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The Temperature of the Cosmic Microwave Background at 10 Ghz D The Astrophysical Journal, 612:86–95, 2004 September 1 # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE TEMPERATURE OF THE COSMIC MICROWAVE BACKGROUND AT 10 GHZ D. J. Fixsen,1 A. Kogut,2 S. Levin,3 M. Limon,1 P. Lubin,4 P. Mirel,1 M. Seiffert,3 and E. Wollack2 Received 2004 February 23; accepted 2004 April 14 ABSTRACT We report the results of an effort to measure the low-frequency portion of the spectrum of the cosmic microwave background (CMB) radiation, using a balloon-borne instrument called the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE). These measurements are to search for deviations from a thermal spectrum that are expected to exist in the CMB as a result of various processes in the early universe. The radiometric temperature was measured at 10 and 30 GHz using a cryogenic open-aperture instrument with no emissive windows. An external blackbody calibrator provides an in situ reference. Systematic errors were greatly reduced by using differential radiometers and cooling all critical components to physical temperatures approxi- mating the antenna temperature of the sky. A linear model is used to compare the radiometer output to a set of thermometers on the instrument. The unmodeled residualsarelessthan50mKpeaktopeakwithaweightedrms of 6 mK. Small corrections are made for the residual emission from the flight train, atmosphere, and foreground Galactic emission. The measured radiometric temperature of the CMB is 2:721 Æ 0:010 K at 10 GHz and 2:694 Æ 0:032 K at 30 GHz. Subject headinggs: cosmic microwave background — cosmology: observations 1. INTRODUCTION The primary cooling mechanism is Compton scattering of hot electrons against a colder background of CMB photons, Since the discovery of the cosmic microwave back- characterized by the dimensionless integral ground (CMB), a key question has been, how does it de- Z ÂÃ viate from a perfect uniform blackbody spectrum? The Far z kT(z) À T (z) dt Infrared Absolute Spectrophotometer (FIRAS) instrument y e n (z)c dz0 1 ¼ 2 T e 0 ð Þ (Fixsen & Mather 2002; Brodd et al. 19975)hasshown 0 mec dz that the spectrum is nearly an ideal blackbody spectrum of the electron pressure n kT along the line of sight, where from 60 to 600 GHz with temperature 2:725 Æ 0:001 K. e e m , n ,andT are the electron mass, spatial density, and At lower frequencies the spectrum is not so tightly con- e e e temperature, respectively, T is the photon temperature, k is strained; plausible physical processes (e.g., reionization, Boltzmann’s constant, z is redshift, and denotes the particle decay) could generate detectable distortions below T Thomson cross section (Sunyaev & Zel’dovich 1970). For 10 GHz while remaining undetectable by the FIRAS instru- recent energy releases z < 104, the gas is optically thin, ment. The Absolute Radiometer for Cosmology, Astro- resulting in a uniform decrement ÁT T (1 2y)inthe physics, and Diffuse Emission (ARCADE) experiment RJ ¼ À Rayleigh-Jeans part of the spectrum, where there are too few observes the CMB spectrum at frequencies a decade below photons, and an exponential rise in temperature in the Wien FIRAS to search for potential distortions from a blackbody region, with too many photons. The magnitude of the distortion spectrum. is related to the total energy transfer The frequency spectrum of the CMB carries a history of energy transfer between the evolving matter and radiation ÁE ¼ e4y À 1 4y: ð2Þ fields in the early universe. Energetic events in the early uni- E verse (e.g., particle decay, star formation) heat the diffuse matter, which then cools via interactions with the background Energy transfer at higher redshift 104 < z < 107 approaches radiation, distorting the radiation spectrum away from a the equilibrium Bose-Einstein distribution, characterized by blackbody. The amplitude and shape of the resulting distortion the dimensionless chemical potential 0 ¼ 1:4(ÁE=E ). Free- depend on the magnitude and redshift of the energy transfer free emission thermalizes the spectrum at long wavelengths. (Burigana et al. 1991, 1995). Including this effect, the chemical potential becomes fre- quency dependent, 2x (x) ¼ exp À b ; ð3Þ 0 x 1 Science Systems and Applications, Inc., NASA Goddard Space Flight Center, Code 685, Greenbelt, MD 20771; fi[email protected]. where xb is the transition frequency at which Compton scat- 2 Laboratory for Astronomy and Solar Physics, NASA Goddard Space tering of photons to higher frequencies is balanced by free- Flight Center, Code 685, Greenbelt, MD 20771. free creation of new photons. The resulting spectrum has a 3 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak sharp drop in brightness temperature at centimeter wave- Grove Drive, Pasadena, CA 91109. 