arXiv subjects

Bohdan Novosyadlyj

Publications and source records attributed to Bohdan Novosyadlyj.

At least 19 recordsLinked to original sources

arXiv2026

Absorption effects in the expanding Universe: spectral transmittance functions of the intergalactic medium for distant sources

We construct two self-consistent analytic approximations to the neutral hydrogen fraction, $x_{\rm HI}(z)$, and the helium ionization fractions, $x_{\rm HeI}(z)$, $x_{\rm HeII}(z)$, and $x_{\rm HeIII}(z)$, that are consistent with current constraints inferred from quasar spectra, galaxy surveys, and CMB polarization measurements. These approximations describe observationally motivated early- and late-reionization scenarios. Using these histories, we analyse the formation of broad absorption troughs in the continuum spectra of high-redshift sources over $1\leq z_{\rm s}\leq15$. We assume that neutral hydrogen and helium in a homogeneous diffuse intergalactic medium reside predominantly in their ground states and absorb radiation through the Lyman-series lines and continua of HI, HeI, and HeII. We compute the wavelength-dependent optical depths for the first 39 Lyman-series lines of HI and HeII, the first 10 lines of HeI, and the corresponding continua, and use them to derive spectral transmittance functions, $T(λ;z_{\rm s})$. As illustrative applications, we apply them to toy-model continuum spectra of starless haloes and to model spectra of a low-metallicity dwarf galaxy at different redshifts. Spectral features in sources at $5\lesssim z_{\rm s}\lesssim7$ caused by intergalactic absorption are found to be particularly sensitive to the adopted hydrogen and helium ionization histories

astro-ph.CO↗

arXiv2025

Sensitivity of the redshifted 21 cm signal from the Dark Ages to parameters of primordial magnetic fields

We analyse the impact of the decaying magnetic turbulence of primordial magnetic fields (PMFs) and ambipolar diffusion on the ionisation and thermal history of the Dark Ages Universe ($30\le z\le300$), and its imprint on the spectral profile of the global signal in the redshifted 21 cm hydrogen line. The heating function caused by decaying magnetic turbulence monotonically decreases after cosmological recombination; its amplitude strongly depends on the strength of the PMFs' $B_0$ and weakly depends on the spectral index of the initial power spectrum of the PMFs' $n_{\rm B}$. The heating function caused by ambipolar diffusion, in contrast, noticeably depends on the spectral index in the range $-3\lesssim n_{\rm B}\lesssim4$ but is subdominant in the Dark Ages epoch for PMF models with $B_0\lesssim0.5$ nG. We computed the ionisation and thermal history of intergalactic gas from the cosmological recombination up to the end of the Dark Ages epoch for a range of PMF parameters, $0.05\lesssim B_0\lesssim0.5$ nG, $-2.9\lesssim n_{\rm B}\lesssim4$, and show the essentially distinguished thermal evolution from one in the Lambda cold dark matter ($Λ$CDM) model. We also show that the profile of the redshifted 21 cm hydrogen line is very sensitive to the PMF parameters from this range and can be used for their constraints in conjunction with other observational data.

astro-ph.CO↗

arXiv2025

Global Signals of the First Molecules from the Dark Ages in the Presence of Primordial Magnetic Fields

We investigate how primordial magnetic fields (PMFs) affect the formation kinetics of the first molecules, H$_2$, HD, and HeH$^+$, as well as the populations of rovibrational levels and the global signals in the rovibrational transitions of H$_2$ and HD. We show that PMFs can significantly speed up the formation and destruction of the first molecules, leading to an increase in the number density of H$_2$ and HD molecules and a decrease in the number density of HeH$^+$ ion-molecules compared to the case without PMFs. We demonstrate that more frequent collisions of the gas particles in such models alter the ortho-to-para ratio of hydrogen molecules, making it a potential probe of the thermal history of gas in the early Universe. In contrast to the standard cosmological model, where the global signal from the first molecules appears as an absorption feature in the cosmic microwave background spectrum, cosmological models with PMFs can produce an emission signal. Specifically, for non-helical PMFs with $n_B = -2.9$ and a strength of $\sim 1$~nG, the signal transforms into emission with an amplitude of about $\sim 0.5$~Jy/sr. This signal is comparable in magnitude to other known CMB spectral distortions and falls within the detection capabilities of several proposed missions, including Super-PIXIE, Multi-SIMBAD (4 units), and Voyage2050. We show that both the amplitude and the spectral range of the global signals from the first molecules are highly sensitive to the spectral index $n_B$, the strength $B_0$, and the helicity of the PMFs. Therefore, the global signals from the first molecules can serve as a potential probe of PMFs.

