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Published January 16, 2014 | Submitted + Published
Journal Article Open

Baryons do trace dark matter 380,000 years after the big bang: Search for compensated isocurvature perturbations with WMAP 9-year data


Primordial isocurvature fluctuations between photons and either neutrinos or nonrelativistic species such as baryons or dark matter are known to be subdominant to adiabatic fluctuations. Perturbations in the relative densities of baryons and dark matter (known as compensated isocurvature perturbations or CIPs), however, are surprisingly poorly constrained. CIPs leave no imprint in the cosmic microwave background (CMB) on observable scales, at least at linear order in their amplitude and zeroth order in the amplitude of adiabatic perturbations. It is thus not yet empirically known if baryons trace dark matter at the surface of last scattering. If CIPs exist, they would spatially modulate the Silk damping scale and acoustic horizon, causing distinct fluctuations in the CMB temperature/polarization power spectra across the sky: this effect is first order in both the CIP and adiabatic mode amplitudes. Here, temperature data from the Wilkinson Microwave Anisotropy Probe (WMAP) are used to conduct the first CMB-based observational search for CIPs, using off-diagonal correlations and the CMB trispectrum. Reconstruction noise from weak lensing and point sources is shown to be negligible for this data set. No evidence for CIPs is observed, and a 95% confidence upper limit of 1.1×10^(−2) is imposed to the amplitude of a scale-invariant CIP power spectrum. This limit agrees with CIP sensitivity forecasts for WMAP and is competitive with smaller-scale constraints from measurements of the baryon fraction in galaxy clusters. It is shown that the root-mean-squared CIP amplitude on 5–100° scales is smaller than ∼0.07–0.17 (depending on the scale) at the 95% confidence level. Temperature data from the Planck satellite will provide an even more sensitive probe for the existence of CIPs, as will the upcoming ACTPol and SPTPol experiments on smaller angular scales.

Additional Information

© 2014 American Physical Society. Received 23 June 2013; published 16 January 2014. We acknowledge useful conversations with D. N. Spergel, C. Dvorkin, and K. M. Smith. D. G. was supported at the Institute for Advanced Study by the National Science Foundation (AST-0807044) and NASA (NNX11AF29G). M. K. was supported by the Department of Energy (DoESC-0008108) and NASA (NNX12AE86G). 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. This work was supported by a CITA National Fellowship at McGill. Some of the results in this paper have been derived using HEALPIX [92]. This work was begun during the 2011 winter conference "Inflationary theory and its confrontation with data in the Planck era" at the Aspen Center for Physics (NSF Grant No. 1066293). The authors are very grateful for the hospitality of the Aspen Center.

Attached Files

Published - PhysRevD.89.023006.pdf

Submitted - 1306.4319v1.pdf


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