Ginnungagap is a fully parallel (MPI+OpenMP) code designed to generate cosmological initial conditions for simulations involving very large numbers of particles. It operates in several modes, including the creation of initial conditions with either uniform or spatially varying resolution (for "zoom-in" simulations). The initial conditions can be fully random or derived by extending the resolution of existing ones while preserving the large-scale structures. Ginnungagap is open source and modular, consisting of a collection of independent tools that can be used for a variety of tasks. In this paper, we describe the main features of Ginnungagap and present test results for different types of simulations prepared with it.

We analyze the possibility of using observational data on nearby dwarf galaxies—their luminosity functions and spatial distributions—to constrain deviations of the cosmological power spectrum from the standard one. Specifically, we consider a cosmological model with a “bump” in the power spectrum at a wavelength of 1.3 Mpc and a dimensionless amplitude 𝒜 = 2.0 . Such a spectrum is motivated by observations of an excess number of galaxies at high redshifts. The bump leads to a noticeable increase in the luminosity function in the range - 13 > M_B > - 17 at z = 0 . Comparison with observations constrains the bump amplitude to 𝒜 < 0.25 at a 3σ significance level for a wavelength of 1.3 Mpc. For wavelengths smaller than 0.8 Mpc, the bump manifests only in the luminosity function of dwarfs with M_B > - 14 .

The initial power spectrum of density perturbations, generated during the inflationary epoch, is now constrained by observations on scales of λ>5 Mpc and has a power-law form. The peculiarities of the inflationary process can lead to the appearance of non-power-law contributions to this spectrum, such as peaks. The exact size and shape of the peak cannot be predicted in advance. In this paper, we propose methods for searching for such peaks in the region of the spectrum with λ<5 Mpc. Perturbations on these scales enter the nonlinear stage at z≳ 10 , which is now becoming accessible to observations. Our studies of numerical models of the large-scale structure with peaks in the initial spectrum have shown that spectral features on scales of λ>0.1 Mpc manifest in the clustering of galaxies, as well as affect their mass function, sizes, and density. Studying these characteristics of distant galaxies will allow us to constrain cosmological models with peaks.

Investigations of trajectories of various objects orbiting the Milky Way (MW) halo with modern precision, achievable in observations by Gaia, requires sophisticated, nonstationary models of the Galactic potential. In this paper we analyze the evolution of the spherical harmonics expansion of MW analogs potential in constrained simulations of the Local Group (LG) from the HESTIA suite. We find that at distances r >= 100 kpc, the nonspherical part of the potential demonstrates a significant impact of the environment: ignoring the mass distribution outside the virial radius of the MW results in > 20% errors in the potential quadrupole at these distances. Account of the environment results in a noticeable change of the angular momenta of objects orbiting MW analogs. Spherical harmonics vary significantly during the last 6 Gyr. We attribute variations of the potential at r >= 30 kpc to the motions of MW satellites and LG galaxies. We also predict that the nonsphericity of the real MW potential should grow with distance in the range r(vir) < r < 500 kpc, since all realizations of simulated MW-like objects demonstrate such a trend.

In the current galaxy formation paradigm, collisions play a crucial role. A fraction of galaxy collisions results in fly-by events, and a galaxy that has passed through another galaxy is called a backsplash galaxy. Such fly-by events are of particular interest for explaining the quenching of isolated galaxies. One signature of backsplash galaxies is that they have high velocities relative to their environment, since they do not move in the same flow as surrounding galaxies. This feature can be studied in simulations, but it is also useful to have a theory that can predict the velocities of backsplash galaxies. In this paper, we develop such a theory based on the Zel'dovich approximation and use it to determine the maximal expected velocity of a backsplash galaxy in a given volume.

We investigate the prospects for experimentally determining the Sun’s angular momentum by measuring the gravitomagnetic frequency shift of signals exchanged by two satellites in heliocentric orbits. When using modern optical clocks with stability of ∼10–18, the accuracy of such an experiment can reach 5

The initial power spectrum of density perturbations, generated during the inflationary epoch, is now constrained by observations on scales λ>5 Mpc and has a power-law form. The peculiarities of the inflationary process can lead to the appearance of non-power-law contributions to this spectrum, such as peaks. The exact size and shape of the peak cannot be predicted in advance. In this paper, we propose methods for searching for such peaks in the region of the spectrum with λ<5 Mpc. Perturbations on these scales enter the nonlinear stage at z≳10, which is now becoming accessible to observations. Our studies of numerical models of large-scale structure with peaks in the initial spectrum have shown that spectral features on scales with λ>0.1 Mpc manifest in the clustering of galaxies, as well as affect their mass function, sizes, and density. Studying these characteristics of distant galaxies will allow us to constrain cosmological models with peaks.

