scholarly journals Mimetic inflation

2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
Seyed Ali Hosseini Mansoori ◽  
Alireza Talebian ◽  
Hassan Firouzjahi
Keyword(s):  
Dispersion Relation ◽  
Speed Of Light ◽  
Consistency Relation ◽  
Higher Derivative ◽  
Tensor Perturbations ◽  
Mimetic Gravity ◽  
Scalar Perturbations

Abstract We study inflationary solution in an extension of mimetic gravity with the higher derivative interactions coupled to gravity. Because of the higher derivative interactions the setup is free from the ghost and gradient instabilities while it hosts a number of novel properties. The dispersion relation of scalar perturbations develop quartic momentum correction similar to the setup of ghost inflation. Furthermore, the tilt of tensor perturbations can take either signs with a modified consistency relation between the tilt and the amplitude of tensor perturbations. Despite the presence of higher derivative interactions coupled to gravity the tensor perturbations propagate with the speed equal to the speed of light as required by the LIGO observations. Furthermore, the higher derivative interactions induce non-trivial interactions in cubic Hamiltonian, generating non-Gaussianities in various shapes such as the equilateral, orthogonal and squeezed configurations with observable amplitudes.

scholarly journals Stability of Einstein static universe over Lyra geometry

2015 ◽  
Vol 93 (12) ◽  
pp. 1566-1570 ◽  
Author(s):  
F. Darabi ◽  
Y. Heydarzade ◽  
F. Hajkarim
Keyword(s):  
State Parameter ◽  
Lyra Geometry ◽  
Physical Parameters ◽  
Static Universe ◽  
Equation Of State Parameter ◽  
Existence And Stability ◽  
Einstein Static Universe ◽  
Tensor Perturbations ◽  
The Stability ◽  
Scalar Perturbations

The existence and stability conditions of Einstein static universe against homogeneous scalar perturbations in the context of Lyra geometry is investigated. The stability condition is obtained in terms of the constant equation of state parameter ω = p/ρ depending on energy density ρ0 and scale factor a0 of the initial Einstein static universe. Also, the stability against vector and tensor perturbations is studied. It is shown that a stable Einstein static universe can be found in the context of Lyra geometry against scalar, vector, and tensor perturbations for suitable range and values of physical parameters.

scholarly journals Electromagnetic waves propagating in the string axiverse

2018 ◽  
Vol 2018 (4) ◽  
Author(s):  
Daiske Yoshida ◽  
Jiro Soda
Keyword(s):  
Dark Matter ◽  
Electromagnetic Field ◽  
Dispersion Relation ◽  
Resonance Frequency ◽  
Parametric Resonance ◽  
Electromagnetic Waves ◽  
Coupling Constant ◽  
Speed Of Light ◽  
Coherent Oscillation ◽  
Chern Simons

Abstract It is widely believed that axions are ubiquitous in string theory and could be dark matter. The peculiar features of axion dark matter are coherent oscillations and a coupling to the electromagnetic field through the Chern–Simons term. In this letter, we study the consequences of these two features of axions with mass in the range $10^{-13}\,{\rm eV}$ to $10^{3}\,{\rm eV}$. First, we study the parametric resonance of electromagnetic waves induced by the coherent oscillation of the axion. Since the resonance frequency is determined by the mass of the axion dark matter, if we detect this signal, we can get information on the mass of the axion dark matter. Second, we study the velocity of light in the background of the axion dark matter. In the presence of the Chern–Simons term, the dispersion relation is modified and the speed of light will oscillate in time. It turns out that the change in the speed of light would be difficult to observe. We argue that future radio wave observations of the resonance can give rise to a stronger constraint on the coupling constant and/or the density of the axion dark matter.

scholarly journals EFT compatible PBHs: effective spawning of the seeds for primordial black holes during inflation

2021 ◽  
Vol 2021 (7) ◽  
Author(s):  
Amjad Ashoorioon ◽  
Abasalt Rostami ◽  
Javad T. Firouzjaee
Keyword(s):  
Black Holes ◽  
Dispersion Relation ◽  
Power Spectrum ◽  
Effective Field ◽  
Primordial Black Holes ◽  
Instability Mechanism ◽  
Solar Mass ◽  
Order Polynomial ◽  
Matter Energy ◽  
Scalar Perturbations

