350 research outputs found
Aspects of Dark Matter Annihilation in Cosmology
Cosmic microwave background (CMB) constraints on dark matter annihilation are
a uniquely powerful tool in the quest to understand the nature of dark matter.
Annihilation of dark matter to Standard Model particles between recombination
and reionization heats baryons, ionizes neutral hydrogen, and alters the CMB
visibility function. Surprisingly, CMB bounds on dark matter annihilation are
not expected to improve significantly with the dramatic improvements in
sensitivity expected in future cosmological surveys. In this paper, we will
present a simple physical description of the origin of the CMB constraints and
explain why they are nearly saturated by current observations. The essential
feature is that dark matter annihilation primarily affects the ionization
fraction which can only increase substantially at times when the universe was
neutral. The resulting change to the CMB occurs on large angular scales and
leads to a phenomenology similar to that of the optical depth to reionization.
We will demonstrate this impact on the CMB both analytically and numerically.
Finally, we will discuss the additional impact that changing the ionization
fraction has on large scale structure.Comment: 22 pages, 11 figure
Phases of New Physics in the CMB
Fluctuations in the cosmic neutrino background are known to produce a phase
shift in the acoustic peaks of the cosmic microwave background. It is through
the sensitivity to this effect that the recent CMB data has provided a robust
detection of free-streaming neutrinos. In this paper, we revisit the phase
shift of the CMB anisotropy spectrum as a probe of new physics. The phase shift
is particularly interesting because its physical origin is strongly constrained
by the analytic properties of the Green's function of the gravitational
potential. For adiabatic fluctuations, a phase shift requires modes that
propagate faster than the speed of fluctuations in the photon-baryon plasma.
This possibility is realized by free-streaming relativistic particles, such as
neutrinos or other forms of dark radiation. Alternatively, a phase shift can
arise from isocurvature fluctuations. We present simple models to illustrate
each of these effects. We then provide observational constraints from the
Planck temperature and polarization data on additional forms of radiation. We
also forecast the capabilities of future CMB Stage IV experiments. Whenever
possible, we give analytic interpretations of our results.Comment: 39 pages, 10 figures, 5 tables; v2: minor corrections, references
added; v3: corrected Planck parameter constraints, conclusions unchange
Dark Radiation Emerging After Big Bang Nucleosynthesis?
We show how recent data from observations of the cosmic microwave background
may suggest the presence of additional radiation density which appeared after
big bang nucleosynthesis. We propose a general scheme by which this radiation
could be produced from the decay of non-relativistic matter, we place
constraints on the properties of such matter, and we give specific examples of
scenarios in which this general scheme may be realized.Comment: v3: 5 pages, 1 figure. References added, typos corrected, notation
changed throughout. v2: 5 pages, 1 figure. Reformatted, references added,
acknowledgments updated, effect of radiation on CMB clarified. v1: 11 pages,
1 figur
Single-Field Inflation and the Local Ansatz: Distinguishability and Consistency
The single-field consistency conditions and the local ansatz have played
separate but important roles in characterizing the non-Gaussian signatures of
single- and multifield inflation respectively. We explore the precise
relationship between these two approaches and their predictions. We demonstrate
that the predictions of the single-field consistency conditions can never be
satisfied by a general local ansatz with deviations necessarily arising at
order . This implies that there is, in principle, a minimum
difference between single- and (fully local) multifield inflation in
observables sensitive to the squeezed limit such as scale-dependent halo bias.
We also explore some potential observational implications of the consistency
conditions and its relationship to the local ansatz. In particular, we propose
a new scheme to test the consistency relations. In analogy with delensing of
the cosmic microwave background, one can deproject the coupling of the long
wavelength modes with the short wavelength modes and test for residual
anomalous coupling.Comment: 17 page
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