Cold Dark Matter Distribution Research Articles

MHz to TeV expectations from scotogenic WIMP dark matter

ABSTRACT The indirect search for dark matter is typically restricted to individual photon bands and instruments. In the context of multiwavelength observations, finding a weak signal in large foreground and background at only one wavelength band is hampered by systematic uncertainties dominating the signal strength. Dark matter particle annihilation is producing Standard Model particles of which the prompt photon emission is searched for in many studies. However, also the secondary emission of charged particles from dark matter annihilation in the TeV range results in comparable or even stronger fluxes in the GHz–GeV range. In this study, we calculate the prompt and secondary emission of a scotogenic weakly interacting massive particle (WIMP) with a mass of 1 TeV in 27 dwarf galaxies of the Milky Way. For the secondary emission, we include inverse Compton scattering, bremsstrahlung, and synchrotron radiation, which results in a ‘triple hump’ structure characteristic for only dark matter and no other astrophysical source. In order to determine the best candidates for multi-instrument analyses, we estimate the diffuse emission component of the Milky Way itself, including its own dark matter halo from the same scotogenic WIMP model. We find signal-to-background ratios of individual sources on the order of 10−3 to 10−2 across X-ray to γ-ray assuming J factors for the cold dark matter distribution inferred from observations and no additional boosting due to small-scale clumping. We argue that a joint multiwavelength analysis of all nearby galaxies and the extension towards the cosmic gamma-ray background is required to disentangle possible dark matter signals from astrophysical background and foreground.

Multi-stream radial structure of cold dark matter haloes from particle trajectories: deep inside splashback radius

Abstract By tracking trajectories of dark matter (DM) particles accreting onto haloes in cosmological N-body simulations, we investigate the radial phase-space distribution of cold dark matter (CDM) haloes, paying attention to their inner regions deep inside the halo boundary called the splashback radius, where the particles undergo multi-stream flows. Improving the analysis by Sugiura et al., we classify DM particles by the number of apocenter passages, p, and count it up to p = 40 for each halo over a wide mass range. Quantifying the radial density profile for particles having the same value of p, we find that it generally exhibits a double-power law feature, whose indices of inner and outer slopes are well-described by −1 and −8, respectively. Its characteristic scale and density are given as a simple fitting function of p, with a weak halo mass dependence. Interestingly, summing up these double-power law profiles beyond p = 40 reproduces well the total density profile of simulated haloes. The double-power law nature is persistent and generic not only in mass-selected haloes but also in haloes selected in different criteria. Our results are compared with self-similar solutions that describe the stationary and spherical accretion of DM. We find that even when introducing a non-zero angular momentum, none of them explain the radial multi-stream structure. The analysis with particle trajectories tracing back to higher redshifts suggests that the double-power law nature has been established during an early accretion phase and remains stable.

Can a variation of fine structure constant influence the fate of gravitational collapse?

We show that it is possible to steer clear of a spacetime singularity during gravitational collapse by considering time-variation of a fundamental coupling, in this case, the fine structure constant alpha . We study a spherical distribution of cold dark matter coexisting with other fluid elements, collapsing under its own gravity. The dark matter is written as a scalar field interacting with electrically charged matter. This leads to a time variation of alpha and as a consequence, a breakdown of local charge conservation within the sphere. The exterior has no such field and therefore, Einstein’s GR and standard equivalence principles remain valid. We derive the lowest possible bound on the collapse of this sphere beyond which there is a bounce and dispersal of most of the accumulated matter. We discuss the critical behavior of the system around this point and show that the bound is connected to a length scale of the order of Planck, introduced in the theory for dimensional requirements.

