Dark Matter Clumps Research Articles

Black Hole in a Radiation-Dominated Universe

We study a black hole in an expanding Universe during the radiation-dominated stage. In the case when the black hole radius is much smaller than the cosmological horizon, we present a solution of the Einstein equations for the metric, the matter density and velocity distributions. At distances much smaller than the cosmological horizon the solution features quasi-stationary accretion, while at large distances the solution asymptotes the homogeneous radiation dominated FLRW. We discuss how our solution is related to the McVittie solution. The obtained results can be applied, in particular, for the formation of dark matter density spikes around primordial black holes, and for the evolution of dark matter clumps during the radiation-dominated stage.

Scalar dark matter clumps with angular momentum

The behavior of light scalar dark matter has been a subject of much interest recently as it can lead to interesting small scale behavior. In particular, this can lead to the formation of gravitationally bound clumps for these light scalars, including axions. In ref. [1] we analyzed the behavior of these clumps, assuming spherical symmetry, allowing for both attractive and repulsive self-interactions. There is a maximum allowed mass for the clumps in the case of attractive interactions, and a minimum radius for the clumps in the case of repulsive interactions, which is saturated at large mass. Here we extend this work to include non-spherically symmetric clumps. Since the system tries to re-organize into a BEC ground state, we consider configurations with a conserved non-zero angular momentum, and construct minimum energy configurations at fixed particle number and fixed angular momentum. We find generalizations of the previous spherically symmetric results. In particular, while there is still a maximum mass for the attractive case, its value increases with angular momentum. Also, the minimum radius in the repulsive case is raised to higher radii. We clarify how a recent claim in the literature of an upper bound on angular momentum is due to inaccurate numerics. In a forthcoming paper we shall investigate the possibility of resonance of axion clumps into both visible and hidden sector photons, and analyze how the altered mass and radius from non-zero angular momentum affects the resonance.

Properties of Merger Shocks in Merging Galaxy Clusters

Abstract X-ray shocks and radio relics detected in the cluster outskirts are commonly interpreted as shocks induced by mergers of subclumps. We study the properties of merger shocks in merging galaxy clusters, using a set of cosmological simulations for the large-scale structure formation of the universe. As a representative case, we focus on the simulated clusters that undergo almost head-on collisions with mass ratio ∼2. Due to the turbulent nature of the intracluster medium, shock surfaces are not smooth, but composed of shocks with different Mach numbers. As the merger shocks expand outward from the core to the outskirts, the average Mach number, , increases in time. We suggest that the shocks propagating along the merger axis could be manifested as X-ray shocks and/or radio relics. The kinetic energy through the shocks, F ϕ , peaks at ∼1 Gyr after their initial launching, or at ∼1–2 Mpc from the core. Because of the Mach number dependent model adopted here for the cosmic-ray (CR) acceleration efficiency, their CR-energy-weighted Mach number is higher with , compared to the kinetic-energy-weighted Mach number, . Most energetic shocks are to be found ahead of the lighter dark matter (DM) clump, while the heavier DM clump is located on the opposite side of clusters. Although our study is limited to the merger case considered, the results such as the means and variations of shock properties and their time evolution could be compared with the observed characteristics of merger shocks, constraining interpretations of relevant observations.

Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk.

Our Galaxy is thought to have an active evolutionary history, dominated over the past ten billion years or so by star formation, the accretion of cold gas and, in particular, the merging of clumps of baryonic and dark matter. The stellar halo-the faint, roughly spherical component of the Galaxy-reveals rich 'fossil' evidence of these interactions, in the form of stellar streams, substructures and chemically distinct stellar components. The effects of interactions with dwarf galaxies on the content and morphology of the Galactic disk are still being explored. Recent studies have identified kinematically distinct stellar substructures and moving groups of stars in our Galaxy, which may have extragalactic origins. There is also mounting evidence that stellar overdensities (regions with greater-than-average stellar density) at the interface between the outer disk and the halo could have been caused by the interaction of a dwarf galaxy with the disk. Here we report a spectroscopic analysis of 14 stars from two stellar overdensities, each lying about five kiloparsecs above or below the Galactic plane-locations suggestive of an association with the stellar halo. We find that the chemical compositions of these two groups of stars are almost identical, both within and between these overdensities, and closely match the abundance patterns of stars in the Galactic disk. We conclude that these stars came from the disk, and that the overdensities that they are part of were created by tidal interactions of the disk with passing or merging dwarf galaxies.

