http://arxiv.org/abs/2112.12159

We derive some of the central equations governing quantum fluctuations in gravitational waves, making use of general relativity as a sensible effective quantum theory at large distances. We begin with a review of classical gravitational waves in general relativity, including the energy in each mode. We then form the quantum ground state and coherent state, before then obtaining an explicit class of squeezed states. Since existing gravitational wave detections arise from merging black holes, and since the quantum nature of black holes remains puzzling, one can be open-minded to the possibility that the wave is in an interesting quantum mechanical state, such as a highly squeezed state. We compute the time and space two-point correlation functions for the quantized metric perturbations. We then constrain its amplitude with LIGO observations. Using existing LIGO data, we place a bound on the (exponential) squeezing parameter of the quantum gravitational wave state of $\zeta<41$.

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M. Hertzberg and J. Litterer
Fri, 24 Dec 21
31/58

Comments: 10 pages, 3 figures, in double column format

http://arxiv.org/abs/2112.12159

We derive some of the central equations governing quantum fluctuations in gravitational waves, making use of general relativity as a sensible effective quantum theory at large distances. We begin with a review of classical gravitational waves in general relativity, including the energy in each mode. We then form the quantum ground state and coherent state, before then obtaining an explicit class of squeezed states. Since existing gravitational wave detections arise from merging black holes, and since the quantum nature of black holes remains puzzling, one can be open-minded to the possibility that the wave is in an interesting quantum mechanical state, such as a highly squeezed state. We compute the time and space two-point correlation functions for the quantized metric perturbations. We then constrain its amplitude with LIGO observations. Using existing LIGO data, we place a bound on the (exponential) squeezing parameter of the quantum gravitational wave state of $\zeta<41$.

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M. Hertzberg and J. Litterer
Fri, 24 Dec 21
28/58

Comments: 10 pages, 3 figures, in double column format

http://arxiv.org/abs/2110.15488

Infrared dressing of bosonic or fermionic heavy particles by a cloud of massless particles to which they couple is studied as a possible production mechanism of ultra light dark matter or dark radiation in a radiation dominated cosmology. We implement an adiabatic expansion valid for wavelengths much smaller than the Hubble radius combined with a non-perturbative and manifestly unitary dynamical resummation method to study the time evolution of an initial single heavy particle state. We find a striking resemblance to the process of particle decay: the initial amplitude of the single particle decays in time, not exponentially but with a power law with anomalous dimension $\propto t^{-\Delta/2}$ featuring a crossover to $t^{-\Delta}$ as the heavy particle becomes non-relativistic in both bosonic and fermionic cases suggesting certain universality. At long time the asymptotic state is an entangled state of the heavy and massless particles. The entanglement entropy is shown to grow under time evolution describing the flow of information from the initial single particle to the final multiparticle state. The expectation value of the energy momentum tensor in the asymptotic state is described by two indpendent fluids each obeying covariant conservation, one of heavy particles and the other of relativistic (massless) particles (dark radiation). Both fluids share the same frozen distribution function and entropy as a consequence of entanglement.

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D. Boyanovsky, M. Rai and L. Chen
Mon, 1 Nov 21
42/58

Comments: 47 pages, 2 figures

http://arxiv.org/abs/2110.01557

\noindent We formulate Friedmann’s equations as second-order linear differential equations. This is done using techniques related to the Schwarzian derivative that selects the $\beta$-times $t_\beta:=\int^t a^{-2\beta}$, where $a$ is the scale factor. In particular, it turns out that Friedmann’s equations are equivalent to the eigenvalue problems $$ O_{1/2} \Psi=\frac{\Lambda}{12}\Psi \ , \qquad O_1 a =-\frac{\Lambda}{3} a \ , $$ which is suggestive of a measurement problem. $O_{\beta}(\rho,p)$ are space-independent Klein-Gordon operators, depending only on energy density and pressure, and related to the Klein-Gordon Hamilton-Jacobi equations. The $O_\beta$’s are also independent of the spatial curvature, labeled by $k$, and absorbed in $$ \Psi=\sqrt a e^{\frac{i}{2}\sqrt{k}\eta} \ . $$ The pair of the above equations is the unique possible linear form of Friedmann’s equations unless $k=0$, in which case there are infinitely many pairs of linear equations. Such a uniqueness just selects the conformal time $\eta\equiv t_{1/2}$ among the $t_\beta$’s, which is the key to absorb the curvature term. An immediate consequence of the linear form is that it reveals a new symmetry of Friedmann’s equations in flat space.

