212 research outputs found
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
CMB spectral distortions from small-scale isocurvature fluctuations
The damping of primordial perturbations at small scales gives rise to
distortions of the cosmic microwave background (CMB). Here, the dependence of
the distortion on the different types of cosmological initial conditions is
explored, covering adiabatic, baryon/cold dark matter isocurvature, neutrino
density/velocity isocurvature modes and some mixtures. The radiation transfer
functions for each mode are determined and then used to compute the dissipative
heating rates and spectral distortion signatures, utilizing both analytic
estimates and numerical results from the thermalization code CosmoTherm. Along
the way, the early-time super-horizon behavior for the resulting fluid modes is
derived in conformal Newtonian gauge, and tight-coupling transfer function
approximations are given. CMB spectral distortions caused by different
perturbation modes can be estimated using simple k-space window functions which
are provided here. Neutrinos carry away some fraction of the primordial
perturbation power, introducing an overall efficiency factor that depends on
the perturbation type. It is shown that future measurements of the CMB
frequency spectrum have the potential to probe different perturbation modes at
very small scales (corresponding to wavenumbers 1 Mpc^{-1} < k < few x 10^4
Mpc^{-1}). These constraints are complementary to those obtained at large
scales and hence provide an exciting new window to early-universe physics.Comment: 16 pages, 5 figures, minor changes, accepted versio
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