Could quantum gravity be tested with high intensity Lasers?

In quantum gravity theories Planckian behavior is triggered by the energy of {\it elementary} particles approaching the Planck energy, $E_P$, but it's also possible that anomalous behavior strikes systems of particles with total energy near $E_P$. This is usually perceived to be pathological and has been labelled ``the soccer ball problem''. We point out that there is no obvious contradiction with experiment if {\it coherent} collections of particles with bulk energy of order $E_P$ do indeed display Planckian behavior, a possibility that would open a new experimental window. Unfortunately field theory realizations of deformed special relativity never exhibit a ``soccer ball problem''; we present several formulations where this is undeniably true. Upon closer scrutiny we discover that the only chance for Planckian behavior to be triggered by large coherent energies involves the details of second quantization. We find a formulation where the quanta have their energy-momentum (mass-shell) relations deformed as a function of the bulk energy of the coherent packet to which they belong, rather than the frequency. Given ongoing developments in Laser technology, such a possibility would be of great experimental interest

New non-Gaussian feature in COBE-DMR Four Year Maps

We extend a previous bispectrum analysis of the Cosmic Microwave Background temperature anisotropy, allowing for the presence of correlations between different angular scales. We find a strong non-Gaussian signal in the ``inter-scale'' components of the bispectrum: their observed values concentrate close to zero instead of displaying the scatter expected from Gaussian maps. This signal is present over the range of multipoles $\ell=6 -18$, in contrast with previous detections. We attempt to attribute this effect to galactic foreground contamination, pixelization effects, possible anomalies in the noise, documented systematic errors studied by the COBE team, and the effect of assumptions used in our Monte Carlo simulations. Within this class of systematic errors the confidence level for rejecting Gaussianity varies between 97% and 99.8%.Comment: Replaced with revised version. Two typos in and around equation (3) have been corrected (components of bispectrum are of the form (l-1, l, l+1) with l even). Published in Ap.J.Let

On the bispectrum of COBE and WMAP

The COBE-DMR 4-year maps displayed a strong non-Gaussian signal in the ``inter-scale'' components of the bispectrum: their observed values did not display the scatter expected from Gaussian maps. We re-examine this and other suggested non-Gaussian features in the light of WMAP. We find that they all disappear. Given that it was proved that COBE-DMR high noise levels and documented systematics could at most {\it dilute} the observed non-Gaussian features, we conclude that this dataset must have contained non-negligible undocumented systematic errors. It turns out that the culprit is a combination of QuadCube pixelization and data collected during the ``eclipse season''.Comment: 4 pages, 4 figure, MNRAS submissio

Solutions to the Quasi-flatness and Quasi-lambda Problems

Big Bang models of the Universe predict rapid domination by curvature, a paradox known as the flatness problem. Solutions to this problem usually leave the Universe exactly flat for every practical purpose. Explaining a nearly but not exactly flat current Universe is a new problem, which we label the quasi-flatness problem. We show how theories incorporating time-varying coupling constants could drive the Universe to a late-time near-flat attractor. A similar problem may be posed with regards to the cosmological constant $\Lambda$, the quasi-lambda problem, and we exhibit a solution to this problem as well.Comment: 9 pages, no figures. Minor changes corresponding to version to be published in Physics Letters

DSR as an explanation of cosmological structure

Deformed special relativity (DSR) is one of the possible realizations of a varying speed of light (VSL). It deforms the usual quadratic dispersion relations so that the speed of light becomes energy dependent, with preferred frames avoided by postulating a non-linear representation of the Lorentz group. The theory may be used to induce a varying speed of sound capable of generating (near) scale-invariant density fluctuations, as discussed in a recent Letter. We identify the non-linear representation of the Lorentz group that leads to scale-invariance, finding a universal result. We also examine the higher order field theory that could be set up to represent it

Quantum analysis of the recent cosmological bounce in comoving Hubble length

We formulate the transition from decelerated to accelerated expansion as a bounce in connection space and study its quantum cosmology, knowing that reflections are notorious for bringing to the fore quantum effects. We use a formalism for obtaining a time variable via the demotion of the constants of Nature to integration constants, and focus on a toy Universe containing only radiation and a cosmological constant $\Lambda$ for its simplicity. We find that, beside the usual factor ordering ambiguities, there is an ambiguity in the order of the quantum equation, leading to two distinct theories: one second, the other first order. In both cases two time variables may be defined, conjugate to $\Lambda$ and to the radiation constant of motion. We make little headway with the second order theory, but are able to produce solutions to the first order theory. They exhibit the well-known "ringing" whereby incident and reflected waves interfere, leading to oscillations in the probability distribution even for well-peaked wave packets. We also examine in detail the probability measure within the semiclassical approximation. Close to the bounce, the probability distribution becomes double-peaked, one peak following a trajectory close to the classical limit but with a Hubble parameter slightly shifted downwards, the other with a value of $b$ stuck at its minimum. An examination of the effects still closer to the bounce, and within a more realistic model involving matter and $\Lambda$, is left to future work.Comment: 15 pages, 13 figures; v2: accepted for publication; minor changes in presentation, unchanged result

Quantum resolution of the cosmological singularity

We study a quantum Hot Big Bang with matter characterized by a constant of motion m, whose conjugate defines time. A superposition in m suggests a natural, conserved inner product. For two quantum theories in connection and metric variables, unitarity resolves the classical singularity. For connection variables, the most likely value for the curvature saturates at a finite maximum, followed by a regular transition between contraction and expansion. In metric variables unitarity implies a boundary condition reflecting a contracting Universe into an expanding one. No appeal to new physics is needed.Comment: 5 pages, 4 figures, APS styl