Questions tagged [gravity]

Gravity is an attractive force that affects and is effected by all mass and - in general relativity - energy, pressure and stress. Prefer newtonian-gravity or general-relativity if sensible.

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212 views

Criteria for being able to work with gravity in quantum mechanics, without a full theory of quantum gravity?

It's common to see people oversimplify by saying that physics currently lacks the tools to describe any situation involving both quantum mechanics and gravity. Clearly this is not the case. For ...
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558 views

What are Galileons good for?

Lately I've seen many papers (for example "The galileon as a local modification of gravity"; 292 total hits on the arXiv) on types of field theories known as Galileons, and I'm wondering what the ...
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Variation of the Einstein-Hilbert action in D dimensions without the Gibbons-Hawking-York term

Consider the standard Einstein-Hilbert action in $D \ne 2$ dimensions spacetimes : \begin{equation} S_{EH} = \frac{1}{2 \kappa} \int_{\Omega} R \; \sqrt{- g} \; d^D x, \end{equation} where $\Omega$ is ...
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Does the Gibbons-Hawking boundary action have an anomaly inflow interpretation?

The Einstein-Hilbert action on a manifold $M$ with boundary is $$\frac{-1}{16\pi G}\int_M d^n x \sqrt{-g} R +\frac{1}{8\pi G} \int_{\partial M} d^{n-1}x \sqrt{|h|} K$$ where $K$ is the extrinsic ...
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63 views

Different features of Gravity and Yang-Mills

I am reading a famous paper by S.Hawking - "Quantum gravity and path integrals" https://doi.org/10.1103/PhysRevD.18.1747. On the third page left column there is a statement, after the ...
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109 views

Entropy and equilibrium concepts at astronomic scales

I am always puzzled to read here and there discussions dealing with thermodynamic concepts applied to astronomic scales where gravity matters. To my opinion, there is a certain carelessness to go into ...
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185 views

Metric transformation, polygons and gravitons

I'm trying to understand the paper by Hitchin called: ''Polygons and gravitons". I'm stuck at page 471. At this point, he does some computations and obtains a metric: $$ \gamma dz d\bar{z}+\gamma^{...
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490 views

Will a black hole cause scattering of a gravitational wave?

In my GR textbook, it states that gravitational waves can undergo interference but not scattering. I am just starting the chapter on linearised gravity concepts (weak field approximation) and my ...
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110 views

How do we decode the image formed by a gravitational lens?

Using our own sun as a gravitational lens, we can scrutinize planetary surfaces in distant solar systems with a good deal of accuracy. How do we translate this smeared out and curved image into a ...
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Why is the group of gauge transformations on the frame bundle isomorphic to $\text{Diff}(M)$?

Consider the frame bundle $LM \to M$ for given Lorentzian manifold $M$. The group $\mathcal{G}$ of gauge transformations of the second kind are automorphisms $\phi:LM \to LM$ covering the identity $\...
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Propagator in Brans-Dicke Gravity

Consider an action of the form $$ S = -\frac{2}{\kappa^2}\int d^4x\sqrt{-g}~\left(\phi R + \phi\mathcal{L}_{matter}\right). $$ Expanding this to second order in $h_{\mu\nu}$ and including a harmonic ...
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304 views

Definition of gravity path integral?

In a non-abelian gauge theory there is a "fundamental" gauge field $A_\mu^a$ with gauge index $a$ often called connection. Although $ A_\mu^a$ is not gauge invariant, gauge invariant quantities can be ...
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Spin 3 vs spin 2 vs spin 1

I wanna to understand, why when one gonna to construct interacting theory of spin 3, one need also include infinite tower of spins 4, 5, 6 , ... As I know, this statement correct even in classical ...
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1answer
758 views

String theory and trace anomaly in semiclassical gravity?

what does string theory have to say about the trace anomaly in the expectation value of the stress energy tensor of massless quantum fields on a curved background and its interpretation as the ...
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107 views

Modified Lie bracket

In a paper by Barnich, they use a different definition of the Lie bracket for vector fields at null infinity. Can somebody please give me the intuition behind using this Lie bracket in Equation 4.12: ...
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1answer
226 views

Proportionality Constant in Einstein Field Equations

The Einstein Field Equations: $$G_{ab}~=~8\pi T_{ab}.$$ I am familiar with how to obtain the $8\pi$ proportionality factor through correspondence with Newtonian gravity, but am wondering if this ...
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51 views

Bargmann–Wigner equations in NP formalism

Bargmann-Wigner equations describe free particles of arbitrary spin $j$, namely $$(-\gamma^{\mu}\partial_{\mu}+m)_{\alpha_r \alpha_{r’}}\Psi_{\alpha_1,..,\alpha_{r’},...,\alpha_{2j}}=0$$ where we have ...
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Is there any observable change in Earth's magnetism to an observer in geosynchronous orbit?

