Using the Most Violent Explosions in the Universe to Measure Its Size

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Jun 29, 2016
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One of the most difficult aspects of astronomy is distance. Even the Earth’s Moon is 380,000 kilometers away, a four-day trip by rocket, and that’s the closest object in the Universe to us!

Remote galaxies are a hundred million billion times farther away than that. That’s a soul-crushing distance, almost beyond human grasp.

Almost. We’re pretty clever, we humans, and we’ve found ways to figure out how far away cosmic objects are. One powerful tool we’ve found are “standard candles”: bright objects that all shine at the same luminosity. Think of it this way: Take two light bulbs you know are the same brightness (say, 100 Watt bulbs) and place them at different distances. The more distant one will be fainter, of course. But if you can measure how much fainter it is, you can calculate their relative distances. If you can measure the distance to the nearer one, you automatically know how much farther away the second one is.

We have various types of standard candles at our disposal. Supernovae are good ones; there’s a special kind of exploding star (called a Type Ia) that all explode with more or less the same energy. These are very bright and can be seen to tremendous distances. It turns out they have their quirks, so they’re not perfect candles, but we could adjust for inconsistencies. Once that was done, they allowed astronomers to measure distances all the way across the Universe, and even determine that the universal expansion is accelerating.

Still, it would be nice to have more than one ruler. Waiting around for a supernova takes a while, and even then they’re so faint we need huge telescopes to spot them. And while they let us see out to about 11 billion light-years—a very long way—that still falls short of the roughly 14 billion light-year distance to the edge of the observable Universe.

Is there anything that could fill that gap? The problem is you need hugely energetic events to be bright enough to see from all those billions of light-years away.

The good news is there are such hugely energetic events: gamma-ray bursts (or GRBs). These are ridiculously powerful blasts, so luminous they can even dwarf the brightness of a supernova. They were discovered in the 1960s, and we’ve learned a lot about them since then, including the amazing idea that they are the birth cries of black holes! If you want some background, I have a whole episode of Crash Course Astronomy about GRBs:

There’s a problem, though: No two GRBs appear to be alike. They explode with different energy, they have different brightnesses, they fade differently. Some have explosions that last for seconds, others for hours. There’s an expression in the GRB community: “If you’ve seen one GRB, you’ve seen one GRB.” That makes them terrible standard candles.

maybe. Astronomers have been trying for quite some time to find some way to standardize them, account for their differences, so that they can be used as cosmic metersticks. And it appears that the first solid step toward that has been done: Maria Dainotti and a team of astronomers have just published a paper where it looks like they cracked the GRB code! They found a set of 122 GRBs that, when examined carefully, display a unique set of observable characteristics that allow them to be used to determine their distance.

The GRBs they examined were all of the long variety, with detectable energy that lingers for weeks. Each got very bright and then faded rapidly for minutes. After that they stopped fading, “plateauing” for a few minutes. Then, finally, they begin fading again, this time more slowly. Each of these GRBs also had an independent measurement of its distance (using its redshift; see Crash Course Astronomy Episodes 24 and 42 for more on that), which then allowed for the total energy emitted to be calculated.

What Dainotti and her team found is that when you look at that sample of GRBs and plot three of their characteristics—how luminous they are at their peak, how luminous they are during their plateau stage, and when the plateau phase ends—they all behaved in a nice, orderly way. Instead of being randomly scattered around, they all fall into an obvious pattern. They also found that if they exclude a few subtypes of GRBs the correlation is even tighter.

The beauty of this is what you can do with it. If you observe a distant GRB with these characteristics, all you need to do is plot it against the others, and the distance to it pops right out.

Well, it’s not that easy, but the point is this may be a new way to use GRBs to measure distances across the cosmos (and it’s telling us some physics about the GRBs themselves which is interesting). That’s pretty exciting. This is still new, but it’s a promising step toward being able to use these fickle explosions to probe the conditions of the Universe at much larger distances than we’ve been able to up this point.

I’ll note that this work would not have been possible at all were it not for the existence of Swift, a NASA satellite launched in 2004 designed specifically to rapidly find and observe GRBs. It’s cataloged more than 1,000 bursts now, providing a huge database that allows astronomers to look for trends. And it’s still going strong after all these years. I worked on the education and public outreach for Swift for many years, and seeing it supporting important work like this makes me pretty proud.

We didn’t understand GRBs much at all until the late 1990s, and now we’re close to being able to use them to take the measure of the Universe itself. That’s almost as amazing as GRBs themselves.