arXiv subjects

S. Ramazanov

Publications and source records attributed to S. Ramazanov.

15 recordsLinked to original sources

arXiv2026

Domain walls through different cosmologies

We study properties of domain walls (DWs) arising in the model with a double well potential assuming different early universe cosmologies: from dust through stiff matter domination to the limit of effective Minkowski space. Using lattice simulations we demonstrate that evolution of DW networks exhibits an approximate universality with respect to the cosmological equation of state (EoS). Namely, the wall area inside a given large volume is mainly determined by the particle horizon, while details of cosmic expansion play a subdominant role. As it follows, particle horizon rather than the inverse Hubble rate is pivotal in DW evolution defining the network correlation length and hence its phenomenology, e.g., the characteristic wavelength of emitted gravitational waves (GWs). Formation of closed DWs is shown to be very sensitive to the cosmological EoS and hence violate the universality, but their contribution to the total network is too small to change the overall picture. The universality breaking is also observed in the spectral properties of GWs from annihilating (biased) DWs: i) the IR slope of the spectrum depends on the EoS parameter; ii) there is a plateau in the UV part of the spectrum, which gets more pronounced as one stiffens the EoS. However, the dependence on the EoS is rather weak in the near peak region, which is most relevant from the viewpoint of pulsar timing arrays and other searches for the stochastic GW background. For a selection of primordial cosmologies, we provide fitting formulae for the spectra near the peak.

astro-ph.CO↗

arXiv2026

Caustic formation in DBI models: Wave propagation on planar domain walls

We investigate propagation of generic waves on thin planar domain walls effectively described by the scalar Dirac-Born-Infeld model (DBI). We pay a particular attention to the possibility of caustic formation - the process, which may lead to intensive particle emission by domain walls. It is demonstrated that no singularities arise in DBI in 2D flat spacetime in the hyperbolic case, if one starts from smooth initial conditions. Technically, this happens because the same family characteristics of the relevant partial differential equation remain parallel at all the times, albeit not being straight lines generically. Crucially, characteristic curves cease to be parallel beyond the simplified setup of DBI in 2D flat spacetime. In particular, this is shown to be the case in $D>2$ for spherical waves, in an expanding Universe, and in the case of a minimal deformation of DBI necessary for avoiding the domain wall problem in cosmology. However, we prove that DBI remains caustic free in the hyperbolic case in all these physically relevant situations. This strongly suggests that caustics can form on planar domain walls only due to the loss of hyperbolicity, and they have a cusp profile. We demonstrate, how the non-trivial structure of DBI characteristics beyond the 2D flat spacetime setup uncovered in this work can significantly affect cusp formation.

hep-th↗

arXiv2025

Cosmic domain walls on a lattice: illusive effects of initial conditions

Evolution of cosmic domain walls (DWs) settles to the scaling solution, which is often assumed to be independent of initial conditions. However, lattice simulations performed in this work reveal a clear dependence of the scaling DW area on the initial configuration of the sourcing scalar field, specifically, its infrared (IR) properties. Namely, the DW area grows as one suppresses IR modes in the initial scalar field spectrum. This growth is saturated, when the area parameter $ξ$ commonly used in the literature reaches the value $ξ_{max} \approx 1.2$. The dependence of $ξ$ on IR modes is argued to be of non-physical origin: it is likely to be due to effects of the lattice boundary. Assuming that physically the memory of initial conditions is erased, one recognizes $ξ\approx 1.2$ obtained in the situation with maximally suppressed IR modes as a genuine universal value of the area parameter in the scaling regime. We demonstrate that ignorance about initial conditions may affect predictions for the energy density of gravitational waves by the factor five. The spectral shape of gravitational waves is also affected by the choice of initial conditions, most notably in the low-frequency part. Likewise, we revisit annihilation of DWs under the influence of a potential bias. It has been previously found in Ref. [19] that the annihilation happens significantly earlier compared to the estimate based on the simple balance between the potential bias and surface energy density. We further support this observation and show that the tendency towards an earlier annihilation gets even stronger upon removing IR modes in simulations.

hep-ph↗

arXiv2025

Biased domain walls: faster annihilation, weaker gravitational waves

We study the evolution of domain wall networks and their phenomenological implications in a model of a real scalar $χ$, where a $Z_2$-symmetry is slightly broken by a potential bias $V_{bias}$. It is demonstrated that the latter triggers domain wall annihilation considerably earlier than previously thought. Namely, we observe that the scaling relation $t_{ann} \propto 1/V^{2/3}_{bias}$ for the annihilation time $t_{ann}$ fits to the simulation data better than a commonly assumed $t_{ann} \propto 1/V_{bias}$. As a result, the energy density of gravitational waves produced by the network of biased domain walls, for a given tiny $V_{bias}$, is suppressed compared to naive expectations. The spectral shape of gravitational waves is similar to that resulting from unbiased domain walls, but with more power in the close-to-maximum ultraviolet part. In the far ultraviolet region, the spectrum of gravitational waves becomes nearly flat; such a plateau has been recognised earlier in the case of unbiased walls. In our investigation we mainly focus on the symmetry breaking potential $V_{breaking} \propto χ^3$, and argue that no significant modifications of the domain walls evolution take place if one includes higher powers of $χ$.

