Research

We call it dark matter because it doesn’t emit light.  It’s a substance different than anything on the periodic table, and undetectable by any human senses. It’s not made of normal atoms, or anything like your desk is made of, or you are made of.  If you had a ball of normal matter, and it was traveling toward a wall, it would bounce off the wall like a tennis ball.  But if you had a ball of dark matter, it would go right through wall. 

So dark matter is some pretty weird stuff. But we have good evidence that there’s about five times as much dark matter as there is normal matter in the universe. 

It was like finding a needle in a haystack. Whenever you look at a galaxy that’s far away from us, you have to look through all the stars that are in our galaxy first, and there’s a billion of them. About 10 years ago, astronomers started something called the Sloan Digital Sky Survey.  They dedicated a telescope and a big charged-couple device camera to make a map of the sky.  Initially, some cosmologists were inclined to throw images of Milky Way stars in a discard pile, because they wanted to see the galaxies farther away. 

Other astronomers started sifting through the star pile, and one of the things they noticed was that every now and then there would be a pile up of stars in a certain area of the sky: more stars than you would expect. It turns out our galaxy has a lot of little, bitty satellite galaxies around it.  These galaxies are tiny; they might have 1,000 stars whereas the Milky Way has something like 10 billion.  So in the last five years, we’ve discovered an entirely new class of galaxies. 

We’ve found that these galaxies have something like 1,000 times as much mass in dark matter, as they have in their stars.  So even if they only have a few stars, they weigh as much as if they had a lot more.  They’re basically balls of dark matter with the dregs of stars floating around in them.

Our team’s role in this research has been to calculate the actual mass of the dark matter in these tiny galaxies.  It turns out they all weigh about 10 million times the mass of the sun.  The strange thing is if you compare them to galaxies around us that shine ten thousand times brighter, they all still have the same mass.  So it’s almost like there’s a dark matter mass threshold.  There may be no galaxies less massive than this.  

Ultimately, we hope to isolate the particle. But it’s so elusive that so far all we have done is place limits on what it can be like, and what it can’t be like. If dark matter was heavy, for example, the particles would behave in a different way than if the particles were light. One thing we’ve learned over the past 30 years is that dark matter is made of particles that are fairly heavy, compared, say, to the mass of a proton. If dark matter was light, the galaxies in the universe wouldn’t cluster the way they do.

Almost never. That’s our poetic image of what an astronomer does. These days most astronomers vie for time on big shared telescopes.  One of the reasons UC attracts so many talented astronomers is because we have access to the Keck telescopes in Hawaii, which are the most powerful optical telescopes in the world.  Some observing opportunities can be handled remotely from our offices in California. But in practice at least one observer from a UC team will usually go to Hawaii to their observing station at sea  level in Waimea when research is underway.  You never observe at the summit anymore.  It’s 14,000 feet up, and the air is thin, and you don’t think straight.

There are a lot of astronomers in the UC system but only two Keck telescopes, so pooling telescope time and going after problems and questions that are bigger than any one person can handle individually.  

The rule of thumb for astronomy is the universe is always weirder than you thought.  Every time you have a telescope that’s slightly bigger, or an instrument that’s more powerful, you find something unexpected. Nobody ever imagined, for example, there would be galaxies with only a thousand stars in them.  Nor did we imagine they would have as much mass as they do! It’s like finding an elephant that’s the size of the kitten, and then finding that the kitten weighs as much as an elephant.

Science doesn’t tell you the meaning of life, it’s all about the quest. One important aspect of life to me is to learn as much as you possibly can, and ask as many questions as you can.  One thing homosapiens do well is think.