arXiv subjects

Kris Sigurdson

Publications and source records attributed to Kris Sigurdson.

63 records · Page 4Linked to original sources

arXiv2005

Cosmic 21-cm Delensing of Microwave Background Polarization and the Minimum Detectable Energy Scale of Inflation

The curl (B) modes of cosmic microwave background (CMB) polarization anisotropies are a unique probe of the primordial background of inflationary gravitational waves (IGWs). Unfortunately, the B-mode polarization anisotropies generated by gravitational waves at recombination are confused with those generated by the mixing of gradient-mode (E-mode) and B-mode polarization anisotropies as CMB photons propagate through the Universe and are gravitationally lensed. We describe here a method for delensing CMB polarization anisotropies using observations of anisotropies in the cosmic 21-cm radiation emitted or absorbed by neutral hydrogen atoms at redshifts 10 to 200. While the detection of cosmic 21-cm anisotropies at high resolution is challenging, a combined study with a relatively low-resolution (but high-sensitivity) CMB polarization experiment could probe inflationary energy scales well below the Grand Unified Theory (GUT) scale of 10^{16} GeV -- constraining models with energy scales below 10^{15} GeV (the detectable limit derived from CMB observations alone). The ultimate theoretical limit to the detectable inflationary energy scale via this method may be as low as 3 \times 10^{14} GeV.

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arXiv2005

How Dark is 'Dark'? Electromagnetic Interactions in the Dark Sector

We review the physical and cosmological consequences of two possible electromagnetic couplings to the dark sector: (i) a neutral lightest dark-matter particle (LDP) with nonzero electric and/or magnetic dipole moments and (ii) a charged next-to-lightest dark-matter particle (NLDP) which decays to a neutral LDP. For scenario (i) we find that a relatively light particle with mass between a few MeV and a few GeV and an electric or magnetic dipole as large as $\sim 3\times 10^{-16}e$ cm (roughly $1.6\times10^{-5} μ_B$) satisfies experimental and observational bounds. In scenario (ii), we show that charged-particles decaying in the early Universe result in a suppression of the small-scale matter power spectrum on scales that enter the horizon prior to decay. This leads to either a cutoff in the matter power spectrum, or if the charged fraction is less than unity, an effect in the power spectrum that might resemble a running (scale-dependent) spectral index in small-scale data.

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arXiv2004

A Running Spectral Index in Supersymmetric Dark-Matter Models with Quasi-Stable Charged Particles

We show that charged-particles decaying in the early Universe can induce a scale-dependent or `running' spectral index in the small-scale linear and nonlinear matter power spectrum and discuss examples of this effect in minimal supersymmetric models in which the lightest neutralino is a viable cold-dark-matter candidate. We find configurations in which the neutralino relic density is set by coannihilations with a long-lived stau, and the late decay of staus partially suppresses the linear matter power spectrum. Nonlinear evolution on small scales then causes the modified linear power spectrum to evolve to a nonlinear power spectrum similar (but different in detail) to models parametrized by a constant running $α_{s}=d n_{s}/d{\rm ln} k$ by redshifts of 2 to 4. Thus, Lyman-$α$ forest observations, which probe the matter power spectrum at these redshifts, might not discriminate between the two effects. However, a measurement of the angular power spectrum of primordial 21-cm radiation from redshift $z \approx 30$--$200$ might distinguish between this charged-decay model and a primordial running spectral index. The direct production of a long-lived charged particle at future colliders is a dramatic prediction of this model.

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arXiv2004

Statistical Imprints of SZ Effects in the Cosmic Microwave Background

We review several aspects of the Sunyaev-Zel'dovich (SZ) effect associated with the large scale baryon distribution and its characteristic signatures in the statistics of cosmic microwave background (CMB) anisotropies. We discuss (1) the contributions to the angular power spectrum from the thermal and kinetic SZ effects, (2) the effect of SZ non-Gaussianities on cosmological parameter estimation, and (3) the SZ-induced CMB polarization towards galaxy clusters. We also discuss the extent to which recent measurements of small angular scale CMB anisotropy can be accounted for by SZ clusters and the potential use of SZ polarization towards large samples of galaxy clusters as a probe of dark energy parameters.

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arXiv2004

Dark-Matter Electric and Magnetic Dipole Moments

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 $\sim 3\times 10^{-16}e$ cm (roughly $1.6\times10^{-5} μ_B$) 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.

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arXiv2004

The shapes of dark matter dominated dwarf spheroidal stellar density profiles

A Bayesian likelihood analysis of the stellar density profile shapes of several dark matter dominated Galactic dwarf spheroidal galaxies (dSphs) has been completed. This analysis indicates that the inner power law, ρ\propto r^{-γ}, of each of these profiles satisfies γ< 1.21 at greater than 99% confidence with a slight preference for γ\sim 0.7 . Speculating that this bound on the stellar profiles may be related to the inner power law of the underlying dark matter distribution reveals a statistical discrepancy with the values favoured by recent N-body simulations of cold dark matter (CDM), γ_{CDM} \sim 1.4-1.5 .

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arXiv2003

Spatial Variation of the Fine-Structure Parameter and the Cosmic Microwave Background

We study the effects on cosmic microwave background (CMB) temperature and polarization anisotropies of spatial fluctuations of the fine-structure parameter between causally disconnected regions of the Universe at the time of recombination. Analogous to weak gravitational lensing, in addition to modifying the mean power spectra and inducing a curl component (B mode) to the polarization, spatial fluctuations of the fine-structure parameter induce higher-order (non-Gaussian) temperature and polarization correlations in the CMB. We calculate these effects for the general case of arbitrary correlation between temperature fluctuations and fine-structure parameter fluctuations, and show the results for a model where these two types of fluctuations are uncorrelated. The formalism we present here may also be applied to other modifications of recombination physics that do not significantly alter the evolution of the dominant density perturbations. We discuss the constraints on the effective Lagrangian for variable fine-structure parameter necessary to realize this scenario.

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arXiv2003

Charged-particle decay and suppression of small-scale power

We study the suppression of the small-scale power spectrum due to the decay of charged matter to dark matter prior to recombination. Prior to decay, the charged particles couple to the photon-baryon fluid and participate in its acoustic oscillations. However, after decaying to neutral dark matter the photon-baryon fluid is coupled only gravitationally to the newly-created dark matter. This generically leads to suppression of power on length scales that enter the horizon prior to decay. For decay times of approximately 3.5 years this leads to suppression of power on subgalactic scales, bringing the observed number of Galactic substructures in line with observation. Decay times of a few years are possible if the dark matter is purely gravitationally interacting, such as the gravitino in supersymmetric models or a massive Kaluza-Klein graviton in models with universal extra dimensions.

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arXiv2000

Morphing the CMB: a technique for interpolating power spectra

The confrontation of the Cosmic Microwave Background (CMB) theoretical angular power spectrum with available data often requires the calculation of large numbers of power spectra. The standard practice is to use a fast code to compute the CMB power spectra over some large parameter space, in order to estimate likelihoods and constrain these parameters. But as the dimensionality of the space under study increases, then even with relatively fast anisotropy codes, the computation can become prohibitive. This paper describes the employment of a "morphing" strategy to interpolate new power spectra based on previously calculated ones. We simply present the basic idea here, and illustrate with a few examples; optimization of interpolation schemes will depend on the specific application. In addition to facilitating the exploration of large parameter spaces, this morphing technique may be helpful for Fisher matrix calculations involving derivatives.

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