TED2008: Pat Burchart explained the 96% of the universe we can’t see

Dr. Roy Gould of Caltech introduces a new product on the TED stage – the WorldWide Telescope. It’s being released in part to celebrate the International Year of Astronomy, which coincides with the 400th anniversary of Gallileo’s telescope. This telescope is a digital one, a service that knits together the best available imagery of space.

The tool, Gould tells us, lets us navigate the universe on our own. But it also lets you tour with astronomers as your guides, or to create your own tours, sharing them with friends. Curtis Wong of Microsoft tells us that it will be available this spring as a free download. The imagery, from what we see here, is quite amazing.

Dr. Patricia Burchat, a particle physicist at Stanford, is in a good position to explain to us what those galaxies shown in the WorldWide Telescope are made of. Most of her work has been with particle accelerators. But the questions answered in those experiments are huge questions, questions about what the universe is made of.

Ordinary matter – atoms and molecules as we know them – make up only 4% of matter in the universe. Dark matter – matter which doesn’t interact at all with the electromagnetic field – makes up 26% of matter in the universe. The rest, 70%, is dark energy.

To understand dark matter, we need to understand gravity, because that’s how we sense it. She shows us a picture of the Andromeda galaxy, pointing out that most of the mass is concentrated in the center, pulling stars into circular orbit. Stars closer to the mass in the middle should rotate faster, and should be slower on the edges. Instead, the speed is pretty constant. Stars are feeling gravitational effects that we don’t see. They’re embedded in a cloud of dark matter that has a much greater extent than the visible galaxy. We can also intuit dark matter based on galactic clustering. Galaxies aren’t random in space – they group together, and in these clusters, they rotate more quickly than they “should”… which again points to giant spheres of dark matter.

How do we “see” dark matter? She shows us a diagram of light travelling from a galaxy to our eyes. We deduce direction based on the direction of the light. But if you put a galaxy, and a huge sphere of dark matter, between us and the galaxy we’re observing, it would distort the light reaching our eyes. The gravitational field, due to mass, will deflect light itself, as Einstein predicted. The galaxy could actually be anywhere in a ring around where we perceive it to be – this ring is called “an Einstein ring”.

This is an example of gravitational lensing. You can see how it works by breaking off the base of a wineglass and holding it over graph paper – the lines on graph paper will turn to arcs. You can see similar lensing in the above image from the Hubble Space telecope. By studying the deflection, we can calculate the mass of the gravitational lenses.

The universe – space itself – is expanding. Each day, the distance between galaxies is getting greater. The galaxies aren’t moving through space – space itself is getting bigger. At the big band, space expanded very rapidly. But gravitation tends to slow this expansion down. There’s an old debate – will universal expansion slow down? Slow down and stop? Slow down, stop and contract?

Scientists have measured spacial expansion, and discovered that, actually, it’s speeding up. There’s no persuasive theoretical argument why this should happen. To try to make this make sense, physicists can add a term to an equation. This term is dark energy. It’s not related to dark matter. Dark matter tends to create structure by increasing density. Dark energy tends to decrease structure, pulling matter away from itself.

Contemporary physics offers the theoretical prediction of a particle. It would be great if that particle were the dark matter particle. In the hopes of finding it, we are building extremely sensitive detectors, a form of crystal that would ring if they encountered dark matter. Her colleague is building a project called Cryogenic Dark Matter Search. Other projects include the Gamma Ray Large Array Space Telescope. The large hadron collider may also produce dark matter particles. They wouldn’t be detected, but the missing energy might be noted… or the missing energy might be the solution to any number of other unanswered problems in physics.