Monday, August 07, 2006

Universe older and bigger?

An article on Space.com today revealed that a new study is stating that the universe might be bigger and older than current estimates predict. Since a few other news sources have picked up the article, I figured I should comment on it.

Let me start off by pointing out that, I'm being my usual skeptic self when I say I don't really find the article to be overly convincing. The main line of evidence is based on a single observation on a single binary system in a single galaxy.

To make this measurement, they looked at a binary system in M33 (the Triangulum Galaxy). These systems are pretty nifty because you can use some relatively simple and well understood math to determine the masses of the stars very accurately.

The assumption from there is that, if you know the mass, you know the luminosity. This isn't a bad assumption. If you remember my discussion of the H-R Diagram earlier, there's a wonderful thing known as the main sequence, that gives a relation between these two.

From there, if you know what the luminosity is supposed to be, you can compare it to what the luminosity is. If the assumption is then made that the only thing dimming the star is distance, there's a nifty little formula that relates the difference between the two to the distance to the star (called the distance modulus equation).

So there's been a few assumptions in this measurement already. As I've already said, most of those assumptions are pretty safe, but in cases like what I'm working on this summer, there are other things that can dim how bright a star appears, thus throwing off your measurement unless you take it into account.

I'm willing to bet that the researches in this study did take such things into account though.

So why do I remain skeptical of the accuracy? The main reason is that they claim it is a "new method". If it's new, chances are it's not terribly well calibrated yet. Other methods of distance indication (Cepheid P-L relation, Supernova Ia brightnesses, etc...) have been undergoing constant calibration to make sure that they are accurate, since the time they were put into use, which is around 100 years at least for these two.

Thus, I'll reserve judgement on this until such a time when things are better calibrated.

My second issue with the study is the fallacy of small sample sizes. So far, this study has a single sample. One. Uno. In comparison, the studies giving the current Hubble constant and age of the universe, have thousands of galaxies contributing to their claim, coming from several different methods of distance indication.

For these reasons, it's more than a bit presumptuous, in my opinion, to say that the commonly accepted value is wrong. What this study does do, however, is raise a red flag and let other astronomers know that there is an issue here that needs to be addressed. Thus, we can look at both the new method to refine it and see if things will iron out, or, assuming this method is revised and discrepancies are still found when there is a statistically meaningful sample size, that the current age of the universe needs to be revised.

2 comments:

Chet said...

I highly recommend reading a terrific science book:

"The View from the Center of the Universe--Discovering Our Place in the Cosmos" by Joel R. Primack and Nancy Ellen Abrams, 2006, Riverhead Books, ISBN 1-59448-914-9, $26.95.

Stephen said...

I agree. It seems that if you're doing a strictly luminance distance thing, you'd want multiple systems in the same galaxy to measure. And, yes, that's what they want to do next. I's what they do for supernovea. But they didn't publish until they had it. The six percent claim is bothersome. Where's it come from? I mean, if they made lots of measurements of the one system, that means something different than if it is a guess on dust variance.

When the Hubble Key project announced that the Universe was 11.7 +/- .4 billion years old, i expected it to be wrong. We know of stars older than that. Well, something had to be wrong. I wanted to know how they computed that error bar too.

There's supposed to be a great galaxy distance idea that involves simple geometry, and can be used with VLBI radio scopes. The initial distance announced was kinda odd too. I haven't heard any followup.