## Monday, March 10, 2014

### Three interesting hep-th papers

First, a comment about the phenomenological hep-ph archive. Three new "primarily hep-ph" papers among twelve, namely the papers #5, #6, #8, are talking about the $$3.5\keV$$ X-ray line that Jester described as a possible dark matter signal. Jester would talk about "sterile neutrinos" but the three new papers try to identify the dark matter particle with a radiative neutrino; decaying moduli; and axinos. If you're intrigued by the $$3.5\keV$$ line, maybe you should bookmark the list of followups to the empirical paper by Bolbul et al..

Off-topic: a new colleague of Bill O'Reilly was hired by Rupert Murdoch. His name is Barack Obama and in this first job, he introduces the new "Cosmos" hosted by Neil deGrasse Tyson 34 years after it was done by Carl Sagan. Incidentally, Obama is likely to name his law school classmate Andrew Schapiro as the new ambassador to Czechia.

Now, hep-th, theory.

Michael Douglas and two Stony-Brook and/or partially Bonn collaborators talk about 8-dimensional F-theory vacua and the fate of vector multiplets in them. Recall that F-theory has formally 12 spacetime dimensions but two of them are infinitesimal and must be compactified on a 2-torus. That allows one to compactify F-theory on a K3 (which has 4 real dimensions), leaving 7+1 large dimensions, as long as the K3 has toroidal (elliptical) fibers.

The base of such an eliptically fibered K3 manifold is a sphere $$S^2$$ or, as we call it in complex geometry, $${\mathbb P}^1$$, the 1-dimensional projective space (a complex one, so we mean $$\mathbb{CP}^1$$). On this sphere, there are at most (if you maximally separate them) 24 singular places – because of the extra 7+1 large dimensions, the loci are the places where 24 $$(p,q)$$ sevenbranes live, and you could expect 24 vector multiplets. However, four of them are effectively "eaten" by some tensor multiplets, they show in detail, in a mechanism known as the Cremmer-Scherk (CS: not to be confused with Chern-Simons or Czecho-Slovak or Computer-Science) mechanism.

Hans Peter Nilles and Patrick Vaudrevange of Bonn and Garching/Munich discuss the string phenomenology – how to use string theory to describe the real world of particle physics – in a way that almost looks like there aren't thousands of papers about it. They are picking the most elegant models respecting "five golden rules" of unification with a good taste: $$SO(10)$$ spinor for SM fermionic matter; incomplete GUT multiplets for the Higgs pair; repetition of families deduced from the compactification manifold's properties; $$\NNN=1$$ SUSY; and R-parity plus discrete symmetries.

They decide that the models respecting these "five golden rules" of good behavior arise in more or less all the major phenomenological descriptions and they are numerous and lots of work has to be done to pick the right model etc.

Alexander Vilenkin and Jun Zhang of Tufts provide us with some evidence that "bounces replacing big crunches" don't really solve anything. Recall that a cosmology with a negative cosmological constant typically wants to end up with a collapse and a "Big Crunch", the time-reversed mirror image of the Big Bang (not quite because the entropy never decreases etc.).

The Cosmos shrinks to (near) zero size near the Big Crunch and it's ugly and deadly, so people have argued that some Planckian effects imply that the world isn't really over during the Big Crunch. Instead, it bounces back and expands again, and we could have had such "bounces" in our own history. However, the authors determine that this scenario still implies that the cosmic history is "past-incomplete" – something is missing in the spacetime if you extrapolate geodesics into the past – if it were past-incomplete in the model without bounces.

It's probably not a hard observation but it surely agrees with what I think about the usefulness (more precisely, uselessness) of the bounces and cyclic cosmologies. You may jump and bounce on a trampoline, for example, but your energy ultimately dissipates and your jumping comes to a halt. You could have stopped jumping right away. So something is inevitably deteriorating about the bounces and "cycles" – they shrink either in the future or in the past. The "balanced" intermediate scenario is as unstable as Einstein's static Universe. So by these bounces, you may at most delay some problems but you don't solve them. I personally tend to think that if everything in the Universe is squeezed and crushed into near-Planckian curvatures etc., the time is really over and the continuation of the "same spacetime" is unphysical from a sufficiently positivist, observation-rooted perspective.

1. No bounce, no cycle, no big bang either. But a slow beginning some fifty trillion years ago.
http://www.alphagalileo.org/ViewItem.aspx?ItemId=139355&CultureCode=en

Solid science... or crackpot theory? Something in between? (Can there be an in-between?)

I took one look at the paper and got immediate equation-shock. No way will I ever be able to fight my way through.

Amusingly, he uses the particle-wave duality analogy.

Wetterich stresses that this in no way renders the previous view of the
Big Bang “invalid”, however. “Physicists are accustomed to describing
observed phenomena using different pictures.” Light, for example, can be
depicted as particles and as a wave. Similarly, his model can be seen
as a picture equivalent to the Big Bang.

2. Very interesting! I look forward to Lumo's analysis and conclusion!

3. Wetterich, enough said.

4. Perhaps it is old age and senility, or an education in which the stooopid were left behind, but I heard "September 31" during Cosmos' turgid calendrical fandango.

Now, something interesting.
https://files.oakland.edu/users/garfinkl/web/mog/
Number 43, Winter 2014,
"http://files.oakland.edu/users/garfinkl/MOG/mog43/mog43.pdf
Top of page 8: "Prof. Piotr Bizon presented his striking result
that the anti-de Sitter space-time is unstable in full, non-linear general relativity
" Pookie pookie.

5. I am very unimpressed with Cosmos, first because it isn't as good without Sagen, second because Through the Wormhole with Morgan Freeman is much better,

6. It’s utter nonsense, Eugene.

7. Dear Eugene, I don't think that any positive progress has occurred since that time but I have discussed this thing here:

http://motls.blogspot.com/2013/08/imagine-that-universe-is-not-expanding.html?m=1

8. Why all the drooling over Sagan? He spoke as if his capacities were diminished by codeine (they were), he came up with wacky theories about methane and an early Greenhouse atmosphere that gave rise to life (forgetting about geothermal heat), and he wasted a lot of money looking for some "signal" sent to him by "aliens" with an antenna array with a probability of something like 1 in 10**10**10 of seeing something.

Lost in space.