Partridge, Bruce

Wilkinson, David T.

English

Haverford College: Haverford Scholarship

Haverford College Haverford Scholarship Faculty Publications Astronomy 1967 Isotropy and Homogeneity of the Universe from Measurements of the Cosmic Microwave Background Bruce Partridge Haverford College, bpartrid@haverford.edu David T. Wilkinson Follow this and additional works at: https://scholarship.haverford.edu/astronomy_facpubs Repository Citation (with D. T. Wilkinson) Isotropy and Homogeneity of the Universe from Measurements of the Cosmic Microwave Background, Physical Review Letters, Vol. 18, No. 14, 557, April (1967). This Journal Article is brought to you for free and open access by the Astronomy at Haverford Scholarship. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Haverford Scholarship. For more information, please contact nmedeiro@haverford.edu. Haverford CollegeHaverford ScholarshipFaculty Publications Astronomy1967Isotropy and Homogeneity of the Universe fromMeasurements of the Cosmic MicrowaveBackgroundR. Bruce PartridgeHaverford CollegeDavid T. WilkinsonFollow this and additional works at: http://scholarship.haverford.edu/astronomy_facpubsThis Journal Article is brought to you for free and open access by the Astronomy at Haverford Scholarship. It has been accepted for inclusion in FacultyPublications by an authorized administrator of Haverford Scholarship. For more information, please contact nmedeiro@haverford.edu.Repository Citation(with D. T. Wilkinson) Isotropy and Homogeneity of the Universe from Measurements of the Cosmic Microwave Background,Physical Review Letters, Vol. 18, No. 14, 557, April (1967).VOLUME 18, NUMBER 14 P H Y S I C A I. R E V I E %" L E T T E R S $ APRIL 1967ISOTROPY AND HOMOGENEITY OF THE UNIVERSE FROM MEASUREMENTSOF THE COSMIC MICROWAVE BACKGROUND~R. B. Partridge and David T. Wilkinson)Palmer Physical Laboratory, Princeton, New Jersey(H,eceived 2 March 1967)A Dicke radiometer (3.2-cm wavelength) was used to make daily scans near the celes-tia, l equator to look for possible anisotropy in the cosmic blackbody radiation. Afterabout one year of intermittent operation we find no 24-h asymmetry with an amplitudegreater than +0.1% (of 3 K). There is, however, a possibly significant 12-h anisotropywith an amplitude of about 0.2Vo.Measurements' of the intensity of the cosmicbackground radiation at several wavelengthssupport the suggestion' that this radiation hasa blackbody spectrum. If further measurements,particularly at short wavelengths, continueto fit a blackbody spectrum, then we must takeseriously the idea' that the universe is filledwith blackbody radiation which is a relic ofa hot, highly contracted phase of the universe.A measurement of the isotropy of this radia-tion is of considerable cosmological interest,for the radiation serves as a probe to studythe isotropy of the expansion of the universe.Thorne~ has shown that an anisotropy in theexpansion subsequent to the last interactionof the radiation with matter can contribute tothe anisotropy of the background radiation.This last scattering occurs at a redshift of atleast hX/A = 7, and may occur at a much ear-lier time if the density of the intergalactic plas-ma is low. ' Another effect has recently beensuggested by Sachs and Wolfe. They show thata large-scale (-3 && 10' light yr) density inho-mogeneity, of magnitude 5p/p-10%, could pro-duce inhomogeneity and anisotropy in the back-ground-radiation temperature as large as —,'%.For the last year, we have looked for anisot-ropy in the microwave background along thecelestial equator.The instrument used in this work is shownschematically in Fig. 1. The Dicke radiome-ter is a modified version of the one' used tomake an absolute intensity measurement ofthe background radiation. The principal mod-ifications were these: (1) The cold referencetermination was replaced by a small horn an-tenna pointed to the zenith, and (2) the radiom-eter was tipped to an elevation of 42' abovethe southern horizon. As the earth rotates,the antenna beam (width -10 to -10 dB) scansa region running parallel to the celestial equa-tor at a declination of about -8 . If the inten-sity of the background radiation is varying alongthis region, we