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Exploration

The universe is expanding faster than we expected. These astronomers want to know why.

A hilltop telescope in Texas is charting two million galaxies — and trying to figure out if one number explains everything.

By Matt Crossman

The Hobby-Eberly Telescope, the world’s third largest, sits high atop a hill in rural southwest Texas, one of the darkest places in America, all the better for stargazing. To get there, visitors follow Highway 118 to Dark Sky Drive — a winding, twisty road that leads up, up, left, right, until it arrives at the visitors’ center. A little farther up the way is the McDonald Observatory, which stands like an altar to a celestial congregation. From the University of Texas-Austin, which co-owns the telescope, it’s a seven-hour drive.

That’s an easy journey compared to the path light follows to get there. Individual particles of light, called photons, left their galaxies up to 13 billion years ago. They dodged stars and space dust, planets and asteroids, dark matter (whatever that is) and dark energy (same), before arriving in the Milky Way. They skirted Saturn’s rings, slipped by the moon, survived entering our atmosphere, and finally arrived at that hill in southwest Texas.

Perhaps 50 or 100 of those photons will hit one of the 91 hexagonal segments that comprise the Hobby-Eberly Telescope’s 11-meter mirror. They’ll bounce from the mirror into one of 35,000 optical fibers. From there the photons will run through one of 156 spectrographs, which split those photons into individual wavelengths, called spectra.

And that arrival makes them part of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), an ambitious project with a goal of nothing less than understanding the state of the universe. Analysis of each spectrum will reveal much about the galaxy that sent the photons, including how fast that galaxy is moving. All of which results in a dot — one dot, per galaxy — on a map full of other dots.

Once the map is complete, probably three years from now, all the dots will be measured against each other. The goal is to generate a single number that will define the rate of increase of the universe’s expansion.

“That’s what blows my mind,” says Gary Hill, who helped design the telescope and is co-founder of HETDEX. “We’re doing all of this, spending many tens of millions [of dollars] to do this, and many, many people’s work over a decade or more, and we’ll get one number out of this.”

That one number could reshape our understanding of the universe.

The scientists working on HETDEX believe that the future of understanding space lies in understanding the past. Studying light that is up to 13 billion years old, they are seeking to answer the eternal question of why are we here — not in the metaphysical sense, but in the sense of why are we here and not over there.


Astronomers have known for 70 years that the universe is expanding, and has been since the Big Bang, 13.8 billion years ago. Until 1998, they thought the rate of expansion was slowing because of gravity, like when you take your foot off the gas in your car. But empirical observations showed otherwise: the rate of expansion is increasing, as if the universe’s throttle is stuck.

Really, really stuck: The universe is expanding at a rate at least 10 to the 56th power faster than astronomers think it should. They have no idea why. Actually, scratch that. They have a ton of ideas. They just don’t know which one, if any, is right.

That is, in part, because we understand only four percent of what makes up the universe. Scientists call everything else dark matter (presumably because “stuff” was already taken) and dark energy (even though it might be neither dark nor energy). Dark energy is the term used to explain the universe’s inexplicable behavior.

HETDEX is unique in its use of spectrographs, the massive amount of galaxy data it is collecting, and the vast epochs of time it is studying.

“The problem with dark energy is there are too many possible theories for it,” Hill says in a video produced to explain HETDEX. “The theorists know no bounds. We’ve got to provide them with some boundaries. In fact, what we need to do is provide a really precise measurement of how the universe is expanding through time, against which they can test ideas.” Once HETDEX finds that measurement, Hill thinks it’ll probably disprove nearly all of the ideas, “and we [will] find out it’s something else that we haven’t thought of.”

HETDEX is not unique in attempting to understand dark energy and the expansion rate of the universe. But it is unique in its use of spectrographs, the massive amount of galaxy data it is collecting, and the vast epochs of time it is studying, from the modern age back to a few hundred thousand years after the Big Bang.

The project is so intriguing, its goals so ambitious, that it has drawn collaborators from all over the world. The telescope is co-owned by the University of Texas-Austin, Penn State, and Texas A&M, along with Oxford and three German universities.

Karl Gebhardt, a renowned astronomer at the University of Texas-Austin who is HETDEX’s project scientist, likens the project to a study of fingerprints. If you examined the ridges on a baby’s fingerprints, and that same person’s ridges when she is an adult, there would be a predictable difference in the distance between the ridges. Applying that analogy to the universe, the difference in the distance between the ridges doesn’t make sense; it’s too big. HETDEX aims to give astronomers new data to show why the ridges in the universe’s fingerprints are farther apart than they think they should be.

