Vera Gluscevic

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 nonzero 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 the case of a high-signal-to-noise detection of TB/EB correlations. Appendix evaluates the relative sensitivities of the BB, TB, and EB signals for detecting a chiral gravitational-wave background.

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 that a uniform rotation produces in the cosmic microwave background 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 cosmic microwave background constraint on the rotation-angle power spectrum C^(αα)_L for multipoles between L=0 and L=512. We also obtain a 68% confidence-level upper limit of √(C^(αα)_(2)/(4π))≲1° on the quadrupole of a scale-invariant rotation-angle power spectrum.

Derotation 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.

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.

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 10^(−19)  G at a reference redshift of ∼20 (or 10^(−21)  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-α 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.

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.

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.

New probe of magnetic fields in the pre-reionization epoch. II. Detectability

In the first paper of this series, we proposed a novel method to probe large–scale intergalactic magnetic fields during the cosmic Dark Ages, using 21–cm tomography. This method relies on the effect of spin alignment of hydrogen atoms in a cosmological setting, and on the effect of magnetic precession of the atoms on the statistics of the 21-cm brightness-temperature fluctuations. In this paper, we forecast the sensitivity of future tomographic surveys to detecting magnetic fields using this method. For this purpose, we develop a minimum-variance estimator formalism to capture the characteristic anisotropy signal using the two-point statistics of the brightness-temperature fluctuations. We find that, depending on the reionization history, and subject to the control of systematics from foreground subtraction, an array of dipole antennas in a compact-grid configuration with a collecting area slightly exceeding one square kilometer can achieve a 1σ detection of ∼10^(−21)  Gauss comoving (scaled to present-day value) within three years of observation. Using this method, tomographic 21–cm surveys could thus probe ten orders of magnitude below current cosmic microwave background constraints on primordial magnetic fields, and provide exquisite sensitivity to large-scale magnetic fields in situ at high redshift.