Testing string theory with inflation July 5, 2007Posted by dorigo in astronomy, news, physics, science.
Yesterday morning there were several important talks scheduled in the plenary session at PASCOS, and the main conference hall was quite full.
I arrived late for the first talk, having had to pack and leave my room at the Radisson hotel. Sadly, in fact, my stay in London was designed to be short: basically limited to the three days bracketing the afternoon of my talk, since due to travel fund shortages I had originally intended to pay this trip with money coming out of my own pocket (it ended up being fully financed anyway, but that’s another story).
So I made a fashionably late entrance in the hall, found a seat that did not involve bothering more than three people (there has to be a more rational way to arrange seats in a conference hall than a dumb laying them down in rows!), and set out to try and grasp something from the second talk, virtual paper and pencil in hand (my faithful vaio): Renata Kallosh, “Testing string theory by CMB“. Her thoughts on inflation and String theory are also described in a recent proceedings paper: hep-th0702059.
I have to admit that most of what I heard, and frantically wrote down as if in a trance, does not really make much sense to me as I read it back now. However, one has to take into account the fact that as I write these lines I am blogging from a pub on Cromwell road, and I am well into my second pint of Foster’s. So read the following with caution, and be aware that your reporter knows this matter at a *very* superficial level!
Renata gave an overview of inflation and dark energy as viewed from string theory. She noted that it was not possible to explain those features in string theory until recently, when the 2003 the new idea of flux compactification and moduli stabilization came forward, providing the solution through a landscape of vacua, some of them de Sitter vacua.
Having so many (here’s a understatement!) possible vacua is due to the ease with which you can make anti-de Sitter minima, which are supersymmetric. The problem of the negative cosmological constant can be circumvented by lifting the minima to de Sitter ones with a anti-D3 potential. I understood the graph she showed, but I admit I do not know how to write a anti-D3 potential. But brane theory was not part of my instruction, so I just admit defeat and move on.
Renata then went as far as to proclaim the following:
“String theory has now one explanation of dark energy: a metastable cosmological constant with an equation of state w=-1. So far it is in agreement with data, and no compelling alternatives are available. Is it possible to rule out experimentally the positive cosmological constant as an explanation of dark energy ? This is an open question”.
To me, that seems a bit far-fetched, because it looks like string theory is getting tuned to experimental observations (a positive cosmological constant) and then claim it is “in agreement with data”. What detail am I missing ? I guess more than just a detail.
She then said that at present there are several models of inflation in string theory. They are flexible enough to describe a spectral index n_s = 0.95 (the spectral index is a number describing the tilt in the power spectrum of scalar perturbations from the cosmic microwave background – a value different from unity implying a violation of scale invariance), but they typically predict a low level of gravitational waves and low “non-gaussianity” (I admit to not know what non-gaussianity is, although I may have my own ideas). They may explain light cosmic strings.
She would expect that SUSY will eventually be discovered, and if that is so, one will be able to use the framework of 4-dimensional supergravity, and if possible derive it from ten- or eleven-dimensional M-string theory. In case SUSY is found, the crucial future data for fundamental physics is the scale of gravitino mass: is it going to be at the typical scale of a LSP (light supersymmetric particle), at 1 TeV, or at 10^13 GeV ?
Then, another important unit is the detection or non-detection of tensor B-modes: the primordial gravity waves from inflation. One other sentence that startled me was the following:
“This model is extremely falsifiable, because it predicts no cosmic strings and n_s=0.95”.
It looks remarkable to me because the model in question seems to have been tailored around the findings of cosmology, and because falsifying a model by finding cosmic strings seems paradoxical to me. I feel that if we were to find cosmic strings, then ok, one particular model of string theory would be preferred rather than the others – but to me that would look like a detail. Again, I think I am missing something.
Then the speaker said there are other models, inflationary ones with large volume compactification. They require less fine tuning, but have more moduli, and more parameters. They also appear to have “nice predictive power”: they allow no gravity waves, no cosmic strings, and a value n_s=0.96. Well… Again, from my ignorant viewpoint, it looks to me as if string theorists, being unable to prove their construction through hard data, trick the rest of us into accepting the game of deciding which one among a class of string theory models better fits the experimental data which is used to construct them. Please now shoot me if I am letting too much of my negative thinking out…
The talk went on. My notes are hard to decypher, and I will refrain from adding uncooked meat to this already confused post, and jump to a few concluding remarks.
She sees several possibilities for the future: after Planck produces its data, we will have to fit the spectral index n_s, and may find no tensor modes, no cosmic strings, no non-gaussianity: in such case string inflation will already be in a good shape! Otherwise, if tensor modes are detected, it is a great challenge for string theory. If cosmic strings are detected, that appears to be not a problem: a welcome effect, and a potential window into physics at string scale. If no gaussianity is detected, some solution may be possible.
Renata concluded by recalling that when we learned that our universe is accelerating it was a creative crisis, which forced physicists to reconsider many issues in string theory, including the one of moduli stabilization and metastable vacua. If tensor modes are found in the future, this may be equally important. It may provide us with information about the Kahler potential from the sky and force us to consider SUSY phenomenology with a superheavy gravitino, or invent new methods of moduli stabilization.