Friday, February 3, 2017

Confessions of a Cowboy Cosmologist

We recently watched The Magnificent Seven (the original, not the remake). It's a good movie, even if Yul Brynner, with his bald head and vaguely Eastern European accent, sometimes gives the impression that he wandered in by mistake from an adjacent movie set.

Which way to The King and I?
There's an odd similarity between the closing of the western frontier and my own research field of cosmology. In the early 1980s cosmology was the crazy no-holds-barred Wild West of science.  Cosmologists knew that the Big Bang theory was correct: the universe started out incredibly hot and dense and then expanded and cooled to form the space we inhabit today. But there was so much that we didn’t know. What was the universe made of?  Would it expand forever, or collapse back down and crush us all into an atomic soup? Where did all of the galaxies come from? We didn’t even know how fast the universe was expanding: the two groups measuring the expansion rate kept getting answers that differed by a factor of two! But life on the lawless frontier was great fun for theoretical physicists like me. With so little data to go on, we were free to speculate endlessly -- no theory was too outlandish to publish. We roamed the scientific landscape like cowboys, drifting from one new idea to the next. And theories sprouted like tumbleweeds, only to blow away when the next hot idea came along.

But then the experimentalists came to town and started fencing us in. First came the astonishing discoveries by astronomers mapping out the expansion of the universe on the largest scales. These investigators used distant supernovae, so far away that the light from these cosmic explosions took billions of years to reach us. These supernovae allowed the scientists to peer back in time and measure the expansion rate of the universe billions of years ago.  And they made a shocking discovery: the expansion of the universe isn’t slowing down under the force of gravity; instead, it’s speeding up! Next came the precision measurements of the radiation left over from the early stages of the Big Bang. This radiation contains tiny ripples that encode information about the universe:  its age, how much matter it contains, and what kind of matter it’s made of.

Together with observations of the way that galaxies cluster, these discoveries have given birth to the era of precision cosmology. In the space of just three decades, we’ve gone from knowing almost nothing about the true nature of the universe to understanding it in detail. We know what it’s made of: about 5% ordinary matter, the stuff that makes up our bodies and the world around us, about 25% dark matter, which forms an enormous sphere around our galaxy and binds it together, and 70% dark energy, which drives the universe to expand faster and faster. We know how old the universe is (13.8 billion years) and we know how fast it’s expanding. And we even have a good idea of the eventual fate of the universe: an ever-faster expansion, until the universe becomes dark, cold, and inhospitable to life.

The success story of cosmology over the past 30 years is a textbook example of the way that science is supposed to work: as scientists gather more and better experimental data, incorrect theories are pruned away until only one remains. A scientist is rarely lucky enough to experience this entire process within his own lifetime, so I feel fortunate to have lived at just the right time and worked in the right field to have seen it happen. And yet I can’t help feeling nostalgic sometimes for the old frontier days of cosmology. The legend of Daniel Boone claims that as soon as he could see the smoke from his neighbor’s fireplace, he knew that the neighborhood was getting too crowded, and it was time to move on. My own research field is starting to look awfully smoky.

But a few deep mysteries remain, so at least for now, there’s still room in cosmology for theorists like me. We don’t yet know what the dark matter is made of, and dark energy is even more puzzling.  But scientists are hunting for both of them. Deep beneath the earth’s surface, shielded from the cosmic rays constantly sleeting down on us, scientists have constructed vast laboratories to search for dark matter. They’ve filled these underground spaces with enormous vats of liquid xenon, hoping to detect a few feeble collisions between the dark matter and the xenon, collisions that would give us precise information about the particles making up the dark matter. The nature of the dark energy remains even more elusive, but astronomers are laboring to pin down exactly how quickly the expansion of the universe is speeding up. With this information in hand, we might be able to determine what makes up the dark energy driving this expansion.
Nothing lasts forever: just as our children inevitably grow up, leaving us with a mixture of pride and wistful regret, so science advances from pure speculation to textbook knowledge. I may eventually have to face the day when the frontier closes, and there’s nothing left to do in cosmology but cross the t’s and dot the i’s. And then, like Daniel Boone, I’ll either have to join civilization or move on to something else. Maybe Buffalo Bill is still hiring.


Plarry said...

Well, we know a lot, you say: 5% ordinary matter, 25% dark matter, and 70% dark energy. But I think the "dark" things are just words for things that we don't know anything (or not much of anything) about, because the experimentalists don't have any experiments to tell us much of anything about them, and theory seems pretty bereft too. Don't you agree?

Robert Scherrer said...

I don't entirely agree. It's true that these areas represent the current unknowns in cosmology, and we are partially hiding our ignorance by giving them these names. But we actually have a lot of experimental evidence giving us clues as to what is going on. For example, with respect to dark matter, we can measure very precisely the orbits at the outer edges of galaxies, telling us how much mass is exerting gravity. We also know how this matter would affect the fluctuations in the cosmic microwave background, which we can also measure very precisely. So there's clearly something there, and we can describe its properties quite well. Is it possible that we're barking up the wrong tree entirely? That's always possible in science -- nothing is certain until you actually observe it.