OUT THERE (Continued from page 62) 



is straddled by a doughnut-shaped cloud 

 of dense, hot gas. If we here on Earth 

 see it side-on, the doughnut "hole" 

 points at right angles to our line of sight; 

 all we see is the obscuring gas, and we 

 can't see down toward the black hole it- 

 self. But if light coming out of the 

 doughnut hole shines into clouds of 

 dusty gas farther away, some of that light 

 will bounce our way off the dust in those 

 clouds — the same way that smoke or fog 

 in a flashlight beam reflects 

 the light sideways into view. 

 That reflected light is polar- 

 ized, so by putting a polar- 

 izing filter in our telescope 

 camera, we astronomers can 

 isolate the light bouncing oft 

 those clouds — in effect, us- 

 ing them as giant angled mir- 

 rors to look into the dough- 

 nut and indirectly study the 

 black hole within. 



What about light that 

 permeates the cosmos as a 

 whole? The universe has 

 been expanding at a more or 

 less constant rate for billions 

 of years. According to cur- 

 rent theory, though, it 

 swelled by the staggering factor of at 

 least ten trillion trillion in size between 

 10 3:5 and 10 ,2 second after the big 

 bang. To distinguish it from ordinary 

 expansion, that expansion-on-steroids 

 is called the inflationary epoch, or just 

 inflation. Inflation is strongly support- 

 ed by circumstantial evidence, but un- 

 til now it had not been confirmed by 

 any observational data. 



For decades, though, astronomers 

 predicted that inflation might have left 

 a telltale imprint in the energy distribu- 

 tion of the early universe. That energy, 

 observable today as the leftover heat 

 from those early times, is called the cos- 

 mic microwave background (CMB) — 

 the oldest direct signature of the big bang 

 [see "Sharper Focus," by Charles Liu, May 

 2003]. I )epending on how inflation ac- 

 tually happened, the amount of polar- 

 ization in the background light would 

 vary according to fluctuations in th.it 

 light at any given location in space. 



Here's how it all went down. Before 



inflation happened, the big bang sent 

 powerful gravitational Shockwaves 

 through the infant universe. By then, 

 the energy that filled space was already 

 mottled with barely perceptible quan- 

 tum fluctuations — the seeds of today's 

 large-scale cosmic structure. So the in- 

 terplay between the gravitational waves 

 and the quantum fluctuations (within 

 the first trillionth of a trillionth of a tril- 

 lionth of a second after the big bang!) 



Polarization of light is depicted in the schematic diagram. An ordinary 

 light beam (a) is made up of many waves that oscillate in any random 

 plane that makes a right angle to the beam direction (b). A polarizing 

 filter (c) passes only those wave components that oscillate in the same 

 orientation as the filter. The resultant beam is polarized (d). 



gave rise to patterns of electric and 

 magnetic fields that in turn partly po- 

 larized the cosmic background light. 



If, at this point, inflation kicked in, a 

 whopping amount of energy flooded 

 the universe, seemingly out of nowhere; 

 then the energy flow must have shut off 

 by the time the observed, normal rate 

 of expansion resumed. In the basic in- 

 flation model, that shut-off would hap- 

 pen naturally if the broader, wider 

 quantum fluctuations had bigger tem- 

 perature variations than the smaller 

 ones. Since the energy of inflation 

 would have interacted with all of the 

 still-traveling gravity waves and all of the 

 still-present quantum fluctuations, that 

 temperature-varying signature would 

 have left a strong imprint in the polar- 

 ization pattern of the cosmic back- 

 ground — many times greater than what 

 had existed before inflation began. 



The measurement — discerning the 

 overall polarization pattern, and then 

 isolating its various components — is 



brutally difficult. The structure of the 

 CMB is subtle enough as it is, varying 

 in temperature by less than one part in 

 10,000 across the entire sky. The varia- 

 tion in background polarization, in turn, 

 is only a tenth to a hundredth the 

 strength of those temperature variations. 

 Amazingly, though, the WMAP team 

 made the measurement. 



By combining more than three years 

 of data, the team measured the back- 

 ground temperature of the 

 universe to a precision of 

 several millionths of a de- 

 gree, on angular distance 

 scales varying from less than 

 the width of the Moon to 

 the breadth of the entire sky. 

 Then they extracted the var- 

 iation in polarization from 

 that background tempera- 

 ture, and measured how 

 much the polarization varies 

 from one scale to another. 

 The results appear to con- 

 firm two things: Larger-size 

 fluctuations do seem to have 

 greater temperature varia- 

 tions than smaller-size ones. 

 And the temperature varia- 

 tions in the polarized light caused by 

 gravity waves from the big bang are less 

 than a third the strength of the varia- 

 tions unrelated to those waves. Both 

 results suggest — for the first time with 

 some observational confidence — that 

 inflation did indeed take place. 



That is a lot to wrap your head 

 around, but then, the origin of the uni- 

 verse should be a little deep, right? And 

 remember, it is all being studied 

 through polarizing filters. So the next 

 time you're at the beach, enjoying a 

 sunset that is particularly sublime, you 

 may experience a momentary oneness 

 with the world. And maybe, as you re- 

 flect that the light passing through your 

 glare-reducing shades is polarized, just 

 like the oldest light in the cosmos, that 

 sense of oneness will extend to the uni- 

 verse as well. 



Charles Liu is a professor of astrophysics at the 

 City I Wversity of New York and an associate 

 with the American Museum of Natural History. 



NATURAL HISTORY June 2006 