astro-ph.CO↗

arXiv2025

Ukrainian Wartime Astronomy and its Prospects

The Russian invasion of Ukraine damaged or compromised astronomical facilities and has prompted the displacement of researchers. A plan to restore Ukrainian astronomy, rooted in a deeper integration with the international community, is now being developed.

astro-ph.IM↗

arXiv2025

Global signal in the redshifted hydrogen 21-cm line from the Dark Ages and Cosmic Dawn: Dependence on the nature of dark matter and modeling of first light

We estimate the global signal in the redshifted hyperfine structure line 21 cm of hydrogen atoms formed during the Dark Ages and Cosmic Dawn epochs. The evolution of the brightness temperature in this line was computed to study its dependence on the physical conditions in the intergalactic medium. We show that the profile of this line crucially depends on the temperature and ionization of baryonic matter as well as the spectral energy distribution of radiation from the first sources. The cosmological models with the self-annihilating and decaying dark matter with allowable parameters by current observational data, as well as the model of the first light which is consistent with the observational data on reionization were considered. The results show that the Dark Ages part of profile is very sensitive to the parameters of self-annihilating and decaying dark matter particles, while the Cosmic Dawn part of profile is very sensitive also to the spectral energy distribution of radiation from the first sources. It was concluded that only compatible observations of the redshifted 21 cm line in the decameter and meter wavelength range, formed during the Dark Ages and Cosmic Dawn, will make it possible to constrain the parameters of dark matter models and astrophysical models of the first sources based on the radiotomography of the young Universe.

astro-ph.CO↗

arXiv2025

Distortion of the CMB spectrum by the first molecules of the Dark Ages

The formation of first stars and galaxies at the Cosmic Dawn had been preceded by the chain of primordial chemistry reactions during Dark Ages which generated first molecules, mostly H$_2$, HD, and HeH$^+$, so crucial for first stars to emerge. These molecules absorbed and scattered CMB quanta this way distorting CMB spectrum. Estimate how much bound-bound transitions between the rovibrational levels of H$_2$, HD, and HeH$^+$ molecules contribute to the distortion of CMB spectrum in the standard $Λ$CDM cosmology. We describe the kinetics of the formation of the first molecules with system of 166 chemical reactions involving 20 reagents. The system of differential equations is solved to describe the processes of spontaneous and collisional transitions between rovibrational levels of H$_2$, HD, and HeH$^+$ molecules. The populations of rovibrational levels and optical thickness of the gas in transition lines between these levels were used to estimate the differential brightness produced by the first molecules on the cosmic microwave background. It is shown that the signal from the first molecules in the standard $Λ$CDM cosmology takes the form of an absorption profile in the CMB spectrum and originates from the Dark Ages. H$_2$ shows multiple peaks, peaking at $\sim 10^{-3}$ Jy/sr between 50-120 GHz, with absorption from $300>z>200$. HD has double peaks, reaching $\sim 10^{-5}$ Jy/sr between 40-70 GHz, with absorption from $300>z>30$. HeH$^+$ has no features, peaks at $\sim 10^{-7}$ Jy/sr between 200-800 GHz, and absorbs mainly from $100>z>4$.

astro-ph.CO↗

arXiv2025

Impact of the cosmological expansion on spectral energy density of radiation in the intergalactic medium and once more about Olbers' paradox

We analyse the impact of the expansion of the Universe on the formation of the total spectral energy density of radiation in the intergalactic medium. Assuming the same proper thermal spectrum of sources, we show how the expansion of the Universe changes the nature of the energy distribution of the thermal spectrum: a decrease of the energy density in the Wien range and an increase in the Rayleigh--Jeans range with increasing the redshift of the bulk filled by sources. This is due to the cosmological redshift and the growing contribution of large number of distant sources. The numerical estimations also illustrate the main factors that resolve the Olbers' paradox in the expanding Universe: i) the particle horizon, ii) the finiteness of the volume filled by luminous objects, and iii) the cosmological redshift. Applying the obtained expressions to the epoch of reionization made it possible to estimate the concentration of objects of various classes (stars, globular clusters, dwarf galaxies) necessary for complete reionization of hydrogen at $z=6$. It is shown that even a small part of globular clusters or dwarf galaxies with a thermal spectrum of moderate temperature, from those predicted by Press-Schechter formalism and its improvements, is able to completely reionize hydrogen in the intergalactic medium at $z=6$.

