Article
Astronomy & Astrophysics
Yolbeiker Rodriguez Baez, Manuel Gonzalez-Espinoza
Summary: In a scalar-vector-tensor theory, the stability of static spherically symmetric black holes under linear odd-parity perturbations is studied. The action is calculated to second order in the linear perturbations to obtain a master equation for these perturbations. No-ghost and Laplacian instability conditions are derived for this general class of models. The stability conditions for particular cases are examined using the generalized Regge-Wheeler potential.
CLASSICAL AND QUANTUM GRAVITY
(2023)
Article
Astronomy & Astrophysics
Keisuke Inomata
Summary: This study focuses on scalar perturbations induced by non-linear interactions at second order in perturbations. Analytic solutions for induced scalar perturbations in a perfect fluid are derived, with specific consideration for perturbations in radiation-dominated and matter-dominated eras. The evolution of induced scalar perturbations from outside to inside the horizon is also discussed using the analytic solutions.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Inyong Cho, Jinn-Ouk Gong, Seung Hun Oh
Summary: By investigating the scalar field and cosmological perturbations, we find that the second-order effective energy-momentum tensor (2EMT) is gauge dependent. In the slow-roll stage of inflation, 2EMT exhibits an effective fluid of w 1/4 -1=3 in various gauge conditions.
Article
Astronomy & Astrophysics
Xin-Yang Wang, Jie Jiang
Summary: The applicability of the Wald entropy to the second law of black hole thermodynamics is discussed, and a general expression for the entropy of black holes that satisfies the linearized second law is obtained. However, more research is needed to determine whether the Wald entropy of black holes with nonminimal coupling matter fields satisfies the second law sufficiently.
Article
Physics, Nuclear
Zhe Chang, Sai Wang, Qing-Hua Zhu
Summary: This study demonstrates the existence of infinite families of gauge invariant variables in cosmological perturbations and verifies conversion formulae among them. The equations of motion for cosmological perturbations induced by perturbations of different orders are explicitly represented.
Article
Astronomy & Astrophysics
Zhe Chang, Xukun Zhang, Jing-Zhi Zhou
Summary: We study the second order gravitational waves produced by the primordial scalar and tensor perturbations in a radiation-dominated era. We provide explicit expressions for the power spectra of the second order GWs. The energy density spectra of the second order GWs are calculated for the monochromatic primordial power spectra P-zeta = A(zeta)k(*)delta(k - k(*)) and P-h = A(h)k(*)delta(k - k(*)). The primordial tensor perturbation significantly affects the total power spectrum of second order GWs for large k (k > k(*)). For f(*) = 1.3 x 10(-3) and A(zeta) = 0.02, the effects of the primordial tensor perturbation result in an approximately 100% increase in the signal-to-noise ratio (SNR) for LISA observations if the tensor-to-scalar ratio r = A(h)/A(zeta) > 0.4.
Article
Astronomy & Astrophysics
Alessandro Casalino, Lorenzo Sebastiani
Summary: This paper investigates the perturbation theory of Regularized Lovelock Gravity on curved FLRW spacetime, deriving a general expression for the velocity of gravitational waves. Application of the theory to slow-roll inflation on flat FLRW background yields power spectra of scalar and tensor perturbations, and equations for inflation observable quantities.
PHYSICS OF THE DARK UNIVERSE
(2021)
Article
Astronomy & Astrophysics
Daisuke Yamauchi, Naonori S. Sugiyama
Summary: In this study, we investigate the galaxy bispectrum induced by nonlinear gravitational evolution as a way to constrain degenerate higher-order scalar tensor (DHOST) theories. We find that the signal obtained from the leading kernel of second-order density fluctuations is partially hidden by the uncertainty in the nonlinear galaxy bias. However, the kernel of second-order velocity fields provides unbiased information on the modification of gravity theory. Based on these findings, we propose new time-dependent functions that are expected to trace the higher-order growth history and provide new constraints on the parameter space of DHOST theories.
Article
Mathematics, Applied
Alessandro Calamai, Maria Patrizia Pera, Marco Spadini
Summary: In this paper, the structure of the set of forced oscillations of a reducible second order functional delayed differential equation with periodic forcing is investigated using a topological approach. Specifically, a delay-type functional dependence involving a gamma probability distribution is considered, and a first order system of ODEs is formulated using a linear chain trick, with its T-periodic solutions corresponding to those of the functional equation.
ELECTRONIC JOURNAL OF QUALITATIVE THEORY OF DIFFERENTIAL EQUATIONS
(2023)
Article
Astronomy & Astrophysics
Srijita Sinha, Narayan Banerjee
Summary: In this study, perturbations in a scalar field model driving recent acceleration were investigated, revealing distinctive qualitative features compared to the traditional ΛCDM model, particularly in reducing the amplitude of the matter power spectrum at an 8h(-1) Mpc scale.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
David Langlois, Karim Noui, Hugo Roussille
Summary: A novel and remarkably simple formulation of degenerate higher-order scalar-tensor (DHOST) theories is presented, which can be simplified to the usual Einstein-Hilbert term through disformal transformations. This classification of all quadratic DHOST theories becomes transparent in a geometric reformulation that applies also to scalar-tensor theories degenerate only in the unitary gauge.
Article
Astronomy & Astrophysics
Jing-Zhi Zhou, Xukun Zhang, Qing-Hua Zhu, Zhe Chang
Summary: This paper investigates the third order induced gravitational waves and presents the two point function and energy density spectrum of these waves using observations on a monochromatic primordial power spectrum. The study finds that the shape of the energy density spectrum of the third order gravitational waves is different from that of the second order scalar induced gravitational waves. Additionally, it is discovered that the third order gravitational waves sourced by the second order scalar perturbations dominate the two point function and energy density spectrum of third order scalar induced gravitational waves.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Luca Marchetti, Daniele Oriti
Summary: This research derives effective dynamics for scalar cosmological perturbations from quantum gravity in the framework of group field theory condensate cosmology. It establishes a connection between fundamental quantum gravity models and cosmological observations.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Mian Zhu, Amara Ilyas, Yunlong Zheng, Yi-Fu Cai, Emmanuel N. Saridakis
Summary: In this study, the bounce realization within the framework of DHOST cosmology was investigated, with a focus on the relation with observables. It was found that cosmological perturbations generated by thermal fluctuations are the most efficient, with predictions in perfect agreement with observational bounds, particularly in terms of the tensor-to-scalar ratio.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Keisuke Nakashi, Masashi Kimura, Hayato Motohashi, Kazufumi Takahashi
Summary: We propose a physically sensible formulation for the initial value problem of black hole perturbations in higher-order scalar-tensor theories. As a first application, we study monopole perturbations around stealth Schwarzschild solutions in a subclass of degenerate higher-order scalar-tensor (DHOST) theories with shift- and reflection-symmetry. Our analysis of the time evolution of the monopole perturbations using a two-dimensional wave equation reveals that the stealth Schwarzschild solutions in the DHOST theory are dynamically stable against monopole perturbations with wavelengths comparable or shorter than the black hole horizon size. Unlike the standard case of general relativity, we find that there are no damped oscillations during the late time.
CLASSICAL AND QUANTUM GRAVITY
(2022)
