In the context of models where the dark energy density $\rD$ is a random variable, anthropic selection effects may explain both the "old" cosmological constant problem and the "time coincidence". We argue that this type of solution to both cosmological constant problems entails a number of definite predictions, which can be checked against upcoming observations. In particular, in models where the dark energy density is a discrete variable, or where it is a continuous variable due to the potential energy of a single scalar field, the anthropic approach predicts that the dark energy equation of state is $p_D=-\rho_D$ with a very high accuracy. It is also predicted that the dark energy density is greater than the currently favored value $\Omega_D\approx 0.7$. Another prediction, which may be testable with an improved understanding of galactic properties, is that the conditions for civilizations to emerge arise mostly in galaxies completing their formation at low redshift, $z\approx 1$. Finally, there is a prediction which may not be easy to test observationally: our part of the universe is going to recollapse eventually. However, the simplest models predict that it will take more than a trillion years of accelerated expansion before this happens.

J. Garriga A. Vilenkin

We discuss the large-scale structure of the universe in inflationary cosmology and the implications that it may have for the long-term future of civilizations. Although each civilization is doomed to perish, it may be possible to transmit its accumulated knowledge to future civilizations. We consider several scenarios of this sort. If the cosmological constant is positive, it eventually dominates the universe and bubbles of inflationary phase begin to nucleate at a constant rate. Thermalized regions inside these inflating bubbles will give rise to new galaxies and civilizations. It is possible in principle to send a message to one of them. It might even be possible to send a device whose purpose is to recreate an approximation of the original civilization in the new region. However, the message or device will almost certainly be intercepted by black holes, which nucleate at a much higher rate than inflating bubbles. Formation of new inflating regions can also be triggered by gravitational collapse, but again the probability is low, and the number of attempts required for a positive outcome is enormous. The probability can be higher if the energy scale of inflation is closer to the Planck scale, but a high energy scale produces a tight bound on the amount of information that can be transmitted. One can try to avoid quantum tunneling altogether, but this requires a violation of quantum inequalities which constrain the magnitude of negative energy densities. However, the limits of validity of quantum inequalities are not clear, and future research may show that the required violation is in fact possible. Therein lies the hope for the future of civilizations.

J. Garriga V. F. Mukhanov K. D. Olum A. Vilenkin

We critically review several recent approaches to solving the two cosmological constant problems. The "old" problem is the discrepancy between the observed value of $\Lambda$ and the large values suggested by particle physics models. The second problem is the "time coincidence" between the epoch of galaxy formation $t_G$ and the epoch of $\Lambda$-domination $t_\L$. It is conceivable that the "old" problem can be resolved by fundamental physics alone, but we argue that in order to explain the "time coincidence" we must account for anthropic selection effects. Our main focus here is on the discrete-$\Lambda$ models in which $\Lambda$ can change through nucleation of branes. We consider the cosmology of this type of models in the context of inflation and discuss the observational constraints on the model parameters. The issue of multiple brane nucleation raised by Feng {\it et. al.} is discussed in some detail. We also review continuous-$\L$ models in which the role of the cosmological constant is played by a slowly varying potential of a scalar field. We find that both continuous and discrete models can in principle solve both cosmological constant problems, although the required values of the parameters do not appear very natural. M-theory-motivated brane models, in which the brane tension is determined by the brane coupling to the four-form field, do not seem to be viable, except perhaps in a very tight corner of the parameter space. Finally, we point out that the time coincidence can also be explained in models where $\Lambda$ is fixed, but the primordial density contrast $Q=\delta\rho/\rho$ is treated as a random variable.

J. Garriga A. Vilenkin

The rate of merger events observed by LIGO/Virgo can be used in order to probe the fraction $f$ of dark mater in the form of Primordial Black Holes. Here, we consider the merger rate of PBH binaries, accounting for the effect of cosmological perturbations on their initial eccentricity $e$. The torque on the binaries receives significant contributions from a wide range of scales, that goes from the size of the horizon at the time when the binary forms, down to the co-moving size of the binary. In scenarios where PBH are formed from adiabatic perturbations, it is natural to expect an enhancement of the power spectrum $P_\Phi$ at small scales, where it is poorly constrained observationally. The effect can then be quite significant. For instance, a nearly flat spectrum with amplitude $P_\Phi \gtrsim 10^{-7}$ on scales smaller than $\sim 10 Mpc^{-1}$ gives a contribution $\langle j^2 \rangle \sim 10^3 P_\Phi$, where $j= (1-e^2)^{1/2}$ is the dimensionless angular momentum parameter of the binaries. This contribution can dominate over tidal torques from neighboring PBHs for any value of $f$. Current constraints allow for a power spectrum as large as $P_\Phi \sim 10^{-5}$ at the intermediate scales $10^3-10^5 Mpc^{-1}$, comparable to the co-moving size of the binaries at the time of formation. In particular, this can relax current bounds on the PBH abundance based on the observed LIGO/Virgo merger rate, allowing for a fraction $f\sim 10\%$ of dark matter in PBH of mass $\sim 30 M_\odot$. We investigate the differential merger rate $\Delta\Gamma(m_1,m_2)$, as a function of the masses of the binary components, and the corresponding ``universality" coefficient $\alpha = -(m_1+m_2)^2 \partial^2 \ln \Delta\Gamma/\partial m_1\partial m_2$, concluding that merger rates may provide valuable information on the spectrum of primordial cosmological perturbations at currently uncharted lengthscales.