astro-ph.CO↗

arXiv2024

Signal in the Hyperfine Structure Line of the Ground State of Atomic Hydrogen from the Dark Ages as a Cosmological Test

We analyze the formation of the signal in the 21 cm hydrogen line of the Dark Ages ($30\le z\le300$) in different cosmological models and discuss the possibility of its detection by decameter-wavelength radio telescopes. To study the dependence of the intensity and profile of the line on the values of cosmological parameters and physical conditions in the intergalactic medium, the evolution of the global (averaged over the sky) differential brightness temperature in this line was calculated in standard and non-standard cosmological models with different parameters. The standard $Λ$CDM model with cosmological parameters predicts a value of the differential brightness temperature in the center of the absorption line $δT_{br}\approx-35$ mK at $z\approx87$. The frequency of the line at the absorption maximum is 16 MHz, the effective width of the line is $\approx$25 MHz. The depth of the line is moderately sensitive to $Ω_b$ and $H_0$, weakly sensitive to $Ω_{dm}$ and insensitive to other parameters of the standard $Λ$CDM model. However, the line is very sensitive to additional mechanisms of heating or cooling of baryonic matter during the Dark Ages, so it can be an effective test of non-standard cosmological models. In models with decaying and self-annihilating dark matter, as well as with an initial global stochastic magnetic field, the temperature of baryonic matter in this period is higher, the higher the density of these dark matter components and the magnetic field strength. The absorption line becomes shallower, disappears, and turns into emission at values of the component parameters lower than the upper bounds on them, which follow from observational data. The estimations show that such spectral features can be detected by decameter-wavelength radio telescopes in the near future.

astro-ph.CO↗

arXiv2024

On the Possibility of Detecting a Global Signal in the Line of the Hyperfine Structure of Hydrogen from the Dark Ages

We analyze the possibilities of detecting a signal in the hydrogen 21~cm line, which was formed in the early Universe during the the Dark Ages cosmological epoch, using the Ukrainian radio telescopes UTR-2 and GURT of the National Academy of Sciences of Ukraine. As a result of cosmological expansion, this line is shifted to the decameter range of wavelengths ($λ_{obs}\approx18$ m, $ν_{obs}\approx16$ MHz) and is in the band of operational frequencies of these telescopes. The brightness temperature of the predicted sky-averaged global signal ranges from $\sim-0.08$ to $\sim0.02$ K, depending on the cosmological model. Such a faint signal is a challenge even for the world's largest radio telescope in the decameter wavelength range, UTR-2, since the signal level of the foreground synchrotron radiation of the Galaxy at these wavelengths is 20000--40000~K. The paper highlights the peculiarities of spectroscopy at the decameter waves, interfering factors of natural and instrumental origin and the ways of eliminating them in order to reliably detect the signal in the 21~cm line, which can become an important source of information both about the environment in which the first stars and galaxies were born, and about the nature of dark matter particles and the magnitude of primordial magnetic fields. It was concluded that the detection of such a signal using the most sensitive radio telescopes at the decameter wavelength range is possible (with the signal integration over the frequency band of 25~MHz), the detection time will be $\sim50$~days) and can be implemented in the coming years of peace in Ukraine.

astro-ph.CO↗

arXiv2023

Formation of the hydrogen line 21-cm in Dark Ages and Cosmic Dawn: dependences on cosmology and first light

We analyze the formation of the redshifted hyperfine structure line 21-cm of hydrogen atom in the Dark Ages, Cosmic Dawn, and Reionization epochs. The evolution of the global differential brightness temperature in this line was computed to study its dependence on the values of cosmological parameters and physical conditions in the intergalactic medium. Variations of the depth of the Dark Ages absorption line at $z\sim80$ with variations of the cosmological parameters $Ω_b$, $Ω_{cdm}$, $Ω_Λ$, $Ω_K$ and $H_0$ are studied. The standard model with post-Planck parameters predicts a value of the differential brightness temperature in the center of the absorption line $\sim$30-50 mK. The profile of this line can be quite another in the non-standard cosmological models, which include the annihilating or decaying dark matter, a primordial stochastic magnetic field, etc. It can be shallower or be an emission bump instead of an absorption trough. It is also shown that the position and depth of the Cosmic Dawn absorption line formed at 10<z<30, due to the Wouthuysen-Field effect, is mainly defined by the spectral energy distribution of the first sources of light. If reionization occurs at $z_{ri}=7\pm1$, then the differential brightness temperature in the center of this line is $\sim$80 mK. During the reionization, the emission with an amplitude of $\sim$20 mK is possible. It is also shown that the temperature, density, and degree of ionization of the baryonic component are decisive in calculating the intensity of the 21-cm absorption/emission line from these epochs.

