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Published December 16, 2011 | Published
Journal Article Open

Compensated isocurvature perturbations and the cosmic microwave background


Measurements of cosmic microwave background (CMB) anisotropies constrain isocurvature fluctuations between photons and nonrelativistic particles to be subdominant to adiabatic fluctuations. Perturbations in the relative number densities of baryons and dark matter, however, are surprisingly poorly constrained. In fact, baryon-density perturbations of fairly large amplitude may exist if they are compensated by dark-matter perturbations, so that the total density remains unchanged. These compensated isocurvature perturbations (CIPs) leave no imprint on the CMB at observable scales, at linear order. B modes in the CMB polarization are generated at reionization through the modulation of the optical depth by CIPs, but this induced polarization is small. The strongest known constraint ≲10% to the CIP amplitude comes from galaxy-cluster baryon fractions. Here, it is shown that modulation of the baryon density by CIPs at and before the decoupling of Thomson scattering at z∼1100 gives rise to CMB effects several orders of magnitude larger than those considered before. Polarization B modes are induced, as are correlations between temperature/polarization spherical-harmonic coefficients of different lm. It is shown that the CIP field at the surface of last scatter can be measured with these off-diagonal correlations. The sensitivity of ongoing and future experiments to these fluctuations is estimated. Data from the WMAP, ACT, SPT, and Spider experiments will be sensitive to fluctuations with amplitude ∼5–10%. The Planck satellite and Polarbear experiment will be sensitive to fluctuations with amplitude ∼3%. SPTPol, ACTPol, and future space-based polarization methods will probe amplitudes as low as ∼0.4%–0.6%. In the cosmic-variance limit, the smallest CIPs that could be detected with the CMB are of amplitude ∼0.05%.

Additional Information

© 2011 American Physical Society. Received 28 July 2011; published 16 December 2011. We acknowledge useful conversations with C. Chiang, C. Dvorkin, G. Holder, M. LoVerde, K. M. Smith, T. L. Smith, D. N. Spergel, and M. Zaldarriaga. We thank B. Jones and A. Fraisse for providing updated parameters for Spider forecasting. D. G. was supported at the Institute for Advanced Study by the National Science Foundation (AST-0807044) and is grateful for the hospitality of the Aspen Center for Physics, where part of this work was completed. M. K. thanks the support of the Miller Institute for Basic Research in Science and the hospitality of the Department of Physics at the University of California, where part of this work was completed. This work was supported at Caltech by DoE DE-FG03-92- ER40701, NASA NNX10AD04G, and the Gordon and Betty Moore Foundation. Part of the research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

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