Ask The Bad Astronomer

Astronomer, author, columnist, and successful populizer of science Phil Plait, perhaps best known as The Bad Astronomer, is a regular sight on Slashdot for his unusual ability to find lucid explanations of esoteric scientific claims and controversies. Phil has graciously agreed to answer Slashdot readers' questions, so ask him below about space, science, debunking conspiracy claims, and anything else that makes sense. Asking more than one question is fine (and encouraged!), but please separate unrelated questions into separate posts, lest your questions be moderated down.

Comments

Light pollution

[Frenzied Apathy]

There are a large number of light pollution articles to be found on the Sky and Telescope website. We amateur astronomers are keenly aware that light pollution isn't just about being able to see more stars from our backyard. Yet, when I mention the subject to friends, family, co-workers, etc, I often get a blank stare. "What's 'light pollution'?" What do you think can/should be done to improve widespread public awareness of light pollution and its effects?

What do you think is the bigger threat?

[UberOogie]

Given your unique position, I'd like to know your answer to this question:

What do you think is the currently a bigger threat to legitimate science:
- The growing wave of anti-intellectualism and anti-science that seemingly rejects science outright on certain issues
- Or the growing wave of pseudo-science that undercuts science by adopting the trappings of science but none of its procedures?

Thank you for your time.

An awfully long time

[pavon]

Whenever I research this, I come back to the "galactic rotation problem" as the most solid evidence.

That isn't the most solid evidence. It was just the first evidence, and if it was the only evidence then Dark Matter would not be preferred over the idea of a modified gravitational theory. Since then there have been two major additional forms of evidence for dark matter. The first is gravitational lensing.

According to General Relativity, mass bends bends light that passes by it. We have measured this effect before with objects of known mass, and the predictions of GR are dead on. Astronomers have looked at how strongly light is bent when passing by large galaxies, and used this to compute the mass of the galaxy. The numbers they get are much greater than the mass of the visible material, and more importantly match up very closely with the estimates of dark matter mass obtained by looking at rotational curves.

The most outstanding evidence of this is from the Bullet Cluster. Here two galaxies had passed through each other, and you can see how the different types of matter were slowed down by different amounts depending on how likely they were to collide. The gas clouds were slowed the most, and the large structures (stars, etc) slowed down less. But if you look at the gravitational lensing, you see that there is a big chunk of non-visible mass that was not slowed down by the collision at all. This is exactly what you would expect to see if the galaxies contained non-baryonic dark matter, and can't be explained by modified gravity at all.

The second major evidence is the cosmic microwave background radiation. I don't pretend to understand this, and thus won't try to expound, except to note that the ratio of baryonic matter to non-baryonic matter found using the CMBR also agrees with the dark matter estimates found using galaxy rotation curves and gravitational lensing.

So we have three drastically different ways of indirectly measuring the same thing, and they all come up with the same result. That is pretty strong evidence in my book.

How long do we have to put up with the notion of "Dark Matter"?

My guess is quite a long time, because it is almost certainly correct. Hopefully though, we will have direct evidence of dark matter with the next few decades, which should make it less annoying :)