hep-ph↗

arXiv2025

Numerical analysis of melting domain walls and their gravitational waves

We study domain walls (DWs) arising in field theories where $Z_2$-symmetry is spontaneously broken by a scalar expectation value decreasing proportionally to the Universe temperature. The energy density of such melting DWs redshifts sufficiently fast not to overclose the Universe. For the first time, evolution of melting DWs and the resulting gravitational waves (GWs) is investigated numerically using lattice simulations. We show that formation of closed melting DWs during radiation domination is much more efficient compared to the scenario with constant tension DWs. This suggests that it can be the main mechanism responsible for reaching the scaling regime similarly to the case of cosmic strings. However, the scaling behaviour of melting DWs is observed, provided only that the initial scalar field fluctuations are not very large. Otherwise, simulations reveal violation of the scaling law, potentially of the non-physical origin. The spectrum of GWs emitted by melting DWs is also significantly different from that of constant tension DWs. Whether the system has reached scaling or not, the numerical study reveals a GW spectrum described in the infrared by the spectral index $n \approx 1.6$ followed by the causality tail. We attribute the difference from the value $n=2$ predicted in our previous studies to a finite lifetime of the DW network. Notably, the updated index is still in excellent agreement with the recent findings by pulsar timing arrays, which confirms that melting DWs can be responsible for the observed (GW) signal. We also point out that results for evolution of melting DWs in the radiation-dominated Universe are applicable to constant tension DW evolution in the flat spacetime.

hep-ph↗

arXiv2024

Revisiting evolution of domain walls and their gravitational radiation with CosmoLattice

Employing the publicly available CosmoLattice code, we conduct numerical simulations of a domain wall network and the resulting gravitational waves (GWs) in a radiation-dominated Universe in the $Z_2$-symmetric scalar field model. In particular, the domain wall evolution is investigated in detail both before and after reaching the scaling regime, using the combination of numerical and theoretical methods. We demonstrate that the total area of closed walls is negligible compared to that of a single long wall stretching throughout the simulation box. Therefore, the closed walls are unlikely to have a significant impact on the overall network evolution. This is in contrast with the case of cosmic strings, where formation of loops is crucial for maintaining the system in the scaling regime. To obtain the GW spectrum, we develop a technique that separates physical effects from numerical artefacts arising due to finite box size and non-zero lattice spacing. Our results on the GW spectrum agree well with Refs. [29, 30], which use different codes. Notably, we observe a peak at the Hubble scale, an exponential falloff at scales shorter than the wall width, and a plateau/bump at intermediate scales. We also study sensitivity of obtained results on the choice of initial conditions. We find that different types of initial conditions lead to qualitatively similar domain wall evolution in the scaling regime, but with important variations translating into different intensities of GWs.

astro-ph.CO↗

arXiv2023

NANOGrav spectral index $γ=3$ from melting domain walls

We discuss cosmic domain walls described by a tension red-shifting with the expansion of the Universe. These melting domain walls emit gravitational waves with the low-frequency spectral shape $Ω_{gw}\propto f^{2}$ corresponding to the spectral index $γ=3$ favoured by the recent NANOGrav 15 yrs data. We discuss a concrete high-energy physics scenario leading to such a melting domain wall network in the early Universe. This scenario involves a feebly coupled scalar field, which can serve as a promising dark matter candidate. We identify parameters of the model matching the gravitational wave characteristics observed in the NANOGrav data. The dark matter mass is pushed to the ultra-light range below $10^{-11}-10^{-12}\,\text{eV}$ which is accessible through planned observations thanks to the effects of superradiance of rotating black holes.

hep-ph↗

arXiv2022

Gravitational shine of dark domain walls

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.

hep-ph↗

arXiv2022

Beyond freeze-in: Dark Matter via inverse phase transition and gravitational wave signal

