May 12th: Michael Turner on Dark Energy

Date: May 12, 2009

Title: Michael Turner on Dark Energy

Full, unedited interview available at Slacker Astronomy!

Podcaster: Michael Koppelman of Slacker Astronomy

Organization: Slacker Astronomy

Description: Slacker Astronomy talks with cosmologist Dr. Michael Turner about Dark Energy.

Bio: Slacker Astronomy is ranked as one of the top 10 astronomy podcasts.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Physics Department at Eastern Illinois University: “Caring faculty guiding students through teaching and research” at

Michael Koppelman: Hello, welcome to Slacker Astronomy. We’re at the AAS meeting in Long Beach, California with the esteemed Dr. Michael Turner, Cosmologist from the Kavli Institute for Cosmological Physics. Welcome!

Dr. Michael Turner: Glad to be here.

Michael: We’re honored to have you here. So, it’s true that you coined the term ‘Dark Energy’?

Dr. Turner: That is, yes, I’m guilty as charged. [Laughter]

Michael: What was the context or what were you thinking, was that like a light bulb going off or did that just evolve slowly that idea?

Dr. Turner: There was a reason to do it because at the time I think many people said aha we’ve got it, it’s just Einstein’s cosmological constant, end of story. That could be the end of the story. That could be the way that it plays out but I think not. Rather than have it called the cosmological constant and then there is just one number to measure and if it really were just a cosmological constant we would have measured it about as well as you can.

To say this could be something much more interesting, we don’t really have a good reason to believe that it is the cosmological constant. Give it a name that is not the cosmological constant. Just like dark matter instead of saying it is this or that, or saying it is faint stars, which Zwicky wouldn’t have known. Here you say it could be something really interesting, here’s what we know about it. It is extremely elastic. It doesn’t seem to cluster. It is uniformly spread out, it is really weird. It is not particles, it is more like energy.

Michael: Is it a constant?

Dr. Turner: No it’s not necessarily constant so it is something that changes more slowly with time than the matter density. I think it was to shift — the herd was moving towards it is just lambda, we’re done let’s go on to the next thing and say wait a second.

This is very profound. I’m also fond of saying that I think that this is the most profound mystery in all of science. Maybe it is not the most important mystery, but the most profound.

Michael: Were you keying off of dark matter in terms of

Dr. Turner: Yeah and then because guess what? In the last ten years in my career as an astronomer we’ve discovered that most of the universe is stuff that we can’t see. Ninety-nine percent of the universe is stuff we can’t see with optical telescopes. Ninety-six percent is stuff that we can’t see with any telescopes. That’s quite amazing.

What we do know is that it is parceled into stuff that is like matter that just doesn’t give off light, can be divided into particles, has ordinary attractive gravity, we call that dark matter. The dark energy is the stuff that is very weird. We think it can’t be broken into particles that it is more like a medium. Its gravity is repulsive.

When you tell the story of our universe it has really been the battle of the dark titans that for the first 10 billion years or so it was dark matter that controlled things and dark matter formed the cosmic infrastructure and formed galaxies and clusters of galaxies. Then the other dark titan, dark energy took over and now the universe is accelerating, no more structures are forming and just learning about the dark side has completely changed the story of how we got here.

Michael: Naysayers kind of don’t like dark energy and dark matter. They think that we just got it all wrong. Does dark energy represent any sort of failure of our cosmology right now?

Dr. Turner: It represents a piece of our cosmology we would like to understand better. You know the march of science, you understand a little bit. It takes you to a higher plane where you can see a little further and ask better questions.

There are people, and they might even be right but I don’t happen to agree with them, that see dark matter and dark energy as cycles and epicycles and indications that we’re just completely off track.

I think on dark matter those people would say that dark matter isn’t really new particles out there it is just gravity behaving differently. I think that path has been tried for twenty-some years and I think it hasn’t really yielded fruit and I think we’re really close to showing that dark matter is just particles. I think the new machine in Geneva, the Large Hadron Collider; we like to think of it as a dark matter factory could actually produce a dark matter particle.

On the other hand, for dark energy I think we’re really early in the game. One of the paths that people are actively pursuing is that there is no dark energy out there, that it is something wrong with the gravity theory. This is pointing us to a modification of Einstein and what is nice about that is that very few people think that Einstein got the last word on gravity.

This could be the clue to go beyond Einstein. There might be some truth that the dark energy might be an epicycle. Trying to ascribe it to a substance might turn out to be wrong. It might actually be an indication that we’ve got the wrong gravity theory.

Michael: That brings me to our next question which is what new results do you see on the horizon that we might be talking about in the next few years like from the LHC and others?

Dr. Turner: I think there is a good chance we’ll close out the dark matter puzzle in the next ten years. The reason is that we really have a full-court press on it. We talked about the Large Hadron Collider as a dark matter factory and you can actually produce and bottle the particles – figuratively.

These very sensitive detectors that are being deployed underground that aspire to detect the dark matter particles that hold together our galaxy. Dark matter particles if they are in our halo they should occasionally annihilate and they could annihilate into photons, positrons or neutrinos.

Michael: There are some right here, right?

Dr. Turner: In your coffee cup over there [Laughter] there’s about one dark matter particle on average at any time. The problem is that because they don’t couple electromagnetically because they don’t participate in the electromagnetic interaction they are essentially like a neutrino. They are actually harder to detect than neutrinos so I kind of feel like we really have the full-court press on which is not a guarantee but I think there is a really good shot.

Dark energy on the other hand I think a reasonable goal would be progress in understanding it. Some people have called the 2010s the decade dark energy. There will be a couple of big experiments. The Joint Dark Energy Mission and LSST the Large Synoptic Survey Telescope that are called the Stage 4 experiments that will address in a serious way the question is dark energy consistent with the energy of the vacuum.

Michael: Right now, you alluded to earlier it’s off by like 80 orders of magnitude or something.

Dr. Turner: That’s trying to calculate it. We’re not trying to calculate it and we really get particularly – I’m a theorist – and we really get all tongue-tied because we say sometimes that it is 50 orders of magnitude off, sometimes 120. The truth of the matter is when we try to calculate how much nothing weighs we get infinity.

Actually that’s better than being 50 orders of magnitude off because if you can calculate something and you actually get a number and it is 50 orders of magnitude off you’re really in trouble.

If you can’t calculate it and getting infinity means you can’t calculate it – didn’t everybody learn that in math class – infinity is not a real answer, you actually have a little bit of hope. You know you’re wrong.

So, I think in the next decade we might be able to rule out the idea that dark energy is the energy of the quantum vacuum by seeing it slowly vary with time. We might be able to show that general relativity can’t self-consistently account for the fact that the universe is accelerating. So I think the goal for the next decade would be a better understanding of dark energy.

Of course if you’re irrationally exuberant you might say and we’ll close out the problem. But I somehow, something tells me that it may take a little longer than that. It is a very important problem and I think at the very least 10 years from now we will have the problem framed a lot better.

Michael: Dr. Michael Turner, thank you very much.

Dr. Turner: Thank you guys, it was fun.

This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.

End of podcast:

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