Punxsutawney Phil emerges February 2 and looks around apprehensively. He ignores the intimidating crowd of cold, loud humans, and, instead, obsesses on his own shadow. This alone so spooks him that every low-pressure weather system for the next 6 weeks dumps snow on the continent. No, Groundhog Day does not exactly mark the high point of human logic.
But shadows do have a serious, legitimate place in astronomy. By blocking the Sun in May 1918, the Moon’s shadow proved that space bends and Einstein was right. The shadows of Jupiter’s moons help astronomers establish those satellites’ orbital parameters. Earth’s million-mile-long shadow so varies its color that when it hits the Moon during an eclipse, it delivers a report on our global air quality, as observers in eastern North and South America will witness a few weeks from now, on March 3. Now shadows are in the news again, to help clarify the universe’s origin. Except, these shadows are not cast by visible light.
There are all sorts of unseen shadows out there; most of us have dealt with some of them. If you’ve ever sat around a fire on a cold night, you faced the flames to keep your skin nice and toasty. If big people stepped between you and the fire, 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 dangle an eraser attached to a thread above white cardboard on a sunny day, you’ll see the eraser’s shadow, duh. Hold the cardboard perpendicular to the Sun so the shadow isn’t distorted. Slowly move the eraser away, and the shadow no longer matches the shape of the eraser. Instead it’s perfectly round. You are blocking out exactly one image of the Sun. In essence, you’re projecting a negative picture of the Sun onto the cardboard.
Rewind to the Big Bang when, in a flash, unimaginable energy permeated all of space and created protons and electrons. There were — and still are — a billion photons for every particle in the universe, but these photons, despite their overwhelming numbers, couldn’t penetrate through the glowing fog of ionized particles.
Then, 350,000 years after the Big Bang, the universe expanded and cooled enough for normal hydrogen atoms to form. The ionization ended, and the universe became transparent. Light stopped bouncing around off things and now headed in straight paths through the cosmos.
That brief period, when ionization ended everywhere and light scattered for the final time, still can be observed far 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. This energy comes from all directions equally. It emanates from where the ionized universe was located just before it became transparent.
As space expanded, light waves stretched like saltwater taffy. Today, they consist of lazy microwaves that are as omnipresent as Alaskan mosquitoes. Because it all emanates from the same precise time, and therefore the same distance from us, large foreground objects ought to block out these microwaves — in effect, producing microwave shadows.
But this past September, astronomers published analyses of data from the sophisticated Wilkinson Microwave Anisotropy Probe observatory, and found that many galaxy clusters cast no shadows at all. No shadows? How could this be?
The most logical answer is 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. Or maybe those galaxies give off their own microwaves, thus “filling in” their shadow. But why would so many galaxies emit microwaves of exactly the same precise 2.725-Kelvin temperature? It makes no sense. Or maybe something’s wacky with the data.
We need a shadow expert. We could call Punxsutawney Phil, but the little groundhog isn’t talking.