astro-ph.CO↗

arXiv2022

Samuil Kaplan and the development of astrophysical research at the Lviv University (dedicated to the 100th anniversary of his birth)

Samuil Kaplan (1921-1978) was a productive and famous astrophysicist. He was affiliated with a number of scientific centers in different cities of former Soviet Union. The earliest 13 years of his career, namely in the 1948-1961 years, he worked in Lviv University in Ukraine (then it was called the Ukrainian Soviet Socialist Republic). In the present paper, the Lviv period of his life and scientific activity is described on the basis of archival materials and his published studies. Kaplan arrived in Lviv in June 1948, at the same month when he obtained the degree of Candidate of science. He was a head of the astrophysics sector at the Astronomical Observatory of the University, was a professor of department for theoretical physics as well as the founder and head of a station for optical observations of artificial satellites of Earth. He was active in the organization of the astronomical observational site outside of the city. During the years in Lviv, Kaplan wrote more than 80 articles and 3 monographs in 9 areas. The focus of his interests at that time was on stability of circular orbits in the Schwarzschild field, on white dwarf theory, on space gas dynamics, and cosmic plasma physics, and turbulence, on acceleration of cosmic rays, on physics of interstellar medium, on physics and evolution of stars, on cosmology and gravitation, and on optical observations of Earth artificial satellites. Some of his results are fundamental for development of theory in these fields as well as of observational techniques. The complete bibliography of his works published during the Lviv period is presented. Respective scientific achievements of Samuil Kaplan are reviewed in the light of the current state of research in these areas.

astro-ph.CO↗

arXiv2022

The first molecules in the intergalactic medium and halos of the Dark Ages and Cosmic Dawn

We study the formation and destruction of the first molecules at the epochs of the Dark Ages and Cosmic Dawn to evaluate the luminosity of the protogalaxy clumps (halos) in the molecular lines. The cosmological recombination is described using the model of an effective three-level atom, while the chemistry of the molecules is examined using the relevant basic kinetic equations. We then studied the effect of collisional and radiative excitation of molecules on the intensity of molecular emission in both warm and hot halos. Using the Planck data on the reionization of the intergalactic medium at z~6-8, we evaluated the upper limits of the light energy density for four models of thermal light from the first sources that appeared in the Cosmic Dawn epoch. Assuming that in the halos, the light energy density may essentially be even higher, we estimated the impact of the light from the first sources on the formation and destruction of the first molecules. We show that the molecules H2 and HD are destroyed by photodissociation processes shortly before the full reionization in the inter-halo medium, in the medium of both types of halos and for all models of the first light. At the same time, the number density of helium hydride ions, HeH+, shows essentially more complicated dependences on the kinetic temperature of halos and the models of the first light. Furthermore, we estimated the differential brightness temperature of the individual halo in the rotational lines of H2, HD and HeH+ molecules at redshifts corresponding to the Dark Ages and Cosmic Dawn epochs. It does not exceed the microkelvin, but its detection may be an important source of information about the physical processes taking place at the beginning of the formation of the first stars and galaxies at the epochs of the Dark Ages and Cosmic Dawn.

astro-ph.CO↗

arXiv2020

Large-scale structures in the $Λ$CDM Universe: network analysis and machine learning

We perform an analysis of the Cosmic Web as a complex network, which is built on a $Λ$CDM cosmological simulation. For each of nodes, which are in this case dark matter halos formed in the simulation, we compute 10 network metrics, which characterize the role and position of a node in the network. The relation of these metrics to topological affiliation of the halo, i.e. to the type of large scale structure, which it belongs to, is then investigated. In particular, the correlation coefficients between network metrics and topology classes are computed. We have applied different machine learning methods to test the predictive power of obtained network metrics and to check if one could use network analysis as a tool for establishing topology of the large scale structure of the Universe. Results of such predictions, combined in the confusion matrix, show that it is not possible to give a good prediction of the topology of Cosmic Web (score is $\approx$ 70 $\%$ in average) based only on coordinates and velocities of nodes (halos), yet network metrics can give a hint about the topological landscape of matter distribution.

