It’s plain to see that the night sky isn’t as bright as the surface of the sun. Last week’s column ended in a cliff hanger because I ran out of space after arguing that Olbers’ Paradox is not resolved by assuming something in space absorbs starlight.
Any light absorbed by matter in space must be re-radiated in the form of infrared light, and the sky in Olbers’ universe would glow warmly as a result. Which is also at odds with observation.
So we’re left to question whether we can tell just by looking if the universe is infinite and uniformly filled with stars and galaxies. The simple answer is: It’s not so simple.
The so-called big bang model of the universe — which successfully accounts for just about every observational clue we have about the nature and origin of the universe — holds as a central tenant that once, long ago, the universe was incredibly hot and dense; and filled not with individual stars and galaxies, but only with the most fundamental bits of matter.
The early universe was like the inside of a star itself; a star as big as the entire universe.
As the universe aged, expanded and cooled, individual stars formed, then galaxies — and eventually things became as they are now.
A curious truth of our ability to sense the universe around us is that the farther away we look the further back in time we see. This is a consequence of the fast but finite speed of light.
Lines of sight from our eyes to nearby things see into the past only a short time. The sun, for example, is about 8 light-minutes away. Nearby stars are tens of light-years away. Nearby galaxies are hundreds of millions of light-years away.
The edge of the universe is about 15 billion light-years away because the universe is just that old.
The night sky is not filled with visible light because the universe is not infinite.
But the night sky is not completely dark.
What do we see along lines of sight that miss distant stars and extend to the edge of the seeable universe? Not darkness, but a nearly uniform warm glow — the so-called cosmic microwave background (CMB).
The entire universe glows in microwave radiation at a temperature a few degrees above absolute zero, what’s left of the big bang.
Physicists Arno Penzias and Robert Wilson, working at Bell Labs to develop microwave systems to support telephony, accidentally discovered the CMB in 1964. Both were awarded the Nobel Prize in physics for their discovery in 1978.
Penzias and Wilson were the first to see to the edge of the universe. We can only imagine what’s beyond.
— By Doug Furton, a member of the physics faculty at GVSU. Send questions and suggestions to firstname.lastname@example.org.