The radial acceleration relation in a ΛCDM universe

ABSTRACT We study the radial acceleration relation (RAR) between the total (atot) and baryonic (abary) centripetal acceleration profiles of central galaxies in the cold dark matter (CDM) paradigm. We analytically show that the RAR is intimately connected with the physics of the quasi-adiabatic relaxation of dark matter in the presence of baryons in deep potential wells. This cleanly demonstrates how the mean RAR and its scatter emerge in the low-acceleration regime ($10^{-12}{\rm \, m\, s}^{-2}\lesssim a_{\rm bary}\lesssim 10^{-10}{\rm \, m\, s}^{-2}$) from an interplay between baryonic feedback processes and the distribution of CDM in dark haloes. Our framework allows us to go further and study both higher and lower accelerations in detail, using analytical approximations and a realistic mock catalogue of ${\sim}342\, 000$ low-redshift central galaxies with Mr ≤ −19. We show that, while the RAR in the baryon-dominated high-acceleration regime ($a_{\rm bary}\gtrsim 10^{-10}{\rm \, m\, s}^{-2}$) is very sensitive to details of the relaxation physics, a simple ‘baryonification’ prescription matching the relaxation results of hydrodynamical CDM simulations is remarkably successful in reproducing the observed RAR without any tuning. And in the (currently unobserved) ultra-low-acceleration regime ($a_{\rm bary}\lesssim 10^{-12}{\rm \, m\, s}^{-2}$), the RAR is sensitive to the abundance of diffuse gas in the halo outskirts, with our default model predicting a distinctive break from a simple power-law-like relation for H i-deficient, diffuse gas-rich centrals. Our mocks also show that the RAR provides more robust, testable predictions of the ΛCDM paradigm at galactic scales, with implications for alternative gravity theories than the baryonic Tully–Fisher relation.

The Schrödinger-Poisson method for Large-Scale Structure

We study the Schrödinger-Poisson (SP) method in the context of cosmological large-scale structure formation in an expanding background. In the limit 0ℏ → , the SP technique can be viewed as an effective method to sample the phase space distribution of cold dark matter that remains valid on non-linear scales. We present results for the 2D and 3D matter correlation function and power spectrum at length scales corresponding to the baryon acoustic oscillation (BAO) peak. We discuss systematic effects of the SP method applied to cold dark matter and explore how they depend on the simulation parameters. In particular, we identify a combination of simulation parameters that controls the scale-independent loss of power observed at low redshifts, and discuss the scale relevant to this effect.

Cold keV dark matter from decays and scatterings

We explore ways of creating cold keV-scale dark matter by means of decays and scatterings. The main observation is that certain thermal freeze-in processes can lead to a cold dark matter distribution in regions with small available phase space. In this way the free-streaming length of keV particles can be suppressed without decoupling them too much from the Standard Model. In all cases, dark matter needs to be produced together with a heavy particle that carries away most of the initial momentum. For decays, this simply requires an off-diagonal DM coupling to two heavy particles; for scatterings, the coupling of soft DM to two heavy particles needs to be diagonal, in particular in spin space. Decays can thus lead to cold light DM of any spin, while scatterings only work for bosons with specific couplings. We explore a number of simple models and also comment on the connection to the tentative 3.5 keV line.

Cosmic voids in coupled dark energy cosmologies: the impact of halo bias

In this work we analyse the properties of cosmic voids in standard and coupled dark energy cosmologies. Using large numerical simulations, we investigate the effects produced by the dark energy coupling on three statistics: the filling factor, the size distribution and the stacked profiles of cosmic voids. We find that the bias of the tracers of the density field used to identify the voids strongly influences the properties of the void catalogues, and, consequently, the possibility of using the identified voids as a probe to distinguish coupled dark energy models from the standard $\Lambda $CDM cosmology. In fact, on one hand coupled dark energy models are characterised by an excess of large voids in the cold dark matter distribution as compared to the reference standard cosmology, due to their higher normalisation of linear perturbations at low redshifts. Specifically, these models present an excess of large voids with $R_{eff}>20, 15, 12$ Mpc h^{-1}, at $z=0, 0.55, 1$, respectively. On the other hand, we do not find any significant difference in the properties of the void detected in the distribution of collapsed dark matter halos. These results imply that the tracer bias has a significant impact on the possibility of using cosmic void catalogues to probe cosmology.

