In our last tour of the universe, you learned about the afterglow of the Big Bang, more properly called the cosmic microwave background (CMB). These microwaves, which come from all directions, sprayed forth about 380,000 years after the Big Bang when protons and electrons settled down to form hydrogen, allowing light to travel freely through space. The CMB started out as visible and infrared light, but has since been stretched into microwaves by the relentless expansion of space itself.
When you look into the night sky, you look back in time. Moonlight bounced off the lunar surface nearly two seconds ago. Light from the bright star Vega left about 26 years ago. And the light from the Andromeda galaxy began its journey to our telescopes about 2 million years ago.
But how far back in time can you see? Could you see, with a big enough telescope, the first stars and galaxies? And could you see the light from the great flash of the Big Bang itself? The answer to the third question is… almost. With radio telescopes, astronomers have detected faint light that fills the universe, and which has all the telltale signs of the first light to emerge from the hot universe just 380,000 years after the Big Bang.
You learned how Vesto Slipher discovered nearby galaxies flying apart at high speed. You learned how Edwin Hubble and Milton Humason found the speed at which galaxies recede is directly related to their distance. And you heard the story of the Georges Lemaitre, the modest priest and mathematician who used Einstein’s general theory of relativity to predict Hubble’s measurements and suggest our universe itself is expanding.
But the idea of an expanding universe led to another shocking idea: that our universe was once smaller, perhaps at one time as small as a “primordial atom”, as Lemaitre called it, that exploded in a gigantic “Big Bang” some 13.7 billion years ago. This expansion caused the matter in the universe to cool and condense into the atoms, stars, and galaxies we see today.
In the first article of this series, you learned of Vesto Slipher’s discovery that galaxies were speeding away from us at astonishing speeds.
In the second, you learned of Edwin Hubble’s epic discovery, first reported in 1929, of an expanding universe where the speed at which galaxies receded from our point of view was directly proportional to their distance, a relationship we now call Hubble’s Law.
Now let’s turn to the explanation of why the universe expands, and the implications of its expansion for its beginning…
Vesto Slipher startled astronomers in the early 20th century when he discovered the astonishing speed at which “spiral nebulae” moved through space. While his measurements suggested these nebulae lay outside our own galaxy, the true nature and distance to these objects remained unknown.
But in 1919, a young American just back from the First World War found himself assigned to the world’s largest telescope, the 100-inch reflector on Mount Wilson in California. With patience, dogged effort, and a keen and open mind, the young man made a discovery which expanded the size of the cosmos a hundred-fold, and lent observational evidence to the expansion of the fabric of space itself.
The young man’s name was Edwin Hubble.