From the discovery of gravitational waves to locating our nearest Earth-like planetary neighbor, 2016 was a banner year for astronomy. In fact, it's not only been a good year for space science, but a great few decades. Many of the biggest puzzles in astronomy and cosmology have been solved.
Take the age of the universe. A century ago, we could say only that the universe was very old. There was no way to find a precise number. Now, thanks to detailed maps showing the faint echo of the Big Bang — what astronomers call the "cosmic microwave background" — we know the universe is 13.82 billion year old, give or take 10 million years. It's a staggering achievement in "precision cosmology."
But we don't have all the answers about our universe. Despite all the data streaming in from the observatories around the world and from particle physics experiments like the Large Hadron Collider in Switzerland — and despite the countless hours that astronomers and physicists spend at the blackboard or running computer simulations, a handful of cosmic questions continue to keep scientists up at night (for those who aren't up at night already, peering skyward).
1. What is Dark Matter?
At the Gran Sasso underground laboratory, deep beneath the Apennine Mountains of central Italy, scientists are keeping watch over a giant tank filled with 3.5 metric tons of liquid xenon. Their hope is that exotic particles from deep space will whiz through the liquid, emitting a telltale signal. So far, that hasn't happened. But scientists hunting for so-called "dark matter" have learned to be patient.
It's been nearly a century since astronomers studying distant galaxies first noticed something odd: The galaxies seemed to hold more matter than could be accounted for by the visible material — stars and gas clouds. This missing mass, dubbed dark matter, is now believed to make up more than a quarter of the total mass and energy in the visible universe.
What is this stuff? The best guess is that it's made up of some kind of fast-moving particle that barely interacts with the ordinary matter that makes up the stars and the planets. Theoretically, these "weakly interacting" particles can pass unimpeded through miles of ordinary matter — which is why we spent millions of dollars on detectors like the one at Gran Sasso.
But scientists have been searching for these exotic particles for decades now, with no luck. And so some are beginning to wonder if dark matter exists at all. Instead, the reasoning goes, Einstein's theory of gravity may require some tweaking. A number of alternative theories of gravity have been put forward in recent years, but all remain controversial. And so the particle quest has continued.
"It would be nice to know what the dark matter particle is — or even to have some reassurance that it is a particle," says University of Toronto physicist Roberto Abraham. "I think that's the most likely thing, but I'm open to the possibility that we need modified gravity."
While there's no hard evidence that Einstein's equations are wrong, he says, "we should keep an open mind."
2. What is Dark Energy?
In the 1990s, data from the Hubble Space Telescope revealed that distant galaxies aren't just moving away from our home galaxy, the Milky Way, they're speeding away from us (and from each other) at an accelerating rate. That came as a big surprise — one that scientists have been struggling to explain ever since. What mysterious force is giving the galaxies this extra push? No one knows. But it's been dubbed "dark energy," and as with dark matter, Einstein is a key figure in the story.
In the early years of the 20th Century, scientists believed the universe was static — that, on average, galaxies remained the same distance from their neighbors. But the equations of general relativity seemed to indicate that the universe must be either expanding or contracting. That made no sense to Einstein, so he gave his theory a fudge factor that he called the "cosmological constant."
A few years later, when astronomers discovered that the universe is expanding, it seemed the fudge factor was no longer needed. Yet now that we know that the universe's expansion is accelerating, the cosmological constant may be making a comeback.
Whatever its true nature, dark energy plays an even greater role in cosmic evolution than dark matter. Our best estimate is that dark energy accounts for a more than two-thirds of the total energy of the visible universe. Taken together, dark matter and dark energy are an enormous mystery — and a bit of an embarrassment for the scientific community.
"I would give anything to know what dark matter and dark energy are," Abraham says. "And I intend to devote the next couple of decades of my life to looking into it."
3. What Came Before the Big Bang?
Whenever a cosmologist gives a public lecture, someone in the audience inevitably raises a hand to ask, "Yes, but what came before the Big Bang?"
"There's this textbook answer that we're supposed to give," says Glenn Starkman, a physicist at Case Western Reserve University. "We say that the question is meaningless, just as it's meaningless to ask what's south of the South Pole."
The idea is this: If time itself began with the Big Bang, then it makes no sense ask what came before. There simply was no "before." And yet Starkman knows that hardly anyone finds that answer satisfying.
