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      Cosmic Polarization Rotation: an Astrophysical Test of Fundamental Physics

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          Abstract

          Possible violations of fundamental physical principles, e.g. the Einstein Equivalence Principle on which all metric theories of gravity are based, including General Relativity, would lead to a rotation of the plane of polarization for linearly polarized radiation traveling over cosmological distances, the so-called cosmic polarization rotation (CPR). We review here the astrophysical tests which have been carried out so far to check if CPR exists. These are using the radio and UV polarization of radio galaxies and the polarization of the cosmic microwave background (both E-mode and B-mode). These tests so far have been negative, leading to upper limits of the order of one degree on any CPR angle, thereby increasing our confidence in those physical principles, including General Relativity. We also discuss future prospects in detecting CPR or improving the constraints on it.

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          Is the Universe rotating?

          P. Birch (1982)
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            Q & A Experiment to Search for Vacuum Dichroism, Pseudoscalar-Photon Interaction and Millicharged Fermions

            A number of experiments are underway to detect vacuum birefringence and dichroism -- PVLAS, Q & A, and BMV. Recently, PVLAS experiment has observed optical rotation in vacuum by a magnetic field (vacuum dichroism). Theoretical interpretations of this result include a possible pseudoscalar-photon interaction and the existence of millicharged fermions. Here, we report the progress and first results of Q & A (QED [quantum electrodynamics] and Axion) experiment proposed and started in 1994. A 3.5-m high-finesse (around 30,000) Fabry-Perot prototype detector extendable to 7-m has been built and tested. We use X-pendulums and automatic control schemes developed by the gravitational-wave detection community for mirror suspension and cavity control. To polarize the vacuum, we use a 2.3-T dipole permanent magnet, with 27-mm-diameter clear borehole and 0.6-m field length,. In the experiment, the magnet is rotated at 5-10 rev/s to generate time-dependent polarization signal with twice the rotation frequency. Our ellipsometer/polarization-rotation-detection-system is formed by a pair of Glan-Taylor type polarizing prisms with extinction ratio lower than 10-8 together with a polarization modulating Faraday Cell with/without a quarter wave plate. We made an independent calibration of our apparatus by performing a measurement of gaseous Cotton-Mouton effect of nitrogen. We present our first experimental results and give a brief discussion of our experimental limit on pseudo-scalar-photon interaction and millicharged fermions.
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              Bayesian Analysis of the Polarization of Distant Radio Sources: Limits on Cosmological Birefringence

              A recent study of the rotation of the plane of polarization of light from 160 cosmological sources claims to find significant evidence for cosmological anisotropy. We point out methodological weaknesses of that study, and reanalyze the same data using Bayesian methods that overcome these problems. We find that the data always favor isotropic models for the distribution of observed polarizations over counterparts that have a cosmological anisotropy of the type advocated in the earlier study. Although anisotropic models are not completely ruled out, the data put strong lower limits on the length scale \(\lambda\) (in units of the Hubble length) associated with the anisotropy; the lower limits of 95% credible regions for \(\lambda\) lie between 0.43 and 0.62 in all anisotropic models we studied, values several times larger than the best-fit value of \(\lambda \approx 0.1\) found in the earlier study. The length scale is not constrained from above. The vast majority of sources in the data are at distances closer than 0.4 Hubble lengths (corresponding to a redshift of \(\approx\)0.8); the results are thus consistent with there being no significant anisotropy on the length scale probed by these data.
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                Author and article information

                Journal
                2015-01-26
                2015-02-16
                Article
                10.1142/S0218271815300165
                1501.06460
                2c61c042-5e08-4577-893f-40746913d48e

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

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                International Journal of Modern Physics D, Vol. 24, No. 5 (2015) 1530016
                Accepted for publication by the International Journal of Modern Physics D. To be published also as a chapter in the book "One Hundred Years of General Relativity: Cosmology and Gravity", edited by Wei-Tou Ni (World Scientific, Singapore, 2015). DOI and Journal reference added, a few typos corrected
                astro-ph.CO

                Cosmology & Extragalactic astrophysics
                Cosmology & Extragalactic astrophysics

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