Compact clumps of dark matter near the solar surface

The solar surface oscillations observed in the Crimean Astrophysical Observatory (CrAO) at the frequency 104.1890 μHz and in the Solar and Heliospheric Observatory (SoHO) at 220.72 μHz are considered as a result of existence of Compact Clumps of Dark Matter (CCDM) at orbits near the solar surface. These CCDM have to emit Gravitational Waves (GW) which are estimated to be the most intensive ones expected in the vicinity of the Earth and can be easily detected in the near future by means of the Evolved Laser Interferometer Space Antenna (eLISA). In addition to CCDMCrAO and CCDMSoHO some other CCDM may exist in the solar structure. It is shown that GW radiated by most of these CCDM could be detected by eLISA even if the respective solar surface oscillations are too small to be observed.

Tidal deformability of boson stars and dark matter clumps

In this work we consider minimally-coupled boson stars immersed in a tidal environment and compute their tidal deformability to leading order. We also describe an approximate correspondence between Newtonian boson star configurations (described by the Schrödinger–Poisson equations) and dynamical dark matter clumps (described by the collisionless Boltzmann equation). This allows us to map our results for the tidal deformability of boson stars to approximate statements for dark matter clumps.

Linear perturbation theory for tidal streams and the small-scale CDM power spectrum

Tidal streams in the Milky Way are sensitive probes of the population of dark-matter subhalos predicted in cold-dark-matter (CDM) simulations. We present a new calculus for computing the effect of subhalo fly-bys on cold tidal streams based on the action-angle representation of streams. The heart of this calculus is a line-of-parallel-angle approach that calculates the perturbed distribution function of a given stream segment by undoing the effect of all impacts. This approach allows one to compute the perturbed stream density and track in any coordinate system in minutes for realizations of the subhalo distribution down to 10^5 Msun, accounting for the stream's internal dispersion and overlapping impacts. We study the properties of density and track fluctuations with suites of simulations. The one-dimensional density and track power spectra along the stream trace the subhalo mass function, with higher-mass subhalos producing power only on large scales, while lower mass subhalos cause structure on smaller scales. The time-dependence of impacts and of the evolution of the stream after an impact gives rise to bispectra. We further find that tidal streams are essentially corrugated sheets in the presence of subhalo perturbations: different projections of the track all reflect the same pattern of perturbations, facilitating their observational measurement. We apply this formalism to density data for the Pal 5 stream and make a first rigorous determination of 10^{+11}_{-6} dark-matter subhalos with masses between 3x10^6 and 10^9 Msun within 20 kpc from the Galactic center (corresponding to 1.4^{+1.6}_{-0.9} times the number predicted by CDM-only simulations or to f_{sub}(r<20 kpc) ~ 0.2%). Improved data will allow measurements of the subhalo mass function down to 10^5 Msun, thus definitively testing whether dark matter clumps on the smallest scales relevant for galaxy formation.

Gravitational microlensing as a probe for dark matter clumps

Extended dark matter (DM) substructures may play the role of microlenses in the Milky Way and in extragalactic gravitational lens systems (GLSs). We compare microlensing effects caused by point masses (Schwarzschild lenses) and extended clumps of matter using a simple model for the lens mapping. A superposition of the point mass and the extended clump is also considered. For special choices of the parameters, this model may represent a cusped clump of cold DM, a cored clump of self-interacting dark matter (SIDM) or an ultra compact minihalo of DM surrounding a massive point-like object. We built the resulting micro-amplification curves for various parameters of one clump moving with respect to the source in order to estimate differences between the light curves caused by clumps and by point lenses. The results show that it may be difficult to distinguish between these models. However, some region of the clump parameters can be restricted by considering the high amplification events at the present level of photometric accuracy. Then we estimate the statistical properties of the amplification curves in extragalactic GLSs. For this purpose, an ensemble of amplification curves is generated yielding the autocorrelation functions (ACFs) of the curves for different choices of the system parameters. We find that there can be a significant difference between these ACFs if the clump size is comparable with typical Einstein radii; as a rule, the contribution of clumps makes the ACFs less steep.