Abstract Most of the inflationary scenarios that try to explain the origin of Primordial Black Holes (PBHs) from the enhancements of the power spectrum to values of order one, at the relevant scales, run into clashes with the Effective Field Theory (EFT) criteria or fail to enhance the power spectrum to such large amplitudes. In this paper, we unravel a mechanism for enhancing the power spectrum during inflation that does not use the flattening of the potential or reduction of the sound speed of scalar perturbations. The mechanism is based on this observation in the formalism of Extended EFT of inflation (EEFToI) with the sixth order polynomial dispersion relation for scalar perturbations that if the quartic coefficient in the dispersion relation is negative and smaller than a certain threshold, the amplitude of the power spectrum is enhanced substantially. The instability mechanism must arrange to kick in at the scales of interest related to the mass of the PBHs one would like to produce, which can be ten(s) of solar mass PBHs, suitable for LIGO events, or 10−17− 10−13 solar mass PBHs, which can comprise the whole dark matter energy density. We argue that the strong coupling is avoided for the range of parameters that the mechanisms enhance the power spectrum to the required amount.

scholarly journals First-Order Formalism and Thick Branes in Mimetic Gravity

Symmetry ◽  
2021 ◽  
Vol 13 (8) ◽  
pp. 1345
Author(s):  
Qun-Ying Xie ◽  
Qi-Ming Fu ◽  
Tao-Tao Sui ◽  
Li Zhao ◽  
Yi Zhong
Keyword(s):  
Conformal Transformation ◽  
Zero Mode ◽  
Conformal Symmetry ◽  
Newtonian Potential ◽  
Field Equations ◽  
Effective Potentials ◽  
First Order ◽  
Mimetic Gravity ◽  
Thick Branes ◽  
Scalar Perturbations

In this paper, we investigate thick branes generated by a scalar field in mimetic gravity theory, which is inspired by considering the conformal symmetry under the conformal transformation of an auxiliary metric. By introducing two auxiliary super-potentials, we transform the second-order field equations of the system into a set of first-order equations. With this first-order formalism, several types of analytical thick brane solutions are obtained. Then, tensor and scalar perturbations are analyzed. We find that both kinds of perturbations are stable. The effective potentials for the tensor and scalar perturbations are dual to each other. The tensor zero mode can be localized on the brane while the scalar zero mode cannot. Thus, the four-dimensional Newtonian potential can be recovered on the brane.

scholarly journals Higher derivative mimetic gravity

2018 ◽  
Vol 2018 (01) ◽  
pp. 020-020 ◽  
Author(s):  
Mohammad Ali Gorji ◽  
Seyed Ali Hosseini Mansoori ◽  
Hassan Firouzjahi
Keyword(s):  
Higher Derivative ◽  
Mimetic Gravity

scholarly journals Scalar cosmological perturbations in M-theory with higher-derivative corrections

2020 ◽  
Vol 2020 (11) ◽  
Author(s):  
Kazuho Hiraga ◽  
Yoshifumi Hyakutake
Keyword(s):  
Power Spectrum ◽  
Equations Of Motion ◽  
Cosmological Perturbations ◽  
Curvature Perturbation ◽  
Linearized Equations ◽  
Higher Derivative ◽  
Expansion Of The Universe ◽  
M Theory ◽  
The Universe ◽  
Scalar Perturbations

Abstract We investigate the inflationary expansion of the universe induced by higher-curvature corrections in M-theory. The inflationary evolution of the geometry is discussed in K. Hiraga and Y. Hyakutake, Prog. Theor. Exp. Phys. 2018, 113B03 (2018), which we follow to analyze metric perturbations around the background. We especially focus on scalar perturbations and analyze linearized equations of motion for the scalar perturbations. By solving these equations explicitly, we evaluate the power spectrum of the curvature perturbation. The scalar spectrum index is estimated under some assumptions, and we show that it becomes close to 1.

scholarly journals The dispersion relation for matter waves in a two-phase vacuum

2014 ◽  
Vol 29 (31) ◽  
pp. 1450168 ◽  
Author(s):  
Paul S. Wesson
Keyword(s):  
Dispersion Relation ◽  
Cosmological Constant ◽  
High Frequency ◽  
Large Scale ◽  
Three Dimensional ◽  
Speed Of Light ◽  
Cosmological Constant Problem ◽  
Two Phase ◽  
Matter Waves ◽  
3D Space

The cosmological constant Λ of general relativity is a natural consequence of embedding Einstein's theory in a five-dimensional (5D) theory of the type needed for unification. The exact 5D solution for Λ<0 shows waves in ordinary three-dimensional (3D) space with properties similar to those of de Broglie or matter waves. Here the dispersion relation is derived for matter waves in a toy two-phase model, where regions with Λ<0 and Λ>0 average on the large scale to Λ≃0, thus providing in principle a resolution of the cosmological-constant problem. A striking result of the analysis is that the dispersion relation is bimodal, with a well-defined window of high-frequency transmission which effectively defines the speed of light.

Sign in / Sign up

Export Citation Format

Share Document