141 research outputs found
Ultrafast effective multi-level atom method for primordial hydrogen recombination
Cosmological hydrogen recombination has recently been the subject of renewed
attention because of its importance for predicting the power spectrum of cosmic
microwave background anisotropies. It has become clear that it is necessary to
account for a large number n >~ 100 of energy shells of the hydrogen atom,
separately following the angular momentum substates in order to obtain
sufficiently accurate recombination histories. However, the multi-level atom
codes that follow the populations of all these levels are computationally
expensive, limiting recent analyses to only a few points in parameter space. In
this paper, we present a new method for solving the multi-level atom
recombination problem, which splits the problem into a computationally
expensive atomic physics component that is independent of the cosmology, and an
ultrafast cosmological evolution component. The atomic physics component
follows the network of bound-bound and bound-free transitions among excited
states and computes the resulting effective transition rates for the small set
of "interface" states radiatively connected to the ground state. The
cosmological evolution component only follows the populations of the interface
states. By pre-tabulating the effective rates, we can reduce the recurring cost
of multi-level atom calculations by more than 5 orders of magnitude. The
resulting code is fast enough for inclusion in Markov Chain Monte Carlo
parameter estimation algorithms. It does not yet include the radiative transfer
or high-n two-photon processes considered in some recent papers. Further work
on analytic treatments for these effects will be required in order to produce a
recombination code usable for Planck data analysis.Comment: Version accepted by Phys. Rev. D. Proof of equivalence of effective
and standard MLA methods moved to the main text. Some rewording
Metals at the surface of last scatter
Standard big-bang nucleosynthesis (BBN) predicts only a trace abundance of lithium and no heavier elements, but some alternatives predict a nonzero primordial metallicity. Here we explore whether CMB measurements may set useful constraints to the primordial metallicity and/or whether the standard CMB calculations are robust, within the tolerance of forthcoming CMB maps, to the possibility of primordial metals. Metals would affect the recombination history (and thus CMB power spectra) in three ways: (1) Lyα photons can be removed (and recombination thus accelerated) by photoionizing metals; (2) The Bowen resonance-fluorescence mechanism may degrade Lyβ photons and thus enhance the Lyβ escape probability and speed up recombination; (3) Metals could affect the low-redshift tail of the CMB visibility function by providing additional free electrons. The last two of these provide the strongest CMB signal. However, the effects are detectable in the Planck satellite only if the primordial metal abundance is at least a few hundredths of solar for (2) and a few tenths of solar for (3). We thus conclude that Planck will not be able to improve upon current constraints to primordial metallicity, at the level of a thousandth of solar, from the Lyman-α forest and ultra-metal-poor halo stars, and that the CMB power-spectrum predictions for Planck suffer no uncertainty arising from the possibility that there may be primordial metals
Flaring of tidally compressed dark-matter clumps
We explore the physics and observational consequences of tidal compression
events (TCEs) of dark-matter clumps (DMCs) by supermassive black holes (SMBHs).
Our analytic calculations show that a DMC approaching a SMBH much closer than
the tidal radius undergoes significant compression along the axis perpendicular
to the orbital plane, shortly after pericenter passage. For DMCs composed of
self-annihilating dark-matter particles, we find that the boosted DMC density
and velocity dispersion lead to a flaring of the annihilation rate, most
pronounced for a velocity- dependent annihilation cross section. If the end
products of the annihilation are photons, this results in a gamma-ray flare,
detectable (and possibly already detected) by the Fermi telescope for a range
of model parameters. If the end products of dark-matter annihilation are
relativistic electrons and positrons and the local magnetic field is large
enough, TCEs of DMCs can lead to flares of synchrotron radiation. Finally, TCEs
of DMCs lead to a burst of gravitational waves, in addition to the ones
radiated by the orbital motion alone, and with a different frequency spectrum.
These transient phenomena provide interesting new avenues to explore the
properties of dark matter.Comment: 11 pages, 6 figures; Minor changes; Version as published in PR
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