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A ¯At Universe from High-Resolution Maps of the Cosmic Microwave Background Radiation articles A ¯at Universe from high-resolution maps of the cosmic microwave background radiation P. de Bernardis1,P.A.R.Ade2, J. J. Bock3, J. R. Bond4, J. Borrill5,12, A. Boscaleri6, K. Coble7, B. P. Crill8, G. De Gasperis9, P. C. Farese7, P. G. Ferreira10, K. Ganga8,11, M. Giacometti1, E. Hivon8, V. V. Hristov8, A. Iacoangeli1, A. H. Jaffe12, A. E. Lange8, L. Martinis13, S. Masi1, P. V. Mason8, P. D. Mauskopf14,15, A. Melchiorri1, L. Miglio16, T. Montroy7, C. B. Netter®eld16, E. Pascale6, F. Piacentini1, D. Pogosyan4, S. Prunet4,S.Rao17, G. Romeo17, J. E. Ruhl7, F. Scaramuzzi13, D. Sforna1 & N. Vittorio9 ............................................................................................................................................................................................................................................................................ The blackbody radiation left over from the Big Bang has been transformed by the expansion of the Universe into the nearly isotropic 2.73 K cosmic microwave background. Tiny inhomogeneities in the early Universe left their imprint on the microwave background in the form of small anisotropies in its temperature. These anisotropies contain information about basic cosmological parameters, particularly the total energy density and curvature of the Universe. Here we report the ®rst images of resolved structure in the microwave background anisotropies over a signi®cant part of the sky. Maps at four frequencies clearly distinguish the microwave background from foreground emission. We compute the angular power spectrum of the microwave background, and ®nd a peak at Legendre multipole l peak 197 6 6, with an amplitude DT 200 69 6 8 mK. This is consistent with that expected for cold dark matter models in a ¯at (euclidean) Universe, as favoured by standard in¯ationary models. 1=2 Photons in the early Universe were tightly coupled to ionized matter lpeak < 200=­0 . A precise measurement of lpeak can ef®ciently through Thomson scattering. This coupling ceased about 300,000 constrain the density and thus the curvature of the Universe. years after the Big Bang, when the Universe cooled suf®ciently to Observations of CMB anisotropies require extremely sensitive form neutral hydrogen. Since then, the primordial photons have and stable instruments. The DMR5 instrument on the COBE travelled freely through the Universe, redshifting to microwave satellite mapped the sky with an angular resolution of ,78, yielding frequencies as the Universe expanded. We observe those photons measurements of the angular power spectrum at multipoles l , 20. today as the cosmic microwave background (CMB). An image of the Since then, experiments with ®ner angular resolution6±16 have early Universe remains imprinted in the temperature anisotropy of detected CMB ¯uctuations on smaller scales and have produced the CMB. Anisotropies on angular scales larger than ,28 are evidence for the presence of a peak in the angular power spectrum at dominated by the gravitational redshift the photons undergo as lpeak < 200. they leave the density ¯uctuations present at decoupling1,2. Aniso- Here we present high-resolution, high signal-to-noise maps of tropies on smaller angular scales are enhanced by oscillations of the the CMB over a signi®cant fraction of the sky, and derive the angular photon±baryon ¯uid before decoupling3. These oscillations are power spectrum of the CMB from l 50 to 600. This power driven by the primordial density ¯uctuations, and their nature spectrum is dominated by a peak at multipole lpeak 197 6 6 depends on the matter content of the Universe. (1j error). The existence of this peak strongly supports in¯ationary In a spherical harmonic expansion of the CMB temperature ®eld, models for the early Universe, and is consistent with a ¯at, euclidean the angular power spectrum speci®es the contributions to the Universe. ¯uctuations on the sky coming from different multipoles l, each corresponding to the angular scale v p=l. Density ¯uctuations The instrument over spatial scales comparable to the acoustic horizon at decoupling The Boomerang (balloon observations of millimetric extragalactic produce a peak in the angular power spectrum of the CMB, radiation and geomagnetics) experiment is a microwave telescope occurring at multipole lpeak. The exact value of lpeak depends on that is carried to an altitude of ,38 km by a balloon. Boomerang both the linear size of the acoustic horizon and on the angular combines the high sensitivity and broad frequency coverage pio- diameter distance from the observer to decoupling. Both these neered by an earlier generation of balloon-borne experiments with quantities are sensitive to a number of cosmological parameters the long (,10 days) integration time available in a long-duration (see, for example, ref. 4), but lpeak primarily depends on the total balloon ¯ight over Antarctica. The data described here were density of the Universe, ­0. In models with a density ­0 near 1, obtained with a focal plane array of 16 bolometric detectors cooled to 0.3 K. Single-mode feedhorns provide two 189 full- 1 Dipartimento di Fisica, Universita' di Roma ``La Sapienza'', P.le A. Moro 2, 00185 Roma, Italy. width at half-maximum (FWHM) beams at 90 GHz and two 109 2 Department of Physics, Queen Mary and West®eld College, Mile End Road, London E1 4NS, UK. FWHM beams at 150 GHz. Four multi-band photometers each 3 Jet Propulsion Laboratory, Pasadena, California 91109, USA. 4 CITAUniversity of Toronto, TorontoM5S 3H8, Canada. 