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COBE) (Microwave/Infrared) C Proc. Natl. Acad. Sci. USA Vol. 90, pp. 4766-4773, June 1993 Colloquium Paper This paper was presented at a colloquium entitled "Physical Cosmology," organized by a committee chaired by David N. Schramm, held March 27 and 28, 1992, at the National Academy of Sciences, Irvine, CA. Scientific results from the Cosmic Background Explorer (COBE) (microwave/infrared) C. L. BENNETT*, N. W. BOGGESS*, E. S. CHENG*, M. G. HAUSER*, T. KELSALL*, J. C. MATHER*, S. H. MOSELEY, JR.*, T. L. MURDOCKt, R. A. SHAFER*, R. F. SILVERBERG*, G. F. SMOOT*, R. WEISS§, AND E. L. WRIGHT$ *National Aeronautics and Space Administration/Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Greenbelt, MD 20771; tGeneral Research Corporation, 5 Cherry Hill Drive, Danvers, MA 01923; tBuilding 50-351, Lawrence Berkeley Laboratories, Berkeley, CA 94720; §Department of Physics, Room 20F-001, Massachusetts Institute of Technology, Cambridge, MA 02139; and 1Astronomy Department, University of California, Los Angeles, CA 90024 ABSTRACT The National Aeronautics and Space Admin- precision of 10- around the peak.... It will also look for the istration (NASA) has flown the COBE sateflite to observe the Big emission from cold dust clouds and from infrared galaxies"; Bang and the subsequent formation of galaxies and large-scale (ii) "measure the large scale isotropy of the background structure. Data from the Far-Infrared Absolute Spectrophotom- radiation ... to a precision of 10-5.... Measurements at eter (FIRAS) show that the spectrum of the cosmic microwave several wavelengths are required in order to distinguish background is that of a black body of temperature T = 2.73 ± anisotropy in the background radiation itselffrom anisotropy 0.06 K, with no deviation from a black-body spectrum greater due to discrete sources"; and (iii) ". search for diffuse than 0.25% of the peak brightness. The data from the Differ- radiation in the 5-30 micron wavelength range, expected to ential Microwave Radiometers (DMR) show statistically signif- arise from interplanetary dust, interstellar dust, and, in icant cosmic microwave background anisotropy, consistent with particular, from the integrated luminosity of very early gal- a scale-invariant primordial density fluctuation spectrum. Mea- axies. The experiment is designed to separate these contri- surements from the Diffuse Infrared Background Experiment butions by their spectral and directional properties." Addi- (DIRBE) provide new conservative upper limits to the cosmic tional proposals were also submitted for large angular scale infrared background. Extensive modeling of solar system and microwave isotropy experiments by Sam Gulkis et al. (3) galactic infrared foregrounds is required for further improve- from the Jet Propulsion Laboratory and by Luis Alvarez et al. ment in the cosmic infrared background limits. (4) from University ofCalifornia at Berkeley. NASA selected six investigators from these proposals and formed the core of what was to become the COBE Science Working Group. Introduction to the COBEII and Mission Objectives The three scientific instruments on COBE are the Far Infrared Absolute Spectrophotometer (FIRAS), the Differen- The observables of modern cosmology include the Hubble tial Microwave Radiometers (DMR), and the Diffuse Infrared expansion of the universe; the ages of stars and clusters; the Background Experiment (DIRBE). The FIRAS objective is to distribution and streaming motions ofgalaxies; the content of make a precision measurement of the spectrum of the CMB the universe (its mass density and composition and the from 1 cm to 100 ,um. The DMR objective is to search for CMB abundances of the light elements); the existence, spectrum, anisotropies on angular scales larger than 70 at frequencies of and anisotropy of the cosmic microwave background (CMB) 31.5, 53, and 90 GHz. The DIRBE objective is to search for a in the infrared, CIB by making absolute brightness measurements of the radiation; and other potential backgrounds diffuse infrared radiation in 10 photometric bands from 1 to 240 ultraviolet, x-ray, t-ray, etc. The purpose of the COBE gm and polarimetric measurements from 1 to 3.5 p.m. The mission is to make definitive measurements of two of these FIRAS and DIRBE instruments are located inside a 650-liter observable cosmological fossils: the CMB radiation and the superfluid liquid helium Dewar. A full description of the cosmic infrared background (CIB) radiation. Since the dis- COBE emission is given by Boggess et al. (5). Many papers covery of the CMB in 1964 (1), many experiments have been giving overviews, implications, and additional detailed infor- performed to measure the CMB spectrum and spatial an- mation about the COBE have been presented (6-17). isotropies over a wide range of wavelengths and angular scales. Fewer attempts have been made to conduct a sensi- from tive search for a CIB radiation, expected to result from