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A NEW MEASUREMENT OF THE COSMIC BLACK BODY RADIATION
D. Woody, N. Nishioka, P. Richards
To cite this version:
D. Woody, N. Nishioka, P. Richards. A NEW MEASUREMENT OF THE COSMIC BLACK BODY RADIATION. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-1629-C6-1630.
�10.1051/jphyscol:19786610�. �jpa-00218106�
JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-1629
A NEW MEASUREMENT OF THE COSMIC BLACK BODY RADIATION
D.P. Woody, N.S. Nishioka and P.L. Richards
Department of Physics, University of California, and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720, U.S.A.
Résumé.- Nous décrivons une expérience qui a permis de mesurer le spectre du rayonnement cosmique fondamental dans l'intervalle des nombres d'onde allant de 2 à 40 cm" . Diverses techniques de bas- ses températures furent utilisées, parmi lesquelles des pompes à effet fontaine et un détecteur re- froidi par 3He. Le spectre est un très bon accord avec celui d'un corps noir à 2,9 K.
Abstract.- An experiment to measure the spectrum of the cosmic background radiation over the wave- number range from 2 to 40 cm-1 is described. A variety of low temperature techniques including foun- tain effect pumps and a 3He cooled detector were used to obtain high accuracy and sensitivity. The resulting spectrum is a close fit to a 2.9 K black body.
The universe is filled with essentially iso- tropic IM homogeneous black body radiation with a characteristic temperature of ^ 3 K. This radiation is understood to be a redshifted remnant of the pri- mordial fireball, which "has not interacted with mat- ter for the last 2 x 1010 years. It is the fundamen- tal experimental fact which underlies modern "big bang" cosmological theory.
In this paper we describe the experiment which has verified the black body character of the spectrum of the cosmic background radiation on the high frequency side of the peak (which occurs at a wavenumber of 6 cm-1 or wavelength of 1.7 mm) /2/.
This experiment uses low temperature optical techni- ques to measure the emission spectrum of the night sky over the range from 2 to 40 cm-1 from a balloon platform at an altitude of 43 km. Below 12 cm-1 the observed spectrum shown in figure 1(a) is dominated, by the "V 3 K CBR. At higher frequencies the molecu-
lar line emission from H O , 0 , and 0 in the atmos- phere dominates the spectrum. A simple model of the atmospheric emission based on known molecular, at- mosphereic, and instrumental parameters is shown in figure 1(b). Two adjustable parameters in the model, the column densities of H 0 and 0 . have been fitted
2 3'
to the observed spectrum. A 1.7 K black body curve is subtracted from the atmospheric model spectrum to take account fo the finite reference temperature of the system. Figure 1(c) shows our measurements of the spect&um of the cosmic background radiation, which is obtained by subtracting 1(b) from 1(a).
This is the spectrum of more than 90 percent of the radiant energy in teh universe. Laboratory calibra- tions of the instrumental response to black body '
radiators operated in the temperature range from 5 to 300 K show that the spectrum is very close to that of a 2.9 K black body.
Night Sky Emission
V c o
£• I Calculated Atmospheric
<u I Emission o I I I
Cosmic Background Radiation
0<f^— _ 2 ^ a ~AA-^—'"-Vv .*jV:y •"•••-VV'-^^W -l* * ' l - " " ' - « . v 1 1 1 I
O 10 2 0 3 0 40 5 0 - Frequency fcnrf'J
Fig. 1 : (a) Measured spectrum of the night sky from 43 km. (b) Computer simulation of the emission of atmospheric 02, 03, and H20 as well as the effects of the spectrometer reference temperature. The co- lumn densities of 03 and H20 we're obtained by fitting to the data in 1(a).
