August 29, 2005

August 29

Tirthabir Biswas, Anupam Mazumdar and Warren Siegel analyze in their paper Bouncing Universes in String-inspired Gravity interesting cosmological questions by putting together general relativity, Newtonian gravity, inflation, Yang-Mills field theory and string theory.

They start by adding to the Einstein-Hilbert action an infinite sum of higher order derivative terms (Eqn. (1.5)).
They find approximate and exact solutions such that that (i) the action can give rise to a ghost free and asymptotically free theory of gravity, and (ii) bouncing cosmological solutions for this type of actions.

In the usual FRW cosmology, it is the attractive force of gravity which makes the universe contract, eventually to a singularity. Now in the presence of asymptotically free gravity spacetime contracts but the internal pressure of matter resists, therefore causing the universe to bounce from the contracting phase into an expanding one (Eqn.
(4.6), hyperbolic cosine bounce). Their cosmological constant is mainly required for the late-time consistency for their bounce ansatz (4.6), it is not necessary for having a bounce. They argue that its absence solves the graceful exit problem.

They divide the evolution into two distinct regimes: (i) near the bounce when the higher curvature terms dominate the evolution (ii) away from (both before and after) the bounce when the higher order terms can be ignored as compared to ordinary gravity and there is normal FRW evolution. They say they are unable to clearly identify a connection of these constraints to the necessity or sufficiency of asymptotic freedom. The write the most likely reason is that they only looked at a very specific bounce solution, namely the hyperbolic cosine bounce, and therefore are missing more complicated bounces which may be present in some of the asymptotically free theories. "We reserve a more detailed study of these issues to future research." The transition from the bounce should lead to a radiation dominated epoch before and after the bounce. During these phases, they claim to have ordinary gravity coupled to an ideal gas of matter/radiation fluid satisfying the usual omega, rho and Hubble equations (Eqn. (5.13)).

I find the paper very inspiring. A number of interesting calculations have been performed. Perhaps I have not grasped it right so I ask is the addition of higher (infinite) order derivative terms in the Einstein-Hilbert action something like adding resonances and Regge poles (in the 1960's) to get the full amplitude. What would the proper duality entities be, and the relationship between them? I do not have the answer but my favorite thought is in the direction of preons having color-like interaction near Planckian distances (there is only an uncompleted manuscript of it, which only Peter has seen, and one withdrawn from arxiv last year).

August 25, 2005

August 25

A. Bouchareb, M. Ramon Medrano and N.G. Sanchez have studied Semiclassical (QFT) and Quantum (String) Rotating Black Holes and their Evaporation. Quoting freely, the authors compute the quantum emission cross section of strings by a Kerr-Newman black hole. In the early stage of evaporation, the string cross section shows the Hawking part of the emission with temperature Tsem, the semiclassical regime. For Tsem → Ts, the massive string modes dominate the emission, the string cross section shows a Hagedorn phase transition at Tsem = Ts. The last state of evaporation of a semiclassical Kerr-Newman black hole with mass M > mPl, Tsem(J,Q) < Ts, angular momentum J and charge Q is a string state of string temperature Ts, string mass Ms, J = 0 and Q = 0, which decays by the usual quantum string decay into all kinds of particles. Besides the classical/ semiclassical known bounds on J and Q, new bounds emerge in the quantum string regime.

A central object is ρ(m, j), the microscopic string density of states of mass m and spin mode j. They find for the extremal string states j → m^2α′ , a new phase transition at a temperature Tsj = sqrt(j/h)Ts, higher than Ts. They call it extremal transition. The characteristic behavior of this transition is a square root branch point near Tsj . It manifests as a logarithmic singularity in the string entropy S(m, j). This extremal behavior is universal and is analogous to the transition found for the thermal self-gravitating gas of point particles and for strings in de Sitter background.

By identifying the semiclassical and quantum (string) gravity regimes, the authors find a new formula for the Kerr black hole entropy Ssem(M, J), which is a function of the usual Bekenstein-Hawking entropy S(0)sem. For M ≫ mPl
^2 and J < GM^2/c, S(0)sem is the leading term of this expression, but for high angular momentum a gravitational phase transition operates and the whole entropy Ssem is drastically different from the Bekenstein-Hawking entropy S(0)sem. This new phase transition takes place at a temperature TsemJ = sqrt(J/h)Tsem, higher than the Hawking temperature Tsem.

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August 13, 2005

August 13

Read "Rumors of new forces" at Sean Carroll's blog. Its an experiment by Eric Adelberger's group at  the University of Washington.

August 12, 2005

August 12

At Strings2005 (and many times before that conference!):"Is there any experiment which would falsify the theory?" Paul Frampton seems to give the first answer: an experiment involving a Josephson junction may detect the effect of dark energy, by observing a cut-off in the frequency of zero-point oscillations at about 1.7 THz. If such a cut-off were discovered, the consequences would be far reaching including the possible demise of the string landscape.