In his many public presentations and media interviews, Gebhardt has displayed a knack for explaining incredibly complex concepts so the general public can understand them. That’s part passion, part job security: He knows that if he can’t explain a $42 million project so laypeople can understand it, he probably won’t be running a $42 million project for very long. But he also wants people to be as excited about it as he is, because this is cool stuff.

Cool, and also profound — a way to merge the massive and the small. Gebhardt often thinks about a time when he was working on his thesis at Cerro Tololo Inter-American Observatory in Chile. The Large and Small Magellanic Clouds — two irregular dwarf galaxies, visible in the Southern Hemisphere — looked like knife slits in black fabric. There were so many stars that constellations faded into obscurity, indistinct among the uncountable freckles of light. His mind boggled at the immensity of what spread out before him.

His reverie was broken when a massive Andean condor, the national symbol of Chile, soared across his line of vision, its wings spread seemingly as wide as a jet’s. In an instant, Gebhardt returned from the cosmos to that mountaintop in Chile.

“It was so freaky and so special,” Gebhardt says. “I love that moment.”

HETDEX’s scientists are usually too caught up in the nitty-gritty of the data to get slack-jawed at the enormity of what that data represents, but it’s as freaky and special as Gebhardt says. One hundred years ago, astronomers thought the Milky Way was the only galaxy in the universe. Now, every day, Gebhardt gets an email containing records with 30, 50, maybe 100 or more previously unknown galaxies that HETDEX has found. “This is the essence of discovery,” he says.

By the time HETDEX ends, it will have charted perhaps two million new galaxies, far more than any other project. Of those, roughly one million will be of a particular type, called a Lyman-alpha emitting galaxy, which will be more than any other project has charted by a factor of 1,000, Hill said.

“Sometimes you have to stop and remind yourself that each dot on this plot represents an entire galaxy that we didn’t even know existed before this survey,” says Steven Finkelstein, who heads HETDEX’s study of galaxies. “We probably don’t stop and do that enough.”

As big as that number is — 2 million newly charted galaxies — it’s still only a fraction of what’s out there. Estimates of the total number of galaxies range from 100 billion to 2 trillion. Those numbers are almost incomprehensible, and HETDEX’s scientists have their moments like Gebhardt’s with the condor in Chile, only usually with printouts or computer screens instead of birds.

It’s hard to wrap your brain around all that, and Gebhardt leavens HETDEX’s profundity with Monty Python references and dad jokes (“astronomy is out of this world”). Gebhardt often asks his undergraduate students to imagine scientists on some far-flung planet in some far-flung galaxy, pondering the future of space, doing their own version of HETDEX, seeing little ol’ Earth as a dot on a chart and wondering what’s going on there.


Whatever the number HETDEX finds, it will have to be studied, parsed, analyzed. The theorists will have a lot to chew on. Gebhardt, Hill, and Finkelstein all say they don’t care what that final number is; they just want to find it. The seeking has brought them great joy, regardless of what is found.

Still, they ponder the possibilities, the implications if they find something huge. Gebhardt lets his mind wander. The explorer in him, the dreamer, the man who loves to gaze at the stars, wants to find something big, something mind-blowing, something nobody has thought of or even dreamed about, something that will set the course of the future of space for generations.

HETDEX could answer a great unknown — what on God’s green Earth is the universe doing? — with an even greater unknown, the rough equivalent of Newton’s apple falling sideways instead of down. For example, HETDEX is looking in different directions. To put it perhaps too simply and from an Earth-centric perspective, one range of study is in the area of the Big Dipper, and the other is in another direction.

Getting numbers from two different locations in the universe could either prove or disprove the idea of a “cosmological constant,” a concept Einstein proposed. Einstein’s math said the universe should be contracting. But since it appeared to be static, he theorized the “cosmological constant” to explain the difference. What that cosmological constant was, he didn’t know. He abandoned that theory and called it his biggest mistake when Edwin Hubble discovered, in 1931, that the universe was actually expanding.

But the cosmological constant has enjoyed a rebirth, not least because something has to explain this mysterious expansion. It is a foundational principle that the cosmological constant would be just that — constant, the same here, there and everywhere. That means the number that HETDEX finds from the Big Dipper should be the same as the number that it finds from the other direction.

Gebhardt wonders: What if it’s not the same? HETDEX should have enough data to know in another six months or a year. If it’s not the same — if there’s no ultimate number, but multiple numbers, revealing that one side is moving slower than the other — that would be a game-changer, an absolutely enormous discovery, bringing up questions nobody even knows to ask yet.

And to answer them, the future of space would begin anew.

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Matt Crossman is a writer based in St. Louis.

 

Illustration by Lorenzo Gritti

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