10:05 AM Dark Energy Guy:
Clusters which we used in our study are BIG. They're millions of light-years across.
As for collapsed: they began as very small "bumps" in energy density in the early universe, and over time -- about 13 billion years -- became denser. Gravity "collapsed" it and brought all of the matter in the cluster together.
There is a smattering of stars in the galaxies, but most of the normal matter is in the hot gas we see via X-rays.
10:06 AM Still, although we can only observe directly the normal matter (the
elements we know on love from the Earth), the bulk of the mass of clusters is DARK MATTER -- not dark energy.
10:08 AM Dave on Earth: Thanks for that -- I'd agree that millions of light-years = BIG.
Now if you were to explain how to look for dark energy -- or should I say signs of its existence, since we can't "see" it -- to a seventh grader, what would you tell the kid?
Dark Energy Guy: Much of the matter that astronomers observe is seen directly by its light in many different wavelengths.
Dark matter we "see" by its gravitational effects on galaxies and groups of galaxies.
Physicists are actively looking for the dark matter and expect that it is some kind of exotic particle, but still a normal kind of matter.
10:12 AM Dark energy is a much more exotic "thing" -- it has energy (and since we know that E=mc^2) it has mass. But it also makes space behave in a strange way -- it accelerates it.
We only indirectly see its effects by performing some pretty complex observations -- watching clusters grow over billions of years, and measuring distances to very young galaxies.
10:13 AM Dave on Earth: You say "watching clusters grow over billions of years" -- I presume you say that because the more distant light is, the "older" the objects emitting that light are?
10:15 AM Dark Energy Guy: We astronomers talk a little loosely about time and space. But you are exactly correct.
Astronomy is a little different than most sciences. We can rarely do an experiment in a laboratory, and we can't set up a cluster or a galaxy or a star and watch it evolve in a controlled environment.
10:16 AM We have
to take the universe as we see it and tease out the information we need. So when we say "watch clusters grow," we look at more and more distant objects which means we are looking
10:17 AM at these clusters when they were younger. We can then compare what we see from this younger generation to what we see now. Combined with some theory, one can then say how the clusters - or stars or galaxies - evolved over time.
10:18 AM So just to clarify your question -- the light we see from distant objects is "old" in a sense since it has been traveling to us for a long time. But the view that we get is of an earlier time!
10:19 AM We see a younger version of the universe and look far away to look back in time.
10:20 AM Dave on Earth: Thanks for the explanation. Now, I've heard rumors that some people are skeptical of dark energy's existence.
Are there really any contenders at this point, and if not why might people "disagree" with dark energy?
10:21 AM Dark Energy Guy: For me, there seems to be little doubt that the universe is doing something a little unusual -- accelerating.
10:22 AM The term dark energy
can be used to describe the cause of origin of this expansion. With the work by Alexey Vikhlinin and his team, there are now two totally independent experiments that show the acceleration.
So there seems little doubt that something unusual is happening.
10:24 AM "Dark energy" is a term that some might apply only to one particular formulation or cause of the acceleration. I think that one can "doubt" the existence of dark energy -- but the case for the acceleration of the universe is getting stronger.
Dave on Earth: I see. So there's really no disagreement on what's happening out there -- just what is causing it.
10:25 AM Dark Energy Guy: Also, the new detailed measurements show that the actual behavior of the acceleration is as Einstein predicted when he introduced a cosmological constant.
So Einstein's theory is holding up well, which supports the idea of the dark energy.
Dave on Earth: Now, you say "the case for acceleration". So it seems like we're not 100 percent sure the universe is accelerating, after all?
10:26 AM And the follow-up: Is it a coincidence that we're at the right point in the evolution of the universe that we can now begin to see dark energy's effect on the universe?
I think Nancy Kerrigan said it best:
"Why me? Why now?"
10:27 AM Dark Energy Guy: It is hard to be 100 percent certain about anything.
But there are now two very strong measurements that show the acceleration.
That is not to say that someone won't come up with a theory that differs from Albert Einstein's General Relativity, which will have a different explanation.
10:28 AM But Einstein's' theory explains a lot: bending of light, behavior of black holes, stretching of time to make your Global Positioning System devices work properly, etc.
In the context of Einstein's theory, it certainly looks like the universe is accelerating.
10:29 AM Dave on Earth: Hmm. I guess the disagreement -- if Einstein was wrong -- is that relativity might not "work" on huge scales (which is what I think some are arguing).
Dark Energy Guy: There are theories that differ from General Relativity. So far, however, there is no compelling reason to abandon General Relativity.
10:32 AM What makes
all of this so exciting is that as new observations are made, one might find an inconsistency -- something that CANNOT be explained with General Relativity.
That would be truly an astounding new discovery.
Certainly we have no clue what the dark energy actually is -- so there must be some major new revelation waiting to be made.
Theory needs to be tested with new observations. But clearly some kind of major new advance awaits a brilliant theorist or a very clever observer.
10:30 AM Dave on Earth: Circling back to Alexey and the team's findings: What might our universe look like now had dark energy not existed?
10:35 AM Would our sun have developed as it is today? Would life have evolved?
Dark Energy Guy: This is a hard question to answer since for astronomy.
Thinking -- back on one second.
Dave on Earth: Think away!
10:37 AM Dark Energy Guy: I think that the universe we live in would not be so different.
The effects
are subtle -- that's why it has taken as long as it has to understand the little that we do know about dark energy. Certainly we would still be here.
10:38 AM Our sun would be shining. On larger scales beyond our galaxy, the universe would undoubtedly be different -- but we would still be here with our telescopes and space observatories to study and try to understand what we are seeing.
Dave on Earth: Now for my final question, then I'll release you from online purgatory:
Does dark energy ever keep you up at night?
10:40 AM Dark Energy Guy: Dark energy certainly does keep me up at night.
But perhaps not in the way you are asking the question -- my interest in astronomy is compulsive -- so when I should be sleeping, I am instead working!
Many other astronomers are up at night doing there observing -- it's only possible at night.
10:41 AM We X-ray astronomers are lucky in that regard... The Chandra observatory stays up late while we think about what to observe next.
Dave on Earth: Thanks very much, and have a great day!
Dark Energy Guy: Now that I've done this once... I'd love to try it again another time.
Article posted December 16, 2008.
Got something to say? E-mail your questions, comments or concerns to discoveryspace@discovery.com.
MORE INTERVIEWS
