arXiv subjects

Alexander Vilenkin

Publications and source records attributed to Alexander Vilenkin.

At least 109 records · Page 6Linked to original sources

arXiv2003

Dark energy equation of state and anthropic selection

We explore the possibility that the dark energy is due to a potential of a scalar field and that the magnitude and the slope of this potential in our part of the universe are largely determined by anthropic selection effects. We find that, in some models, the most probable values of the slope are very small, implying that the dark energy density stays constant to very high accuracy throughout cosmological evolution. In other models, however, the most probable values of the slope are such that the slow roll condition is only marginally satisfied, leading to a re-collapse of the local universe on a time-scale comparable to the lifetime of the sun. In the latter case, the effective equation of state varies appreciably with the redshift, leading to a number of testable predictions.

hep-th↗

arXiv2003

Inflating magnetically charged braneworlds

Numerical solutions of Einstein, scalar, and gauge field equations are found for static and inflating defects in a higher-dimensional spacetime. The defects have $(3+1)$-dimensional core and magnetic monopole configuration in $n=3$ extra dimensions. For symmetry-breaking scale $η$ below the critical value $η_c$, the defects are characterized by a flat worldsheet geometry and asymptotically flat extra dimensions. The critical scale $η_c$ is comparable to the higher-dimensional Planck scale and has some dependence on the gauge and scalar couplings. For $η=η_c$, the extra dimensions degenerate into a `cigar', and for $η>η_c$ all static solutions are singular. The singularity can be removed if the requirement of staticity is relaxed and defect cores are allowed to inflate. The inflating solutions have de Sitter worldsheets and cigar geometry in the extra dimensions. Exact analytic solutions describing the asymptotic behavior of these inflating monopoles are found and the parameter space of these solutions is analyzed.

hep-th↗

arXiv2003

Cosmic Attractors and Gauge Hierarchy

We suggest a new cosmological scenario which naturally guarantees the smallness of scalar masses and VEVs, without invoking supersymmetry or any other (non-gravitationaly coupled) new physics at low energies. In our framework, the scalar masses undergo discrete jumps due to nucleation of closed branes during (eternal) inflation. The crucial point is that the step size of variation decreases in the direction of decreasing scalar mass. This scenario yields exponentially large domains with a distribution of scalar masses, which is sharply peaked around a hierarchically small value of the mass. This value is the "attractor point" of the cosmological evolution.

hep-th↗

arXiv2003

Creation of massive particles in a tunneling universe

We examine the particle production during tunneling in quantum cosmology. We consider a minisuperspace model with a massive, conformally coupled scalar field and a uniform radiation background. In this model, we construct a semiclassical wave function describing a small recollapsing universe and a nucleated inflating universe (``tunneling from something''). We find that the quantum states of the scalar field in both the initial and the nucleated universe are close to the adiabatic vacuum, the number of created particles is small, and their backreaction on the metric is negligible. We show that the use of the semiclassical approximation is justified for this wave function. Our results imply that the creation of the universe from nothing can be understood as a limit of tunneling from a small recollapsing universe.

gr-qc↗

arXiv2003

Gravity of superheavy higher-dimensional global defects

Numerical solutions of Einstein's and scalar-field equations are found for a global defect in a higher-dimensional spacetime. The defect has a $(3+1)$-dimensional core and a ``hedgehog'' scalar-field configuration in $n=3$ extra dimensions. For sufficiently low symmetry-breaking scales $η$, the solutions are characterized by a flat worldsheet geometry and a constant solid deficit angle in the extra dimensions, in agreement with previous work. For $η$ above the higher-dimensional Planck scale, we find that static-defect solutions are singular. The singularity can be removed if the requirement of staticity is relaxed and defect cores are allowed to inflate. We obtain an analytic solution for the metric of such inflating defects at large distances from the core. The three extra dimensions of the nonsingular solutions have a ``cigar'' geometry. Although our numerical solutions were obtained for defects of codimension $n=3$, we argue that the conclusions are likely to apply to all $n\geq 3$.

hep-th↗

arXiv2003

Inflationary spacetimes are not past-complete

Many inflating spacetimes are likely to violate the weak energy condition, a key assumption of singularity theorems. Here we offer a simple kinematical argument, requiring no energy condition, that a cosmological model which is inflating -- or just expanding sufficiently fast -- must be incomplete in null and timelike past directions. Specifically, we obtain a bound on the integral of the Hubble parameter over a past-directed timelike or null geodesic. Thus inflationary models require physics other than inflation to describe the past boundary of the inflating region of spacetime.