Flaring of tidally compressed dark-matter clumps

We explore the physics and observational consequences of tidal compression events (TCEs) of dark-matter clumps (DMCs) by supermassive black holes (SMBHs). Our analytic calculations show that a DMC approaching a SMBH much closer than the tidal radius undergoes significant compression along the axis perpendicular to the orbital plane, shortly after pericenter passage. For DMCs composed of self-annihilating dark-matter particles, we find that the boosted DMC density and velocity dispersion lead to a flaring of the annihilation rate, most pronounced for a velocity- dependent annihilation cross section. If the end products of the annihilation are photons, this results in a gamma-ray flare, detectable (and possibly already detected) by the Fermi telescope for a range of model parameters. If the end products of dark-matter annihilation are relativistic electrons and positrons and the local magnetic field is large enough, TCEs of DMCs can lead to flares of synchrotron radiation. Finally, TCEs of DMCs lead to a burst of gravitational waves, in addition to the ones radiated by the orbital motion alone, and with a different frequency spectrum. These transient phenomena provide interesting new avenues to explore the properties of dark matter.

Can the dark matter annihilation signal be significantly boosted by substructures?

A very general cosmological consideration suggests that, along with galactic dark matter halos, much smaller dark matter structures may exist. These structures are usually called `clumps', and their mass extends to 10−6 M⊙ or even lower. The clumps should give the main contribution into the signal of dark matter annihilation, provided that they have survived until the present time. Recent observations favor a cored profile for low-mass astrophysical halos. We consider cored clumps and show that they are significantly less firm than the standard NFW ones. In contrast to the standard scenario, the cored clumps should have been completely destroyed inside ∼ 20 kpc from the Milky Way center. The dwarf spheroidals should not contain any dark matter clumps. On the other hand, even under the most pessimistic assumption about the clump structure, the clumps should have survived in the Milky Way at a distance exceeding 50 kpc from the center, as well as in low-density cosmic structures. There they significantly boost the dark matter annihilation. We show that at least 70% of the clumps endured the primordial structure formation should still exist untouched in the present-day Universe.

On the matter–antimatter asymmetry

Although the big bang should have produced equal amounts of matter and antimatter, there is evidence that the universe does not contain significant amounts of antimatter. The usual explanations for this matter–antimatter asymmetry involve finding causes for Sakharov’s three conditions to be satisfied. However, if the composite photon theory is correct, antimatter galaxies should appear to us as dark matter, neither emitting light (that we can detect) or reflecting ordinary light. Thus the presence of antimatter galaxies may be harder to detect than previously thought. The large clumps of dark matter that have been observed by weak gravitation lensing could be clusters of antimatter galaxies. “Dark photons,” that are hypothesized to cause self-interactions between dark matter particles, are identified as antiphotons in the composite photon theory. The possibility of a patchwork universe, that had been previously excluded, is also re-examined.

Dark matter may feel a "dark force" that the rest of the Universe does not

Image from colliding galaxies hints at nongravitational attractions between dark matter clumps.

SUSY dark matter annihilation in the Galactic halo

Neutralino annihilation in the Galactic halo is the most definite observational signature proposed for indirect registration of the SUSY Dark Matter (DM) candidate particles. The corresponding annihilation signal (in the form of gamma-rays, positrons and antiprotons) may be boosted for one or three orders of magnitude due to the clustering of cold DM particles into the small-scale and very dense self-gravitating clumps. We discuss the formation of these clumps from the initial density perturbations and their successive fate in the Galactic halo. Only a small fraction of these clumps, ∼ 0.1%, in each logarithmic mass interval Δ log M ∼ 1 survives the stage of hierarchical clustering. We calculate the probability of surviving the remnants of dark matter clumps in the Galaxy by modelling the tidal destruction of the small-scale clumps by the Galactic disk and stars. It is demonstrated that a substantial fraction of clump remnants may survive through the tidal destruction during the lifetime of the Galaxy. The resulting mass spectrum of survived clumps is extended down to the mass of the core of the cosmologically produced clumps with a minimal mass. The survived dense remnants of tidally destructed clumps provide an amplification (boosting) of the annihilation signal with respect to the diffuse DM in the Galactic halo. We describe the anisotropy of clump distribution caused by the tidal destruction of clumps in the Galactic disk.

