Optimal control of strong-field ionization with time-dependent density-functional theory

We show that quantum optimal control theory (OCT) and time-dependent density-functional theory (TDDFT) can be combined to provide realistic femtosecond laser pulses for an enhanced ionization yield in many-electron systems. Using the H$_2$-molecule as a test case, the optimized laser pulse from the numerically exact scheme is compared to pulses obtained from OCT+TDDFT within the TD exact-exchange (TDEXX) and the TD local-density approximation (TDLDA). We find that the TDDFT-pulses produces an ionization yield of up to 50% when applied to the exact system. In comparison, pulses with a single frequency but the same fluence typically reach to yields around 5-15%, unless the frequency is carefully tuned into a Fano-type resonance that leads to $\sim 30$ yield. On the other hand, optimization within the exact system alone leads to yields higher than 80%, demonstrating that correlation effects beyond the TDEXX and TDLDA can give rise to even more efficient ionization mechanisms

Observational Constraints on the Averaged Universe

Averaging in general relativity is a complicated operation, due to the general covariance of the theory and the non-linearity of Einstein's equations. The latter of these ensures that smoothing spacetime over cosmological scales does not yield the same result as solving Einstein's equations with a smooth matter distribution, and that the smooth models we fit to observations need not be simply related to the actual geometry of spacetime. One specific consequence of this is a decoupling of the geometrical spatial curvature term in the metric from the dynamical spatial curvature in the Friedmann equation. Here we investigate the consequences of this decoupling by fitting to a combination of HST, CMB, SNIa and BAO data sets. We find that only the geometrical spatial curvature is tightly constrained, and that our ability to constrain dark energy dynamics will be severely impaired until we gain a thorough understanding of the averaging problem in cosmology.Comment: 6 pages, 4 figure

Hadron multiplicities, pT-spectra and net-baryon number in central Pb+Pb collisions at the LHC

We compute the initial energy density and net baryon number density in 5% most central Pb+Pb collisions at $\sqrt s=5.5$ TeV from pQCD + (final state) saturation, and describe the evolution of the produced system with boost-invariant transversely expanding hydrodynamics. In addition to the total multiplicity at midrapidity, we give predictions for the multiplicity of charged hadrons, pions, kaons and (anti)protons, for the total transverse energy and net-baryon number, as well as for the $p_T$-spectrum of charged hadrons, pions and kaons. We also predict the region of applicability of hydrodynamics by comparing these results with high-$p_T$ hadron spectra computed from pQCD and energy losses.Comment: 2 pages, 2 figures, to be presented at the workshop "Heavy Ion Collisions at the LHC: Last Call for Predictions" at CERN 29 May - 2 Jun

Airy gas model: From three to reduced dimensions

By using the propagator of linear potential as a main tool, we extend the Airy gas model, originally developed for the three-dimensional ($d=3$) edge electron gas, to systems in reduced dimensions ($d=2,1$). First, we derive explicit expressions for the edge particle density and the corresponding kinetic energy density (KED) of the Airy gas model in all dimensions. The densities are shown to obey the local virial theorem. We obtain a functional relationship between the positive KED and the particle density and its gradients and analyze the results inside the bulk as a limit of the local-density approximation. We show that in this limit the KED functional reduces to that of the Thomas-Fermi model in $d$ dimensions

Supernovae data and perturbative deviation from homogeneity

We show that a spherically symmetric perturbation of a dust dominated $\Omega=1$ FRW universe in the Newtonian gauge can lead to an apparent acceleration of standard candles and provide a fit to the magnitude-redshift relation inferred from the supernovae data, while the perturbation in the gravitational potential remains small at all scales. We also demonstrate that the supernovae data does not necessarily imply the presence of some additional non-perturbative contribution by showing that any Lemaitre-Tolman-Bondi model fitting the supernovae data (with appropriate initial conditions) will be equivalent to a perturbed FRW spacetime along the past light cone.Comment: 8 pages, 3 figures; v2: 1 figure added, references added/updated, minor modifications and clarifications, matches published versio

Light Propagation and Large-Scale Inhomogeneities

We consider the effect on the propagation of light of inhomogeneities with sizes of order 10 Mpc or larger. The Universe is approximated through a variation of the Swiss-cheese model. The spherical inhomogeneities are void-like, with central underdensities surrounded by compensating overdense shells. We study the propagation of light in this background, assuming that the source and the observer occupy random positions, so that each beam travels through several inhomogeneities at random angles. The distribution of luminosity distances for sources with the same redshift is asymmetric, with a peak at a value larger than the average one. The width of the distribution and the location of the maximum increase with increasing redshift and length scale of the inhomogeneities. We compute the induced dispersion and bias on cosmological parameters derived from the supernova data. They are too small to explain the perceived acceleration without dark energy, even when the length scale of the inhomogeneities is comparable to the horizon distance. Moreover, the dispersion and bias induced by gravitational lensing at the scales of galaxies or clusters of galaxies are larger by at least an order of magnitude.Comment: 27 pages, 9 figures, revised version to appear in JCAP, analytical estimate included, typos correcte

Averaging Robertson-Walker Cosmologies

The cosmological backreaction arises when one directly averages the Einstein equations to recover an effective Robertson-Walker cosmology, rather than assuming a background a priori. While usually discussed in the context of dark energy, strictly speaking any cosmological model should be recovered from such a procedure. We apply the Buchert averaging formalism to linear Robertson-Walker universes containing matter, radiation and dark energy and evaluate numerically the discrepancies between the assumed and the averaged behaviour, finding the largest deviations for an Einstein-de Sitter universe, increasing rapidly with Hubble rate to a 0.01% effect for h=0.701. For the LCDM concordance model, the backreaction is of the order of Omega_eff~4x10^-6, with those for dark energy models being within a factor of two or three. The impacts at recombination are of the order of 10^-8 and those in deep radiation domination asymptote to a constant value. While the effective equations of state of the backreactions in Einstein-de Sitter, concordance and quintessence models are generally dust-like, a backreaction with an equation of state w_eff<-1/3 can be found for strongly phantom models.Comment: 18 pages, 11 figures, ReVTeX. Updated to version accepted by JCA

Non-Linear N-Parameter Spacetime Perturbations: Gauge Transformations

We introduce N-parameter perturbation theory as a new tool for the study of non-linear relativistic phenomena. The main ingredient in this formulation is the use of the Baker-Campbell-Hausdorff formula. The associated machinery allows us to prove the main results concerning the consistency of the scheme to any perturbative order. Gauge transformations and conditions for gauge invariance at any required order can then be derived from a generating exponential formula via a simple Taylor expansion. We outline the relation between our novel formulation and previous developments.Comment: 7 pages, no figures, RevTeX 4.0. Revised version to match version published in PR