We consider three cosmological models with non-power-law spectra of primordial density perturbations and test them against Lambda CDM in density profiles of dark matter halos. We found that, despite the significant difference in initial conditions, the mean density profiles of all models are still close to the Navarro-FrenkWhite one, albeit with some dispersion. We demonstrate that the density profile slopes in the innermost part of the halo have a significant evolution with z, which can be used to identify the cosmological model. We also present a toy model resulting in the appearance of core in the central part of the dark matter halo.

For the numerical model in the range of redshifts 0 ⩽ z ⩽ 9 , we examined the properties and evolution of dark matter halos using a previously proposed method of compact analysis that allows separating the influence of random and regular factors on the main characteristics of the dark matter halo. In the investigated range of redshifts, a monotonic evolution of the average values of the basic parameters of small halo structures into a central massive object is observed through sequential hierarchical merging. These basic parameters include the circular velocity v_c , the parameter w_c = v_c/r , and the mass. In the range 3 ⩽ z ⩽ 9 , the parameters evolve slowly, while in the range 0 ⩽ z ⩽ 3 , they evolve rapidly. The evolution of the dark matter halos formed before reionization is characterized by a slow change in their average characteristics and the properties of the halo outskirts. The important role of early-formed massive structural elements is emphasized.

This paper addresses the scientific and technical prospects and potential directions for the development of subterahertz astronomy in the Russian Federation. The concept of creating subterahertz instruments in the form of a universal compact antenna array for placement on the territory of the Russian Federation is proposed. It is possible to implement several space projects in the subterahertz range using such an antenna array, including a space interferometer and a telescope on the surface of the Moon. Ground-based compact antenna arrays will be able to act as a support for the very long baseline interferometer mode of the Millimetron observatory.

Modified matter power spectra with approximately Gaussian bump on sub-Mpc scales can be a result of a complex inflation. We consider five spectra with different Gaussian amplitudes A and locations k_0 and run N-body simulations in a cube (5 Mpc/h)^3 at z>8 to reveal the halo mass functions and their evolution with redshift. We have found that the Sheth-Tormen formula provides a good approximation to a such kind of halo mass functions. In the considered models the dark matter halo formation starts much more earlier than in ΛCDM, which in turn can result in an earlier star formation and a nuclear activity in galaxies and can be detected and tested by, e.g., JWST. At z=0 the halo mass functions are hardly distinguishable from the standard ΛCDM, therefore the models with the bumpy spectra can be identified in observations by their excess in number of bright sources at high redshift only.

We explore the possibility of using measurements of the gravitational redshift effect as a means to constrain wave dark matter—a class of models in which the dark matter is accounted for by light scalar particles that behave like classical waves. We construct a mathematical framework that is appropriate for clock comparison experiments with remote clocks and can be used to determine the values of the coupling constants of such dark matter with particles of the Standard Model. Using this framework, we consider an experiment to detect dark matter of the Galactic halo using two satellites equipped with accurate and stable atomic clocks and placed into elliptical heliocentric orbits. We demonstrate that, in most cases, the accuracy of this experiment turns out to be not better than that of ground-based experiments with colocated clocks. The limitation of the accuracy of the space-based experiment is found to be due to the non-relativistic Doppler compensation system, required when using moving clocks, which decreases the amplitude of the useful signal. Possible solutions to this problem are discussed.

We present a signal -foreground separation algorithm for filtering observational data to extract spectral distortions of the cosmic microwave background (CMB). Our linear method, called the least response method (LRM), is based on the idea of simultaneously minimizing the response to all possible foregrounds with poorly defined spectral shapes and random noise while maintaining a constant response to the signal of interest. This idea was introduced in detail in our previous paper. Here, we have expanded our analysis by taking into consideration all the main foregrounds. We draw a detailed comparison between our approach and the moment internal linear combination method, which is a modification of the internal linear combination technique previously used for CMB anisotropy maps. We demonstrate advantages of LRM and evaluate the prospects for measuring various types of spectral distortions. Besides, we show that LRM suggests the possibility of its improvements if we use an iterative approach with sequential separation and partial subtraction of foreground components from the observed signal. In addition, we estimate the optimal temperature that the telescope's optical system should have in order to detect the chemical type mu distortions. We present a design of an instrument where, according to our estimates, the optimal contrast between its thermal emission and the CMB allows us to measure such distortions.

