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.
There’s an amazing feature of the cosmic microwave background… it is extremely smooth and uniform. In fact, across the entire sky, the CMB is exactly the same to better than 1 part in 10,000. Why? Is there any reason for the universe to appear so smooth on such a large scale? Or is it just a coincidence?
As it turns out, it is not a coincidence. But until 1980, it was impossible to explain. That’s because the standard theory of the Big Bang said that a tiny, hot, region expanded to form all of space and all the matter and energy in the universe, and it continued to expand until the present day. But if this is true, then the bits and pieces of the universe on one side of the very early universe flew apart too quickly to ever come in contact with the bits and pieces on the other side. That made it hard to explain why all parts of the universe appear to look the same on a large scale, and how all parts of the universe could have the same physical laws. It was a big puzzle.
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A solution came from the imagination of the American physicist Alan Guth. He speculated that space itself expanded at a tremendously accelerated rate after the Big Bang for a very short period of time. This exponential expansion, or “inflation”, caused space to expand by a factor of 1078 almost instantaneously until the entire universe was almost perfectly flat and smooth. (Note: If a single proton expanded to the size of the Milky Way, that’s only a factor of 1035). After this “inflationary epoch”, suggested Guth, the matter and energy of the universe precipitated like snowflakes out of a cold small region whose parts were physically connected. All of this matter and energy then began expanding into the smooth and flat universe at the much slower rate we see today, with the same temperature, shape, and physical laws.
The “inflation hypothesis”, as it’s now called, was an astonishing insight and it explained, in theory, why the universe is so smooth and uniform on a large scale. But surely this hypothesis is just a crazy idea, right? I mean, no one could ever show that such a thing as inflation happened, could they?
As a matter of fact, they could. You see, like all good scientific theories, the inflation hypothesis made specific predictions, predictions that could be verified by careful observation. And just ten years ago, a NASA satellite named WMAP (see image at top) began making a series of remarkable observations which suggest inflation did take place in the earliest time of our universe.
But my time is up. So let’s leave the tale of WMAP and its cascade of important discoveries for another day…