Structure and adiabatic compression of dark matter halos: Simple analytic model

An analytical model is considered for describing the inner regions of dark matter halos. This model is based on the assumption that the density of dark matter varies according to the power law. The model concerns the distribution function in phase space that is expressed in terms of the adiabatic invariants (radial action and angular momentum). Two types, narrow and broad, are suggested for the angular part of the distribution function. The model makes it possible to explicitly describe the adiabatic compression of halos due to a change induced in the gravitational potential by the condensation of baryonic matter at the center. The change in the density of the dark matter halos is calculated and it is shown that the standard algorithm for calculating the adiabatic compression overestimates the halo density, particularly for the case of strong radial anisotropy.     

Non-thermal dark matter,high energy cosmic rays and late-decaying particles from inflationary quantum fluctuations

It has been suggested that the origin of cosmic rays above the GZK limit might be explained by the decay of particles, X, with mass of the order of 10¹² GeV. Generation of heavy particles from inflationary quantum fluctuations is a prime candidate for the origin of the decaying X particles. It has also been suggested that the problem of non-singular galactic halos might be explained if dark matter originates non-thermally from the decay of particles, Y, such that there is a free-streaming length of the order of 0.1 Mpc. Here we explore the possibility that quantum fluctuations might account for the Y particles as well as the X particles. For the case of non-thermal WIMP dark matter with unsuppressed weak interactions we find that there is a general problem with deuterium photo-dissociation, disfavouring WIMP dark matter candidates. For the case of more general dark matter particles, which may have little or no interaction with conventional matter, we discuss the conditions under which X and Y scalars or fermions can account for non-thermal dark matter and cosmic rays. For the case where X and Y scalars are simultaneously produced, we show that galactic halos are likely to have a dynamically significant component of X scalar cold dark matter in addition to the dominant non-thermal dark matter component.     

Flat rotation curves in scalar field galaxy halos

We start with a model where the dark matter is of scalar field nature, which condensates and form the dark halos of galaxies. In this work we study Bose–Einstein condensates (BEC) where the scalar field particles are in many different states, and not only in the ground state, as in a realistic BEC. We find that this model is in better agreement with the rotation curves of galaxies than previous models with scalar field dark matter.     

Slowly rotating Bose Einstein condensate galactic dark matter halos,and their rotation curves

If dark matter is composed of massive bosons, a Bose–Einstein condensation process must have occurred during the cosmological evolution. Therefore galactic dark matter may be in a form of a condensate, characterized by a strong self-interaction. We consider the effects of rotation on the Bose–Einstein condensate dark matter halos, and we investigate how rotation might influence their astrophysical properties. In order to describe the condensate we use the Gross–Pitaevskii equation, and the Thomas–Fermi approximation, which predicts a polytropic equation of state with polytropic index \(n=1\). By assuming a rigid body rotation for the halo, with the use of the hydrodynamic representation of the Gross–Pitaevskii equation we obtain the basic equation describing the density distribution of the rotating condensate. We obtain the general solutions for the condensed dark matter density, and we derive the general representations for the mass distribution, boundary (radius), potential energy, velocity dispersion, tangential velocity and for the logarithmic density and velocity slopes, respectively. Explicit expressions for the radius, mass, and tangential velocity are obtained in the first order of approximation, under the assumption of slow rotation. In order to compare our results with the observations we fit the theoretical expressions of the tangential velocity of massive test particles moving in rotating Bose–Einstein condensate dark halos with the data of 12 dwarf galaxies and the Milky Way, respectively.     

Are halos of collisionless cold dark matter collisionless?

We study whether gravitational scattering of halo dark matter particles by subhalos can connect two seemingly independent problems: the abundance of subhalos in dark matter halos and the cuspiness of the halos' inner density profiles. Our numerical experiments indicate that subhalos can cause the collisionless dark matter particles in the centers of main halos to diffuse. Combined with tidal mass loss of the subhalos, this process introduces significant scatter in the inner density profiles and offers an explanation for the range of profiles seen in both observations and cosmological simulations.     

Wormhole supported by dark energy admitting conformal motion

In this article, we study the possibility of sustaining static and spherically symmetric traversable wormhole geometries admitting conformal motion in Einstein gravity, which presents a more systematic approach to search a relation between matter and geometry. In wormhole physics, the presence of exotic matter is a fundamental ingredient and we show that this exotic source can be dark energy type which support the existence of wormhole spacetimes. In this work we model a wormhole supported by dark energy which admits conformal motion. We also discuss the possibility of the detection of wormholes in the outer regions of galactic halos by means of gravitational lensing. Studies of the total gravitational energy for the exotic matter inside a static wormhole configuration are also performed.     