I am in high school, and today my physics teacher was teaching us about the Earth's magnetism. This is the first time I've read about this topic, so take these questions as coming from a complete ...
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Effect (if any) of strong(ish) gravity radiation on stars

Two black holes merge, and a good few percent of their total mass is converted into gravitational radiation. Years or decades later, the resulting gravity wave passes through nearby stars. Does it ...
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100 views

What's the difference between a post-Minkowskian approximation and a post-Newtonian one?

I'm studying the book Gravity by Poisson & Will. Specifically, I'm interested in the post-Newtonian and post-Minkowskian approximations showed in chapters 6-10. The problem I'm having is ...
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119 views

Is the cosmological constant problem real?

The cosmological constant problem assumes that the cosmological constant (determined experimentally) can be identified with the vacuum energy density. Theroretical arguments from quantum gravity ...
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92 views

If gravity turns out to be mediated by exchange particles, would that imply a problem with gravitational fields around a black hole?

In general relativity, gravity is a distortion of spacetime due to mass. Its effects travel (if that's the right word) at the speed of light. In the SM all 3 other known interactions are mediated by ...
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110 views

The (Newton-Laplace-Ivory-Arnold) shell theorem in general relativity

It is well-known that Birkhoff's theorem and the classification of LTB spacetimes proves one version of Newton's shell theorem in the context of GR. Another statement in Newtonian gravity, often ...
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205 views

Why gravity does not have any internal symmetry?

The QFT description of forces other than gravity assumes some internal symmetry such as the $SU(3)$ color symmetry for strong interactions. Gravity is based on spacetime symmetries. What forbids the ...
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1answer
248 views

How much uncertainty has the relic graviton background?

In the paper [1], it is mentioned that inflation predicts that a relic graviton background is about 0.9 K (cf. cosmic neutrino background, 1.945 K, and cosmic microwave background, 2.73 K). How much ...
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1answer
66 views

Cosmological constant phase transition?

I recently watched at a talk by Cumrum Vafa in which he stated that the cosmological constant allows us to define a time-scale $T_\Lambda=1/\sqrt{E_\Lambda}$. The time scale of this time is about 10¹¹ ...
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2answers
359 views

Time dilation in quantum gravity

In General Relativity, time moves slower near massive objects where spacetime is curved stronger. In quantum gravity, the gravitational force is represented by the quantum field that refers to the ...
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154 views

How does the Newton's constant get renormalized in Quantum Gravity with matter?

In quantum gravity with matter, say e.g. $R$ + $\partial\phi_{\mu} \partial\phi^{\mu}$, even the one loop correction is non-renormalizable. I am sure many smart people have already worked on this, so ...
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209 views

Closed trajectories for Kepler problem with classical spin-orbit corrections?

Kepler problem explains closed elliptic trajectories for planetary systems or in Bohr's classical atomic model - let say two approximately point objects, the central one has practically fixed position,...
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429 views

How to find the Hawking temperature for this metric?

I am reading this paper about "Hawking radiation of Kerr-Newman-de Sitter black hole", where the authors find Hawking temperature of this metric The authors state that hawking temperature is given by ...
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209 views

Vafa-Witten theorem for gravitational $\theta$-term

"The Vafa–Witten theorem shows that vector-like global symmetries such as isospin and baryon number in vector-like gauge theories like QCD cannot be spontanteously broken as long as the theta angle is ...
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66 views

gravitational force for a binary of point particles with GR term

i'm trying to simulate the two body problem with 2 equal masses and I want to account for general relativistic effects. I know that the difference in the gravitational force would be an additional ...
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167 views

Will the photon wavelength fluctuate in the presence of a gravitational wave?

