Session K9: Indirect Dark Matter Searches

Growth of structure in a Universe with complex scalar-field dark matter

The nature and distribution of dark matter (DM) in the Universe determine the properties of the structures we observe. In recent years, the exploration of different DM candidates has seen a tremendous rise, partly due to the fact that the canonical DM paradigm of a weakly-interacting massive particle (WIMP), has not yet been confirmed experimentally. Moreover, numerical simulations of structure formation of collisonless WIMP DM are often in contradiction with observations of galaxies on small scales. We will assume that ultra-light, self-interacting bosons are responsible for all of the DM. Owing to their ability to form a Bose-Einstein condensate in the very early Universe, DM can be described as a classical complex scalar field (SFDM). In a previous work, we have established that the background evolution of SFDM with a cosmological constant (LSFDM) is in accordance with the concordance LCDM model if the model parameters are properly constrained by observations of the CMB and BBN. However, not only does LSFDM lead to non-standard expansion histories prior to BBN, it can also help to resolve the problems found in the LCDM model on small scales. In this talk, we will present new results on the linear and nonlinear growth of structure in this LSFDM model, and their implications.   

Constraints on Dark Matter Annihilation by Radio Observations of Milky Way

WIMP annihilation in the Milky Way (MW) halo is expected to produce various energetic stable particles. These particles can manifest themselves through various emission processes. Such an emission spans almost the whole spectrum from radio to gamma bands. In a recent few years several groups reported the significant gamma ray excess at GeV energies in the MW center region, which can't be explained by conventional astrophysical sources. To explain this excess, one needs either an additional population of millisecond pulsars or the annihilating dark matter (DM). In the DM scenario, one may estimate the necessary WIMP properties. And several groups report rather close WIMP parameters needed. Naturally, we expect a radio counterpart of this gamma excess to be present, which originates as a synchrotron radiation of leptons produced by WIMP annihilation. And a comprehensive study of such a counterpart has not been conducted yet. Our work is in progress and focused on the low frequency emission. We are planning to present the general constraints on WIMP properties based on whole sky radio observations of MW (involving various radio surveys and Planck data), and also planning to support or weaken the DM interpretation of the gamma excess through studies of its expected counterpart.   

Resolving Small-Scale Dark Matter Structures Using Multi-Source Indirect Detection

The extragalactic dark matter (DM) annihilation signal depends on the product of the clumping factor, $\langle \delta^{2} \rangle $, and the velocity-weighted annihilation cross section, $\sigma v$. It is important to determine the clumping factor as it depends on the minimum DM halo mass, $M_{\rm min}$, or equivalently the kinetic decoupling temperature of DM. In this work, we demonstrate how to break the ``clumping factor--$\sigma v$'' degeneracy by comparing the Isotropic Gamma Ray Background with tentative DM signals from the Galactic Center. We obtain interesting limits on $M_{\rm min}$ and $\sigma v$. Potential improvements in near future are discussed, which will have significant implications for the tentative DM signals.   

Dark-matter admixed white dwarfs

We study the equilibrium structures of white dwarfs (WD) with dark matter cores formed by non-self-annihilating dark matter (DM) particles with masses ranging from 1 GeV to 100 GeV, assuming in form of an ideal degenerate Fermi gas inside the stars. For DM particles of mass 10 GeV and 100 GeV, we find that stable stellar models exist only if the mass of the DM core inside the star is less than $O$(10$^{-3}) M_{\mathrm{sun}}$ and $O$(10$^{-6}) M_{\mathrm{sun}}$, respectively. The global properties of these stars, and the corresponding Chandrasekhar mass (CM) limits, are essentially the same as those of traditional WD models without DM. Nevertheless, in the 10 GeV case, the gravitational attraction of the DM core is strong enough to squeeze the normal matter in the core region to densities above neutron drip. For the 1 GeV case, the DM core inside the star can be as massive as $O$(0.1) $M_{\mathrm{sun}}$ and affects the global structure of the star significantly. The radius of a stellar model with DM can be about two times smaller than that of a traditional WD. Furthermore, the CM limit can be decreased by as much as 40{\%}. Our results may have implications on the extent to which type Ia supernovae can be regarded as standard candles.   