The impact of baryonic physics on the structure of dark matter haloes: the view from the FIRE cosmological simulations

We study the distribution of cold dark matter (CDM) in cosmological simulations from the FIRE (Feedback In Realistic Environments) project, for $M_{\ast}\sim10^{4-11}\,M_{\odot}$ galaxies in $M_{\rm h}\sim10^{9-12}\,M_{\odot}$ halos. FIRE incorporates explicit stellar feedback in the multi-phase ISM, with energetics from stellar population models. We find that stellar feedback, without "fine-tuned" parameters, greatly alleviates small-scale problems in CDM. Feedback causes bursts of star formation and outflows, altering the DM distribution. As a result, the inner slope of the DM halo profile ($\alpha$) shows a strong mass dependence: profiles are shallow at $M_{\rm h}\sim10^{10}-10^{11}\,M_{\odot}$ and steepen at higher/lower masses. The resulting core sizes and slopes are consistent with observations. This is broadly consistent with previous work using simpler feedback schemes, but we find steeper mass dependence of $\alpha$, and relatively late growth of cores. Because the star formation efficiency $M_{\ast}/M_{\rm h}$ is strongly halo mass dependent, a rapid change in $\alpha$ occurs around $M_{\rm h}\sim 10^{10}\,M_{\odot}$ ($M_{\ast}\sim10^{6}-10^{7}\,M_{\odot}$), as sufficient feedback energy becomes available to perturb the DM. Large cores are not established during the period of rapid growth of halos because of ongoing DM mass accumulation. Instead, cores require several bursts of star formation after the rapid buildup has completed. Stellar feedback dramatically reduces circular velocities in the inner kpc of massive dwarfs; this could be sufficient to explain the "Too Big To Fail" problem without invoking non-standard DM. Finally, feedback and baryonic contraction in Milky Way-mass halos produce DM profiles slightly shallower than the Navarro-Frenk-White profile, consistent with the normalization of the observed Tully-Fisher relation.

Cosmology with massive neutrinos I: towards a realistic modeling of the relation between matter, haloes and galaxies

By using a suite of large box-size N-body simulations that incorporate massive neutrinos as an extra set of particles, with total masses of 0.15, 0.30, and 0.60 eV, we investigate the impact of neutrino masses on the spatial distribution of dark matter haloes and on the distribution of galaxies within the haloes. We compute the bias between the spatial distribution of dark matter haloes and the overall matter and cold dark matter distributions using statistical tools such as the power spectrum and the two-point correlation function. Overall we find a scale-dependent bias on large scales for the cosmologies with massive neutrinos. In particular, we find that the bias decreases with the scale, being this effect more important for higher neutrino masses and at high redshift. However, our results indicate that the scale-dependence in the bias is reduced if the latter is computed with respect to the cold dark matter distribution only. We find that the value of the bias on large scales is reasonably well reproduced by the Tinker fitting formula once the linear cold dark matter power spectrum is used, instead of the total matter power spectrum. We also investigate whether scale-dependent bias really comes from purely neutrino's effect or from nonlinear gravitational collapse of haloes. For this purpose, we address the Ων-σ8 degeneracy and find that such degeneracy is not perfect, implying that neutrinos imprint a slight scale dependence on the large-scale bias. Finally, by using a simple halo occupation distribution (HOD) model, we investigate the impact of massive neutrinos on the distribution of galaxies within dark matter haloes. We use the main galaxy sample in the Sloan Digital Sky Survey (SDSS) II Data Release 7 to investigate if the small-scale galaxy clustering alone can be used to discriminate among different cosmological models with different neutrino masses. Our results suggest that different choices of the HOD parameters can reproduce the observational measurements relatively well, and we quantify the difference between the values of the HOD parameters between massless and massive neutrino cosmologies.