We now have a model for what happened very shortly after the Big Bang. During the first tiny fraction of a second of the universe's existence, the "inflation" model says that the universe expanded like a balloon, doubling in size again and again before slowing down to its "normal" rate of expansion. But if we try to look back before inflation — all the way back to "time zero" — general relativity breaks down.
Some physicists now think that time didn't begin with the Big Bang, but somehow emerged when the universe reached a certain level of complexity. Others theorize that the universe runs in cycles, in a possibly endless series of expansions and contractions. If this "cyclic" model is right, the Big Bang wasn't the beginning, but just a transition from an earlier era. Another possibility is that our universe is just one of countless "bubble universes" that pop up repeatedly in a "multiverse."
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Are we any nearer to answering the "what came before" question that we were a generation ago? Starkman says no. And it's unclear whether astronomical observations can settle the matter. Our best bet might be to build an enormous gravity wave detector in space — with the hope that we could detect gravitational waves created by the Big Bang itself.
But don't hold your breath. Starkman says such an enormous project could take many decades to build.
4. What's Inside a Black Hole?
Black holes are regions of space in which gravity exerts such an enormous pull that nothing — not light or any other signal of any kind — can escape. Since nothing can get out, it's as if the inside of every black hole is permanently "pinched off" from the rest of the universe.
"We have no idea what goes on inside a black hole — unless we're willing to jump into one," says Starkman. Even then, you'd have no way to get out to tell anyone what you'd found — or even to send a message.
In the 1970s, physicists Stephen Hawking and the late Jacob Bekenstein showed that black holes emit a form of radiation and slowly "evaporate" as they do. Unfortunately, black hole evaporation seems to violate the rules of quantum mechanics, which means that something's got to give way. (The details are quite technical, but they involve the loss of "quantum information"; physicists call it the "information paradox.")
Physicists have come up with various ideas to explain this puzzle. All are controversial. The real problem is that, at the "event horizon" — the outer boundary of a black hole — both general relativity and quantum mechanics come into play. And so far at least, these two theories are irreconcilable.
"It's possible that quantum mechanics and general relativity somehow 'shake hands' at the event horizon, and work in a different way than here on Earth," Starkman says. "That's an exciting prospect."
Our best bet is probably to study the region immediately outside the event horizon. That's where a radio telescope array known as the Event Horizon Telescope comes in. The EHT is a sort of electronic hookup of dozens of telescopes around the world — from California, Arizona, and Hawaii to Chile, Spain, and Antarctica.
An enhanced version of the EHT will soon start collecting data. Its first target will be a "supermassive" black hole at the center of our galaxy. Astronomers expect the EHT to yield a detailed picture of the radiation emitted by gas and dust in the final moments before it crosses the black hole's event horizon — perhaps shedding some light on the exotic physics of the black hole event horizon.
5. Are We Alone?
Are we the only intelligent creatures in the cosmos? The only beings to wonder what other thinking, wondering beings might be out there?
Our galaxy contains several hundred billion stars, many of which are likely to have planets orbiting them. As if that weren't mind-boggling enough, astronomers believe there at least a trillion galaxies in the visible universe. Given the likely profusion of planets, it seems unlikely that we're alone in the universe. And so scientists around the world have embarked on what they call SETI, or the search for extraterrestrial intelligence.
Seth Shostak, senior astronomer at the SETI Institute near San Francisco, suspects "E.T." is out there somewhere. He points to the data collected by NASA's Kepler Space Observatory, which suggests that as many as one out of every five planets is habitable. If that's right, the cosmos could contain 10²¹ (that's a billion trillion) habitable planets.
But even if life is plentiful in the universe, what about intelligent life? So far, SETI scientists have turned up nothing even after years of scanning the heavens for radio signals that might signify such life. Shostak points out that so far we've aimed our radio telescopes at just a few thousand stars — and thus it's too soon to tell.
At a recent conference in Germany, Shostak bet the scientists in attendance that we'd find an alien signal within 24 years. (It wasn't a huge wager — he only offered to buy each scientist a coffee if he turned out to be wrong.) By then, thanks to more efficient search techniques, we'll likely have checked a million star systems.
In the meantime, our radio telescopes will continue to be eavesdrop on the universe, with astronomers around the world waiting and listening.
Dan Falk is a science journalist based in Toronto. His books include "The Science of Shakespeare" and "In Search of Time."
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