Statistics of light curves of a distant source microlensed by a system of point and extended masses

Gravitational microlensing of a distant source by a stochastic system of point masses (stars) and extended masses that model dark-matter clumps is analyzed. The size of each clump is set to be equal to 5 or 10 Einstein radii. A total of 100 realizations of the magnification map are generated under the assumption of a spatially homogeneous distribution of lenses with the Salpeter mass function for each set of parameters (optical microlensing depth and the clump size). Autocorrelation functions of light curves for different relative contributions of clumps to the total microlensing optical depth σtot = 0.3 are calculated on this basis. It is demonstrated that the dependence of autocorrelation functions on the optical depth of clumps is nonmonotonous.

Disc dark matter in the Galaxy and potential cycles of extraterrestrial impacts, mass extinctions and geological events

A cycle in the range of 26–30Myr has been reported in mass extinctions, and terrestrial impact cratering may exhibit a similar cycle of 31 ± 5Myr. These cycles have been attributed to the Sun’s vertical oscillations through the Galactic disc, estimated to take from ∼30 to 42Myr between Galactic plane crossings. Near the Galactic mid-plane, the Solar system’s Oort Cloud comets could be perturbed by Galactic tidal forces, and possibly a thin dark matter (DM) disc, whichmightproduceperiodiccometshowersandextinctionsontheEarth.PassageoftheEarth through especially dense clumps of DM, composed of Weakly Interacting Massive Particles (WIMPs) in the Galactic plane, could also lead to heating in the core of the planet through capture and subsequent annihilation of DM particles. This new source of periodic heating in the Earth’s interior might explain a similar ∼30Myr periodicity observed in terrestrial geologic activity, which may also be involved in extinctions. These results suggest that cycles of geological and biological evolution on the Earth may be partly controlled by the rhythms of Galactic dynamics.

A unified solution to the small scale problems of the ΛCDM model

We study, by means of the model proposed in Del Popolo (2009), the effect of baryon physics on the small scale problems of the CDM model. We show that, using this model, the cusp/core problem, the missing satellite problem (MSP), the Too Big to Fail (TBTF) problem, and the angular momentum catastrophe can be reconciled with observations. Concerning the cusp/core problem, the interaction among dark matter (DM) and baryonic clumps of 1% the mass of the halo, through dynamical friction (DF), is able to flatten the inner cusp of the density profiles. We moreover assume that haloes form primarily through quiescent accretion, in agreement with the spherical collapse model (SCM)-secondary infall model (SIM) prescriptions. The results of this paper follow from the two assumptions above.Concerning the MSP and TBTF problem, applying to the Via Lactea II (VL2) subhaloes a series of corrections similar to those of Brooks et al. (2013), namely applying a Zolotov et al. (2012)-like correction obtained with our model, and further correcting for the UV heating and tidal stripping, we obtain that the number of massive, luminous satellites is in agreement with the number observed in the MW. The model also produces an angular momentum distribution in agreement with observations, that is with the distribution of the angular spin parameter and angular momentum of the dwarfs studied by van den Bosch, Burkert, & Swaters (2001). In conclusion, the small scale problems of the CDM model can all be solved by introducing baryon physics.

