Final Frontier

Using Black Holes to Measure Dark Energy, Like a BOSS

Thanks to something called “dark energy,” our Universe is expanding. We don’t understand much about it yet, but a project called BOSS is aiming to change that.


The Universe is expanding—the space between galaxies is growing larger all the time. Not only that, but the rate of expansion is getting faster, a phenomenon we call “dark energy.” Right now, we don’t know what dark energy is, but thanks to detailed astronomical observations, we’re getting a better idea of how it behaves.

One of those observations is BOSS: the Baryon Oscillation Spectroscopic Survey. Baryon oscillation is basically sound waves in the early Universe. (Ordinary matter particles, like atoms, are perversely known as “baryons” to people who study the Universe.) BOSS studies those sound waves by mapping the positions and distances to huge numbers of galaxies, stretching back as far in time as possible. The oscillations, in turn, are a way to measure the structure and expansion rate of the cosmos, providing a detailed look at dark energy.

Last week, BOSS researchers revealed they had mapped 164,000 galaxies an average of 11 billion light-years away. One light-year is the distance light travels in a year, so light left these galaxies when the Universe was less than 3 billion years old—about 20 percent of its current age of 13.8 billion years. Those are early galaxies, providing a beautiful map of the cosmos in the old days.

So what does this have to do with cosmic acceleration?

First, “dark energy” is a lame-ass name. For one thing, it sounds like it has to do “dark matter,” but they are almost complete opposites. The only things they have in common: they’re both invisible, and we don’t know what either of them really is. (I’ll write a piece about dark matter soon—stay tuned!)

Dark matter is the invisible mass holding galaxies together and shaping the distribution of stuff on the biggest scales. Except for the “invisible” bit, it mostly acts like atoms and other ordinary matter: it helps keep things together by gravitational attraction. Dark energy, on the other hand, pushes everything apart. As the Universe expands, dark energy makes the speed of expansion get bigger, while a cosmos with only dark matter in it would slow down. From what we can tell, the total amount of dark energy seems to increase as the Universe expands. It’s a feedback cycle: the more expansion we have, the more dark energy; the more dark energy, the faster the Universe grows.

We want to know if dark energy has always been this way, or if it has changed over history—and if it will stay the same forever. We’re also curious about whether dark energy pushes expansion the same way everywhere in the Universe, or if it’s stronger some places than others. Those are important mysteries: they tell us about the nature of dark energy, but also inform us about how our Universe began, and what its future will be like.

If dark energy will be the same in billions of years as it seems to be today, the future will be dark and empty, as galaxies continue to move apart from each other at ever-faster rates. If dark energy comes and goes, though, maybe the rate of expansion will slow down again. All of this is a long time from now—trillions of years after the death of the Sun—but we might see hints about it today. We hope to see signs of what is to come by looking at how dark energy behaves now, and how it has acted in the past. Similarly, if dark energy is stronger in some parts of the cosmos, then certain pockets of the Universe would grow faster than in others. That also has implications for how the future cosmos looks.

And that’s where BOSS comes in. If dark energy was different in the past, then galaxies in the early cosmos would be closer together (for less dark energy) or farther apart (for more). And if the effect of acceleration was stronger in some patches than others, that would mean less or more clumping up of galaxies.

Galaxies that distant are very faint, so BOSS looks for quasars: the powerful massive black hole at the centers of many early galaxies. As matter falls toward these black holes, it accelerates close to the speed of light, heating up and sending a lot of energy back into space. Contrary to stereotypes, black holes don’t devour everything—they can be some of the brightest objects in the Universe! And that helps BOSS: quasars are bright enough to be seen and mapped from 11 billion light-years away.

I visited the Apache Point Observatory in New Mexico two years ago, where the telescope taking data for BOSS is located. Unlike many, this telescope doesn’t have a dome to cover it, so to compensate, it has a square metal box around it to deflect wind. And let me say: those baffles make the telescope ugly, like its own mama puts a bag over its head before kissing it goodnight.

But the results coming out of BOSS are beautiful, even if the telescope is hideous. The new results provide the most accurate measure yet of the expansion rate of the cosmos 11 billion years ago. As researchers sift through the data, they’ll compare it to the outcomes of other observations—and try to answer some of those profound questions about the nature of dark energy.