5 NERSC-LBNL, Berkeley, California 94720, USA. 6 IROE±CNR, Via Panciatichi 64, 50127 provide a 10.59,149 and 139 FWHM beam at 150, 240 and 400 GHz Firenze, Italy. 7 Department of Physics, University of California at Santa Barbara, Santa Barbara, respectively. The average in-¯ight sensitivity to CMB anisotropies California 93106, USA. 8 California Institute of Technology, Mail Code 59-33, Pasadena, California was 140, 170, 210 and 2,700 mKs1/2 at 90, 150, 240 and 400 GHz, 91125, USA. 9 Dipartimento di Fisica, Universita' di Roma Tor Vergata, Via della Ricerca Scienti®ca 1, 00133 Roma, Italy. 10 Astrophysics, University of Oxford, Keble Road, OX1 3RH, UK. 11 PCC, College de respectively. The entire optical system is heavily baf¯ed against France, 11 pl. Marcelin Berthelot, 75231 Paris Cedex 05, France. 12 Center for Particle Astrophysics, terrestrial radiation. Large sunshields improve rejection of radiation University of California at Berkeley, 301 Le Conte Hall, Berkeley, California 94720, USA. 13 ENEA Centro Ricerche di Frascati, Via E. Fermi 45, 00044 Frascati, Italy. 14 Physics and Astronomy Department, Cardiff from .608 in azimuth from the telescope boresight. The rejection University, Cardiff CF2 3YB, UK. 15 Department of Physics and Astronomy, University of Massachusetts, has been measured to be greater than 80 dB at all angles occupied by Amherst, Massachusetts 01003, USA. 16 Department of Physics and Astronomy, University of Toronto, the Sun during the CMB observations. Further details on the Toronto M5S 3H8, Canada. 17 Istituto Nazionale di Geo®sica, Via di Vigna Murata 605, 00143, Roma, Italy. instrument can be found in refs 17±21. NATURE | VOL 404 | 27 APRIL 2000 | www.nature.com © 2000 Macmillan Magazines Ltd 955 articles Observations stellar dust22 and that is approximately opposite the Sun during the Boomerang was launched from McMurdo Station (Antarctica) on austral summer. We mapped this region by repeatedly scanning the 29 December 1998, at 3:30 GMT. Observations began 3 hours later, telescope through 608 at ®xed elevation and at constant speed. Two and continued uninterrupted during the 259-hour ¯ight. The scan speeds (18 s-1 and 28 s-1 in azimuth) were used to facilitate tests payload approximately followed the 798 S parallel at an altitude for systematic effects. As the telescope scanned, degree-scale varia- that varied daily between 37 and 38.5 km, returning within 50 km of tions in the CMB generated sub-audio frequency signals in the the launch site. output of the detector23. The stability of the detector system was We concentrated our observations on a target region, centred at suf®cient to allow sensitive measurements on angular scales up to roughly right ascension (RA) 5h, declination (dec.) -458, that is tens of degrees on the sky. The scan speed was suf®ciently rapid with uniquely free of contamination by thermal emission from inter- respect to sky rotation that identical structures were observed by –300 –200–100 0 100 200 300 µK b = –10 b = –10 –30 b = –20 b = –20 –35 –40 –45 Dec. (deg.) –50 –55 –60 90 GHz 90 GHz - 150 GHz b = –10 b = –10 –30 b = –20 b = –20 –35 –40 –45 Dec. (deg.) –50 –55 –60 150 GHz 240 GHz - 150 GHz b = –10 b = –10 –30 b = –20 b = –20 –35 –40 –45 Dec. (deg.) –50 –55 –60 240 GHz 400 GHz 30 45 60 75 90 105 120 135 30 45 60 75 90 105 120 135 RA (deg.) RA (deg.) Figure 1 Boomerang sky maps (equatorial coordinates). The sky maps at 90, 150 and ISD emission mapped by IRAS/DIRBE22. This emission is strongly concentrated towards 240 GHz (left panels) are shown with a common colour scale, using a thermodynamic the right-hand edge of the maps, near the plane of the Galaxy. The same structures are temperature scale chosen such that CMB anisotropies will have the same amplitude in the evident in the 90, 150 and 240 GHz maps at Galactic latitude b .2158, albeit with an three maps. Only the colour scale of the 400 GHz map (bottom right) is 14 times larger amplitude that decreases steeply with decreasing frequency. The large-scale gradient than the others: this has been done to facilitate comparison of emission from interstellar evident especially near the right edge of the 240 GHz map is a result of high-pass-®ltering dust (ISD), which dominates this map, with ISD emission present in the lower-frequency the very large signals near the Galactic plane (not shown).
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