the Spectral Results FIRAS cumulative emissions of luminous objects formed after the Spectrum of the Primeval Radiation. The discovery of the universe cooled sufficiently to permit the first stars and Penzias and Wilson (1) provided strong galaxies to form. CMB radiation by In 1974 NASA issued Announcements of Opportunity (AO-6 and AO-7) for new Explorer-class space missions. A Abbreviations: COBE, Cosmic Background Explorer; CMB, cosmic microwave background; CIB, cosmic infrared background; NASA, proposal for a Cosmic Background Radiation Satellite was National Aeronautics and Space Administration; FIRAS, Far Infra- submitted by John Mather et al. (2) from NASA/Goddard red Absolute Spectrophotometer; DMR, Differential Microwave Space Flight Center. The objectives of this mission were as Radiometers; DIRBE, Diffuse Infrared Background Experiment; follows: (i) make "definitive measurement of the spectrum CL, confidence limit; IRAS, Infrared Astronomical Satellite. (of the 2.7 K CBR [cosmic background radiation]) . .. with "The National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) is responsible for the design, development, and operation of the COBE. Scientific guidance is The publication costs of this article were defrayed in part by page charge provided by the COBE Science Working Group. GSFC is also payment. This article must therefore be hereby marked "advertisement" responsible for the development ofthe analysis software and for the in accordance with 18 U.S.C. §1734 solely to indicate this fact. production of the mission data sets. 4766 Downloaded by guest on September 25, 2021 CoRoquium Paper: Bennett et al. Proc. Nati. Acad. Sci. USA 90 (1993) 4767 evidence for Big Bang cosmology. Radiation produced in the In 10 months of cryogenic operation the FIRAS obtained very early universe was frequently scattered until about over two million interferograms. This complete data set is 300,000 years after the Big Bang. At this point, the "recom- now undergoing careful analysis. bination," the characteristic energy in the universe fell to the FIRAS Results. Analysis ofthe FIRAS data to date confirm point where previously free electrons could combine with the prediction of the simplest Big Bang model that the CMB nuclei to form neutral atoms. The 2.7-K radiation we see must have a thermal spectrum. Initial results based on only to us unimpeded since that time. The 9 min of data showed that there is no deviation from a today has been traveling black-body spectrum B,(T) as large as 1% of the peak rapid production and destruction of photons within the first brightness (19, 24) over the spectral range from 500 ,um to 1 year after the Big Bang forced the radiation to have a Planck cm. The temperature of the CMB in the direction of the north (black-body) spectrum. Any mechanism that injected energy galactic pole is 2.735 ± 0.060 K, where 60 mK is the initial into the Universe (e.g., a particle decay) between a year after conservative uncertainty in the calibration of the thermom- the Big Bang and =2000 years after the Big Bang would give etry of the absolute calibrator. These data also ruled out the rise to a radiation spectrum characterized by a nonzero existence of a hot smooth intergalactic medium that could chemical potential. Thus there would be a Bose-Einstein emit more than 3% of the observed x-ray background. The spectral distortion with the photon occupation number thermal character of the CMB spectrum was subsequently confirmed by Gush et al. (25), who obtained virtually the 1 same temperature over the spectral range 2-30 cm-1. Neither N(E) (e-,)/kT[1] mean CMB temperature quoted above is corrected for the dipole distortion. These experiments found no submillimeter where e is the photon energy, u is the chemical potential, k excess as previously reported by Matsumoto et al. (26). is the Boltzmann constant, and Tis the absolute temperature. More recently, Shafer et al. (27) and Cheng et al. (28) have A Compton distortion is usually parameterized in terms of a examined FIRAS spectra in a direction known previously to Compton y-parameter, be very low in interstellar material (1 = 1420, b = 55°). In this direction, known as Baade's Hole, the temperature is 2.730 C'Tr + 0.060 K and there is no deviation from a black-body Y= 2 Inek(Te- TCMB)c dt, [2] spectrum greater than 0.25% of the peak brightness. The lack MecJ of deviations from a Planck spectrum translates to a limit on a chemical potential (see Eq. 1) of j/kT < 0.005 [95% where UT iS the Thomson scattering cross section, me is the confidence limit (CL)] and a limit on the Compton y-param- mass of an electron, c is the velocity of light, and the integral eter (Eq. 2) of y < 0.0004 (95% CL). These results rule out a is the electron pressure along the line of sight. A Comp- hot smooth intergalactic medium that could emit more than ton distortion of the spectrum can become important when 1% of the observed x-ray background.
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