We propose a novel scenario of Dark Matter production naturally connected with generation of gravitational waves. Dark Matter is modelled as a real scalar, which interacts with the hot primordial plasma through a portal coupling to another scalar field. For a particular sign of the coupling, this system exhibits an inverse second order phase transition. The latter leads to an abundant Dark Matter production, even if the portal interaction is so weak that the freeze-in mechanism is inefficient. The model predicts domain wall formation in the Universe, long time before the inverse phase transition. These domain walls have a tension decreasing with time, and completely disappear at the inverse phase transition, so that the problem of overclosing the Universe is avoided. The domain wall network emits gravitational waves with characteristics defined by those of Dark Matter. In particular, the peak frequency of gravitational waves is determined by the portal coupling constant, and falls in the observable range for currently planned gravitational wave detectors.

hep-ph↗

arXiv2021

Non-magnetic tight binding disorder effects in the $γ$ sheet of $Sr_2RuO_4$

Inspired by the physics of the Miyake - Narikiyo model (MN) for superconductivity in the γ sheet of $Sr_2RuO_4$ to investigate numerically the behavior caused by a non-magnetic disorder in the imaginary part of the elastic scattering matrix for an anisotropic tight-binding model. We perform simulations by going from the Unitary to the Born scattering limit, varying the parameter c which is inverse to the strength of the impurity potential. It is found that the unitary and intermedia limits persist for different orders of magnitude in simulating the disorder concentration. Subsequently and in order to find the MN tiny gap, we perform a numerical study of the unitary limit as a function of disorder concentration, to find the tiny anomalous gap.

cond-mat.supr-con↗

arXiv2020

Gravitational reheating and superheavy Dark Matter creation after inflation with non-minimal coupling

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: $ξφ^2 R/2$. We show that for large constants $ξ>> 1$, there can be abundant production of particles $χ$ with masses largely exceeding the inflationary Hubble rate $H_{infl}$, up to $(a~few) \times ξH_{infl}$, 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.

hep-ph↗

arXiv2017

General quadrupolar statistical anisotropy: Planck limits

Several early Universe scenarios predict a direction-dependent spectrum of primordial curvature perturbations. This translates into the violation of the statistical isotropy of cosmic microwave background radiation. Previous searches for statistical anisotropy mainly focussed on a quadrupolar direction-dependence characterised by a single multipole vector and an overall amplitude $g_*$. Generically, however, the quadrupole has a more complicated geometry described by two multipole vectors and $g_*$. This is the subject of the present work. In particular, we limit the amplitude $g_*$ for different shapes of the quadrupole by making use of Planck 2015 maps. We also constrain certain inflationary scenarios which predict this kind of more general quadrupolar statistical anisotropy.

astro-ph.CO↗

arXiv2016

Living with ghosts in Horava-Lifshitz gravity

We consider the branch of the projectable Horava-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 $Λ\sim 10$ TeV. At lower momenta, the projectable Horava-Lifshitz gravity is equivalent to General Relativity supplemented by a fluid with a small positive sound speed squared ($10^{-42}\lesssim$) $c^2_s \lesssim 10^{-20}$, 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 Horava's proposal on quantum gravity. Apart from the Horava-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.

hep-th↗

arXiv2012

Can residuals of the Solar system foreground explain low multipole anomalies of the CMB ?

The low multipole anomalies of the Cosmic Microwave Background has received much attention during the last few years. It is still not ascertained whether these anomalies are indeed primordial or the result of systematics or foregrounds. An example of a foreground, which could generate some non-Gaussian and statistically anisotropic features at low multipole range, is the very symmetric Kuiper Belt in the outer solar system. In this paper, expanding upon the methods presented by Maris et al. (2011), we investigate the contributions from the Kuiper Belt objects (KBO) to the WMAP ILC 7 map, whereby we can minimize the contrast in power between even and odd multipoles in the CMB, discussed discussed by Kim & Naselsky (2010). We submit our KBO de-correlated CMB signal to several tests, to analyze its validity, and find that incorporation of the KBO emission can decrease the quadrupole-octupole alignment and parity asymmetry problems, provided that the KBO signals has a non-cosmological dipole modulation, associated with the statistical anisotropy of the ILC 7 map. Additionally, we show that the amplitude of the dipole modulation, within a 2 sigma interval, is in agreement with the corresponding amplitudes, discussed by Lew (2008).

astro-ph.CO↗

arXiv2011

Scalar perturbations in conformal rolling scenario with intermediate stage

Scalar cosmological perturbations with nearly flat power spectrum may originate from perturbations of the phase of a scalar field conformally coupled to gravity and rolling down negative quartic potential. We consider a version of this scenario whose specific property is a long intermediate stage between the end of conformal rolling and horizon exit of the phase perturbations. Such a stage is natural, e.g., in cosmologies with ekpyrosis or genesis. Its existence results in small negative scalar tilt, statistical anisotropy of all even multipoles starting from quardupole of general structure (in contrast to the usually discussed single quadrupole of special type) and non-Gaussianity of a peculiar form.

hep-th↗