Gamma-ray evidence for dark matter clumps

We discuss the possibility of identification of point-like gamma-ray sources (PGS) with small scale dark matter (DM) clumps in our Galaxy. Gamma-rays are supposed to originate from annihilation of DM particles in the clumps, where annihilation rate is supposed to be enhanced, besides higher density, due to smaller relative velocities $v$ of DM particles. We parameterized the annihilation cross section $\sigma_\text{ann}(v)$ in the form of an arbitrary power law dependence on the relative velocity $v$ with/without factor of Sommerfeld-Gamow-Sakharov, implying existence of a new Coulomb-like interaction. Adopting different parameters of cross section and clump, satisfying condition $\Omega\lesssim 0.2$ on density of DM particles of question, they are constrained from comparison with Fermi/LAT data on unidentified PGS as well as on diffuse $\gamma$-radiation; results are applied to concrete DM candidates. Such analysis is found to be sensitive enough to existing uncertainty in the density profiles of DM in the clump what can provide a tool for their test. Also we discuss possibilities when gamma-radiating clump changes visibly its position on celestial sphere and it is seen as a spatially extended gamma-source (EGS), what can be probed in future experiments like Gamma-400.

Gamma-Ray Effects of Dark Forces in Dark Matter Clumps

Existence of new gauge U(1) symmetry possessed by dark matter (DM) particles implies the existence of a new Coulomb-like interaction, which leads to Sommerfeld-Gamow-Sakharov enhancement of dark matter annihilation at low relative velocities. We discuss a possibility to put constraints on such dark forces of dark matter from the observational data on the gamma radiation in our Galaxy. Gamma-rays are supposed to originate from annihilation of DM particles in the small scale clumps, in which annihilation rate is supposed to be enhanced, besides higher density, due to smaller relative velocitiesvof DM particles. For possible cross sections, mass of annihilating particles, masses of clumps, and the contribution of annihilating particles in the total DM density we constrain the strength of new dark long range forces from comparison of predicted gamma-ray signal with Fermi/LAT data on unidentified point-like gamma-ray sources (PGS) as well as on diffuseγ-radiation. Both data on diffuse radiation and data on PGS put lower constraints on annihilation cross section at any dark interaction constant, where diffuse radiation provides stronger constraint at smaller clump mass. Density of annihilating DM particles is conventionally supposed to be defined by the frozen annihilation processes in early Universe.

Formation and internal structure of superdense dark matter clumps and ultracompact minihaloes

We discuss the formation mechanisms and structure of the superdensedark matter clumps (SDMC) and ultracompact minihaloes (UCMH),outlining the differences between these types of DM objects. Wedefine as SDMC the gravitationally bounded DM objectswhich have come into virial equilibrium at the radiation-dominated(RD) stage of the universe evolution. Such objects can be formed from theisocurvature (entropy) density perturbations or from the peaks in thespectrum of curvature (adiabatic) perturbation. The axionminiclusters (Kolb and Tkachev 1994) are the example of the formermodel. The system of central compact mass (e.g. in the form of SDMCor primordial black hole (PBH)) with the outer DM envelope formed inthe process of secondary accretion we refer to as UCMH. Therefore, theSDMC can serve as the seed for the UCMH in some scenarios. Recently,the SDMC and UCMH were considered in the many works,and we try to systematize them here. Weconsider also the effect of asphericity of the initial densityperturbation in the gravitational evolution, which decreases the SDMCamount and, as the result, suppresses the gamma-ray signal from DMannihilation.

Astrophysical manifestations of clumps of cold dark matter

Small-scale structures (clumps) of dark matter may manifest themselves owing to the annihilation of dark-matter particles in them as pointlike gamma-ray sources. In view of this, investigation into respective effects on the basis of data on unidentified pointlike gamma-ray sources is of importance. It is shown that the existing uncertainties in the description of physical properties of dark-matter particles (their annihilation cross section) and in the distribution of their density in the clumps are of crucial importance; therefore, an analysis of data from the observation of pointlike gamma-ray sources makes it possible to impose constraints on the values of respective uncertain parameters (that is, to single out preferable ones). It is considered that the rate of annihilation of dark-matter particles in the clumps may be enhanced both owing to a higher density and owing to the growth of the cross section at low relative velocities of dark-matter particles in the clumps. In particular, dark-matter particles may have self-interaction of the Coulomb type, and this leads to the enhancement of the annihilation rate because of the Sommerfeld-Sakharov effect. It is shown that the heavy-neutrino model featuring an extra interaction can explain partly Fermi and EGRET data on unidentified pointlike gamma-ray sources. It is indicated that the motion of gamma-ray sources over the celestial sphere can be noticed for several clumps within several years of observations.