Semiclassical Limit of Causal Dynamical Triangulations (Ambjorn, Loll et al)

[marcus]

http://arxiv.org/abs/1102.3929
The Semiclassical Limit of Causal Dynamical Triangulations
J. Ambjorn, A. Gorlich, J. Jurkiewicz, R. Loll, J. Gizbert-Studnicki, T. Trzesniewski
30 pages, 10 figures
(Submitted on 18 Feb 2011)
"Previous work has shown that the macroscopic structure of the theory of quantum gravity defined by causal dynamical triangulations (CDT) is compatible with that of a de Sitter universe. After emphasizing the strictly nonperturbative nature of this semiclassical limit we present a detailed study of the three-volume data, which allows us to re-confirm the de Sitter structure, exhibit short-distance discretization effects, and make a first detailed investigation of the presence of higher-order curvature terms in the effective action for the scale factor. Technically, we make use of a novel way of fixing the total four-volume in the simulations."

 

[Schreiberdk]

Hi there Marcus.. I am fairly new to this forum, and my science knowledge aint that great.. But could you please tell me, what does this article mean for the situation of CDT? I recently started reading about CDT, and after the summer I am going to the Niels Bohr Institute in Copenhagen to study physics (as i am going to finish high school in a couple of months). I know Jan Ambjorn has a professorship there, so I am really excited to go there, and talk to him :)

I have a spectacular fascination of quantum gravity, and the whole thing about unifying the forces with the missing quantum theory for gravity. So my other question here is, what is CDT actually, and how far is it from being a complete theory of quantum gravity? Does it incorporate matter yet? Does it describe black holes and near "big bang"-cosmology? And what are the weaknesses of the theory?

\Schreiber

 

[marcus]

... I recently started reading about CDT, and after the summer I am going to the Niels Bohr Institute in Copenhagen to study physics (as i am going to finish high school in a couple of months). I know Jan Ambjorn has a professorship there, so I am really excited to go there, and talk to him :)

how far...from being a complete theory of quantum gravity? Does it incorporate matter yet? Does it describe black holes and near "big bang"-cosmology? And what are the weaknesses of the theory?

I interpret what you say to mean that (soon, in a few months) you enter the 3-year Bachelor's program in physics at NBI. I think you are fortunate to be going there.
Here is a webpage, for other people who don't know about the educational side of NBI
http://www.nbi.ku.dk/english/

It says that the bachelor's courses in physics are (block 3 and block 4)

Thermodynamics and project (Termo)
Mathematics for Physicists (MatF)
Planetary Systems and the formation of stars (Planetsystem)
Earth and the terrestrial planets (JordPlanet)
Dynamic Meteorology and numerical weather prognoses (MetVejr)
Quantum Mechanics 2 (KM2)
Optics

Dynamical Systems and Chaos
Biophysics of Membranes (Membran)
Condensed matter physics 2 (CMP2)
Evolution of the Universe (UU)

Electromagnetism (EM1)
Cosmology (Kosmos)
Biofysik: Introduction to biophysics
Geodesy and geostatistics
Introduction to Atomic Physics
Mathematics F2 (MatF2)

Computational methods
Experimental physics (EF)
Climate physics (Klima)
StatFys: Statistical physics
============================

Let me think if there is something useful that I can say.

To answer your question, I don't think that CDT, at least in the 4D version (not a 2D toy version) has little or nothing about including matter, or BH, or resolving the BB singularity.

But I think of it as a valuable line of qg investigation. It does not require a fixed geometric background---it has made interesting progress. We can get ideas, and learn, from several approaches to quantum geometry/gravity. It is a revolutionary step, to get away from using a fixed spacetime geometry, decided in advance. I think of QG as most importantly quantum geometry (interacting with matter) and how to represent an uncertain geometry is a big wall to climb. CDT is a good introduction to QG and it communicates insights and inspiration to other approaches.

I am not advising you. I simply want to be clear that at this point it does not makes sense to "pick winners" among the different background-independent QG approaches. They are all trying different paths up the mountain and, by communicating, they help each other.

Even though CDT does not have much about BH or BB or including matter, it is very valuable, exciting, and enlightening part of the overall QG effort.

I also am not advising you to cultivate an interest in QG at this point, when you are beginning a 3 year intensive conventional physics course. I would rather focus on the standard curriculum and make the very highest scores that you can, on the exams.

There is a MASTERS program at U Nottingham that involves QG. That is the very earliest level that I would think of getting into QG

That is, 3 years from now, if you work very hard on the standard physics program at NBI.

Maybe by that time other universities will have MS programs like the Nottingham one. So there will be more choice.

CDT is probably the easiest QG to understand intuitively (so it is a very good intro) and it has made significant progress in the past. But right now I think that the NCG and LQG programs are each making more rapid progress. A good thing about Nottingham is that the senior guy there (John Barrett) has done important research in both LQG and NCG. He knows both of those QG programs from the inside. The students there do not have a closed-off horizon.

Don't take me as an authority. I am just guessing. But my guess is that you will do better not to think of QG for 3 years and concentrate on making the best of the NBI standard physics courses.

Other people here could have different ideas and give different advice.