The Luminous Convolution Model-The light side of dark matter

We present a heuristic model for predicting the rotation curves of spiral galaxies. The Luminous Convolution Model (LCM) utilizes Lorentz-type transformations of very small changes in the photon's frequencies from curved space-times to construct a dynamic mass model of galaxies. These frequency changes are derived using the exact solution to the exterior Kerr wave equation, as opposed to a linearized treatment. The LCM Lorentz-type transformations map between the emitter and the receiver rotating galactic frames, and then to the associated flat frames in each galaxy where the photons are emitted and received. This treatment necessarily rests upon estimates of the luminous matter in both the emitter and the receiver galaxies. The LCM is tested on a sample of 22 randomly chosen galaxies, represented in 33 different data sets. LCM fits are compared to the Navarro, Frenk \& White (NFW) Dark Matter Model and to the Modified Newtonian Dynamics (MOND) model when possible. The high degree of sensitivity of the LCM to the initial assumption of a luminous mass to light ratios (M/L), of the given galaxy, is demonstrated. We demonstrate that the LCM is successful across a wide range of spiral galaxies for predicting the observed rotation curves.   

Luminogenesis RG Flow

Using the constraints from the Planck satellite on inflation models and renormalization-group flow, we present constraints on the mass of dark-matter particles in a unification model with the gauge group $SU(3) \times SU(6) \times U(1)$, which breaks to the standard model with an extra gauge group for dark matter when the inflaton rolls into the true vacuum. In this model, inflaton decay gives rise to dark matter, which in turn decays to luminous matter in the right proportion that agrees with cosmological data. Some attractive features of this model include self-interacting dark matter, which may resolve the problems of dwarf-galaxy structures and dark-matter cusps at the centers of galaxies, and the absence of proton decay, which has evaded experimental detection to this day.   

Flattened Velocity Dispersion in Globular Clusters; A Perspective From Modified Gravity Schemes

Recent observations have confirmed the flattening of the radial velocity dispersion profiles for stars invarious nearby globular clusters. Under Newtonian gravity this is explained by invoking tidal heating from the overall Milky Way potential on the outer more loosely bound stars. From the point of view of modified gravity theories, such an outer flattening is expected on crossing the critical acceleration threshold $a_{0}$, beyond which, a transition to MONDian dynamicsis expected. From an empirical point of view, we determine Newtonian tidal radii using masses accurately calculated through stellar population modeling, and hence independent of any dynamical assumptions for a sample of globular clusters. Crucially, we find that the asymptotic values of the velocity dispersion profiles scale with the fourth root of the total masses in accordance with the galactic Tully-Fisher relation. Also, in all cases, Newtonian tidal radii at perigalacticon are larger that the radii at which the flattening in the velocity dispersion profiles occurs, which correlate with the radii where the $a_{0}$ threshold is crossed, as expected under modified gravity scenarios.   

Hunting for topological dark matter with atomic clocks

The cosmological applications of atomic clocks so far have been limited to searches of the uniform-in-time drift of fundamental constants. In this paper, we point out that a transient in time change of fundamental constants can be induced by dark matter objects that have large spatial extent, and are built from light non-Standard Model fields. The stability of this type of dark matter can be dictated by the topological reasons. We point out that correlated networks of atomic clocks, some of them already in existence, can be used as a powerful tool to search for the topological defect dark matter, thus providing another important fundamental physics application to the ever-improving accuracy of atomic clocks. During the encounter with a topological defect, as it sweeps through the network,initially synchronized clocks will become desynchronized. Time discrepancies between spatially-separated clocks are expected to exhibit a distinct signature, encoding defect's space structure and its interaction strength with the Standard Model fields.   

Study of vortices in Axion BEC dark matter

We present an analytic study of the vortices in the axion BEC dark matter and their effects on the galactic angular momentum distribution of baryons and dark matter in disk galaxies.