arXiv subjects

Mohammad Ali Gorji

Publications and source records attributed to Mohammad Ali Gorji.

At least 37 records · Page 2Linked to original sources

arXiv2024

Nonlinear gravitational waves in Horndeski gravity: Scalar pulse and memories

We present and analyze a new non-perturbative radiative solution of Horndeski gravity. This exact solution is constructed by a disformal mapping of a seed solution of the shift-symmetric Einstein-Scalar system belonging to the Robinson-Trautman geometry describing the gravitational radiation emitted by a time-dependent scalar monopole. After analyzing in detail the properties of the seed, we show that while the general relativity solution allows for shear-free parallel transported null frames, the disformed solution can only admit parallel transported null frames with a non-vanishing shear. This result shows that, at the nonlinear level, the scalar-tensor mixing descending from the higher-order terms in Horndeski dynamics can generate shear out of a pure scalar monopole. We further confirm this analysis by identifying the spin-0 and spin-2 polarizations in the disformed solution using the Penrose limit of our radiative solution. Finally, we compute the geodesic motion and the memory effects experienced by two null test particles with vanishing initial relative velocity after the passage of the pulse. This exact radiative solution offers a simple framework to witness nonlinear consequences of the scalar-tensor mixing in higher-order scalar-tensor theories.

gr-qc↗

arXiv2023

Spin-2 dark matter from inflation

The seed of dark matter can be generated from light spectator fields during inflation through a similar mechanism that the seed of observed large scale structures are produced from the inflaton field. The accumulated energy density of the corresponding excited modes, which is subdominant during inflation, dominates energy density of the universe later around the time of matter and radiation equality and plays the role of dark matter. For spin-2 spectator fields, Higuchi bound may seem to prevent excitation of such light modes since deviation of the inflationary background from the exact de Sitter spacetime is very small. However, sizable interactions with the inflaton field breaks (part of) isometries of the de Sitter space in the inflationary background and relaxes the Higuchi bound. Looking for this possibility in the context of effective field theory of inflation, we suggest a dark matter model consisting of spin-2 particles that produce during inflation.

astro-ph.CO↗

arXiv2023

Primordial-tensor-induced stochastic gravitational waves

Cosmological stochastic gravitational waves (GWs) induced by a spectator field are usually expected to have an amplitude very small compared with those generated by the curvature perturbation, or equivalently by a field dominating the universe. On the contrary to this expectation, we show that a spectator field that provides a tensor perturbation, on top of the metric tensor perturbation, can generate a significant amount of GWs. The amplitude and frequency of the generated GWs may lie within the sensitivity range of future GW detectors. In particular, if the sound velocities of the two tensor perturbations coincide, the induced GW amplitude may become very large due to resonance by forced oscillation, even in the limit of small coupling between them. A distinct feature of this scenario is that, since tensor modes can hardly lead to the formation of primordial black holes (PBHs), we expect no presence of PBHs, in contrast to the usual scalar-induced case, in which the detection of strong enough induced GWs suggests the existence of PBHs.

gr-qc↗

arXiv2023

Extra-tensor-induced origin for the PTA signal: No primordial black hole production

Recently, pulsar timing array (PTA) collaborations announced evidence for an isotropic stochastic gravitational wave (GW) background. The origin of the PTA signal can be astrophysical or cosmological. In the latter case, the so-called secondary scalar-induced GW scenario is one of the viable explanations, but it has a potentially serious issue of the overproduction of primordial black holes (PBHs) due to the enhanced curvature perturbation. In this letter, we present a new interpretation of the PTA signal. Namely, it is originated from an extra spectator tensor field that exists on top of the metric tensor perturbation. As the energy density of the extra tensor field is always subdominant, it cannot lead to the formation of PBHs. Thus our primordial-tensor-induced scenario is free from the PBH overproduction issue.

astro-ph.CO↗

arXiv2022

Effective Field Theory of Gravitating Continuum: Solids, Fluids, and Aether Unified

We investigate the relativistic effective field theory (EFT) describing a non-dissipative gravitating continuum. In addition to ordinary continua, namely solids and fluids, we find an extraordinary more symmetric continuum, aether. In particular, the symmetry of the aether concludes that a homogeneous and isotropic state behaves like a cosmological constant. We formulate the EFT in the unitary/comoving gauge in which the dynamical degrees of freedom of the continuum (phonons) are eaten by the spacetime metric. This gauge choice, which is interpreted as the Lagrangian description in hydrodynamics, offers a neat geometrical understanding of continua. We examine a thread-based spacetime decomposition with respect to the four-velocity of the continuum which is different from the foliation-based Arnowitt-Deser-Misner one. Our thread-based decomposition respects the symmetries of the continua and, therefore, makes it possible to systematically find invariant building blocks of the EFT for each continuum even at higher orders in the derivative expansion. We also discuss the linear dynamics of the system and show that both gravitons and phonons acquire "masses" in a gravitating background.