Model for gravitational interaction between dark matter and baryons

We propose a phenomenological model where the gravitational interaction between dark matter and baryons is suppressed on small, subgalactic scales. We describe the gravitational force by adding a Yukawa contribution to the standard Newtonian potential and show that this interaction scheme is effectively suggested by the available observations of the inner rotation curves of small mass galaxies. Besides helping in interpreting the cuspy profile of dark matter halos observed in N-body simulations, this potential regulates the quantity of baryons within halos of different masses.     

Annihilating cold dark matter

Structure formation with cold dark matter (CDM) predicts halos with a central density cusp, which are observationally disfavored. If CDM particles have an annihilation cross section sigmav approximately 10(-29)(m/GeV) cm(2), then annihilations will soften the cusps. We discuss plausible scenarios for avoiding the early Universe annihilation catastrophe that could result from such a large cross section. The predicted scaling of core density with halo mass depends upon the velocity dependence of sigmav, and s-wave annihilation leads to a core density nearly independent of halo mass, which seems consistent with observations.     

Nonthermal production of weakly interacting massive particles and the subgalactic structure of the universe

There is increasing evidence that conventional cold dark matter (CDM) models lead to conflicts between observations and numerical simulations of dark matter halos on subgalactic scales, which rules out the favored candidates for CDM, namely weakly interacting massive particles (WIMPs). We propose a mechanism of nonthermal production of WIMPs and study its implications on the power spectrum. Our results show that, in this context, WIMPs as candidates for dark matter can work well both on large scales and on subgalactic scales.     

Mirror matter, mirror gravity and galactic rotational curves

We discuss astrophysical implications of the modified gravity model in which the two matter components, ordinary and dark, couple to separate gravitational fields that mix to each other through small mass terms. There are two spin-2 eigenstates: the massless graviton, which induces universal Newtonian attraction, and the massive one, which gives rise to the Yukawa-like potential which is repulsive between the ordinary and dark bodies. As a result for distances much smaller than the Yukawa radius r _m the gravitation strength between the two types of matter becomes vanishing. If r _m ～10 kpc, the typical size of a galaxy, there are interesting implications for the nature of dark matter. In particular, one can avoid the problem of the cusp that is typical for the cold dark matter halos. Interestingly, the flat shape of the rotational curves can be explained even in the case of the collisional and dissipative dark matter (as e.g. mirror matter), which cannot give the extended halos but instead must form galactic discs similarly to the visible matter. The observed rotational curves for the large, medium-size and dwarf galaxies can be nicely reproduced. We also briefly discuss possible implications for the direct search of dark matter.     

The spacetime associated with galactic dark matter halos

We show how an adequate post-Newtonian generalization can be obtained for Newtonian dark matter halos associated with an empiric density profile. Applying this approach to halos that follow from the well known numerical simulations of Navarro, Frenk and White (NFW), we derive all dynamical variables and show that NFW halos approximately follow an ideal gas type of equation of state which fits very well to a polytropic relation in the region outside the core. This fact suggests that outer regions of NFW halos might be related to equilibrium states in the non-extensive Statistical Mechanics formalism proposed by Tsallis.     

Weak lensing,dark matter and dark energy

Weak gravitational lensing is rapidly becoming one of the principal probes of dark matter and dark energy in the universe. In this brief review we outline how weak lensing helps determine the structure of dark matter halos, measure the expansion rate of the universe, and distinguish between modified gravity and dark energy explanations for the acceleration of the universe. We also discuss requirements on the control of systematic errors so that the systematics do not appreciably degrade the power of weak lensing as a cosmological probe.     

Probing gravity at cosmological scales by measurements which test the relationship between gravitational lensing and matter overdensity

The standard cosmology is based on general relativity (GR) and includes dark matter and dark energy and predicts a fixed relationship between the gravitational potentials responsible for gravitational lensing and the matter overdensity. Alternative theories of gravity often make different predictions. We propose a set of measurements which can test this relationship, thereby distinguishing between dark energy or matter models and models in which gravity differs from GR. Planned surveys will be able to measure E(G), an observational quantity whose expectation value is equal to the ratio of the Laplacian of the Newtonian potentials to the peculiar velocity divergence, to percent accuracy. This will easily separate alternatives such as the cold dark matter model with a cosmological constant, Dvali-Gabadadze-Porrati, TeVeS, and f(R) gravity.     

