33,441 research outputs found
The evolution of CMB spectral distortions in the early Universe
The energy spectrum of the cosmic microwave background (CMB) allows
constraining episodes of energy release in the early Universe. In this paper we
revisit and refine the computations of the cosmological thermalization problem.
For this purpose a new code, called CosmoTherm, was developed that allows
solving the coupled photon-electron Boltzmann equation in the expanding,
isotropic Universe for small spectral distortion in the CMB. We explicitly
compute the shape of the spectral distortions caused by energy release due to
(i) annihilating dark matter; (ii) decaying relict particles; (iii) dissipation
of acoustic waves; and (iv) quasi-instantaneous heating. We also demonstrate
that (v) the continuous interaction of CMB photons with adiabatically cooling
non-relativistic electrons and baryons causes a negative mu-type CMB spectral
distortion of DI_nu/I_nu ~ 10^{-8} in the GHz spectral band. We solve the
thermalization problem including improved approximations for the double Compton
and Bremsstrahlung emissivities, as well as the latest treatment of the
cosmological recombination process. At redshifts z <~ 10^3 the matter starts to
cool significantly below the temperature of the CMB so that at very low
frequencies free-free absorption alters the shape of primordial distortions
significantly. In addition, the cooling electrons down-scatter CMB photons
introducing a small late negative y-type distortion at high frequencies. We
also discuss our results in the light of the recently proposed CMB experiment
Pixie, for which CosmoTherm should allow detailed forecasting. Our current
computations show that for energy injection because of (ii) and (iv) Pixie
should allow to improve existing limits, while the CMB distortions caused by
the other processes seem to remain unobservable with the currently proposed
sensitivities and spectral bands of Pixie.Comment: 22 pages, 19 figures, 1 table, accepted by MNRA
CMB at 2x2 order: the dissipation of primordial acoustic waves and the observable part of the associated energy release
Silk damping of primordial small-scale perturbations in the photon-baryon
fluid due to diffusion of photons inevitably creates spectral distortions in
the CMB. With the proposed CMB experiment PIXIE it might become possible to
measure these distortions and thereby constrain the primordial power spectrum
at comoving wavenumbers 50 Mpc^{-1} < k < 10^4 Mpc^{-1}. Since primordial
fluctuations in the CMB on these scales are completely erased by Silk damping,
these distortions may provide the only way to shed light on otherwise
unobservable aspects of inflationary physics. A consistent treatment of the
primordial dissipation problem requires going to second order in perturbation
theory, while thermalization of these distortions necessitates consideration of
second order in Compton scattering energy transfer. Here we give a full 2x2
treatment for the creation and evolution of spectral distortions due to the
acoustic dissipation process, consistently including the effect of polarization
and photon mixing in the free streaming regime. We show that 1/3 of the total
energy (9/4 larger than previous estimates) stored in small-scale temperature
perturbations imprints observable spectral distortions, while the remaining 2/3
only raises the average CMB temperature, an effect that is unobservable. At
high redshift dissipation is mainly mediated through the quadrupole
anisotropies, while after recombination peculiar motions are most important.
During recombination the damping of the higher multipoles is also significant.
We compute the average distortion for several examples using CosmoTherm,
analyzing their dependence on parameters of the primordial power spectrum. For
one of the best fit WMAP7 cosmologies, with n_S=1.027 and n_run=-0.034, the
cooling of baryonic matter practically compensates the heating from acoustic
dissipation in the mu-era. (abridged)Comment: 40 pages, 17 figures, accepted by MNRA
Pre-recombinational energy release and narrow features in the CMB spectrum
Energy release in the early Universe (z<~ 2x10^6) should lead to some broad
spectral distortion of the cosmic microwave background (CMB) radiation field,
which can be characterized as y-type distortion when the injection process
started at redshifts z<~ 5x10^4. Here we demonstrate that if energy was
released before the beginning of cosmological hydrogen recombination (z~1400),
closed loops of bound-bound and free-bound transitions in HI and HeII lead to
the appearance of (i) characteristic multiple narrow spectral features at dm
and cm wavelengths, and (ii) a prominent sub-millimeter feature consisting of
absorption and emission parts in the far Wien tail of CMB spectrum. The
additional spectral features are generated in the pre-recombinational epoch of
HI (z>~1800) and HeII (z>~7000), and therefore differ from those arising due to
normal cosmological recombination in the undisturbed CMB blackbody radiation
field. We present the results of numerical computations including 25 atomic
shells for both HI and HeII, and discuss the contributions of several
individual transitions in detail. As examples, we consider the case of
instantaneous energy release (e.g. due to phase transitions) and exponential
energy release because of long-lived decaying particles. Our computations show
that due to possible pre-recombinational atomic transitions the variability of
the CMB spectral distortion increases when comparing with the distortions
arising in the normal recombination epoch. The existence of these narrow
spectral features would open an unique way to separate y-distortions due to
pre-recombinational ($1400<~ z <~5x10^4) energy release from those arising in
the post-recombinational era at redshifts z<~800. (abridged)Comment: 17 pages, 12 Figures, 1 Table, submitted to A&
Rethinking CMB foregrounds: systematic extension of foreground parameterizations
Future high-sensitivity measurements of the cosmic microwave background (CMB)
anisotropies and energy spectrum will be limited by our understanding and
modeling of foregrounds. Not only does more information need to be gathered and
combined, but also novel approaches for the modeling of foregrounds,
commensurate with the vast improvements in sensitivity, have to be explored.
Here, we study the inevitable effects of spatial averaging on the spectral
shapes of typical foreground components, introducing a moment approach, which
naturally extends the list of foreground parameters that have to be determined
through measurements or constrained by theoretical models. Foregrounds are
thought of as a superposition of individual emitting volume elements along the
line of sight and across the sky, which then are observed through an
instrumental beam. The beam and line of sight averages are inevitable. Instead
of assuming a specific model for the distributions of physical parameters, our
method identifies natural new spectral shapes for each foreground component
that can be used to extract parameter moments (e.g., mean, dispersion,
cross-terms, etc.). The method is illustrated for the superposition of
power-laws, free-free spectra, gray-body and modified blackbody spectra, but
can be applied to more complicated fundamental spectral energy distributions.
Here, we focus on intensity signals but the method can be extended to the case
of polarized emission. The averaging process automatically produces
scale-dependent spectral shapes and the moment method can be used to propagate
the required information across scales in power spectrum estimates. The
approach is not limited to applications to CMB foregrounds but could also be
useful for the modeling of X-ray emission in clusters of galaxies.Comment: 19 pages, 8 figures, accepted by MNRAS, minor revision
- …