Among my recent peregrinations was a jaunt up to Santa Barbara, where I gave two talks in a row (although in different buildings, and to somewhat different audiences). Both were about attempts to weasel out of the need for dark stuff in the universe by trying to modify gravity.
The first talk, a high-energy theory seminar, was on trying to do away with dark energy by modifying gravity. I used an antiquated technology called “overhead transparencies” to give the talk itself, so there is no electronic record. If I get a chance sometime soon, I’ll post a summary of the different models I talked about.
The subsequent talk was over at the Kavli Institute for Theoretical Physics. There was a program on gravitational lensing going on, and they had asked Jim Hartle to give an overview of attempts to replace dark matter with modified gravity. Jim decided that he would be happier if I gave the talk, so it was all arranged to happen on a day I’d be visiting SB anyway. (Don’t feel bad for me; it was fun to give the talks, and they took me to a nice dinner afterwards.) I’m not really an expert on theories of gravity that do away with dark matter, but I’ve dabbled here and there, so I was able to put together a respectable colloquium-level talk.
And here it is. You can see the slides from the talk, as well as hear what I’m saying. I started somewhat lethargically, as it’s hard to switch gears quickly from one talk to another, but we built up some momentum by the end. I started quite broadly with the idea of different “gravitational degrees of freedom,” and worked my up to Bekenstein’s TeVeS model (a relativistic version of Milgrom’s MOND), explaining the empirical difficulties with clusters of galaxies, the cosmic microwave background, and most recently the Bullet Cluster. We can’t say that the idea is ruled out, but the evidence that dark matter of some sort exists is overwhelming, which removes much of the motivation for modifying gravity.
The KITP is firmly in the vanguard of putting talks online, both audio/video and reproductions of the slides. By now they have quite the extensive collection of past talks, from technical seminars to informal discussions to public lectures. Some recent categories of interest:
- String Phenomenology Program
- Applications of Gravitational Lensing Program
- Public Lectures
- Blackboard Lunches (informal introductions at a slightly more accessible level)
- Journalist in Residence (including a couple of discussions on “The String Wars,” led by George Johnson)
On Friday I’ll be at Villanova, my alma mater, giving a general talk to undergraduates on what science is all about. I’m not sure if it will be recorded, but if the yet-to-be-written slides turn out okay, I’ll put them online.
You’re right. Except that the gravity-self interaction contribution to the energy-momentum tensor gets moved over to the left side of the Einstein equations. It is not part of ususal definition of this tensor (which is restricted to the matter contribution).
So when you say that gravitons couple to energy-momentum, you mean that in the broad sense of an energy-momentum tensor augmented by a gravitational-self interaction piece. [This is a just a silly quibble on my part that boils down to whether “energy-momentum” = “standard energy-momentum tensor”.]
Sean, what’s the latest story on apsidal motion of DI Hercules and its observed discrepancy with GR? I can’t recall any recent papers on this? Since you have worked on this system ,
could you give a brief overview of what accoridng to you (and also the general
community) is the consensus on this issue?
Thanks
Hi, Belizean. No, I didn’t listen to the talk. Indeed, I probably would not have understood it. I came across this posting on the Web, and asked the question that’s been bothering me. I figured asking a question was okay, and maybe I’d get an answer.
And indeed I did. Thank you, Collin. I much appreciate the pointer.
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I’ve had a thought for quite a while that the universe may expand and contract, rather than expand endlessly – and we wuld never be able to tell which part of the cycle we were in.
In a contracting universe, bodies and matter would be accelerating toward the center of the universe, but bodies further out would be going slower. This would give the appearance that from any observation point, bodies closer to the center would be moving faster – giving the illusion of an expanding universe. And bodies further out would be moving slower – also giving the illusion of an expanding universe.
So as the universe expands, stops, and then collapses on itself we would have have an endless series of “big bangs”. Is this possible??
Dark matter is exactly that it is dark to the naked eye
Because it is faster than the speed of light and can only be observed through Gravity
As an object approaches the speed of light, it condenses and becomes invisible
Undetectable is more like it. Matter is neither created nor destroyed.
Dark matter can pass through galaxies, because it is moving just past the speed of light.
It is hard to detect light in the dark
Dark matter and light enjoy a dance that to some is like doing the two – step for others the waltz
Just fast enough to be seen by gravitation but not fast enough to be observed without.
So we do see shapes and shadows, without light and dark matter we would not.
The reason dark matter appears slow because of the observation point.
Gravity is a bit more substantial than we thought
A famous man once stumped many with a theory called relativity
Dark matter too will be bent by space we just haven’t seen that effect yet.
I think maybe Einstein was on to something…
Einstein’s theory still holds. It is all relative.
Smokyjinx 2006
Use Germanium Calthrates (university of Houston) to capture dark matter at cryogenic temperatures. More surface area equates to more particles seen. Gotta slow down the train to see it.
Smokyjinx 2006