122 research outputs found
Testing Parity-Violating Mechanisms with Cosmic Microwave Background Experiments
Chiral gravity and cosmological birefringence both provide physical
mechanisms to produce parity-violating TB and EB correlations in the cosmic
microwave background (CMB) temperature/polarization. Here, we study how well
these two mechanisms can be distinguished if non-zero TB/EB correlations are
found. To do so, we evaluate the correlation matrix, including new TB-EB
covariances. We find that the effects of these two mechanisms on the CMB are
highly orthogonal, and can thus be distinguished fairly well in case of a
high--signal-to-noise detection of TB/EB correlations. An Appendix evaluates
the relative sensitivities of the BB, TB, and EB signals for detecting a chiral
gravitational-wave background.Comment: 8 pages, 6 figures; to be submitted to PR
Statistics of 21-cm fluctuations in cosmic reionization simulations: PDFs and difference PDFs
In the coming decade, low-frequency radio arrays will begin to probe the epoch of reionization via the redshifted 21-cm hydrogen line. Successful interpretation of these observations will require effective statistical techniques for analysing the data. Due to the difficulty of these measurements, it is important to develop techniques beyond the standard power-spectrum analysis in order to offer independent confirmation of the reionization history, probe different aspects of the topology of reionization and have different systematic errors. In order to assess the promise of probability distribution functions (PDFs) as statistical analysis tools in 21-cm cosmology, we first measure the 21-cm brightness temperature (one-point) PDFs in six different reionization simulations. We then parametrize their most distinct features by fitting them to a simple model. Using the same simulations, we also present the first measurements of difference PDFs in simulations of reionization. We find that while these statistics probe the properties of the ionizing sources, they are relatively independent of small-scale, subgrid astrophysics. We discuss the additional information that the difference PDF can provide on top of the power spectrum and the one-point PDF
Reconstructing the early-universe expansion and thermal history
We present a model-independent reconstruction of the early expansion and
thermal histories of the universe, obtained from light element abundance
measurements. The expansion history is tightly constrained around the onset of
the Big Bang Nucleosynthesis (BBN). The temperature of photons is additionally
constrained around the time of neutrino decoupling. Allowing for perturbations
to the standard expansion rate, we find that the radiation energy density is
constrained to within 15% of its CDM value, and only 1% extra matter
energy density is allowed around the epoch of BBN. We introduce a new and
general analytic fitting formula for the temperature variation, which is
flexible enough to reproduce the signal of large classes of beyond-CDM particle
models that can alter the temperature through early-time energy injection. We
present its constraints from BBN data and from the measurements of effective
number of relativistic species and helium-4 abundance probed by the Cosmic
Microwave Background radiation anisotropy. Our results provide clarity on the
most fundamental properties of the early universe, reconstructed with minimal
assumptions about the unknown physics that can occur at keV--MeV energy scales
and can be mapped to broad classes of models of interest to cosmology.Comment: 14 pages, 13 figures, comments are welcom
Cross-Correlation of Cosmological Birefringence with CMB Temperature
Theories for new particle and early-Universe physics abound with
pseudo-Nambu-Goldstone fields that arise when global symmetries are
spontaneously broken. The coupling of these fields to the Chern-Simons term of
electromagnetism may give rise to cosmological birefringence (CB), a
frequency-independent rotation of the linear polarization of photons as they
propagate over cosmological distances. Inhomogeneities in the CB-inducing field
may yield a rotation angle that varies across the sky. Here we note that such a
spatially-varying birefringence may be correlated with the cosmic microwave
background (CMB) temperature. We describe quintessence scenarios where this
cross-correlation exists and other scenarios where the scalar field is simply a
massless spectator field, in which case the cross-correlation does not exist.
