Wednesday, August 09, 2006

Hubble "Constant"

I went through this stage once where I didn't read the news, at all, for about three years. The entire list of reasons is enough for a whole other post, but one contributing factor was certainly that every news article I have ever read about something (or someone) I actually know something about has made me want to scream due to mistakes ranging from raw factual errors or subtle misinterpretations.

The latest hubbub about oh-no-the-universe-is-15%-bigger-than-previously-thought proves to be no exception. Sigh. The article has it all right, far as I can tell, but it's the twist, the buzz, the headline-producing propoganda that has me annoyed.

In a nutshell, the Hubble constant, H0, relates how fast, v, an object (say, a galaxy) is moving away from us to its distance, d, from us in a simple way: v = H0d. The neat part is that, with very few other assumptions thrown in, the Hubble constant also tells us the age of the universe. So, we want to know this constant pretty badly. Velocities are fairly easy to measure, but distances... distances are hard. Relative distances aren't all that bad (e.g., "this galaxy is about twice as far away as that galaxy") but absolute distances are very difficult (e.g., "that galaxy is 50 kiloparsecs away").

For a long time, people have been anchoring their distance scales to the Large Magellenic Cloud, or the LMC. (For those of you who, like me until a year ago, never realized that the southern and northern hemispheres really do see different parts of the sky, the Large and Small Magellenic Clouds are two small dwarf galaxies orbitting our own Milky Way. They are so named because when Magellen's crew went sailing through the southern hemisphere, they were kind of confused as to why these two clouds always seemed to move with the sky, see...) The LMC kind of makes sense: it's close, and therefore easy to see, so it should be fairly easy to get a distance to. Right? Wrong.

So as I don't want to get into a whole discussion of how distances are measured, suffice it to say, for now, that people are always looking for new, independent measurements. The exciting thing about the M33 distance is that it's just that: a direct measurement of the distance to M33, independent of this fuzzy object in the southern sky. Working backwards, they are able to estimate the Hubble constant, which, sure, while 15% lower than what the HST Key Project got, is actually not unreasonable if you take into account the errorbars.

What all of this buzz has been forgetting, though, is that a "new" calculation of the Hubble constant ... isn't that exciting. I mean, it is, because it's the Hubble constant, which is exciting in and of itself, but the fact that people are getting all worked up over it (whereby "people," I clearly mean the media, which doesn't really count, but anyhow) just goes to show how remarkable it is that there has been a fairly agreed upon value for the last five-ish years.

And here you have it: published values of the Hubble constant over the last thirty-ish years. References can be found here. The big dot there on the right is the M33 value; its errorbars should be roughly the same as the HST ones (70% of which are due to systematics from not knowing the distance to the LMC, by the way). The supernovae paper listed several values, only one of which I've plotted, and they specifically state that their errors do not account for systematics. The WMAP results should be taken skeptically, as they fit for many cosmological parameters, and it's rather tricky to sort out all of their priors and other assumptions that go into H0 in particular.

Bottom line: the "new" value of the Hubble constant isn't exactly new, per se; it's just another step that's being taken in figuring out exactly how to measure distances to far away objects.

1 comment:

Stephen said...

There's also the age of the oldest known stars. One expects that the Universe is at least that old.