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M. Matone
Tue, 19 Oct 21
16/98

Comments: 10 pages. Added a new symmetry of Friedmann’s equations that follows by their linear version. Typos corrected

http://arxiv.org/abs/2103.15892

Quantum mechanics allows for states in macroscopic superpositions, but they ordinarily undergo rapid decoherence due to interactions with their environment. A system that only interacts gravitationally, such as an arrangement of dark matter (DM), may exhibit slow decoherence. In this letter, we compute the decoherence rate of a quantum object in general relativity. For axion DM in a superposition of the field’s phase, we find that DM in the Milky Way is robust against decoherence, while a spatial superposition is not. This novel phase behavior may be relevant to direct detection experiments.

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I. Allali and M. Hertzberg
Wed, 31 Mar 2021
27/62

Comments: 7 pages in double column format, 1 figure

http://arxiv.org/abs/2012.12903

It is of great interest to explore matter in nontrivial quantum arrangements, including Schrodinger cat-like states. Such states are sensitive to decoherence from their environment. Recently, in Ref. [1] we computed the rate of decoherence of a piece of superposed matter that primarily only interacts gravitationally, a dark-matter-Schrodinger-cat-state (DMSCS), within the nonrelativistic approximation. In this work we improve this to a general relativistic analysis. We firstly derive a single particle relativistic Schrodinger equation for a probe particle that passes through the DMSCS; the interaction is provided by the weak field metric of general relativity from the source. For a static DMSCS we find a neat generalization of our previous results. We then turn to the interesting new case of a time dependent DMSCS, which can be provided by a coherently oscillating axion field leading to superposed time dependent oscillations in the metric; a truly quantum-general relativistic phenomenon. We use scattering theory to derive the decoherence rate in all these cases. When the DMSCS is in a superposition of distinct density profiles, we find that the decoherence rate can be appreciable. We then consider the novel special case in which the density is not in a superposition, but the phase of its field oscillation is; this is a property that cannot be decohered within the nonrelativistic framework. We find that if the probe particle and/or the DMSCS’s velocity dispersion is slow, then the rate of decoherence of the phase is exponentially suppressed. However, if both the probe and the DMSCS’s velocity dispersion are relativistic, then the phase can decohere more rapidly. As applications, we find that diffuse galactic axions with superposed phases are robust against decoherence, while dense boson stars and regions near black hole horizons are not, and we discuss implications for experiment.

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I. Allali and M. Hertzberg
Fri, 25 Dec 20
34/51

Comments: 50 pages, 1 figure

http://arxiv.org/abs/2012.12903

It is of great interest to explore matter in nontrivial quantum arrangements, including Schrodinger cat-like states. Such states are sensitive to decoherence from their environment. Recently, in Ref. [1] we computed the rate of decoherence of a piece of superposed matter that primarily only interacts gravitationally, a dark-matter-Schrodinger-cat-state (DMSCS), within the nonrelativistic approximation. In this work we improve this to a general relativistic analysis. We firstly derive a single particle relativistic Schrodinger equation for a probe particle that passes through the DMSCS; the interaction is provided by the weak field metric of general relativity from the source. For a static DMSCS we find a neat generalization of our previous results. We then turn to the interesting new case of a time dependent DMSCS, which can be provided by a coherently oscillating axion field leading to superposed time dependent oscillations in the metric; a truly quantum-general relativistic phenomenon. We use scattering theory to derive the decoherence rate in all these cases. When the DMSCS is in a superposition of distinct density profiles, we find that the decoherence rate can be appreciable. We then consider the novel special case in which the density is not in a superposition, but the phase of its field oscillation is; this is a property that cannot be decohered within the nonrelativistic framework. We find that if the probe particle and/or the DMSCS’s velocity dispersion is slow, then the rate of decoherence of the phase is exponentially suppressed. However, if both the probe and the DMSCS’s velocity dispersion are relativistic, then the phase can decohere more rapidly. As applications, we find that diffuse galactic axions with superposed phases are robust against decoherence, while dense boson stars and regions near black hole horizons are not, and we discuss implications for experiment.

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I. Allali and M. Hertzberg
Fri, 25 Dec 20
26/51

Comments: 50 pages, 1 figure