Shadows have a legitimate place in astronomy. In 1918, the Moon’s shadow helped prove that space bends and Einstein was right. The shadows of Jupiter’s moons help refine the orbits of those satellites. The million-mile-long shadow of Earth so varies its color that when it hit the Moon during the recent eclipse, it delivered an instant report on our global air quality. (Verdict: We were fine.)
There are all sorts of unseen shadows out there. If you’ve ever gathered around a bonfire on a chilly night, you faced the flames to keep your skin nice and toasty. If big people stepped in front of you, you felt instantly cold because they blocked the fire’s infrared photons. Put another way, those people were casting infrared shadows.
Shadows can reveal surprising information. If you attach a small eraser to a thread (use chewing gum) and dangle it an inch from some white cardboard on a sunny day, you’ll see the eraser’s shadow. Slowly move the eraser away, and at some distance the shadow no longer matches the shape of the eraser. Instead, it’s perfectly round. At this moment, you are blocking out exactly one image of the Sun. In essence, you’re projecting a negative picture of the Sun onto the cardboard.
Every sunlit surface is made up of countless overlapping solar images. With your suspended eraser, you blocked out one of them. So what did the shadow show – the eraser or the Sun? Bottom line: All by itself, a shadow can reveal hidden information about the light source.
Rewind to the Big Bang when, in a flash, unimaginable energy permeated all space, and soon created particles like protons and electrons. There were – and still are – a billion photons of light for every particle in the universe; but these photons, despite their overwhelming numbers, couldn’t penetrate the glowing fog of ionized particles.
Then, 350,000 years after the Big Bang, the universe expanded and cooled enough for normal atoms of hydrogen to form. The universe became transparent. Light stopped bouncing around things and now headed off in straight paths through the open Cosmos.
That brief period, when light stopped scattering for the final time, can still be seen off in the distance. It’s a wall of light that originated way back when the Cosmos was a glowing fog with an age of just 350,000 years. It comes equally from all directions.
As space expanded, the light’s waves stretched like taffy, until today it consists of lazy microwaves that are as omnipresent as roaches in Rio. Large foreground objects ought to block out these microwaves – in effect, producing microwave shadows.
But five years ago, astronomers using the sophisticated WMAP microwave observatory found that many galaxy clusters cast no shadows at all. No shadows? How could this be?
The most logical answer was that the cosmic microwave background (CMB) originates from space that’s nearer than the galaxies. In that case, the CMB does not come from any wall of light at the edge of the visible universe – and we have to throw out the Big Bang.
While that puzzle still remains, other galaxies do cast shadows. Indeed, a few months ago, Rutgers University astrophysicists discovered ten new massive galaxy clusters by detecting their shadows on the cosmic microwave background radiation.
We need a shadow expert. We could call Punxsutawney Phil. But he’s not talking.