Gravitational shine of dark domain walls Babichev, E.; Gorbunov, D.; Ramazanov, S. ...
Journal of Cosmology and Astroparticle Physics,
04/2022, Letnik:
2022, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Abstract
Cosmic domain walls are harmless, provided that their tension decreases with expansion of the Universe. This setup can be realized, if the scale of spontaneous symmetry breaking is induced ...dynamically through the interaction with hot primordial plasma. In that case, the domain wall tension can attain large values in the early Universe without any conflict with observations. Owing to the large initial tension, these topological defects may serve as a powerful source of gravitational waves. We make a preliminary estimate of the gravitational wave spectrum and argue that it is distinct from the spectrum produced by other sources, in particular by domain walls of a constant tension. The resulting gravitational wave signal is in the range accessible by Einstein Telescope, DECIGO, TianQin, LISA, IPTA, or SKA, if the field constituting the domain walls is very feebly coupled with hot primordial plasma and has tiny self-interactions. In particular, one can consider this field for the role of Dark Matter. We discuss various Dark Matter production mechanisms and properties of the emitted gravitational waves associated with them. We find that the conventional freeze-out and freeze-in mechanisms lead to large and perhaps unobservable frequency of gravitational waves. However, the Dark Matter production is also possible at the second order phase transition leading to the domain wall formation or at the inverse phase transition, when the domain walls get dissolved eventually. In both cases, there is essentially no lower bound on the frequency of emitted gravitational waves.
Shimmering gravitons in the gamma-ray sky Ramazanov, S.; Samanta, R.; Trenkler, G. ...
Journal of Cosmology and Astroparticle Physics,
06/2023, Letnik:
2023, Številka:
6
Journal Article
Recenzirano
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Abstract
What is the highest energy at which gravitons can be observed? We address this question by studying graviton-to-photon conversion — the inverse-Gertsenshtein effect — in the magnetic field ...of the Milky Way. We find that above ∼ 1 PeV the effective photon mass grows large enough to quench the conversion rate.
For sub-PeV energies, the induced photon flux is comparable to the sensitivity of LHAASO to a diffuse
γ
-ray background, but only for graviton abundances of order Ω
gw
h
2
0
∼ 1. In the future, owing to a better understanding of
γ
-ray backgrounds, larger effective areas and longer observation times, sub-PeV shimmering gravitons with a realistic abundance of Ω
gw
h
2
0
∼ 0.01 could be detected. We show how such a large abundance is achieved in a cosmologically-motivated scenario of post-recombination superheavy dark matter decay. Therefore, the sub-PeV range might be the ultimate energy frontier at which gravitons can be observed.
A
bstract
We consider the branch of the projectable Hořava-Lifshitz model which exhibits ghost instabilities in the low energy limit. It turns out that, due to the Lorentz violating structure of the ...model and to the presence of a finite strong coupling scale, the vacuum decay rate into photons is tiny in a wide range of phenomenologically acceptable parameters. The strong coupling scale, understood as a cutoff on ghosts’ spatial momenta, can be raised up to Λ ∼ 10 TeV. At lower momenta, the projectable Hořava-Lifshitz gravity is equivalent to General Relativity supplemented by a fluid with a small positive sound speed squared (10
−42
≲)
c
s
2
≲ 10
−20
, that could be a promising candidate for the Dark Matter. Despite these advantages, the
unavoidable
presence of the strong coupling obscures the implementation of the original Hořava’s proposal on quantum gravity. Apart from the Hořava-Lifshitz model, conclusions of the present work hold also for the mimetic matter scenario, where the analogue of the projectability condition is achieved by a non-invertible conformal transformation of the metric.
In a subclass of scalar-tensor theories, it has been shown that standard general relativity solutions of neutron stars and black holes with trivial scalar field profiles are unstable. Such an ...instability leads to solutions which are different from those of general relativity and have non-trivial scalar field profiles, in a process called scalarization. In the present work we focus on scalarization due to a non-minimal coupling of the scalar field to the Gauss-Bonnet curvature invariant. The coupling acts as a tachyonic mass for the scalar mode, thus leading to the instability of general relativity solutions. We point out that a similar effect may occur for the scalar modes in a cosmological background, resulting in the instability of cosmological solutions. In particular, we show that a catastrophic instability develops during inflation within a period of time much shorter than the minimum required duration of inflation. As a result, the standard cosmological dynamics is not recovered. This raises the question of the viability of scalar-Gauss-Bonnet theories exhibiting scalarization.
We discuss the gravitational creation of superheavy particles χ in an inflationary scenario with a quartic potential and a non-minimal coupling between the inflaton ϕ and the Ricci curvature: ξϕ2R/2. ...We show that for large constants ξ≫1, there can be abundant production of particles χ with masses largely exceeding the inflationary Hubble rate Hinfl, up to (afew)×ξHinfl, even if they are conformally coupled to gravity. We discuss two scenarios involving these gravitationally produced particles χ. In the first scenario, the inflaton has only gravitational interactions with the matter sector and the particles χ reheat the Universe. In this picture, the inflaton decays only due to the cosmic expansion, and effectively contributes to dark radiation, which can be of the observable size. The existing limits on dark radiation lead to an upper bound on the reheating temperature. In the second scenario, the particles χ constitute Dark Matter, if substantially stable. In this case, their typical masses should be in the ballpark of the Grand Unification scale.
Abstract The present study explores the effect of a magnetic field on the thermal conductivity of two-dimensional (2D) Yukawa systems in a wide range of system parameters using the non-equilibrium ...molecular dynamic method (NEMD). We consider an external magnetic field with $$\Omega =\omega _c/\omega _p\le 1$$ Ω = ω c / ω p ≤ 1 (with $$\Omega$$ Ω being the ratio of the cyclotron frequency to plasma frequency) and the coupling parameter values in the range $$1\le \Gamma \le 100$$ 1 ≤ Γ ≤ 100 (with $$\Gamma$$ Γ being the ratio of the Coulomb interaction energy at mean inter-particle distance to the thermal energy of particles). The results show that an external uniform magnetic field results in the reduction of the thermal conductivity at the considered values of the coupling parameter $$\Gamma$$ Γ . Additionally, we found that the effect of the magnetic field on thermal conduction is weaker at larger values of the system coupling parameter. To ensure that calculated results for the thermal conductivity are accurate and reliable, we performed a detailed investigation of the convergence of the results with respect to simulation parameters in NEMD with a strong external magnetic field. We believe that the presented results will serve as useful benchmark data for the theoretical models of (2D) Yukawa systems.
The dynamical structure factor (DSF) of strongly coupled ions in dense plasmas with partially and strongly degenerate electrons is investigated. The main focus is on the impact of electronic ...correlations (nonideality) on the ionic DSF. The latter is computed by carrying out molecular dynamics (MD) simulations with a screened ion-ion interaction potential. The electronic screening is taken into account by invoking the Singwi-Tosi-Land-Sjölander approximation, and it is compared to the MD simulation data obtained considering the electronic screening in the random phase approximation and using the Yukawa potential. We find that electronic correlations lead to lower values of the ion-acoustic mode frequencies and to an extension of the applicability limit with respect to the wave-number of a hydrodynamic description. Moreover, we show that even in the limit of weak electronic coupling, electronic correlations have a nonnegligible impact on the ionic longitudinal sound speed. Additionally, the applicability of the Yukawa potential with an adjustable screening parameter is discussed, which will be of interest, e.g., for the interpretation of experimental results for the ionic DSF of dense plasmas.