A surprisingly large number of galaxies with masses of ~109–1010$${{M}_{ \odot }}$$ at redshifts of $$z \geqslant 9$$ are discovered with the James Webb Space Telescope. A possible explanation for the increase in the mass function can be the presence of a local maximum (bump) in the power spectrum of density perturbations on the corresponding scale. In this paper, it is shown that simultaneously with the growth of the mass function, galaxies from the bump region must have a higher density (compactness) compared to cosmological models without a bump. These more compact galaxies have been partially included in larger galaxies and have been subjected to tidal gravitational disruption. They have been less destructed than “ordinary” galaxies of the same mass, and some of them could survive to z = 0 and persist on the periphery of some galaxies. The formation and evolution of compact halos in a cube with a volume of (47 Mpc)3 with (1024)3 dark matter particles in the redshift range from 120 to 0 have been numerically simulated and observational implications of the presence of such galaxies in the current Universe have been discussed.

This is the second paper in a series presenting the results from a 500 $h^{-1}$Mpc large constrained hydro-dynamical simulation of the local Universe (SLOW). The initial conditions are based on peculiar velocities derived from the CosmicFlows-2 catalogue. The inclusion of galaxy formation treatment, allows to directly predict observable properties of the Intra-Cluster Medium (ICM) within galaxy clusters. Comparing the properties of observed galaxy clusters within the local Universe with the properties of their simulated counterparts, enables us to assess the effectiveness of the initial condition constraints in accurately replicating the non-linear properties of the largest, collapsed objects within the simulation. Based on the combination of several, publicly available surveys, we identified 45 local Universe galaxy clusters in SLOW, including the 13 most massive from the Planck SZ catalog and 70% of those with $M_{500} > 2\times 10^{14}$ M$_{\odot}$. We then derived the probability of the cross identification based on mass, X-ray luminosity, temperature and Compton-y by comparing it to a random selection. In relation to previous constrained simulations of the local volume, we found in SLOW a much larger amount of replicated galaxy clusters, where their simulation based mass prediction falls within the uncertainties of the observational mass estimates. Comparing the median observed and simulated masses of our cross identified sample allows to independently deduce a hydrostatic mass bias of $(1-b)\approx0.87$. The SLOW constrained simulation of the local Universe faithfully reproduces numerous fundamental characteristics of the galaxy clusters within our local neighbourhood, opening a new avenue for studying the formation and evolution of a large set of individual galaxy clusters as well as testing our understanding of physical processes governing the ICM.

ABSTRACT A simple model of spherical dark matter haloes is proposed in terms of two structure functions. In the framework of this model, we analysed the properties of 450 000 simulated haloes with masses Mvir > 1011 M⊙. We compared the mean characteristics of simulated haloes with the corresponding characteristics of 641 galaxies and found that the basic characteristics of the two populations are similar but that their probability distribution functions are different. This disagreement may be caused by technical factors such as the halo identification algorithm, which does not reproduce the observed dark matter haloes correctly.

Recent studies based on numerical models of the Local Group predict the existence of field haloes and galaxies that have visited both the Milky Way and M31 in the past, called Hermeian haloes. We extend this analysis beyond the Local Group using two high-resolution dark matter-only N-body simulations from the MultiDark suite. We define Hermeian haloes as field haloes which had close interactions with two other more massive field haloes in the past, called targets. We find that Hermeian haloes are a more extreme example of field haloes with interactions in the past than the well-known backsplash haloes that experienced only one interaction. Compared to backsplashers, Hermeians have more concentrated density profiles and tend to occupy more overdense regions. They also have higher velocities relative to their target haloes and relative to their neighbours within 1 h−1Mpc. Hermeian haloes can be found around every halo in the simulation (if the resolution is sufficient) and make up 0.4 to 2.3 per cent of the total number of field haloes (for haloes more massive than 1010h−1M⊙ and 3.3×107h−1M⊙, respectively), increasing to 10 per cent in overdense regions. They tend to be distributed close to the line connecting their targets, which may help to identify Hermeian haloes in observations. We also identify Local Group analogues in the simulation and find that about one-third (15 out of 49) of them contain Hermeian haloes if the distance between the two main haloes is below 1 h−1Mpc.