hep-th↗

arXiv2022

Cosmological memory effect in scalar-tensor theories

The cosmological memory effect is a permanent change in the relative separation of test particles located in a FLRW spacetime due to the passage of gravitational waves. In the case of a spatially flat FLRW spacetime filled with a perfect fluid in general relativity, it is known that only tensor perturbations contribute to the memory effect while scalar and vector perturbations do not. In this paper, we show that in the context of scalar-tensor theories, the scalar perturbations associated to the scalar graviton contribute to the memory effect as well. We find that, depending on the mass and coupling, the influence of cosmic expansion on the memory effect due to the scalar perturbations can be either stronger or weaker than the one induced by the tensor perturbations. As a byproduct, in an appendix, we develop a general framework which can be used to study coupled wave equations in any curved spacetime region which admits a foliation by time slices.

gr-qc↗

arXiv2022

Dark Matter from Entropy Perturbations in Curved Field Space

The accumulated energy density of the excited entropy modes in multiple field inflationary scenarios can play the role of dark matter. In the usual case of a flat field space without any turning trajectory, only light superhorizon entropy modes can be excited through the gravitational instability. In the case of a negatively curved field space, we show that subhorizon entropy modes can be excited as well through the tachyonic instability induced by the negative curvature of the field space. The latter allows for production of entropy modes with masses larger than or at the order of the Hubble expansion rate during inflation, leading to a new dark matter scenario. Due to the contribution of subhorizon modes, the corresponding spectral density has a peak at a scale smaller than its counterpart in the models based on a flat field space. This difference makes our model observationally distinguishable.

gr-qc↗

arXiv2022

Disformal map and Petrov classification in modified gravity

Disformal transformation provides a map relating different scalar-tensor and vector-tensor theories and gives access to a powerful solution-generating method in modified gravity. In view of the vast family of new solutions one can achieve, it is crucial to design suitable tools to guide their construction. In this work, we address this question by revisiting the Petrov classification of disformally constructed solutions in modified gravity theories. We provide close formulas which relate the principal nulls directions as well as the Weyl scalars before and after the disformal transformation. These formulas allow one to capture if and how the Petrov type of a given seed geometry changes under a disformal transformation. Finally, we apply our general setup to three relevant disformally constructed solutions for which the seeds are respectively homogeneous and isotropic, static spherically symmetric and stationary axisymmetric. For the first two cases, we show that the Petrov type O and Petrov type D remain unchanged after a disformal transformation while we show that disformed Kerr black hole is no longer of type D but of general Petrov type I. The results presented in this work should serve as a new toolkit when constructing and comparing new disformal solutions in modified gravity.

gr-qc↗

arXiv2022

Inflation with $0 \leq c_{\rm s} \leq 1$

We investigate a novel single field inflationary scenario which allows a transition between a slow-roll k-inflation with $c_{\rm s}$ of order unity and a ghost inflation with $ c_{\rm s} \simeq 0$, where $ c_{\rm s}$ is the sound speed for the curvature perturbations. We unify the two phases smoothly by appropriately taking into account a higher derivative scordatura term, which is always there from the effective field theory point of view but which becomes important only in the $c_{\rm s} \simeq 0$ regime. The model achieves the whole range of $0 \leq c_{\rm s} \leq 1$ avoiding strong coupling and gradient instability, and allows us to access the $c_{\rm s} \simeq 0$ regime in a self-consistent manner. We also discuss implications to the formation of primordial black holes.

hep-th↗

arXiv2022

The Effective Field Theory of Vector-Tensor Theories

We investigate a systematic formulation of vector-tensor theories based on the effective field theory (EFT) approach. The input of our EFT is that the spacetime symmetry is spontaneously broken by the existence of a preferred timelike direction in accordance with the cosmological principle. After clarifying the difference of the symmetry breaking pattern from the conventional EFT of inflation/dark energy, we find an EFT description of vector-tensor theories around the cosmological background. This approach not only serves as a unified description of vector-tensor theories but also highlights universal differences between the scalar-tensor theories and the vector-tensor theories. The theories having different symmetry breaking patterns are distinguished by a phenomenological function and consistency relations between the EFT coefficients. We study the linear cosmological perturbations within our EFT framework and discuss the characteristic properties of the vector-tensor theories in the context of dark energy. In particular, we compute the effective gravitational coupling and the slip parameter for the matter density contrast in terms of the EFT coefficients.