gr-qc↗

arXiv2002

Solitonic D-branes and brane annihilation

We point out some intriguing analogies between field theoretic solitons (topological defects) and D-branes. Annihilating soliton-antisoliton pairs can produce stable solitons of lower dimensionality. Solitons that localize massless gauge fields in their world volume automatically imply the existence of open flux tubes ending on them and closed flux tubes propagating in the bulk. We discuss some aspects of this localization on explicit examples of unstable wall-anti-wall systems. The annihilation of these walls can be described in terms of tachyon condensation which renders the world-volume gauge field non-dynamical. During this condensation the world volume gauge fields (open string states) are resonantly excited. These can later decay into closed strings, or get squeezed into a network flux tubes similar to a network of cosmic strings formed at a cosmological phase transition. Although, as in the $D$-brane case, perturbatively one can find exact time-dependent solutions, when the energy of the system stays localized in the plane of the original soliton, such solutions are unstable with respect to decay into open and closed string states. Thus, when a pair of such walls annihilates, the energy is carried away (at least) by closed string excitations (``glueballs''), which are the lowest energy excitations about the bulk vacuum. Suggested analogies can be useful for the understanding of the complicated D-brane dynamics and of the production of topological defects and reheating during brane collision in the early universe.

hep-th↗

arXiv2002

Quantum cosmology and eternal inflation

This contribution consists of two parts. In the first part, I review the tunneling approach to quantum cosmology and comment on the alternative approaches. In the second part, I discuss the relation between quantum cosmology and eternal inflation. In particular, I discuss whether or not we need quantum cosmology in the light of eternal inflation, and whether or not quantum cosmology makes any testable predictions.

gr-qc↗

arXiv2002

On the size of the smallest scales in cosmic string networks

We present a method for the calculation of the gravitational back reaction cutoff on the smallest scales of cosmic string networks taking into account that not all modes on strings interact with all other modes. This results in a small scale structure cutoff that is sensitive to the initial spectrum of perturbations present on strings. From a simple model, we compute the cutoffs in radiation- and matter-dominated universes.

gr-qc↗

arXiv2002

Quantum tunneling of superconducting string currents

We investigate the decay of current on a superconducting cosmic string through quantum tunneling. We construct the instanton describing tunneling in a simple bosonic string model, and estimate the decay rate. The tunneling rate vanishes in the limit of a chiral current. This conclusion, which is supported by a symmetry argument, is expected to apply in general. It has important implications for the stability of chiral vortons.

hep-ph↗

arXiv2002

Stealth Branes

We discuss the brane world model of Dvali, Gabadadze and Porrati in which branes evolve in an infinite bulk and the brane curvature term is added to the action. If Z_2 symmetry between the two sides of the brane is not imposed, we show that the model admits the existence of "stealth branes" which follow the standard 4D internal evolution and have no gravitational effect on the bulk space. Stealth branes can nucleate spontaneosly in a Minkowski bulk. This process is described by the standard 4D quantum cosmology formalism with tunneling boundary conditions for the brane world wave function. The notorious ambiguity in the choice of boundary conditions is fixed in this case due to the presence of the embedding spacetime. We also point to some problematic aspects of models admitting stealth brane solutions.

hep-th↗

arXiv2001

Cosmological constant problems and their solutions

There are now two cosmological constant problems: (i) why the vacuum energy is so small and (ii) why it comes to dominate at about the epoch of galaxy formation. Anthropic selection appears to be the only approach that can naturally resolve both problems. The challenge presented by this approach is that it requires scalar fields with extremely flat potentials or four-form fields coupled to branes with an extremely small charge. Some recent suggestions are reviewed on how such features can arise in particle physics models.

hep-th↗

arXiv2001

Field theory models for variable cosmological constant

Anthropic solutions to the cosmological constant problem require seemingly unnatural scalar field potentials with a very small slope or domain walls (branes) with a very small coupling to a four-form field. Here we introduce a class of models in which the smallness of the corresponding parameters can be attributed to a spontaneously broken discrete symmetry. We also demonstrate the equivalence of scalar field and four-form models. Finally, we show how our models can be naturally embedded into a left-right extension of the standard model.

