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.
Problems related to X-ray imaging with minimum radiation doses are described, a method for constructing systems for X-ray imaging of people at these doses is proposed, the main components of these ...systems are briefly described, and their optimal applications in medicine and security systems are characterized.
We describe the spherically symmetric steady-state accretion of perfect fluid in the Reissner-Nordstroem metric. We present analytic solutions for accretion of a fluid with linear equations of state ...and of the Chaplygin gas. We also show that under reasonable physical conditions, there is no steady-state accretion of a perfect fluid onto a Reissner-Nordstroem naked singularity. Instead, a static atmosphere of fluid is formed. We discuss a possibility of violation of the third law of black hole thermodynamics for a phantom fluid accretion.
Low-dose digital radiographic device “Siberia” based on a multistrip ionization chamber was developed in Budker Institute of Nuclear Physics. Originally, the detector was filled with Xe gas under
10
...bar
pressure, however, the latest study showed that Kr filled detector at
20
bar
has higher signal-to-noise ratio (SNR) than Xe one. The main factor affecting SNR is amplitude fluctuation of detected signal. Kr has lower value of K-shell binding energy and less K-shell fluorescence yield than Xe gas. It leads to less distortion of registered X-ray spectra and better energy localization. As a result the detector has better channel shape and higher DQE value.
Phys. Rev. D 108, 123529 (2023) 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 $\Omega_{gw}\propto f^{2}$ corresponding
to the spectral index $\gamma=3$ 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.
The new, rapidly developing field of theoretical research --- studies of dark energy interacting with black holes (and, in particular, accreting onto black holes) --- is reviewed. The term `dark ...energy' is meant to cover a wide range of field theory models, as well as perfect fluids with various equations of state, including cosmological dark energy. Various accretion models are analyzed in terms of the simplest test field approximation or by allowing back reaction on the black-hole metric. The behavior of various types of dark energy in the vicinity of Schwarzschild and electrically charged black holes is examined. Nontrivial effects due to the presence of dark energy in the black hole vicinity are discussed. In particular, a physical explanation is given of why the black hole mass decreases when phantom energy is being accreted, a process in which the basic energy conditions of the famous theorem of nondecreasing horizon area in classical black holes are violated. The theoretical possibility of a signal escaping from beneath the black hole event horizon is discussed for a number of dark energy models. Finally, the violation of the laws of thermodynamics by black holes in the presence of noncanonical fields is considered.
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.
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.
High-resolution X-ray computed tomography (HRXCT) is a technology ideally applicable to a wide range of geological investigations. It is an express non-destructive method to produce images ...corresponding to series of slice projections through a sample. In the present study, HRXCT was applied to rock samples with the use of synchrotron radiation from the VEPP-3 storage ring, at the “X-ray microscopy and tomography” station. The method was calibrated and preliminary measurements were carried out on the Low-Dose Digital Radiographic Device “Siberia”. The data obtained have determined the internal structure of rock samples with a spatial resolution of 100
μm. HRXCT has been applied to the xenoliths of diamondiferous eclogites from the Udachnaya kimberlitic pipe located in Yakutia, Russia. It has allowed determination of the distribution of rock-forming (garnet and clinopyroxene) and accessory (diamond, rutile, and sulfide) minerals of different X-ray absorption. This is important to find out the genetic relationship of diamonds with associated minerals and the sequence of crystallization.
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