would expect a sidereal peri-odicity in the recorder output.To provide a reference for the instrument,a large aluminum reflecting sheet was raisedto the vertical to deflect the antenna beam to-ward the celestial pole (near Polaris). Thispoint served as a fixed reference source ofbackground radiation. The reflector was raisedfor 15 min every half -hour to subtract out theeffects of solar heating of the apparatus anddiurnal variations in the atmospheric radiationtemperature (-5'K). Both the measuring beam(near the celestial equator) and the referencebeam (near Polaris) can have the same zenithangle. Thus the two beams pass through thesame thickness of the atmosphere, and atmo-//////ZENITHHORNREF L ECTORBALANCEATTENUATOR IR AND RC*ISOseCOCK IN AMP ~~RECORDERRC$FIG. 1. The isotropometer. The zenith horn and at-tenuator provide a convenient means of balancing thera,diometer inputs, and the sky horn is merely a coldtermination. The undeflected beam actually is pointed8 8 of the equator and the reflected beam misses Po-la,ris by 2'.557VOI.UME 18, NVMSZR 14 PHYSICAL REVIEW LEYYERS $ ApRIL 1/67Table I. The results of averaging -55 runs. Each error is the standard error of the mean. Because of the rela, —tively large error associated with the mean 24-h amplitude, very little weight should be attached to the hour angle.Siderealperiod(}1)Mean amplitude(10-'K) Anisotropy(percent of 3 K)Right a.scension ofmaximum tempera. ture12244.9+2.01.0+ 2.20.16 + 0.070.03+ 0.077 30 and 19 3015h30mspheric temperature drifts largely cancel out.The differences between the equatorial andpolar radiation temperatures, TE-Tp, wereread from the recorder chart at 15-min inter-vals for each 24-h run. The differences werethen analyzed for 24- and 12-h components byleast-squares fitting the functions [A,» cos( at/24+ y,»)+ C,»] and [A» cos(2wt/12+ p»)+ C»]to T~ 7'p (t-= sidereal time in h). The best-fit amplitudes and phases of each of -55 runswere found and a weighted average was made.These results are listed in Table I, and Fig.2 is a polar diagram of the 12-h results.)phX = FULL WEIGHT0 = —WEIGHT23'K9h , ghFIG. 2. The results of analyzing -55 isotropy runsfor 12-h components. Half-weighted runs were inter-rupted by weather or instrument failure so that a, com-plete 24-h series was not obtained. Note that the outercircle represents an a,mplitude of 1% of the backgroundradiation temperature of O'K. The a,vera, ge of all re-sults is indicated by the solid square.The 24-h asymmetry has an average ampli-tude of less than 0.1% (of O'K). This placesan upper limit of about 300 km/sec on the equa-torial component of the velocity of the sun withrespect to the comoving frame of the distantmatter which last scattered the background ra-diation. A 24-h component would also be ex-pected from the daily passage through the an-tenna beam of the central region of the galaxy.To estimate the size of this effect, we haveextrapolated long-wavelength measurementssof gala, ctic radiation to our wavelength (3.2 cm)and convoluted this temperature distributionwith our antenna pattern. The resulting galac-tic pulse should contribute 12- and 24-h ampli-tudes of only about 0.01%.The average amplitude of the 12-h componentis about 2& standard deviations greater than0 and is possibly a real anisotropy. One pos-sible spurious source of a 12-h sidereal com-ponent is the noon-time passage of the sun throughour antenna beam, which occurs only duringtwo seasons of the year. The result is a semi-annual modulation of a series of spikes sepa-rated by 24 h in solar time. Such a series con-tains a 12-h sidereal time component. To checkfor this effect we removed all data taken with-in 1~ h of solar noon and reanalyzed all the runs.The average amplitudes of the 12- and 24-hsidereal components did not change significant-ly.As a further check for possible systematicerrors due to solar heating of the apparatus,including the reflector itself, the daily runswere averaged over the year in solar time.Thus solar heating effects are not averagedout. The 12- and 24-h solar components haveamplitudes of less than or about 5&10 'K.Since our data were averaged over most of ayear, the solar-heating effects in the siderealaverages are at least 10 times smaller. Wefeel confident that the 12-h sidereal result isnot caused by solar-time effects.Finally, all the data in hourly (sidereal) binswere averaged, and the results examined for558Vor.vMz 18, NUMszR 14 PHYSI GAI. RKVIKW I.KTTKRS 3 Apart. 