Jaume Garriga Nikolaos Triantafyllou

Inference of joule-class THz radiation sources from microchannel targets driven with hundreds of joule, picosecond lasers is reported. THz sources of this magnitude are useful for nonlinear pumping of matter and for charged-particle acceleration and manipulation. Microchannel targets demonstrate increased laser-THz conversion efficiency compared to planar foil targets, with laser energy to THz energy conversion up to approximately 0.9% in the best cases.

G. Bruhaug H. G. Rinderknecht K. Weichman M. VanDusen-Gross J. P. Palastro M. S. Wei S. P. Regan Y. E K. Garriga X. -C. Zhang G. W. Collins J. R. Rygg

In order to describe the thermodynamics of the glassy systems it has been recently introduced an extra parameter also called effective temperature which generalizes the fluctuation-dissipation theorem (FDT) to systems off-equilibrium and supposedly describes thermal fluctuations around the aging state. Here we investigate the applicability of a zero-th law for non-equilibrium glassy systems based on these effective temperatures by studying two coupled subsystems of harmonic oscillators with Monte Carlo dynamics. We analyze in detail two types of dynamics: 1) sequential dynamics where the coupling between the subsystems comes only from the Hamiltonian and 2) parallel dynamics where there is a further coupling between the subsystems arising from the dynamics. We show that the coupling described in the first case is not enough to make asymptotically the effective temperatures of two interacting subsystems coincide, the reason being the too small thermal conductivity between them in the aging state. This explains why different interacting degrees of freedom in structural glasses may stay at different effective temperatures without never mutually thermalizing.

Adan Garriga Felix Ritort

We study the most suitable procedure to measure the effective temperature in off-equilibrium systems. We analyze the stationary current established between an off-equilibrium system and a thermometer and the necessary conditions for that current to vanish. We find that the thermometer must have a short characteristic time-scale compared to the typical decorrelation time of the glassy system to correctly measure the effective temperature. This general conclusion is confirmed analyzing an ensemble of harmonic oscillators with Monte Carlo dynamics as an illustrative example of a solvable model of a glass. We also find that the current defined allows to extend Fourier's law to the off-equilibrium regime by consistently defining effective transport coefficients. Our results for the oscillator model explain why thermal conductivities between thermalized and frozen degrees of freedom in structural glasses are extremely small.

Adan Garriga Felix Ritort

We report on the complex dielectric tensor components of four chalcopyrite semiconductors in the optical energy range (1.4-5.2 eV, from 0.9 eV for CuInSe2) determined at room temperature by spectroscopic ellipsometry. Our results were obtained on single crystals of CuInSe2, CuGaSe2, CuInS2, and CuGaS2. Values of refractive indices n, extinction coefficients k and normal-incidence reflectivity R in the two different polarizations are given and compared with earlier data where available. We analyze in detail the structures of the dielectric function observed in the studied energy region. Critical-point parameters of electronic transitions are obtained from fitting of numerically calculated second-derivative spectra. Experimental energies and polarizations are discussed on the basis of published band structure calculations.

M. I. Alonso K. Wakita J. Pascual M. Garriga N. Yamamoto

We study the evolution of cosmic strings taking into account the frictional force due to the surrounding radiation. We consider small perturbations on straight strings, oscillation of circular loops and small perturbations on circular loops. For straight strings, friction exponentially suppresses perturbations whose co-moving scale crosses the horizon before cosmological time $t_*\sim \mu^{-2}$ (in Planck units), where $\mu$ is the string tension. Loops with size much smaller than $t_*$ will be approximately circular at the time when they start the relativistic collapse. We investigate the possibility that such loops will form black holes. We find that the number of black holes which are formed through this process is well bellow present observational limits, so this does not give any lower or upper bounds on $\mu$. We also consider the case of straight strings attached to walls and circular holes that can spontaneously nucleate on metastable domain walls.

J. Garriga M. Sakellariadou

Generally speaking, `almost distance-regular' graphs share some, but not necessarily all, of the regularity properties that characterize distance-regular graphs. In this paper we propose two new dual concepts of almost distance-regularity, thus giving a better understanding of the properties of distance-regular graphs. More precisely, we characterize $m$-partially distance-regular graphs and $j$-punctually eigenspace distance-regular graphs by using their spectra. Our results can also be seen as a generalization of the so-called spectral excess theorem for distance-regular graphs, and they lead to a dual version of it.

Cristina Dalfó Edwin R. van Dam Miquel Angel Fiol Ernest Garriga