Direct detection of self-interacting dark matter

Self-interacting dark matter offers an interesting alternative to collisionless dark matter because of its ability to preserve the large-scale success of the cold dark matter model, while seemingly solving its challenges on small scales. We present here the first study of the expected dark matter detection signal taking into account different self-scattering models. We demonstrate that models with constant and velocity dependent cross sections, which are consistent with observational constraints, lead to distinct signatures in the velocity distribution, because non-thermalised features found in the cold dark matter distribution are thermalised through particle scattering. Depending on the model, self-interaction can lead to a 10% reduction of the recoil rates at high energies, corresponding to a minimum speed that can cause recoil larger than 300 km/s, compared to the cold dark matter case. At lower energies these differences are smaller than 5% for all models. The amplitude of the annual modulation signal can increase by up to 25%, and the day of maximum amplitude can shift by about two weeks with respect to the cold dark matter expectation. Furthermore, the exact day of phase reversal of the modulation signal can also differ by about a week between the different models. In general, models with velocity dependent cross sections peaking at the typical velocities of dwarf galaxies lead only to minor changes in the detection signals, whereas allowed constant cross section models lead to significant changes. We conclude that different self-interacting dark matter scenarios might be distinguished from each other through the details of direct detection signals. Furthermore, detailed constraints on the intrinsic properties of dark matter based on null detections, should take into account the possibility of self-scattering and the resulting effects on the detector signal.

Non-linear dark energy clustering

We consider a dark energy fluid with arbitrary sound speed and equation of state and discuss theeffect of its clustering on the cold dark matter distribution at the non-linear level. We writethe continuity, Euler and Poisson equations for the system in the Newtonian approximation.Then, using the time renormalization group method to resum perturbative corrections at all orders, wecompute the total clustering power spectrum and matter power spectrum.At the linear level, a sound speed of dark energy different from that of light modifies the powerspectrum on observationally interesting scales, such as those relevant for baryonic acousticoscillations. We show that the effect of varying the sound speed of dark energy on the non-linear corrections to the matter power spectrum is below the per cent level, and therefore these corrections can be well modelled by their counterpart in cosmological scenarios with smooth dark energy. We also show that the non-linear effects on the matter growth index can be as large as 10–15 per cent for small scales.

Simulations of Disk Galaxy Formation in their Cosmological Context

AbstractTogether with the discovery of the accelerated expansion of the present Universe and measurements of large-scale structure at low redshift, observations of the cosmic microwave background have established a standard paradigm in which all cosmic structure grew from small fluctuations generated at very early times in a flat universe which today consists of 72% dark energy, 23.5% dark matter and 4.5% ordinary baryons. The CMB sky provides us with a direct image of this universe when it was 400,000 years old and very nearly uniform. The galaxy formation problem is then to understand how observed galaxies with all their regularity and diversity arose from these very simple initial conditions. Although gravity is the prime driver, many physical processes appear to play an important role in this transformation, and direct numerical simulation has become the principal tool for detailed investigation of the complex and strongly nonlinear interactions between them.The evolution of structure in the gravitationally dominant Cold Dark Matter distribution can now be simulated in great detail, provided the effects of the baryons are ignored, and there is general consensus for the results on scales relevant to the formation of galaxies like our own. The basic nonlinear units are so-called “dark matter halos”, slowly rotating, triaxial, quasi-equilibrium systems with a universal cusped density profile and substantial substructure in the form of a host of much less massive subhalos which are concentrated primarily in their outer regions.Attempts to include the baryons, and so to model the formation of the visible parts of galaxies, have given much more diverse results. It has been known for 30 years that substantial feedback, presumably from stellar winds and supernovae, is required to prevent overcooling of gas and excessive star formation in the early stages of galaxy assembly. When realistic galaxy formation simulations first became possible in the early 1990's, this problem was immediately confirmed. Without effective feedback, typical halos produced galaxies which were too massive, too concentrated and had too little disk to be consistent with observation.Simple models for disk formation from the mid 1990's show that the angular momentum predicted for collapsing dark halos is sufficient for them to build a disk population similar to that observed. Direct simulations have repeatedly failed to confirm this picture, however, because nonlinear effects lead to substantial transfer of angular momentum between the various components. In most cases the condensing baryonic material loses angular momentum to the dark matter, and the final galaxy ends up with a disk that is too compact or contains too small a fraction of the stars.These problems have been reduced as successive generations of simulations have dramatically improved the numerical resolution and have introduced “better” implementations of feedback (i.e. more successful at building disks). Despite this, no high-resolution simulation has so far been able to produce a present-day disk galaxy with a bulge-to-disk mass ratio much less than one in a proper ΛCDM context. Such galaxies are common in the real Universe; our own Milky Way is a good example. The variety of results obtained by different groups show that this issue is very sensitive to how star formation and feedback are treated, and all implementations of these processes to date have been much too schematic to be confident of their predictions.The major outstanding issues I see related to disk galaxies and their formation are the following: Do real disk galaxies have the NFW halos predicted by the ΛCDM cosmology? If not, could the deviations have been produced by the formation of the observed baryonic components, or must the basic structure formation picture be changed? How are Sc and later type galaxies made? Why don't our simulations produce them? What determines which galaxies become barred and which not? Can we demonstrate that secular evolution produces the observed population of (pseudo)bulges from pre-existing disks? How does the observed population of thin disks survive bombardment by substructure and the other transient potential fluctuations expected in ΛCDM halos? Is a better treatment of feedback really the answer? If so, can we demonstrate it using chemical abundances as fossil tracers? And how can we best use observations at high redshift to clarify these formation issues?