Context: Several observations of the local Universe (LU) point towards the existence of very prominent structures. The presence of massive galaxy clusters and local super clusters on the one hand, but also large local voids and under-densities on the other hand. However, it is highly non trivial to connect such different observational selected tracers to the underlying dark matter (DM) distribution. Methods (abridged): We used a 500 Mpc/h large constrained simulation of the LU with initial conditions based on peculiar velocities derived from the CosmicFlows-2 catalogue and follow galaxy formation physics directly in the hydro-dynamical simulations to base the comparison on stellar masses of galaxies or X-ray luminosity of clusters. We also used the 2668 Mpc/h large cosmological box from the Magneticum simulations to evaluate the frequency of finding such anomalies in random patches within simulations. Results: We demonstrate that haloes and galaxies in our constrained simulation trace the local DM density field very differently. Thereby, this simulation reproduces the observed 50% under-density of galaxy clusters and groups within the sphere of ~100 Mpc when applying the same mass or X-ray luminosity limit used in the observed cluster sample (CLASSIX), which is consistent with a ~1.5$\sigma$ feature. At the same time, the simulation reproduces the observed over-density of massive galaxy clusters within the same sphere, which on its own also corresponds to a ~1.5$\sigma$ feature. Interestingly, we find that only 44 out of 15635 random realizations (i.e. 0.28%) are matching both anomalies, making the LU to be a ~3$\sigma$ environment. We finally compared a mock galaxy catalogue with the observed distribution of galaxies in the LU, finding also a match to the observed factor of two over-density at ~16 Mpc as well as the observed 15% under-density at ~40 Mpc distance.

Context. Several observations of the Local Universe point toward the existence of very prominent structures: massive galaxy clusters and local superclusters on the one hand, but also large local voids and underdensities on the other. However, it is highly nontrivial to connect such different observational selected tracers to the underlying dark matter (DM) distribution. Aims. Therefore, constructing mock catalogs of such observable tracers using cosmological hydrodynamics simulations is needed. These simulations have to follow galaxy formation physics and also have to be constrained to reproduce the Local Universe. Such constraints should be based on observables that directly probe the full underlying gravitational field, such as the observed peculiar velocity field, to provide an independent test on the robustness of these distinctive structures. Methods. We used a 500 h −1 Mpc constrained simulation of the Local Universe to investigate the anomalies in the local density field, as found in observations. Constructing the initial conditions based on peculiar velocities derived from the CosmicFlows-2 catalog makes the predictions of the simulations completely independent from the distribution of the observed tracer population, and following galaxy formation physics directly in the hydrodynamics simulations also allows the comparison to be based directly on the stellar masses of galaxies or X-ray luminosity of clusters. We also used the 2668 h −1 Mpc large cosmological box from the Magneticum simulations to evaluate the frequency of finding such anomalies in random patches within simulations. Results. We demonstrate that halos and galaxies in our constrained simulation trace the local dark matter density field very differently. Thus, this simulation reproduces the observed 50% underdensity of galaxy clusters and groups within the sphere of ≈100 Mpc when applying the same mass or X-ray luminosity limit used in the observed cluster sample (CLASSIX), which is consistent with a ≈1.5 σ feature. At the same time, the simulation reproduces the observed overdensity of massive galaxy clusters within the same sphere, which on its own also corresponds to a ≈1.5 σ feature. Interestingly, we find that only 44 out of 15 635 random realizations (i.e., 0.28%) match both anomalies, thus making the Local Universe a ≈3 σ environment. We finally compared a mock galaxy catalog with the observed distribution of galaxies in the Local Universe, finding a match to the observed factor of 2 overdensity at ∼16 Mpc as well as the observed 15% underdensity at ∼40 Mpc. Conclusions. Constrained simulations of the Local Universe which reproduce the main features of the local density field open a new window for local field cosmology, where the imprint of the specific density field and the impact on the bias through the observational specific tracers can be investigated in detail.