Wait a sec. How big is the Universe again?

Contributed by
Aug 8, 2006
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A couple of recent news articles have me scratching my head. And if I don't get them, then I'm guessing that folks who aren't astronomers will be even more confused. By themselves the press releases aren't too bad, but taken together they contradict each other.

Sit tight. This'll take a minute.

The Universe is expanding. We're pretty sure about that at this point; we have about a century's worth of observations backing it up. The idea is actually pretty simple: when we look at distant galaxies, they appear to be moving away from us. The farther away the galaxy is, the faster it's receding.

There are a zillion lines of evidence for this, but the most basic one is redshift. You've already experienced this! When a race car goes past you, the sound it makes goes "rrrrrreeeeeeeeeeeeOOOOOWWWWWWRRRRRR". The pitch is higher when it approaches, then drops as it passes you. This is redshift! A similar thing happens with light: when an object approaches you, the "pitch" of the light -- the color -- gets higher, which means the light moves toward the blue end of the spectrum. When an object recedes from you, the color goes toward the red end.

And so it goes. We observe galaxies, and the farther away they are, the more redshift we see. That means the farther away they are, the faster they recede from us. So the Universe is expanding.

The key question is, how fast is it expanding? That is, how does the expansion speed relate to the distance? This is the question Edwin Hubble tried to answer back in the 1920s. He came up with the idea that the distance and speed of a galaxy are related by a simple constant, which we call the Hubble constant. The current value is about 70 kilometers per second per megaparsec (+/- 8 km/s/Mpc). What that means is that a galaxy one megaparsec (about 3 million light years) away will recede from us at 70 km/sec. A galaxy 2 Mpc away will recede at 140 km/sec, and so on.

Still with me? Now, the problem is that this is a BIG deal-- for one thing, using some math, you can get the age of the Universe from the Hubble constant. Scientists are picky, and they like to have lots of independent ways of measuring something when it's this important. So they're always looking for new ways to see how fast the Universe is expanding. If these methods are totally independent of redshift, but get the same results (i.e., a Hubble constant of 70 or so) then you have more confidence we know what we're doing.

There are lots of such methods. Some are more complicated than others. The folks at the Chandra X-ray Observatory just announced that they have independently confirmed the Hubble constant. Their method is a bit obscure for the non-scientist: they look at light coming from the background glow of the Big Bang, and see how the light is distorted by superhot gas floating around in clusters of galaxies (I told you it was obscure). This is called the Sunyaev-Zeldovich effect after the astronomers who figured it out (fun side note: Sunyaev's daughter, who was very young at the time, threw up in a van I was driving once). The image below shows some of those clusters of galaxies. The purple stuff is the hot gas.

The thing to know is that this method really is independent of redshifts, and they get about the same value for the Hubble constant: 77 km/s/Mpc +/- 15%. That's a pretty good value, and agrees with the currently held value.

But there's a problem...

Just last week, another team of astronomers announced that using a different method, they get a Hubble constant of 61 km/s/Mpc (irritatingly, they didn't give an error measurement for this value). That's a lot lower than the currently accepted value, and the way the math works out that means they get a bigger, older Universe.

The method they used (looking at eclipsing binaries, stars that pass in front of each other, in a nearby galaxy called M33, pictured below) is pretty well-established, and should yield solid data. But so should the hot gas method! So we have two methods, both independent, and both generally good, getting different answers. And that's what has me scratching my head. What do we do?

Well, there is more to know. For one, the eclipsing binary method used only one example of such a system in a nearby galaxy, and so it may not be terribly reliable. You want to have lots of binary systems to measure, to make sure you haven't made some mistake. And more distant galaxies are better to use, since that gives you a longer baseline for your measurements. The problem is, stars get pretty hard to detect in distant galaxies, so we're stuck with using ones that are close by. Worse, nearby galaxies are affected by our own Milky Way Galaxy's gravity-- for example, the Andromeda galaxy is actually approaching us, not moving away! The galaxy the binary team used is only a wee bit farther away than Andromeda, so I'm not sure how reliable it is to use-- that alone might account for the 15% problem. But then, the astronomers on that team are smarter than I am, and have probably thought of that.

The hot gas method is weirder, but it also conforms to a lot of other independent methods that yield that 70 km/s/Mpc value for Hubble's constant. And that value, when you crank through the math, gives an age for the Universe of about 14 billion years, which we know is about right from a zillion other lines of evidence!

So what do we do?

Well, as usual, we have to be a bit careful. First, remember we're arguing over details, and not the overall idea that the Universe is expanding. Any creationist who uses this as "evidence" that even scientists can't agree on the Big Bang should be poked in the eye. Second, as usual in science, time will tell. More observations are needed, and that will help nail down some of the specifics of all this. We've had lots of controversies like this in astronomy before, and they have always -- always -- resolved themselves in a logical and coherent way.

The Universe is pretty cool. It obeys a set of rules, and we can watch how it does so. Mysteries like this force us to pay better attention, and to rethink our ideas, our observations, and what we know. And from all that, the rules governing the entire Universe can be inferred. How amazing is that?