the mass of dark matter is used as a free variable to match theory and reality. On the other hand, Verlinde's theory does not use these free variables, and there is the advantage that the predicted value derived directly from the theory agrees with the actual observation result. [3]

○Tasks
・ It is still an open question if and how the standard cosmological picture is incorporated in a theory of emergent gravity. [1]
・Mark Van Raamsdonk calls Verlinde’s idea “definitely an important direction.” But he says it’s too soon to tell whether everything in the paper — which draws from quantum information theory, thermodynamics, condensed matter physics, holography and astrophysics — hangs together. [4]
・One problem, said Brian Swingle, who also works in holography, is that Verlinde lacks a concrete model universe like the ones researchers can construct in AdS space, giving him more wiggle room for making unproven speculations. [4]
・This theory combines a lot of hypotheses that are not well established and only identifies the amount compared with the observation, not precise formulation is done. [5]
• The order is the same as Beckenstein - Hawking entropy, but it is offset by a factor of 1/4. [5]
・This theory introduces the degree of freedom of measurement which describes time and space bending from the beginning. In order to claim that the Einstein equation was properly derived under elastic coordinate representation from the general coordinate transformation, it is necessary to write down the degree of freedom of the measurement from the original degree of freedom of the elastic body and also derive general coordinate transformation invariance . Furthermore, since no correction term to the equation is given, it can not be compared whether gravity wave data observed with LIGO matches entropic gravity theory or not. . [5]
• The theoretical check at the paper [6] written by the observation group of the Netherlands was made only within the range of error of 1σ at most. [5]

○In order to verify this theory and to obtain a definite answer, firstly progress of observation and theory is desired.

【References】
1. Erik P. Verlinde “Emergent Gravity and the Dark Universe” 20161107
https://arxiv.org/abs/1611.02269
2. Galaxy rotation curve – Wikipedia
3.“Verlinde's new theory of gravity passes first test” 20161216
https://phys.org/news/2016-12-verlinde-theory-gravity.html
4. Natalie Wolchover” The Case Against Dark Matter”at Quanta magazine 20161129
https://www.quantamagazine.org/erik-verlindes-gravity-minus-dark-matter-20161129/
5. Masahiro Hotta”エントロピック重力理論” at Quantum Universe 20161225
6. ”First test of Verlinde’s theory of Emergent Gravity using Weak Gravitational Lensing measurements” 20161219
https://arxiv.org/pdf/1612.03034.pdf





【Change log】
20171112  New description
20171209 Description of the strength of entropic gravity and tasks.

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