809 research outputs found
Towards a complete treatment of the cosmological recombination problem
A new approach to the cosmological recombination problem is presented, which
completes our previous analysis on the effects of two-photon processes during
the epoch of cosmological hydrogen recombination, accounting for ns-1s and
nd-1s Raman events and two-photon transitions from levels with n>=2. The
recombination problem for hydrogen is described using an effective 400-shell
multi-level approach, to which we subsequently add all important recombination
corrections discussed in the literature thus far. We explicitly solve the
radiative transfer equation of the Lyman-series photon field to obtain the
required modifications to the rate equations of the resolved levels. In
agreement with earlier computations we find that 2s-1s Raman scattering leads
to a delay in recombination by DN_e/N_e~0.9% at z~920. Two-photon decay and
Raman scattering from higher levels (n>3) result in a small additional
modifications, and precise results can be obtained when including their effect
for the first 3-5 shells. This work is a major step towards a new cosmological
recombination code (CosmoRec) that supersedes the physical model included in
Recfast, and which, owing to its short runtime, can be used in the analysis of
future CMB data from the Planck Surveyor.Comment: 17 pages, 8 figures, minor typos corrected, accepted by MNRAS,
CosmoRec available at http://www.Chluba.de/CosmoRe
Which spectral distortions does CDM actually predict?
Ever refined cosmological measurements have established the CDM
concordance model, with the key cosmological parameters being determined to
percent-level precision today. This allows us to make explicit predictions for
the spectral distortions of the cosmic microwave background (CMB) created by
various processes occurring in the early Universe. Here, we summarize all
guaranteed CMB distortions and assess their total uncertainty within
CDM. We also compare simple methods for approximating them,
highlighting some of the subtle aspects when it comes to interpreting future
distortion measurements. Under simplified assumptions, we briefly study how
well a PIXIE-like experiment may measure the main distortion parameters (i.e.,
and ). Next generation CMB spectrometers are expected to detect the
distortion caused by reionization and structure formation at extremely high
significance. They will also be able to constrain the small-scale power
spectrum through the associated -distortion, improving limits on running
of the spectral index. Distortions from the recombination era, adiabatic
cooling of matter relative to the CMB and dark matter annihilation require a
higher sensitivity than PIXIE in its current design. The crucial next step is
an improved modeling of foregrounds and instrumental aspects, as we briefly
discuss here.Comment: 13 pages, 5 figures, 3 tables, accepted by MNRA
Time-Dependent Corrections to the Ly-alpha Escape Probability During Cosmological Hydrogen Recombination
We consider the effects connected with the detailed radiative transfer during
the epoch of cosmological recombination on the ionization history of our
Universe. We focus on the escape of photons from the hydrogen Lyman-alpha
resonance at redshifts 600<~ z <~ 2000, one of two key mechanisms defining the
rate of cosmological recombination. We approach this problem within the
standard formulation, and corrections due to two-photon interactions are
deferred to another paper. As a main result we show here that within a
non-stationary approach to the escape problem, the resulting correction in the
free electron fraction, N_e, is about ~1.6-1.8% in the redshift range
800<~z<~1200. Therefore the discussed process results in one of the largest
modifications to the ionization history close to the maximum of
Thomson-visibility function at z~1100 considered so far. We prove our results
both numerically and analytically, deriving the escape probability, and
considering both Lyman-alpha line emission and line absorption in a way
different from the Sobolev approximation. In particular, we give a detailed
derivation of the Sobolev escape probability during hydrogen recombination, and
explain the underlying assumptions. We then discuss the escape of photons for
the case of coherent scattering in the lab frame, solving this problem
analytically in the quasi-stationary approximation and also in the
time-dependent case. We show here that during hydrogen recombination the
Sobolev approximation for the escape probability is not valid at the level of
DP/P~5-10%. This is because during recombination the ionization degree changes
significantly over a characteristic time Dz/z~10%, so that at percent level
accuracy the photon distribution is not evolving (abridged)Comment: 18 pages, 12 figures, accepted versio
Could the Cosmological Recombination Spectrum Help Us Understand Annihilating Dark Matter?
In this paper we explore the potential effects of DM annihilations on the
cosmological recombination spectrum. With this example we want to demonstrate
that the cosmological recombination spectrum in principle is sensitive to
details related to possible extra energy release during recombination. We
restrict ourselves to DM models which produce a negligible primordial
distortion of the CMB energy spectrum. However, since during the epoch of
cosmological recombination a large fraction of the deposited energy can
directly go into ionizations and excitations of neutral atoms, both the
cosmological recombination spectrum and ionization history can still be
affected significantly. We compute the modifications to the cosmological
recombination spectrum using our multi-level HI and HeI recombination code,
showing that additional photons are created due to uncompensated loops of
transitions which are induced by DM annihilations. As we illustrate here, the
results depend on the detailed branching of the deposited energy into heating,
ionizations and excitations. This dependence in principle should allow us to
shed light on the nature of the underlying annihilating DM model (or more
generally speaking, the mechanism leading to energy injection) when measuring
the cosmological recombination spectrum. However, for current upper limits on
the potential DM annihilation rate during recombination the cosmological
recombination spectrum is only affected at the level of a few percent.
Nevertheless, we argue here that the cosmological recombination spectrum would
provide another independent and very direct way of checking for the presence of
sources of extra ionizing or exciting photons at high redshifts. This would
open an new window to possible (non-standard) processes occurring (abridged)Comment: 14 pages, 11 figure, submitted to MNRA
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