4 Department of Physics, University of California, Broida Hall, Building lengths (Burigana et al. 1991). A chemical potential distortion 572, Santa Barbara, CA 93106. would arise, for instance, from the late decay of heavy par- 5 AvailableinelectronicformfromtheNSSDC. ticles produced at much higher redshifts. 86 TEMPERATURE OF CMB AT 10 GHz 87 Uncertainties in previous measurements have been domi- nated by systematic uncertainties in the correction for emis- sion from the atmosphere, Galactic foregrounds, or warm parts of the instrument. ARCADE represents a long-term effort to improve measurements at centimeter wavelengths using a cryogenic balloon-borne instrument designed to minimize these systematic errors. This paper presents the first results from the ARCADE program. 2. THE INSTRUMENT ARCADE is a balloon-borne instrument with two radiom- eters at 10 and 30 GHz mounted in a liquid helium Dewar. Each radiometer consists of cryogenic and room temperature components. A corrugated horn antenna, a Dicke switch con- sisting of a waveguide ferrite latching switch, an internal ref- erence load constructed from a waveguide termination, and a Fig. 1.—Current 95% confidence upper limits to distorted CMB spectra. GaAs HEMT amplifier comprise the cryogenic components. Measurements at short wavelengths ( Fixsen et al. 1996) do not preclude The signal then passes to a 270 K section consisting of addi- detectable signals at wavelengths longer than 1 cm. tional amplification and separation into two subbands followed by diode detectors, making four channels in all (Kogut et al. Free-free emission can also be an important cooling 2004a). Helium pumps and heaters allow thermal control of the mechanism. The distortion to the present-day CMB spectrum cryogenic components, which are kept at 2–8 K during the is given by critical observations. There is an external calibrator that can be positioned to fully YA A cover the aperture of either the 10 or 30 GHz horn (Kogut et al. ÁT ¼ T 2 ; ð4Þ x 2004b). The horn is corrugated to limit the field near the wall, and the external target covers the full aperture of the horn. The where x is the dimensionless frequency h/kT , Yff is the op- tical depth to free-free emission gap between the two is only 15% of a wavelength at 10 GHz. The radiation leaking through this gap was measured to be Z ÂÃ À5 z 6 2 2 10 . Most of this radiation leaks in from the sky as the gap kTe(z) À T (z) 8e h ne g dt 0 YA ¼ ÀÁffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dz ; ð5Þ at the aperture plane is surrounded by the cold reflective flares T (z) 3p dz0 0 e 3me kT 6mekTe but is open to the sky. The induced signal on the radiometer is at most 0.1 mK. and g is the Gaunt factor (Bartlett & Stebbins 1991). The Residual reflections from the calibrator are effectively distorted CMB spectrum is characterized by a quadratic rise in trapped within the horn/calibrator system. Within this system temperature at long wavelengths. Such a distortion is an the calibrator absorbs almost all of the radiation because the expected signal of the reionization of the intergalactic medium horn emissivity is low. The effect of reflection from the horn/ by the first collapsed structures. calibrator system is proportional to the difference in temper- Figure 1 shows current upper limits to CMB spectral dis- ature between the calibrator and the antenna throat, both of tortions. Measurements at wavelengths shorter than 1 cm are which remain near 2.7 K throughout the observations. consistent with a blackbody spectrum, limiting y < 14 ; 10À6 To minimize instrumental systematic effects, the horns are and <9 ; 10À5 at 95% confidence (Fixsen et al. 1996; Gush cooled to approximately the temperature of the CMB (2.7 K). et al. 1990). Direct observational limits at longer wavelengths Thehornshavea16 FWHM beam and are pointed 30 from are weak. Reionization is expected to produce a cosmological the zenith to minimize acceptance of balloon and flight train free-free background with amplitude of a few mK at frequency emission. A helium-cooled flare reduces contamination from 3 GHz (Haiman & Loeb 1997; Oh 1999). The most precise ground emission. No windows are used. Air is kept from the observations (Table 1) have uncertainties much larger than the instrument by the efflux of helium gas. predicted signal from reionization. Existing data only constrain The Dicke switch chops between the reference and the horn 5 jYAj < 1:9 ; 10À , corresponding to temperature distortions antenna at 100 Hz. Following the HEMT amplifier, outside the ÁT < 19 mK at 3 GHz (Bersanelli et al. 1994). Dewar, each radiometer has a warm amplifier followed by TABLE 1 Previous Low-Frequency CMB Measurements and Their Uncertainties from the Literature Temperature Uncertainty Frequency (K) (mK) (GHz) Source 2.783...................... 25 25 Johnson & Wilkinson (1987) 2.730...................... 14 10.7 Staggs et al. (1996a) 2.64........................ 60 7.5 Levin et al. (1992) 2.55.......................
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