hep-th↗

arXiv2021

Inflationary gravitational waves in consistent $D\to 4$ Einstein-Gauss-Bonnet gravity

We study the slow-roll single field inflation in the context of the consistent $D\to4$ Einstein-Gauss-Bonnet gravity that was recently proposed in \cite{Aoki:2020lig}. In addition to the standard attractor regime, we find a new attractor regime which we call the Gauss-Bonnet attractor as the dominant contribution comes from the Gauss-Bonnet term. Around this attractor solution, we find power spectra and spectral tilts for the curvature perturbations and gravitational waves (GWs) and also a model-independent consistency relation among observable quantities. The Gauss-Bonnet term provides a nonlinear $k^4$ term to the GWs dispersion relation which has the same order as the standard linear $k^2$ term at the time of horizon crossing around the Gauss-Bonnet attractor. The Gauss-Bonnet attractor regime thus provides a new scenario for the primordial GWs which can be tested by observations. Finally, we study non-Gaussianity of GWs in this model and estimate the nonlinear parameters $f^{s_1s_2s_3}_{\rm NL,\;sq}$ and $f^{s_1s_2s_3}_{\rm NL,\;eq}$ by fitting the computed GWs bispectra with the local-type and equilateral-type templates respectively at the squeezed limit and at the equilateral shape. For helicities $(+++)$ and $( -- )$, $f^{s_1s_2s_3}_{\rm NL,\;sq}$ is larger while $f^{s_1s_2s_3}_{\rm NL,\;eq}$ is larger for helicities $(++-)$ and $(--+)$.

gr-qc↗

arXiv2021

Cosmology and gravitational waves in consistent $D\to 4$ Einstein-Gauss-Bonnet gravity

In a very recent paper [1], we have proposed a novel $4$-dimensional gravitational theory with two dynamical degrees of freedom, which serves as a consistent realization of $D\to4$ Einstein-Gauss-Bonnet gravity with the rescaled Gauss-Bonnet coupling constant $\tildeα$. This has been made possible by breaking a part of diffeomorphism invariance, and thus is consistent with the Lovelock theorem. In the present paper, we study cosmological implications of the theory in the presence of a perfect fluid and clarify the similarities and differences between the results obtained from the consistent $4$-dimensional theory and those from the previously considered, naive (and inconsistent) $D\rightarrow 4$ limit. Studying the linear perturbations, we explicitly show that the theory only has tensorial gravitational degrees of freedom (besides the matter degree) and that for $\tildeα>0$ and $\dot{H}<0$, perturbations are free of any pathologies so that we can implement the setup to construct early and/or late time cosmological models. Interestingly, a $k^4$ term appears in the dispersion relation of tensor modes which plays significant roles at small scales and makes the theory different than not only general relativity but also many other modified gravity theories as well as the naive (and inconsistent) $D\to 4$ limit. Taking into account the $k^4$ term, the observational constraint on the propagation of gravitational waves yields the bound $\tildeα \lesssim (10\,{\rm meV})^{-2}$. This is the first bound on the only parameter (besides the Newton's constant and the choice of a constraint that stems from a temporal gauge fixing) in the consistent theory of $D\to 4$ Einstein-Gauss-Bonnet gravity.

gr-qc↗

arXiv2021

Bounds on the Horndeski Gauge-Gravity Coupling

The Horndeski gauge-gravity coupling is the leading non-minimal interaction between gravity and gauge bosons, and it preserves all the symmetries and the number of physical degrees of freedom of the standard model of particle physics and general relativity. In this paper we study the effects of the non-minimal interaction in astronomy and cosmology, and obtain upper bounds on the associated dimensionless coupling constant $λ$. From the modification of equations of motion of gauge bosons applied to compact astronomical objects, we find upper bounds $|λ| \lesssim 10^{88}$, $|λ| \lesssim 10^{75}$ and $|λ| \lesssim 10^{84}$ from a black hole shadow, neutron stars and white dwarfs, respectively. The bound $|λ| \lesssim 10^{75}$ that is deduced from neutron stars is the strongest and provides twenty orders of magnitude improvement of the previously known best bound on this parameter. On the other hand, the effects of this term on modification of the gravitational Poisson equation lead to a weaker bound $|λ| \lesssim 10^{98}$. From the propagation of gravitational waves we also find $|λ| \lesssim 10^{119}$, which is even weaker.