PROBING COLD DARK MATTER CUSPS BY GRAVITATIONAL LENSING

I elaborate on my prediction that an indirect detection of cold dark matter (CDM) may be possible by observing the gravitational lensing effects of the CDM cusp caustics at cosmological distances. Cusps in the distribution of CDM are plentiful once density perturbations enter the nonlinear regime. Caustic Ring Model of galactic halos provides a well-defined density profile and a geometry near the cusps of the caustic rings. I calculate the lensing effects in this model. As a point-like background source passes behind a cusp of a cosmological foreground halo, the magnification in its image may be detected by present instruments. Depending on the strength of detected effect and the time scale of brightness change, it may even be possible to discriminate between the CDM candidates: axions and weakly interacting massive particles.

Effects of Gravitational Evolution, Biasing, and Redshift Space Distortion on Topology

We have studied the dependence of topology of large-scale structure on tracer, gravitational evolution, redshift space distortion, and cosmology. A series of large N-body simulations of the ΛCDM and SCDM models that have evolved 1.1 or 8.6 billion particles are used in the study. Evolution of the genus statistic, used as a topology measure, from redshift 8 to 0 is accurately calculated over a wide range of smoothing scales using the simulations. The tracers of large-scale structure considered are the cold dark matter (CDM), biased peaks in the initial density field, dark halos, and populating the dark halos in accordance with a halo occupation distribution (HOD) model. We have found that the effects of biasing, gravitational evolution, and initial conditions on topology of large-scale structure are all comparable. The redshift space distortion effects are relatively small down to about 5 h-1 Mpc for all tracers except for the high-threshold part of the genus curve. The gravitational effects are found to be well modeled by analytic perturbation theory when the CDM distribution is considered. But the direction of gravitational evolution of topology can be even reversed for different tracers. For example, the shift parameter of the genus curve evolves in opposite directions for matter and HOD galaxies at large scales. At small scales, there are interesting deviations of the genus curve of dark halos and galaxies from that of matter in our initially Gaussian simulations. The deviations should be understood as due to combined effects of gravitational evolution and biasing. This fact gives us an important opportunity: topology of large-scale structure can be used as a strong constraint on galaxy formation mechanisms. At scales larger than 20 h-1 Mpc all the above effects gradually decrease. With good knowledge of the effects of nonlinear gravitational evolution and galaxy biasing on topology, one can also constrain the Gaussian random phase initial conditions hypothesis to high accuracy.

