OUT THERE 



Shades of the Past 



A clearer view of cosmic inflation, 

 through the polarized light of the big bang 



By Charles Liu 



It's late afternoon 

 on a cloudless 

 day at the 

 beach, and the Sun 

 is hanging just 

 above the horizon. 

 You want to watch 

 the sunset, but you 

 can't bear to look: not on- 

 ly is the Sun itself too bright to 

 view, but the reflected glare from the 

 waves hurts your eyes. No problem: you 

 slip on a pair of polarized shades, and 

 voila! You still can't look directly at the 

 Sun, but the glare off the ocean is gone. 



Polarization is one of those scientific 

 terms that show up in regular speech 

 all the time. Unfortunately, the poli- 

 tical sense of the word is almost the 

 reverse of its optical sense. A polarized 

 citizenry, of course, is one whose 

 opinions are so divided that the camps 

 might as well be coming from opposite 

 poles of the world. Polarized light, by 

 contrast, marches in lock-step; it's uni- 

 form and coherent. 



Polarized light comes at us through 

 polarizing sunglasses and camera filters, 

 as well as from such technogadgets as 

 cell-phone screens, computer moni- 

 tors, and flat-screen TVs. It also em- 

 anates from almost any reflective sur- 

 face — the ocean, for instance. 



You can add the universe itself to your 

 list of polarized-light sources. In fact, 

 polarized light has long been a vital tool 

 for us astronomers. Much of what we 

 know about dust-enshrouded, super- 

 massive black holes in distant galaxies, 

 for instance, comes to Earth because of 

 polarized light. Now, thanks to astron- 



Full-sky map, rendered in false colors, shows 

 the temperature distribution of the cosmic 

 microwave background (CMB), as measured 

 by NASA's Wilkinson Microwave Anisotropy 

 Probe. Relatively cold regions are shown in 

 blue, relatively warm regions in red. Along 

 the white bars are regions in which the 

 orientation of the polarization of the CMB 

 radiation field remains the same. Although 

 imperceptible to the eye, the correlation 

 between temperature "hot spots" and 

 polarization strengths provides the best 

 observational evidence to date in support of 

 the inflationary model of the universe. 



omers working with the Wilkinson Mi- 

 crowave Anisotropy Probe (WMAP) — 

 a satellite that has been orbiting some 

 900,000 miles above the Earth since 

 June 2001 — polarization has been de- 

 tected and measured in the oldest light 

 of the cosmos: the afterglow of the 

 biggest bang ot all. 



Light travels from place to place in 

 waves. In an ocean wave, the water 

 just goes up and down as the wave pass- 

 es by, but the energy moves along the 

 wave until it crashes on the shore. A light 

 wave works the same way: picture a rope 

 wiggling up and down really fast, and 

 you'll get the idea. But unlike waves on 



the ocean, light isn't restricted to up- 

 and-down waves; waves in a beam of 

 light can go up and down, side to side, 

 or any diagonal tilt in between — all as 

 they come toward you. The result is a 

 complex, braidlike beam — like a whole 

 bunch of wiggling ropes, intricately in- 

 terwoven but not interfering with one 

 another, carrying energy forward along 

 their gyrating lengths. 



Every ordinary (unpolarized) beam 

 of light is made up of lots of weak- 

 er light waves, all wiggling 

 back and forth at differ- 

 ent tilts. Each wave can 

 be broken down into 

 i up-and-down and 

 side-to-side com- 

 ponents; so when 

 you add up all those 

 weaker waves, you get 

 the equivalent of two 

 stronger waves added to- 

 gether — one wiggling up and 

 down, the other side to side. In ordi- 

 nary circumstances, light is likely to 

 move forward completely randomly, so 

 the up-down and side-to-side waves 

 are equally strong. But if some physi- 

 cal process makes one wiggle direction 

 of light stronger than the other, the 

 light becomes polarized [see illustration 

 on page 66}. 



So, if you build a filter with long, 

 thin, parallel strips, it'll let through 

 only the light wiggling in one direc- 

 tion (say, up and down), while block- 

 ing the light that wiggles in the other 

 direction (say, side to side). The light 

 that comes through the filter is polar- 

 ized — more orderly and less bright. 

 You can imprint such a filter onto a 

 piece of plastic, and presto! glare- 

 reducing sunglasses! Or you can sand- 

 wich some gelatinous filter material be- 

 tween sheets of glass, and control the 

 amount of polarization with weak elec- 

 trical signals. Wow! Flat-screen TVs! 



Out there, polarization happens 

 naturally where light gets reflect- 

 ed or scattered a lot. For example, a few 

 paragraphs earlier I mentioned super- 

 massive black holes. Such a black hole 

 (Continued on page 66) 



NATURAL HISTORY June 2006 