1967hot and cold spots. No local irregularities great-er than +-, lo were found along the directionsscanned. This result places an upper limit of+10' on density inhomogeneities of the typeconsidered in Ref. 6.Re mould like to thank Professor R. H. Dickeand Professor P. J. E. Peebles for a numberof very helpful discussions of several aspectsof this work. Much of the data analysis masperformed by K. C. Davis and A. A. Mondelli.~This research was supported in part by the U. S.Office of Naval Research and by the National ScienceFounda. tion.)Alfred P. Sloan Foundation Fellow.A. A. Penzias and R. W. Wilson, Astrophys. J. 142,419 (1965); P. G. Roll and D. T. Wilkinson, Phys. Rev.Letters 16, 405 (1966); T. F. Howell and J. R. Shake-shaft, Nature 210, 1318 (1966); G. B. Field and J. L.Hitchcock, Phys. Rev. Letters 16, 817 (1966); P. Thad-deus and J. F. Clauser, Phys. Rev. Letters 16, 819(1966&.2R. H. Dicke, P. J. E. Peebles, P. G. Roll, and D. T.Wilkinson, Astrophys. J. 142, 414 (1965).3G. Ga,mow, Nature 162, 680 (1948); in Vistas in As-tronomy, edited by A. Beers (Pergamon Press, NewYork, 1956), Vol. 2, p. 1726.4K. S. Thorne, to be published.J. E. Gunn and B. A. Peterson, Astrophys. J. 142,1633 {1965).6R. K. Sachs and A. M. Wolfe, Astrophys. J. 147, 73(1967).~P. G. Roll and D. T. Wilkinson, to be published.A. A. Penzias and R. W. Wilson, Astrophys. J. 146,666 (1966).QUANTUM THEORY OF LINEWIDTHS IN BRILLOUIN SCATTERING FROMPURE SOUND FIELDS IN THE LOW-DAMPING LIMIT*R. M. HermanDepartment of Physics, The Pennsylvania State University, University Park, Pennsylvania{Heceived 27 January 1967)Linearized classical hydrodynamic and heat-transport equations describing the collectivebehavior of density and temperature fluctua-tions in liquids have in the past been the sub-ject of intensive study. For example, Moun-tain' recently examined these equations andfrom his solutions deduced the spectral distri-bution of light scattered from liquids (spontane-ous Brillouin scattering). Among other clas-sical predictions is the following. ' If the soundfield is nondispersive and if the thermal coef-ficient of expansion is 0 (in which case the soundexcitations are independent of the thermal fluc-tuations —i.e. , we have a, "pure" sound field),the half-width (angular frequency) of the Bril-louin lines is equal to the inverse lifetime ofthe sound wave,(cia s.) =-'(, q +q )(0'/p ),g~ and q~ being the shear and bulk viscosities,p, being the ambient fluid density, and k beingthe wave vector of the excitation which scattersthe light, whose magnitude is related to thescattering angle 6) and the wave number k; ofthe incident light in vacuum through the equa-tionk = 2rsk. sin26',2n being the index of refraction for the liquid.It is the purpose of the present paper to showthat quantum mechanically, Eq. (1) is decided-ly not valid in low-damping situations; instead,quantum mechanics leads to the startling re-sultin this limit. Accordingly, it is not clear thatclassical calculations of line shapes in lightscattering by thermal sound excitations arevalid under any conditions.We begin by writing the quantum mechanicalexpression for the intensity of scattered radi-ation as a function of the frequency differencebetmeen the incident and scattered light (, andthe point of observation R,Nk. Be )I (R, (a') =I. — 2 2 —,P~(&u) sjn2cp „(4)7 inc ] 6~2+2Iinc representing the incident-light intensity(assumed to be monochromatic), %being thenumber of molecules in the scattering volume,and p being the angle between the polarizationaxis and the direction of observation. &k(&u)

Isotropy and Homogeneity of the Universe from Measurements of the Cosmic Microwave Background

https://scholarship.haverford.edu/cgi/viewcontent.cgi?article=1283&amp;context=astronomy_facpubs

Isotropy and Homogeneity of the Universe from Measurements of the Cosmic Microwave Background

Abstract

Similar works

Full text

Available Versions

Haverford College: Haverford Scholarship