Results of a Search for Cold Flows of Dark Matter Axions

Theoretical arguments predict that the distribution of cold dark matter in spiral galaxies has peaks in velocity space associated with nonthermalized flows of dark matter particles. We searched for the corresponding peaks in the spectrum of microwave photons from axion to photon conversion in a cavity detector for dark matter axions. We found none and place limits on the density of any local flow of axions as a function of the flow velocity dispersion over the axion mass range 1.98 to 2.17 microeV.

The first WIMPy halos

Dark matter direct and indirect detection signals depend crucially on the dark matterdistribution. While the formation of large scale structure is independent of the nature ofthe cold dark matter (CDM), the fate of inhomogeneities on subgalactic scales, and hencethe present day CDM distribution on these scales, depends on the microphysics of theCDM particles. We study the density contrast of weakly interacting massive particles(WIMPs) on subgalactic scales. We calculate the damping of the primordial powerspectrum due to collisional damping and free streaming of WIMPy CDM and show thatfree streaming leads to a CDM power spectrum with a sharp cut-off at about. We also calculate the transfer function for the growth of the inhomogeneities in the linearregime, taking into account the suppression in the growth of the CDM densitycontrast after matter–radiation equality due to baryons and show that our analyticresults are in good agreement with numerical calculations. Combining the transferfunction with the damping of the primordial fluctuations we produce a WMAPnormalized primordial CDM power spectrum, which can serve as an input for highresolution CDM simulations. We find that the smallest inhomogeneities typically havecomoving radius of about 1 pc and enter the non-linear regime at a red-shift of60 ± 20. We study the effect of scale dependence of the primordial power spectrum on thesenumbers and also use the spherical collapse model to make simple estimates of theproperties of the first generation of WIMP halos to form. We find that the very firstWIMPy halos may have a significant impact on indirect dark matter searches.

Probing the Origins of Voids in the Distribution of Galaxies

AbstractIf the voids that we see today in the distribution of galaxies existed at recombination, they will leave an imprint on the cosmic microwave background (CMB). On the other hand, if these voids formed much later, their effect on the CMB will be negligible and will not be observed with the current generation of experiments. In this paper, presented at the 2004 Annual Scientific Meeting of the Astronomical Society of Australia, we discuss our ongoing investigations into voids of primordial origin. We show that if voids in the cold dark matter distribution existed at the epoch of decoupling, they could contribute significantly to the apparent rise in CMB power on small scales detected by the Cosmic Background Imager (CBI) Deep Field. Here we present our improved method for predicting the effects of primordial voids on the CMB in which we treat a void as an external source in the cold dark matter (CDM) distribution employing a Boltzmann solver. Our improved predictions include the effects of a cosmological constant (Λ) and acoustic oscillations generated by voids at early times. We find that models with relatively large voids on the last scattering surface predict too much CMB power in an Einstein–de Sitter background cosmology but could be consistent with the current CMB observations in a ΛCDM universe.

Effect of elastic scattering in the Earth on cold dark matter experiments.

The diurnal modulation of fluxes and velocity distributions of cold dark matter (CDM) candidate particles, due to elastic scattering on the constituent nuclei of the Earth, is predicted. The geology, nuclear physics, the Earth's orientation, rotation, and trajectory through the galactic halo are included as well as the isotropic Maxwellian velocity distribution of CDM in the rest frame of the galactic halo. Observable modulations in detection rates and energy spectra are predicted for some interesting theoretical and experimental scenarios. The effects are seen to be maximal at southern latitudes. The differences in the sensitivity of experiments at southern and northern latitudes are explored. It is shown that it would be possible to detect Dirac neutrinos with densities well below those heretofore experimentally excluded.

Diurnal modulation effects in cold dark matter experiments

The effects of elastic scattering of cold dark matter (CDM) candidate particles on the constituent nuclei of the earth are predicted. The geology, nuclear physics, the earth's orientation, rotation, and trajectory through the galactic halo are included, as well as the isotropic maxwellian velocity distribution of CDM in the rest-frame of the halo. Observable modulations of detection rates and energy spectra are predicted for some interesting theoretical and experimental scenarios.