While, to ensure successful cosmology, dark matter (DM) must kinematically decouple from the standard model plasma very early in the history of the Universe, it can remain coupled to a bath of "dark radiation" until a relatively late epoch. One minimal theory that realizes such a scenario is the Atomic Dark Matter model, in which two fermions oppositely charged under a new U(1) dark force are initially coupled to a thermal bath of "dark photons" but eventually recombine into neutral atom-like bound states and begin forming gravitationally-bound structures. As dark atoms have (dark) atom-sized geometric cross sections, this model also provides an example of self-interacting DM with a velocity-dependent cross section. Delayed kinetic decoupling in this scenario predicts novel DM properties on small scales but retains the success of cold DM on larger scales. We calculate the atomic physics necessary to capture the thermal history of this dark sector and show significant improvements over the standard atomic hydrogen calculation are needed. We solve the Boltzmann equations that govern the evolution of cosmological fluctuations in this model and find in detail the impact of the atomic DM scenario on the matter power spectrum and the cosmic microwave background (CMB). This scenario imprints a new length scale, the Dark-Acoustic-Oscillation (DAO) scale, on the matter density field. This DAO scale shapes the small-scale matter power spectrum and determines the minimal DM halo mass at late times which may be many orders of magnitude larger than in a typical WIMP scenario. This model necessarily includes an extra dark radiation component, which may be favoured by current CMB experiments, and we quantify CMB signatures that distinguish an atomic DM scenario from a standard ΛCDM model containing extra free-streaming particles. We finally discuss the impacts of atomic DM on galactic dynamics and show that these provide the strongest constraints on the model. PACS numbers: 98.80.-k,98.80.Jk

We present the first simulations within an effective theory of structure formation (ETHOS), which includes the effect of interactions between dark matter and dark radiation on the linear initial power spectrum and dark matter self-interactions during non-linear structure formation. We simulate a Milky Way-like halo in four different dark matter models and the cold dark matter case. Our highest resolution simulation has a particle mass of 2.8 × 10 4 M and a softening length of 72.4 pc. We demonstrate that all alternative models have only a negligible impact on large scale structure formation. On galactic scales, however, the models significantly affect the structure and abundance of subhaloes due to the combined effects of small scale primordial damping in the power spectrum and late time self-interactions. We derive an analytic mapping from the primordial damping scale in the power spectrum to the cutoff scale in the halo mass function and the kinetic decoupling temperature. We demonstrate that certain models within this extended effective framework that can alleviate the too-big-to-fail and missing satellite problems simultaneously, and possibly the core-cusp problem. The primordial power spectrum cutoff of our models naturally creates a diversity in the circular velocity profiles, which is larger than that found for cold dark matter simulations. We show that the parameter space of models can be constrained by contrasting model predictions to astrophysical observations. For example, some models may be challenged by the missing satellite problem if baryonic processes were to be included and even over-solve the too-big-to-fail problem; thus ruling them out.

We formulate an effective theory of structure formation (ETHOS) that enables cosmological structure formation to be computed in almost any microphysical model of dark matter physics. This framework maps the detailed microphysical theories of particle dark matter interactions into the physical effective parameters that shape the linear matter power spectrum and the self-interaction transfer cross section of non-relativistic dark matter. These are the input to structure formation simulations, which follow the evolution of the cosmological and galactic dark matter distributions. Models with similar effective parameters in ETHOS but with different dark particle physics would nevertheless result in similar dark matter distributions. We present a general method to map an ultraviolet complete or effective field theory of low energy dark matter physics into parameters that affect the linear matter power spectrum and carry out this mapping for several representative particle models. We further propose a simple but useful choice for characterizing the dark matter self-interaction transfer cross section that parametrizes self-scattering in structure formation simulations. Taken together, these effective parameters in ETHOS allow the classification of dark matter theories according to their structure formation properties rather than their intrinsic particle properties, paving the way for future simulations to span the space of viable dark matter physics relevant for structure formation.

The thermal and expansion history of the Universe before big bang nucleosynthesis is unknown. We investigate the evolution of cosmological perturbations through the transition from an early matter era to radiation domination. We treat reheating as the perturbative decay of an oscillating scalar field into relativistic plasma and cold dark matter. After reheating, we find that subhorizon perturbations in the decay-produced dark matter density are significantly enhanced, while subhorizon radiation perturbations are instead suppressed. If dark matter originates in the radiation bath after reheating, this suppression may be the primary cutoff in the matter power spectrum. Conversely, for dark matter produced nonthermally from scalar decay, enhanced perturbations can drive structure formation during the cosmic dark ages and dramatically increase the abundance of compact substructures. For low reheat temperatures, we find that as much as 50% of all dark matter is in microhalos with M 0.1M⊕ at z ≃ 100, compared to a fraction of ∼ 10 −10 in the standard case. In this scenario, ultradense substructures may constitute a large fraction of dark matter in galaxies today. * Electronic address: erickcek@cita.utoronto.ca † Electronic address: krs@physics.ubc.ca

The Giant Metrewave Radio Telescope Epoch of Reionization experiment is an ongoing effort to measure the power spectrum from neutral hydrogen at high redshift. We have previously reported an upper limit of (70 mK) 2 at wavenumbers of k ≈ 0.65 h Mpc −1 using a basic piecewise-linear foreground subtraction. In this paper we explore the use of a singular value decomposition to remove foregrounds with fewer assumptions about the foreground structure. Using this method we also quantify, for the first time, the signal loss due to the foreground filter and present new power spectra adjusted for this loss, providing a revised measurement of a 2σ upper limit at (248 mK) 2 for k = 0.50 h Mpc −1 . While this revised limit is larger than previously reported, we believe it to be more robust and still represents the best current constraint on reionization at z ≈ 8.6.