The microwave background is due to the expansion of the Universe where the wavelengths of radiations are stretched by spacetime. As in the LIGO experiment, in the presence of gravitational wave, ...
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53 views

Inertial frames and Sagnac interferometers

Let's posit: I am in orbit around a large body, like a planet, and I am close enough to be tidally locked to the large body. Am I in an inertial frame? Even without looking at the stars, couldn't I ...
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87 views

Palatini type gravities: Where could I find them?

I read the article Quantum gravity with torsion and non-metricity. Although I found interesting the analysis in the paper, I found quite interesting an statement in the abstract, The class of ...
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405 views

Choice of framing in Gravitational Chern-Simons

I was trying to understand formula(2.21) in Witten's paper "Quantum Field Theory and Jones Polynomial"(link: https://projecteuclid.org/euclid.cmp/1104178138) (Page 360). There, it was mentioned, the ...
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177 views

$\langle TT\rangle$ correlator of the boundary CFT from metric fluctuations in the bulk gravity

Is there a reference which explains how the $\langle TT\rangle $ correlation of the boundary conformal field theory (CFT) can be holographically calculated from the bulk gravity? (..I am often getting ...
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197 views

How to calculate gravity path integrals about an AdS background?

Suppose I have some Lagrangian of some higher derivative gravity coupled to a may be matter fields. Now I want to fluctuate it to quadratic order about an AdS background and calculate the 1-loop ...
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724 views

Squashed 3-sphere?

What is a squashed 3-sphere? In context of quantum gravity. I stumbled upon a term 'squashed 7 sphere' but that's concerning supersymmetry. Is it just normal 3-sphere metric, that is just 'squashed' ...
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411 views

On-shell action in asymptotically AdS space

Consider a field theory coupled with gravity described by the action: $S=\int d^Dx \sqrt{-g} \left( \mathcal{R}-\Lambda+\mathcal{L}_m[\phi] \right)$, with the requirement that g must be ...
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2answers
965 views

Attractive gravity has negative energy, what about repulsive gravity in the inflation phase?

Alan Guth's cosmic inflation theory posits that about 10^-38 second before the Big Bang which led to our particular universe, a tiny patch of space doubled in size more than 100 times, from sub-atomic ...
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Static Magnetic field in curved spacetime

I am trying to solve the Maxwell equations in curved spacetime and I came across this paper [1] which tries to do just that. In this paper, the author explores the gravitational effects in a long ...
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35 views

Relation between bulk Hamiltonian in AdS and stress energy of CFT

Consider the following two situations: One can define a stress energy for AdS which matches with the expectation value for the CFT stress tensor. Consider bulk metric perturbations of the form: $$g_{...
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63 views

Why is the Weyl tensor square gravity non-unitary?

I want to understand constraints of unitarity in quantum field theory. There is quite folklore statement: gravity with a Weyl tensor square term is non-unitary How to understand this? (I think that ...
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117 views

How does $1/r$ gravity change in de Sitter space?

Classical $1/r$ gravity arises from general relativity when curvature is neglected and speeds are small compared to the speed of light. In de Sitter space with a positive cosmological constant $\...
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Conserved charges without conserved currents?

I recently read (Section 9.3 of J.Kaplan AdS/CFT notes) that we can have QFT's s.t. we have conserved charges but NO associated conserved currents. How is this related to the gauging of global ...
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26 views

Hamiltonian in gravity with $\Lambda =0$ and using it to generate time translations

Time translation is generated by Hamiltonian. In gravity, the bulk Hamiltonian for closed $d$ hypersurfaces (obtained by ADM decomposition of $d+1$ spacetimes) is 0. This is basically a constraint of ...
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105 views

Global symmetry in gravity

There exists "folk-theorem" about the impossibility to have global symmetries in a consistent theory of quantum gravity. For example, see Global symmetries in quantum gravity . Typical argument ...
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68 views

Does this Boltzmann-factor-included metric relate to some known physics phenomena?

When I tried to map the motion of a constant-speed particle with speed $v$ and central force magnitude $k(r)$ to a space-time metric, it turns out the metric is in the form $$d\lambda^2=-\alpha^2 dt^...
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31 views

Gravitational wave energy fluxes and bracket averaging notation

Typically the rate of energy loss due to gravitational waves is given in the form (e.g. MTW or Eq 33 of these notes), $$ \dot{E} = - \frac{1}{5} \langle \dddot{I}_{jk} \dddot{I}_{jk} \rangle$$ where ...

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