Astronomy Buff

The Universe Can’t Be Bothered to Interact With You

by Tony on January 10th, 2007

Some amazing news is pouring out of the 206th meeting of the American Astronomical Society this week. I’ve been following the talks and presentations very closely.

In one of the most exciting results produced since the Hubble’s Ultra Deep Field, collaborating astronomers from around the world produced a three dimensional map of dark matter contained in a 2 degree square area of sky. Almost 100 astronomers from a dozen countries, and using a variety of telescopes, stared at one region of sky for over 1000 hours using everything from the Hubble telescope to the radio telescopes in New Mexico.

There is lot to discuss about this, but here’s the punchline:

The above animation shows one of the big results that was announced during the meeting, that Dark Matter is clumping together with the the protons, neutrons, electrons, and atoms that make up normal matter (also called baryonic matter). I made this animation using these images from the HST press release. As you can see, much of the dark matter does indeed seem to be gathering around the normal matter, but to my eye, I also see a lot there that isn’t.

Understand that these measurements are very hard to make. While I’m not sure what the error bars are, I’ll bet they are significant.

Technorati Tags: astronomy, cosmology, dark matter

There is already an excellent writeup describing some of the details of this research by Phil Plait, the Bad Astronomer (I wish he would have picked a different name, I hate calling him that), and another one at Cosmic Variance, so I won’t duplicate their efforts. Here’s what I’ll bring to the discussion.

Let’s start by asking what the big deal is about dark matter. Well, the short answer is that it’s the stuff that makes up 95% of the universe. When you add up everything you see in the universe through telescopes, you come up with a number that is only 5% of what we know is there. That means the other 95% is made up of stuff we can’t measure, at least directly.

Until very recently, astronomers weren’t even sure dark matter existed, they figured that maybe one of their models had a flaw. Now that they know it’s there, they are finding some really clever ways to measure it, although it is extremely difficult. From the HST Site:

For astronomers, the challenge of mapping the universe has been similar to mapping a city from nighttime aerial snapshots showing only streetlights. Dark matter is invisible, so only the luminous galaxies can be seen directly.

Ok, so let’s see the area of sky everyone was measuring:

This is a pretty big area of sky for this kind of a project, roughly 2 degrees square (the moon is about a half a degree wide).

By staring at this patch of sky for so long (which, by the way was in the constellation Sextans), and doing it at a variety of wavelengths, astronomers gathered an unprecedented amount of data about the deep universe, because each wavelength provides different information, the pieces could all be put together to provide a comprehensive view of this relatively small section of sky.

For example, here is what the Hubble Space Telescope saw when it looked at that area (click on it for a larger view). The wavelength is visible light, the light our eyes can see:

Every galaxy in the above image is at a different distance from us, some are closer, some are farther away. The dark matter that lies between us and any particular galaxy is affecting the path of that galaxy’s light by its gravity. The gravity exerted by dark matter pulls the light from a galaxy from a straight line into a curved one, causing a lensing effect. So by the time the light gets to us, it’s path has been altered by an amount proportional to the pull of its gravity, from which we can infer how much was there. That’s the only way we can see this stuff. It’s a very indirect measurement and one that is extremely hard to make, so the errors in such a measurement are high.

But, we have no choice since most of what makes up our universe won’t have anything to do with us. We simply have no way to interact with it.

And by measuring the distances to each of those galaxies (by looking at their redshifts in their spectra), astronomers can roughly determine where the dark matter was when it bent the light, and they can make a 3D map if all of the dark matter that lay between us and the farthest point in the area of sky shown in the first image above.

Here’s what they came up with:

Now that’s pretty cool.
All images courtesy: HubbleSite, ESA and NASA

Share This

Related Stories

Dark Energy is a Misnomer
Three Most Common Misconceptions About the Big Bang
Edge of the Universe May Be Creating Dark Energy
Six Characteristics of Dark Energy
What is Dark Matter and Dark Energy?

POSTED IN: cosmology