We calculate the kinetic-decoupling temperature for weakly interacting massive particles (WIMPs) in supersymmetric (SUSY) and extra dimensional models that can account for the cold-dark-matter abundance determined from cosmic microwave background measurements. Depending on the parameters of the particle-physics model, a wide variety of decoupling temperatures is possible, ranging from several MeV to a few GeV. These decoupling temperatures imply a range of masses for the smallest protohalos much larger than previously thought -ranging from 10 ÿ6 M to 10 2 M . We expect the range of protohalos masses derived here to be characteristic of most particle-physics models that can thermally accommodate the required relic abundance of WIMP dark matter, even beyond SUSY and extra dimensions. DOI: 10.1103/PhysRevLett.97.031301 PACS numbers: 98.80.Cq, 11.10.Kk, 12.60.Jv, 95.35.+d The physical nature of dark matter remains one of the major unsolved problems in theoretical physics and cosmology. One of the leading candidates for dark matter is a weakly interacting massive particle (WIMP) [1]. In the simplest models, WIMPs (which we denote by ) carry a conserved quantum number that renders them stable. When the temperature in the early Universe drops below m , the WIMP abundance tracks the thermal equilibrium distribution and is Boltzmann suppressed until pairs of WIMPs can no longer find each other within a Hubble time, and the comoving number density of WIMPs becomes constant. Up to factors of a few, this freeze-out of the annihilation channel happens at a temperature T fo m =20 and leads to a relic abundance of dark matter of h 2 ' 3 10 ÿ27 cm 3 s ÿ1 =h a vi, where h a vi is the thermally averaged cross section (times relative velocity) for annihilation of pairs into lighter particles. In typical models, m 100-1000 GeV, and T fo 5-50 GeV.While freeze-out signals the departure of WIMPs from chemical equilibrium, it does not signal the end of WIMP interactions. Elastic and inelastic scattering processes of the form f ! f or f ! 0 f 0 keep the dark matter in kinetic equilibrium until later times (lower temperatures) [2 -4]. Here f and f 0 are standard model particles in the thermal bath (leptons, quarks, gauge bosons) and 0 is an unstable particle that carries the same conserved quantum number as . The temperature T kd of kinetic decoupling sets the distance scale at which linear density perturbations in the dark-matter distribution get washed out-the smallscale cutoff in the matter power spectrum. In turn, this small-scale cutoff sets the mass M c of the smallest protohalos that form when these very small scales go nonlinear at a redshift z 70. There may be implications of this small-scale cutoff for direct detection (detection of WIMPs scattering off ordinary matter in low-background environments) [5] and indirect detection (detection of the byproducts of WIMP-pair annihilations including antimatter, gamma rays, and neutrinos) [6].Some early work assumed that the cross sections for WIMPs to scatter from light particles (...

In this paper we describe the spherical harmonic transit telescope, a novel formalism for the analysis of transit radio telescopes. This all-sky approach bypasses the curved sky complications of traditional interferometry and so is particularly well suited to the analysis of wide-field radio interferometers. It enables compact and computationally efficient representations of the data and its statistics that allow new ways of approaching important problems like mapmaking and foreground removal. In particular, we show how it enables the use of the Karhunen-Loève transform as a highly effective foreground filter, suppressing realistic foreground residuals for our fiducial example by at least a factor twenty below the 21 cm signal even in highly contaminated regions of the sky. This is despite the presence of the angle-frequency mode mixing inherent in real-world instruments with frequency-dependent beams. We show, using Fisher forecasting, that foreground cleaning has little effect on power spectrum constraints compared to hypothetical foreground-free measurements. Beyond providing a natural real-world data analysis framework for 21 cm telescopes now under construction and future experiments, this formalism allows accurate power spectrum forecasts to be made that include the interplay of design constraints and realistic experimental systematics with twenty-first century 21 cm science. † jrs65@cita.utoronto.ca 1 http://gmrt.ncra.tifr.res.in/ 2

We consider the consequences of a neutral dark-matter particle with a nonzero electric and/or magnetic dipole moment. Theoretical constraints, as well as constraints from direct searches, precision tests of the standard model, the cosmic microwave background and matter power spectra, and cosmic gamma rays, are included. We find that a relatively light particle with mass between an MeV and a few GeV and an electric or magnetic dipole as large as ∼ 3 × 10 −16 e cm (roughly 1.6 × 10 −5 µB) satisfies all experimental and observational bounds. Some of the remaining parameter space may be probed with forthcoming more sensitive direct searches and with the Gamma-Ray Large Area Space Telescope. PACS numbers: 95.35+d, 13.40.Em, 95.30.cq, 98.80.Cq