We discuss how the cross-correlation between CB-rotation angle and CMB
temperature may be measured with CMB polarization. This measurement may improve
the sensitivity to the CB signal, and it can help discriminate between
different models of CB.Comment: 9 pages, 1 figure; submitted to PR
First CMB Constraints on Direction-Dependent Cosmological Birefringence from WMAP-7
A Chern-Simons coupling of a new scalar field to electromagnetism may give
rise to cosmological birefringence, a rotation of the linear polarization of
electromagnetic waves as they propagate over cosmological distances. Prior work
has sought this rotation, assuming the rotation angle to be uniform across the
sky, by looking for the parity-violating TB and EB correlations a uniform
rotation produces in the CMB temperature/polarization. However, if the scalar
field that gives rise to cosmological birefringence has spatial fluctuations,
then the rotation angle may vary across the sky. Here we search for
direction-dependent cosmological birefringence in the WMAP-7 data. We report
the first CMB constraint on the rotation-angle power spectrum for multipoles
between L = 0 and L = 512. We also obtain a 68% confidence-level upper limit of
1 degree on the square root of the quadrupole of a scale-invariant
rotation-angle power spectrum.Comment: 14 pages, 12 figures, 4 tables; accepted to PR
Patchy screening of the cosmic microwave background by inhomogeneous reionization
We derive a constraint on patchy screening of the cosmic microwave background from inhomogeneous reionization using off-diagonal TB and TT correlations in WMAP-7 temperature/polarization data. We interpret this as a constraint on the rms optical-depth fluctuation Δτ as a function of a coherence multipole L_C. We relate these parameters to a comoving coherence scale, of bubble size R_C, in a phenomenological model where reionization is instantaneous but occurs on a crinkly surface, and also to the bubble size in a model of “Swiss cheese” reionization where bubbles of fixed size are spread over some range of redshifts. The current WMAP data are still too weak, by several orders of magnitude, to constrain reasonable models, but forthcoming Planck and future EPIC data should begin to approach interesting regimes of parameter space. We also present constraints on the parameter space imposed by the recent results from the EDGES experiment
De-Rotation of the Cosmic Microwave Background Polarization: Full-Sky Formalism
Mechanisms have been proposed that might rotate the linear polarization of
the cosmic microwave background (CMB) as it propagates from the surface of last
scatter. In the simplest scenario, the rotation will be uniform across the sky,
but the rotation angle may also vary across the sky. We develop in detail the
complete set of full-sky quadratic estimators for the rotation of the CMB
polarization that can be constructed from the CMB temperature and polarization.
We derive the variance with which these estimators can be measured and show
that these variances reduce to the simpler flat-sky expressions in the
appropriate limit. We evaluate the variances numerically. While the flat-sky
formalism may be suitable if the rotation angle arises as a realization of a
random field, the full-sky formalism will be required to search for rotations
that vary slowly across the sky as well as for models in which the angular
power spectrum for the rotation angle peaks at large angles.Comment: 13 pages, 7 figures; minor changes, added references and Appendix D;
accepted for publication in PR
New probe of magnetic fields in the prereionization epoch. I. Formalism
We propose a method of measuring extremely weak magnetic fields in the
intergalactic medium prior to and during the epoch of cosmic reionization. The
method utilizes the Larmor precession of spin-polarized neutral hydrogen in the
triplet state of the hyperfine transition. This precession leads to a
systematic change in the brightness temperature fluctuations of the 21-cm line
from the high-redshift universe, and thus the statistics of these fluctuations
encode information about the magnetic field the atoms are immersed in. The
method is most suited to probing fields that are coherent on large scales; in
this paper, we consider a homogenous magnetic field over the scale of the 21-cm
fluctuations. Due to the long lifetime of the triplet state of the 21-cm
transition, this technique is naturally sensitive to extremely weak field
strengths, of order G at a reference redshift of (or
G if scaled to the present day). Therefore, this might open up the
possibility of probing primordial magnetic fields just prior to reionization.
If the magnetic fields are much stronger, it is still possible to use this
method to infer their direction, and place a lower limit on their strength. In
this paper (Paper I in a series on this effect), we perform detailed
calculations of the microphysics behind this effect, and take into account all
the processes that affect the hyperfine transition, including radiative decays,
collisions, and optical pumping by Lyman- photons. We conclude with an
analytic formula for the brightness temperature of linear-regime fluctuations
in the presence of a magnetic field, and discuss its limiting behavior for weak
and strong fields.Comment: 26 pages, 4 figures, updated to match published versio
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