(c) Measurement of the cosmic background radiation obtained by subtracting curve (b) from curve (a)
The overall design of our apparatus /3/ is shown schematically in figure 2. The upper end of a conical radiation collector emerges from the top of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786610
C6- 1630
JOURNAL DE PHYSIQUEa 'He c r y o s t a t . It i s open t o t h e atmosphere a t t h e 2 t o r r ambient p r e s s u r e a t b a l l o o n a l t i t u d e . The helium b o i l o f f g a s i s v e n t e d t h r o u g h t h e a n t e n n a t o keep i t c l e a r of f r o s t . The a n t e n n a shape i s c a r e - f u l l y s e l e c t e d t o p r o v i d e a % '6 beam on t h e s k y , and t o s u p p r e s s r a d i a t i o n d i f f r a c t e d i n from below t h e h o r i z o n by a f a c t o r i n e x c e s s of lo6. The por- t i o n of t h e a n t e n n a which i s i n c o n t a c t w i t h t h e primary ( u n d i f f r a c t e d ) beam i s c o o l e d below 3 K, d e s p i t e t h e c l o s e p r o x i m i t y o f t h e warm t o p of t h e c r y o s t a t and t h e ambient p r e s s u r e of 4 ~ e gas. T h i s i s accomplished by i s o l a t i n g t h e s i d e s o f t h e anten- na from t h e warmer i n t e r i o r w a l l s of t h e c r y o s t a t w i t h a vacuum j a c k e t and u s i n g two s u p e r f l u i d foun- t a i n - e f f e c t pumps t o pour l i q u i d 'He o v e r t h e t o p of t h e antenna a t t h e r a t e o f s 2 l i t e r s l h o u r . The r a d i a t i o n from t h e a n t e n n a was c o l l i m a t e d and passed through a F o u r i e r t r a n s f o r m i n f r a r e d s p e c t r o m e t e r and a r o t a t i n g chopper, b o t h of which were immersed i n l i q u i d ' ~ e .
s u p p o r t e d by n y l o n t h r e a d s i n a vacuum a t a tempera- t u r e of 0.35 K o b t a i n e d w i t h a c h a r c o a l pumped 3 ~ e . The bolometer t e m p e r a t u r e was measured w i t h a s m a l l InSb doped Ge thermometer. The n o i s e i n t h e bolome- t e r system was w i t h i n a f a c t o r two of t h e l i m i t s e t by t h e thermodynamic e n e r g y f l u c t u a t i o n s
< ( A E ) ~ = ~ T ~ c ~ . With i t s o p t i c a l a b s o r p t i v i t y of 0.5, i t s e l e c t r i c a l n o i s e e q u i v a l e n t power NEP = 6 x 10-l6
w/&,
and i t s time c o n s t a n t T = 6 m s , t h i s i s t h e most s e n s i t i v e b o l o m e t r i c i n f r a r e d d e t e c t o r e v e r ma- d e 151.An experiment which i s s i m i l a r t o t h e one d e s c r i b e d h e r e w i l l b e i n c l u d e d i n t h e Cosmic Background E x p l o r e r S a t e l l i t e (COBE) which h a s an expected l a u n c h d a t e of 1983.
ACKNOWLEDGMENT.- T h i s work was s u p p o r t e d by t h e D i v i s i o n o f B a s i c Energy S c i e n c e s , U.S. Department of Energy.
R e f e r e n c e s
/ I / An a n i s o t r o p y o f ATIT % h a s r e c e n t l y been demonstrated. It a r i s e s from t h e motion of t h e e a r t h r e l a t i v e t o t h e m a t t e r o f t h e u n i v e r s e . See f o r example, Smoot, G.F., G o r e n s t e i n , M.V., and M u l l e r , R.A., Phys. Rev. L e t t .
2
(1977) 898 121 The r e s u l t s .of a p r e v i o u s f l i g h t of t h i s expe-r i m e n t a r e g i v e n i n Woody, D.P., Mather, J . C . , N i s h i o k a , N.S., and R i c h a r d s , P.L., Rev. L e t t .
34 (1975) 1036
-
131 The a p p a r a t u s used i n a p r e v i o u s f l i g h t of t h i s experiment i s d e s c r i b e d i n Mather, J . C . ,
R i c h a r d s , P.L., and Woody D.P., IEEE Trans. M i - crow. Theory and Techn.
22
(1974) 1046141 Uoody, D.P., N i s h i o k a , N.S., and R i c h a r d s , P . L . , Appl. Opt. ( t o b e p u b l i s h e d )
Fig. 2 : Schematic diagram o f t h e a p p a r a t u s showing t h e warm b l a c k body c a l i b r a t o r and s e m i - t r a n s p a r e n t window t h a t a r e removed from t h e beam d u r i n g t h e
sky measurement, t h e c o n i c a l Winston a n t e n a w i t h a p o d i z i n g h o r n , t h e F o u r i e r s p e c t r o m e t e r , chopper and b o l o m e t r i c d e t e c t o r
C o n s i d e r a b l e e f f o r t was expended on t h e bblo- m e t r i c r a d i a t i o n d e t e c t o r 141. It was a composite s t r u c t u r e c o n s i s t i n g of a 200