astro-ph.CO↗

arXiv2021

Analytic study of dark photon and gravitational wave production from axion

Axion-like fields heavier than about $10^{-27}$eV are expected to oscillate in the radiation dominated epoch when the Hubble parameter drops below their mass. Considering the Chern-Simons coupling with a dark gauge boson, large amount of dark photons are produced during a short time interval through tachyonic resonance instability. The produced dark photons then source gravitational tensor modes leading to chiral gravitational waves. Through this process, one can indirectly probe a large parameter space of coupled axion-dark photon models. In this work we first find an analytic expression for the number density of the dark photons produced during the tachyonic resonance regime. Second, by using the saddle point approximation we find an analytic expression for the gravitational wave spectrum in terms of the mass, coupling and misalignment angle. Our analytic results can be used for the observational analysis of these types of scenarios.

hep-ph↗

arXiv2021

Vector dark matter production from inflation with symmetry breaking

We present a scenario of vector dark matter production from symmetry breaking at the end of inflation. In this model, the accumulated energy density associated with the quantum fluctuations of the dark photon accounts for the present energy density of dark matter. The inflaton is a real scalar field while a heavy complex scalar field, such as the waterfall of hybrid inflation, is charged under the dark gauge field. After the heavy field becomes tachyonic at the end of inflation, rolling rapidly towards its global minimum, the dark photon acquires mass via the Higgs mechanism. To prevent the decay of the vector field energy density during inflation, we introduce couplings between the inflaton and the gauge field such that the energy is pumped to the dark sector. The setup can generate the observed dark matter abundance for a wide range of the dark photon's mass and with the reheat temperature around $10^{12}$ GeV. The model predicts the formation of cosmic strings at the end of inflation with the tensions which are consistent with the CMB upper bounds.

hep-ph↗

arXiv2021

Stealth dark energy in scordatura DHOST theory

A stealth de Sitter solution in scalar-tensor theories has an exact de Sitter background metric and a nontrivial scalar field profile. Recently, in the context of Degenerate Higher-Order Scalar-Tensor (DHOST) theories it was shown that stealth de Sitter solutions suffer from either infinite strong coupling or gradient instability for scalar field perturbations. The sound speed squared is either vanishing or negative. In the first case, the strong coupling scale is zero and thus lower than the energy scale of any physical phenomena. From the viewpoint of effective field theory, this issue is naturally resolved by introducing a controlled detuning of the degeneracy condition dubbed scordatura, recovering a version of ghost condensation. In this paper we construct a viable dark energy model in the scordatura DHOST theory based on a stealth cosmological solution, in which the metric is the same as in the standard $Λ$CDM model and the scalar field profile is linearly time-dependent. We show that the scordatura mechanism resolves the strong coupling and gradient instability. Further, we find that the scordatura is also necessary to make the quasi-static limit well-defined, which implies that the subhorizon observables are inevitably affected by the scordatura. We derive the effective gravitational coupling and the correction to the friction term for the subhorizon evolution of the linear dark matter energy density contrast as well as the Weyl potential and the gravitational slip parameter. In the absence of the scordatura, the quasi-static approximation would break down at all scales around stealth cosmological solutions even if the issue of the infinite strong coupling is unjustly disregarded. Therefore previous estimations of the subhorizon evolution of matter density contrast in modified gravity in the literature need to be revisited by taking into account the scordatura effect.

gr-qc↗

arXiv2020

Inflation with multiple vector fields and non-Gaussianities

We consider a model of inflation consisting a triplet of $U(1)$ vector fields with the parity violating interaction which is non-minimally coupled to inflaton. The vector field sector enjoys global $O(3)$ symmetry which admits isotropic configuration and provides not only vector modes but also scalar and tensor modes. We decompose the scalar perturbations into the adiabatic, entropy and isocurvature perturbations and compute all power spectra and cross correlations of the scalar and the tensor sectors. The tensor modes associated with the vector fields contribute to the power spectrum of gravitational waves while the parity violating term generates chirality in gravitational power spectra and bispectra. We study nonlinear scalar and tensor perturbations and compute all bispectra and three-point cross-correlations. In particular, it is shown that the non-Gaussianity of curvature perturbations and gravitational waves are enhanced by the vector field perturbations. We show that non-Gaussianities put strong constraints on the model parameters such as the parity violating coupling and the fractional energy of the vector fields.

astro-ph.CO↗

arXiv2020

Dark photon dark matter from charged inflaton

We present a scenario of vector dark matter production during inflation containing a complex inflaton field which is charged under a dark gauge field and which has a symmetry breaking potential. As the inflaton field rolls towards the global minimum of the potential the dark photons become massive with a mass which can be larger than the Hubble scale during inflation. The accumulated energy of the quantum fluctuations of the produced dark photons gives the observed relic density of the dark matter for a wide range of parameters. Depending on the parameters, either the transverse modes or the longitudinal mode or their combination can generate the observed dark matter relic energy density.

hep-ph↗