TY - JOUR

T1 - Taking the Universe's Temperature with Spectral Distortions of the Cosmic Microwave Background

AU - Hill, J. Colin

AU - Battaglia, Nick

AU - Chluba, Jens

AU - Ferraro, Simone

AU - Schaan, Emmanuel

AU - Spergel, David N.

N1 - Publisher Copyright:
© 2015 American Physical Society.

PY - 2015/12/23

Y1 - 2015/12/23

N2 - The cosmic microwave background (CMB) energy spectrum is a near-perfect blackbody. The standard model of cosmology predicts small spectral distortions to this form, but no such distortion of the sky-averaged CMB spectrum has yet been measured. We calculate the largest expected distortion, which arises from the inverse Compton scattering of CMB photons off hot, free electrons, known as the thermal Sunyaev-Zel'dovich (TSZ) effect. We show that the predicted signal is roughly one order of magnitude below the current bound from the COBE-FIRAS experiment, but it can be detected at enormous significance (1000σ) by the proposed Primordial Inflation Explorer (PIXIE). Although cosmic variance reduces the effective signal-to-noise ratio to 230σ, this measurement will still yield a subpercent constraint on the total thermal energy of electrons in the observable Universe. Furthermore, we show that PIXIE can detect subtle relativistic effects in the sky-averaged TSZ signal at 30σ, which directly probe moments of the optical depth-weighted intracluster medium electron temperature distribution. These effects break the degeneracy between the electron density and the temperature in the mean TSZ signal, allowing a direct inference of the mean baryon density at low redshift. Future spectral distortion probes will thus determine the global thermodynamic properties of ionized gas in the Universe with unprecedented precision. These measurements will impose a fundamental "integral constraint" on models of galaxy formation and the injection of feedback energy over cosmic time.

AB - The cosmic microwave background (CMB) energy spectrum is a near-perfect blackbody. The standard model of cosmology predicts small spectral distortions to this form, but no such distortion of the sky-averaged CMB spectrum has yet been measured. We calculate the largest expected distortion, which arises from the inverse Compton scattering of CMB photons off hot, free electrons, known as the thermal Sunyaev-Zel'dovich (TSZ) effect. We show that the predicted signal is roughly one order of magnitude below the current bound from the COBE-FIRAS experiment, but it can be detected at enormous significance (1000σ) by the proposed Primordial Inflation Explorer (PIXIE). Although cosmic variance reduces the effective signal-to-noise ratio to 230σ, this measurement will still yield a subpercent constraint on the total thermal energy of electrons in the observable Universe. Furthermore, we show that PIXIE can detect subtle relativistic effects in the sky-averaged TSZ signal at 30σ, which directly probe moments of the optical depth-weighted intracluster medium electron temperature distribution. These effects break the degeneracy between the electron density and the temperature in the mean TSZ signal, allowing a direct inference of the mean baryon density at low redshift. Future spectral distortion probes will thus determine the global thermodynamic properties of ionized gas in the Universe with unprecedented precision. These measurements will impose a fundamental "integral constraint" on models of galaxy formation and the injection of feedback energy over cosmic time.

UR - http://www.scopus.com/inward/record.url?scp=84953220820&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=84953220820&partnerID=8YFLogxK

U2 - 10.1103/PhysRevLett.115.261301

DO - 10.1103/PhysRevLett.115.261301

M3 - Article

C2 - 26764983

AN - SCOPUS:84953220820

VL - 115

JO - Physical Review Letters

JF - Physical Review Letters

SN - 0031-9007

IS - 26

M1 - 261301

ER -