The scoop: Dark energy might be invisible, but we can still see its influence. William Forman, an astronomer at the Harvard-Smithsonian Center for Astrophysics, explains how X-ray astronomers pinned down the best-yet evidence of the strange stuff.
Dave on Earth: Hi Dr. Forman!
9:29 AM Dark Energy Guy: Hello -- I'll be ready in a couple of minutes.
Dave on Earth: Sure! Take your time.
9:35 AM Dark Energy Guy: Ok.
I'm NERVOUS -- I've never done anything like this before.
9:36 AM Dave on Earth: No worries at all! It can be nerve-wracking at first.
Dark Energy Guy: Nerve-wracking -- only for me. I'm sure you're a pro at this.
9:37 AM Dave on Earth: I try!
So Dr. Forman -- or should I say William? Bill?
9:38 AM Where are you chatting from?
Dark Energy Guy: Bill is fine -- no particular formality is required.
I'm sitting here in my office at the Smithsonian Observatory, which is just outside Harvard Square in Cambridge Massachusetts (up on a small hill called Observatory Hill).
Just outside my window I can see the copper-covered dome of the old Harvard College Observatory telescope.
9:40 AM Dave on Earth: Ever used it?
Dark Energy Guy: The telescope is really only for show now. But it was recently refurbished...
It has a marvelous red velvet chair for the observer that rotates around the dome. It also slides up and down on a mount to get the observe to the right height to peer through the eyepiece.
On open nights the telescope is opened and in the past people had been allowed to try out the observers seat - wide enough for two!
9:42 AM The telescope has another curiosity: it was originally rotated on Civil War-era cannon balls.
9:43 AM Dave on Earth: Thanks for that. Now if I had to guess, I'd say you didn't use Harvard's telescope to uncover some more of the mystery of dark energy.
9:46 AM Can you explain what you found -- and how?
Dark Energy Guy: Correct -- the telescope that we used to study dark energy is farther away than just a peek out my window.
We -- that is the team led by Alexey Vikhlinin, also a scientist here at Smithsonian -- used the Chandra X-ray Observatory.
With Chandra we studied clusters of galaxies that glow in X-rays. They're easy to see to large distances.
By studying clusters and measuring their masses over range of distances (equivalent to time), we watched the growth of clusters. This gave new clues to the nature of the dark energy.
9:48 AM Side note: this observatory is in an orbit that takes it about one-third of the way to the moon. To use it, you have to apply for observing time, and it's difficult to get.
9:50 AM Dave on Earth: How do astronomers get time reserved on Chandra? Is it first-come, first-served?
Or something more exciting -- fistfights? Duels to the death?
9:51 AM Dark Energy Guy: Hardly first-come, first-served! It's rather a very organized, formal process.
Every year in mid-December there's a call for proposals -- all submitted with strict page limits and formats -- due in mid-March.
9:52 AM In the spring, the Chandra Science Center organizes a review, and astronomers from all over the world are selected for panels with different areas of expertise to review and allocate one year's worth of observing time.
It's a frantic time up to that deadline!
9:53 AM Dave on Earth: So it's more like, um... an essay contest to get telescope time on a space observatory.
Dark Energy Guy: Like an essay contest -- but even if the grammar is poor and the ideas
are great, you can still get your observing time.
9:54 AM Dave on Earth: Gotcha. Now to return to what you, Vikhlinin and everyone else did:
You mentioned some new clues about dark energy...
Dark Energy Guy: The goal was to study how clusters -- the largest "collapsed" objects in the universe -- grew over cosmic time.
One of the most surprising recent cosmological discoveries is that the universe has not been decelerating under the attractive gravity of all the matter.
9:57 AM Instead, it has been accelerating. This discovery rests on the study of distances to galaxies using supernovae, or exploding stars.
We wanted to study dark energy in
a different way, and galactic clusters allowed us to do that.
We were able to measure how clusters grew from the time the universe was about half its present age, up until recently.
Dave on Earth: Why study it in a different way? Aren't supernovae good enough to figure out how dark energy works?
9:58 AM Dark Energy Guy:
The previous studies showed that dark energy existed, but that its properties are very hard to precisely measure.
9:59 AM More and more observations of different phenomena are needed to really know that our conclusions are correct.
Let me get a quote for you...
to paraphrase the famous cosmologist John Wheeler:
"Geometry tells matter how to move.
Matter tells geometry how to curve." This was a description of Einstein's theory of General Relativity.
10:01 AM So measuring the distances to galaxies with supernova was a study
of geometry -- with data on the growth of clusters, we could study the other aspect: How gravity, in the presence of dark energy, works to regulate growth.
10:02 AM Since we have really no idea what "dark energy" is, it is really important to study it from every possible aspect and with every possible technique.
The new observations, when combined with all previous observations, are still consistent with Einstein's original conception.
So this "experiment" was partly a test of Einstein's theory of General Relativity.
Dave on Earth: Side questions: how big are these clusters of galaxies, and why call them "collapsed"?
