Emergent gravity paradigm: Recent progress

Padmanabhan, Τ.

doi:10.1142/s0217732315400076

Cited by 83 publications

(97 citation statements)

References 40 publications

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“…Combined with the idea that spacetime is also is an emergent phenomenon [8][9][10][11][12], this paves the way for a computation of some thermodynamical properties of spacetime in quantum-mechanical terms. We would like to remark that a quantum-mechanical approach to the expansion of the Universe was called for long ago in Reference [13], where it was suggested to regard the expansion of the Universe as a scattering problem in quantum mechanics.…”

Section: The Approach Via Emergent Quantum Theorymentioning

confidence: 99%

Boltzmann Entropy of a Newtonian Universe

Cabrera

Córdoba

Isidro

2017

Entropy

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A dynamical estimate is given for the Boltzmann entropy of the Universe, under the simplifying assumptions provided by Newtonian cosmology. We first model the cosmological fluid as the probability fluid of a quantum-mechanical system. Next, following current ideas about the emergence of spacetime, we regard gravitational equipotentials as isoentropic surfaces. Therefore, gravitational entropy is proportional to the vacuum expectation value of the gravitational potential in a certain quantum state describing the matter contents of the Universe. The entropy of the matter sector can also be computed. While providing values of the entropy that turn out to be somewhat higher than existing estimates, our results are in perfect compliance with the upper bound set by the holographic principle.Keywords: newtonian cosmology; emergent quantum theory The Approach via Emergent Quantum TheoryIn this article, we will argue in favour of emergent quantum mechanics as providing an appropriate framework to estimate thermodynamical quantities of the Universe, such as the entropy.The notion that quantum mechanics is an emergent theory has been discussed at length in the literature [1][2][3][4][5][6][7]. Combined with the idea that spacetime is also is an emergent phenomenon [8][9][10][11][12], this paves the way for a computation of some thermodynamical properties of spacetime in quantum-mechanical terms. We would like to remark that a quantum-mechanical approach to the expansion of the Universe was called for long ago in Reference [13], where it was suggested to regard the expansion of the Universe as a scattering problem in quantum mechanics.The expansion of the Universe is a long-standing experimental observation [14] that has in recent years been refined thanks to very precise measurements [15,16]. In the Newtonian approximation, this receding behaviour of the galaxies can be easily modelled by a phenomenological potential-namely, an isotropic harmonic potential carrying a negative sign:As the angular frequency, we take the current value of Hubble's constant H 0 . Thus, U Hubble has the dimensions of energy per unit mass, or velocity squared.In the emergent approach to spacetime presented in Reference [12], gravity qualifies as an entropic force. If gravitational forces are entropy gradients, gravitational equipotential surfaces can be identified with isoentropic surfaces. Recalling the arguments of Reference [12], a classical point particle approaching a holographic screen causes the entropy of the latter to increase by one quantum k B . Here we will analyse a quantum-mechanical model in which the forces driving the galaxies away from each other can be modelled by the Hubble potential (1). We will replace the classical particle of Reference [12] with a collection of quantum particles (the matter contents of the Universe) described

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Section: The Approach Via Emergent Quantum Theorymentioning

confidence: 99%

Boltzmann Entropy of a Newtonian Universe

Cabrera

Córdoba

Isidro

2017

Entropy

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“…The effect of the cosmological constant can be also obtained through extended theories of gravity (see [3] and the references therein). In this article we will concentrate on thermodynamic models of gravity [4][5][6][7][8][9][10][11][12][13], where the cosmological constant arises in the Einstein's equations as a constant of integration.…”

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confidence: 99%

“…Black hole thermodynamics, the Unruh effect and some other evidences suggest that the gravitation has a fundamental connection to thermodynamics (see the reviews [11][12][13]). Within the thermodynamic model spacetime geometry emerges from the properties of the finite unified ensemble of quantum objects (particles -spacetime atoms).…”

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confidence: 99%

“…Within the thermodynamic model spacetime geometry emerges from the properties of the finite unified ensemble of quantum objects (particles -spacetime atoms). In this approach, the Einstein equations can be derived by combining general thermodynamic considerations with the equivalence principle and is written as a single scalar relation [11][12][13][14],…”

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confidence: 99%

“…The scalar equation (1) involves additional vector field u ν , but contains all information content of the ordinary tensorial Einstein equations, because it is demanded that it hold for all u ν . In addition, if one assumes that u ν is an orthogonal to the observers horizon 4-velocity of light (null vector field) [11][12][13],…”

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confidence: 99%

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Cosmological Constant in the Thermodynamic Models of Gravity

Gogberashvili¹,

Chutkerashvili²

2017

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The isolated elliptical galaxy NGC 5812–MOND or dark matter?

Richtler,

Salinas,

Lane

et al. 2023

Astron Nachr

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There exist isolated elliptical galaxies, whose dynamics can be modeled without resorting to dark matter or MOND, for example, NGC 7507. Such objects lack understanding within the current framework of galaxy formation. The isolated elliptical NGC 5812 is another object to investigate a possible role of isolation. We use globular clusters (GCs) and the galaxy light itself as dynamical tracers to constrain its mass profile. We employ Gemini/GMOS mask spectroscopy, apply the GMOS reduction procedures provided within IRAF, measure GC velocities by cross correlation methods and extract the line‐of‐sight kinematics of galaxy spectra using the tool pPXF. We identify 28 GCs with an outermost galactocentric distance of 20 kpc, for which velocities could be obtained. Furthermore, 16 spectra of the integrated galaxy light out to 6 kpc have been used to model the central kinematics. These spectra provide evidence for a disturbed velocity field, which is plausible given the disturbed morphology of the galaxy. We construct spherical Jeans models for the galaxy light and apply tracer mass estimators for the globular clusters. With the assumptions inherent to the mass estimators, MOND is compatible with the mass out to 20 kpc. However, a dark matter free galaxy is not excluded, given the uncertainties related to a possible nonsphericity and a possible nonequilibrium state. We find one globular cluster with an estimated mass of , the first Ultra Compact Dwarf in an isolated elliptical. We put NGC 5812 into the general context of dark matter or alternative ideas in elliptical galaxies. The case for a MONDian phenomenology also among early‐type galaxies has become so strong that deviating cases appear astrophysically more interesting than agreements. The baryonic Tully Fisher relation (BTFR) as predicted by MOND is observed in some samples of early‐type galaxies, in others not. However, in cases of galaxies that deviate from the MONDian prediction, data quality and data completeness are often problematic.

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Emergent gravity paradigm: Recent progress

Cited by 83 publications

References 40 publications

Boltzmann Entropy of a Newtonian Universe

Boltzmann Entropy of a Newtonian Universe

Cosmological Constant in the Thermodynamic Models of Gravity

The isolated elliptical galaxy NGC 5812–MOND or dark matter?

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