Dark matter spikes and annihilation radiation from the galactic center

The annihilation rate of weakly interacting cold dark matter particles at the galactic center could be greatly enhanced by the growth of a density spike around the central supermassive black hole (SBH). Here we discuss the effects of hierarchical mergers on the central spike. Mergers between halos containing SBHs lead to the formation of SBH binaries which transfer energy to the dark matter particles, lowering their density. The predicted flux of annihilation photons from the galactic center is several orders of magnitude smaller than in models that ignore the effects of SBHs and mergers. Measurement of the annihilation radiation could in principle be used to constrain the merger history of the galaxy.     

Analysis of rotation curves in the framework of the gravitational suppression model

We present an analysis of suitable rotation curves (RCs) of eight galaxies, aimed at checking the consistency and universality of the gravitational suppression (GraS) hypothesis, a phenomenological model for a new interaction between dark matter and baryons. Motivated by the puzzle of the core versus cusp distribution of dark matter in the center of halos, this hypothesis claims to reconcile the predictions from N-body Lambda cold dark matter simulations with kinematic observations. The GraS model improves the kinematic fitting residuals, but the mass parameters are unphysical and put the theory in difficulty.     

Gravothermal collapse of self-interacting dark matter halos and the origin of massive black holes

Black hole formation is an inevitable consequence of relativistic core collapse following the gravothermal catastrophe in self-interacting dark matter (SIDM) halos. Very massive SIDM halos form supermassive black holes (SMBHs) > or about 10(6)M(middle dot in circle) directly. Smaller halos believed to form by redshift z = 5 produce seed black holes of (10(2)-10(3))M(middle dot in circle) which can merge and/or accrete to reach the observational SMBH range. This scenario for SMBH formation requires no baryons, no prior star formation, and no other black hole seed mechanism.     

Are galaxies extending?

It is suggested that the recently observed size evolution of very massive compact galaxies in the early universe can be explained, if dark matter is in Bose–Einstein condensate. In this model the size of the dark matter halos and galaxies depends on the correlation length of dark matter and, hence, on the expansion of the universe. This theory predicts that the size of the galaxies increases as the Hubble radius of the universe even without merging, which agrees well with the recent observational data.     

Brief History of a Relativistic Century

More than one century is passed by the publication of special relativity and few less by the birth of general relativity. Despite the great experimental successes of these theories, the study of the universe, is plagued by numerous unsolved problems. For example one of the most problems in cosmology is the cosmological constant, which governs the expansion of the universe, also known as dark energy. A substantial portion, about 60%, of the mass-energy in the universe is in a form of mysterious energy that is pushing the cosmos apart at an accelerating rate. What is this energy, and where does it come from? Cosmologists have no real idea. Although given a similar name, there is another problem in cosmology, the so-called dark matter, which is actually unrelated to dark energy, except insofar as they involve things we don’t understand. About 90% of the mass in the universe is in an apparently invisible form of matter that we call dark matter. This dark matter can only be measured by the gravitational pull it has on objects around it, and all galaxies we observe contain large halos of it, often extending for hundreds of thousands of light years beyond the edge of luminous matter. Is this dark matter actual matter, such as weakly interacting massive particles, or perhaps it is just an observational artifact caused by an improper theory of gravity? Another mystery is why there is so much more matter than antimatter in the universe. According to physical theories, these forms of matter are essentially equivalent, but conventional matter is observed in much greater abundances than antimatter. In this paper we summarily introduce the principal alternative theories proposed during one century of relativity.     

Unitarity bounds and the cuspy halo problem

Conventional cold dark matter cosmological models predict cuspy halos which are in apparent conflict with observations. We show that unitarity arguments imply interesting constraints on two proposals to address this problem: collisional dark matter and strongly annihilating dark matter. Efficient scattering in both implies m less, similar to 12 GeV and m less, similar to 25 GeV, respectively. We also show that the strong annihilation in the second scenario implies the presence of elastic scattering. Recent evidence suggests a collisional scenario where the cross section scales inversely with velocity--we argue superelastic processes are likely involved. Exceptions and implications for searches are discussed.     

Upper bound on the dark matter total annihilation cross section

We consider dark matter annihilation into standard model particles and show that the least detectable final states, namely, neutrinos, define an upper bound on the total cross section. Calculating the cosmic diffuse neutrino signal, and comparing it to the measured terrestrial atmospheric neutrino background, we derive a strong and general bound. This can be evaded if the annihilation products are dominantly new and truly invisible particles. Our bound is much stronger than the unitarity bound at the most interesting masses, shows that dark matter halos cannot be significantly modified by annihilations, and can be improved by a factor of 10-100 with existing neutrino experiments.