Search for Linear Polarization of the Cosmic Background Radiation

W e present pre l iminary measurements of t h e l i n e a r p o l a r i z a t i o n of t h e cosmic microwave background ( 3 ° K blackbody) r a d i a t i o n . These ground-based measurements are made a t 9 mm wavelength. We f i n d no evidence f o r l i n e a r p o l a r i z a t i o n , and se t an upper l i m i t f o r a po la r i zed component of 0.8 m°K with a 95% confidence level. This impl ies t h a t t he p re sen t rate of expansion of t h e Universe is i s o t r o p i c t o one p a r t i n 10 , assuming no r e i o n i z a t i o n of t h e pr imordia l plasma a f t e r recombination. 6 The observed cosmic microwave background r a d i a t i o n i s g e n e r a l l y thought t o be a r e l i c of t h e ho t , dense i n i t i a l phase of t h e Universe. This r ad ia t i o n should then have c h a r a c t e r i s t i c s which r e f l e c t i t s thermal o r i g i n and t h e geometry of the e a r l y Universe. t h a t t h i s r a d i a t i o n has a blackbody (Planckian) spectrum, i s unpolar ized , The simple hot Big Bang model p r e d i c t s e x h i b i t s t h e s t a t i s t i c a l p r o p e r t i e s of blackbody radiation’, and is i s o t r o p i c i n a r e fe rence frame co-moving wi th the expansion of t h e universe . The spectrum of t h i s r a d i a t i o n has been e x t e n s i v e l y inves t iga t ed . The b e s t experiment t o d a t e i s t h a t of D. Woody and P. Richards.3 r e s u l t s q u a l i t a t i v e l y fo l low a blackbody spectrum, they r e p o r t a 50 deviat i o n from t h e bes t f i t blackbody curve. ments of t h e angular d i s t r i b u t i o n of t h i s r a d i a t i o n w i t h r ecen t r e p o r t s of a 3.5 f 0 . 4 m°K amplitude f i r s t o r d e r (cosine) a n i ~ o t r o p y ~ ’ ~ . Although t h e i r There have a l s o been many measure-