astro-ph.CO↗

arXiv2019

White dwarfs as a probe of dark energy

We investigate the radial density distribution of the dynamical dark energy inside the white dwarfs (WDs) and its possible impact on their intrinsic structure. The minimally-coupled dark energy with barotropic equation of state which has three free parameters (density, equation of state and effective sound speed) is used. We analyse how such dark energy affects the mass-radius relation for the WDs because of its contribution to the joint gravitational potential of the system. For this we use Chandrasekhar model of the WDs, where model parameters are the parameter of the chemical composition and the relativistic parameter. To evaluate the dark energy distribution inside a WD we solve the conservation equation in the spherical static metric. Obtained distribution is used to find the parameters of dark energy for which the deviation from the Chandrasekhar model mass-radius relation become non-negligible. We conclude also, that the absence of observational evidence for existence of WDs with untypical intrinsic structure (mass-radius relation) gives us lower limit for the value of effective sound speed of dark energy $c_s^2 \gtrsim 10^{-4}$ (in units of speed of light).

astro-ph.CO↗

arXiv2018

Halos in Dark Ages: formation and chemistry

Formation of halos in the Dark Ages from initial spherical perturbations is analyzed in a four component Universe (dark matter, dark energy, baryonic matter and radiation) in the approximation of relativistic hydrodynamics. Evolution of density and velocity perturbations of each component is obtained by integration of a system of nine differential equations from $z=10^8$ up to virialization, which is described phenomenologically. It is shown that the number density of dark matter halos with masses $M\sim10^8-10^9\,\mathrm{M_{\odot}}$ virialized at $z\sim10$ is close to the number density of galaxies in comoving coordinates. The dynamical dark energy of classical scalar field type does not significantly influence the evolution of the other components, but dark energy with a small value of effective sound speed can affect the final halo state. Simultaneously, the formation/dissociation of the first molecules have been analyzed in the halos which are forming. The results show that number densities of molecules $\rm{H_2}$ and $\rm{HD}$ at the moment of halo virialization are $\sim10^3$ and $\sim400$ times larger, respectively, than on a uniformly expanding background. It is caused by increased density and rates of reactions at quasilinear and nonlinear evolution stages of density and velocity of the baryonic component of halos. It is shown also that the temperature history of the halo is important for calculating the concentration of molecular ions with low binding energy. So, in a halo with virial temperature $\sim10^5$ K the number density of the molecular ion HeH$^+$ is approximately 100 times smaller than that on the cosmological background.

astro-ph.CO↗

arXiv2018

Century of $Λ$

The cosmological constant was proposed 100 years ago in order to make the model of static Universe, imagined then by most scientists, possible. Today it is the main candidate for the physical essence causing the observed accelerated expansion of our Universe. But, as well as a hundred years ago, its nature is unknown. This paper is devoted to the story of invention of $Λ$ by Albert Einstein in 1917, rejection of it by him in 1931 and returning of it into the science by other scientists during the century.

physics.hist-ph↗

arXiv2016

Evolution of density and velocity profiles of dark matter and dark energy in spherical voids

We analyse the evolution of cosmological perturbations which leads to the formation of large isolated voids in the Universe. We assume that initial perturbations are spherical and all components of the Universe (radiation, matter and dark energy) are continuous media with perfect fluid energy--momentum tensors, which interact only gravitationally. Equations of the evolution of perturbations for every component in the comoving to cosmological background reference frame are obtained from equations of energy and momentum conservation and Einstein's ones and are integrated numerically. Initial conditions are set at the early stage of evolution in the radiation-dominated epoch, when the scale of perturbation is much larger than the particle horizon. Results show how the profiles of density and velocity of matter and dark energy are formed and how they depend on parameters of dark energy and initial conditions. In particular, it is shown that final matter density and velocity amplitudes change within range $\sim$4-7\% when the value of equation-of-state parameter of dark energy $w$ vary in the range from --0.8 to --1.2, and change within $\sim$1\% only when the value of effective sound speed of dark energy vary over all allowable range of its values.

astro-ph.CO↗

arXiv2016

Evolution of density and velocity profiles of matter in large voids

We analyse the evolution of cosmological perturbations which leads to the formation of large voids in the distribution of galaxies. We assume that perturbations are spherical and all components of the Universe - radiation, matter and dark energy - are continuous media with ideal fluid energy-momentum tensors, which interact only gravitationally. Equations of the evolution of perturbations in the comoving to cosmological background reference frame for every component are obtained from equations of conservation and Einstein's ones and are integrated by modified Euler method. Initial conditions are set at the early stage of evolution in the radiation-dominated epoch, when the scale of perturbation is mush larger than the particle horizon. Results show how the profiles of density and velocity of matter in spherical voids with different overdensity shells are formed.

astro-ph.CO↗