hep-th↗

arXiv2001

Many worlds in one

A generic prediction of inflation is that the thermalized region we inhabit is spatially infinite. Thus, it contains an infinite number of regions of the same size as our observable universe, which we shall denote as $Ø$-regions. We argue that the number of possible histories which may take place inside of an $Ø$-region, from the time of recombination up to the present time, is finite. Hence, there are an infinite number of $Ø$-regions with identical histories up to the present, but which need not be identical in the future. Moreover, all histories which are not forbidden by conservation laws will occur in a finite fraction of all $Ø$-regions. The ensemble of $Ø$-regions is reminiscent of the ensemble of universes in the many-world picture of quantum mechanics. An important difference, however, is that other $Ø$-regions are unquestionably real.

gr-qc↗

arXiv2001

Gravitational wave bursts from cusps and kinks on cosmic strings

The strong beams of high-frequency gravitational waves (GW) emitted by cusps and kinks of cosmic strings are studied in detail. As a consequence of these beams, the stochastic ensemble of GW's generated by a cosmological network of oscillating loops is strongly non Gaussian, and includes occasional sharp bursts that stand above the ``confusion'' GW noise made of many smaller overlapping bursts. Even if only 10% of all string loops have cusps these bursts might be detectable by the planned GW detectors LIGO/VIRGO and LISA for string tensions as small as $G μ\sim 10^{-13}$. In the implausible case where the average cusp number per loop oscillation is extremely small, the smaller bursts emitted by the ubiquitous kinks will be detectable by LISA for string tensions as small as $G μ\sim 10^{-12}$. We show that the strongly non Gaussian nature of the stochastic GW's generated by strings modifies the usual derivation of constraints on $G μ$ from pulsar timing experiments. In particular the usually considered ``rms GW background'' is, when $G μ\gaq 10^{-7}$, an overestimate of the more relevant confusion GW noise because it includes rare, intense bursts. The consideration of the confusion GW noise suggests that a Grand Unified Theory (GUT) value $ G μ\sim 10^{-6}$ is compatible with existing pulsar data, and that a modest improvement in pulsar timing accuracy could detect the confusion noise coming from a network of cuspy string loops down to $ G μ\sim 10^{-11}$. The GW bursts discussed here might be accompanied by Gamma Ray Bursts.

gr-qc↗

arXiv2001

Dynamics of superconducting strings with chiral currents

We rederive, using an elementary formalism, the general solution to the equations of motion for a superconducting string with a chiral (null) neutral current, earlier obtained by Carter and Peter. We apply this solution to show that the motion of such string loops is strictly periodic and analyze cusp-like behavior and vorton solutions of arbitrary shape. We argue that this solution can be used to approximately describe the dynamics of superconducting strings with small non-chiral currents. We use this description to estimate the electromagnetic radiation power from such strings.

astro-ph↗

arXiv2001

A prescription for probabilities in eternal inflation

Some of the parameters we call ``constants of Nature'' may in fact be variables related to the local values of some dynamical fields. During inflation, these variables are randomized by quantum fluctuations. In cases when the variable in question (call it $χ$) takes values in a continuous range, all thermalized regions in the universe are statistically equivalent, and a gauge invariant procedure for calculating the probability distribution for $χ$ is known. This is the so-called ``spherical cutoff method''. In order to find the probability distribution for $χ$ it suffices to consider a large spherical patch in a single thermalized region. Here, we generalize this method to the case when the range of $χ$ is discontinuous and there are several different types of thermalized region. We first formulate a set of requirements that any such generalization should satisfy, and then introduce a prescription that meets all the requirements. We finally apply this prescription to calculate the relative probability for different bubble universes in the open inflation scenario.

gr-qc↗

arXiv2000

On likely values of the cosmological constant

We discuss models in which the smallness of the effective vacuum energy density $ρ_Ł$ and the coincidence of the time of its dominance $t_Ł$ with the epoch of galaxy formation $t_G$ are due to anthropic selection effects. In such models, the probability distribution for $ρ_Ł$ is a product of an {\it a priori} distribution ${\cal P}_*(ρ_Ł)$ and of the number density of galaxies at a given $ρ_Ł$ (which is proportional to the number of observers who will detect that value of $ρ_Ł$). To determine ${\cal P}_*$, we consider inflationary models in which the role of the vacuum energy is played by a slowly-varying potential of some scalar field. We show that the resulting distribution depends on the shape of the potential and generally has a non-trivial dependence on $ρ_Ł$, even in the narrow anthropically allowed range. This is contrary to Weinberg's earlier conjecture that the {\it a priori} distribution should be nearly flat in the range of interest. We calculate the (final) probability distributions for $ρ_Ł$ and for $t_G/t_Ł$ in simple models with power-law potentials. For some of these models, the agreement with the observationally suggested values of $ρ_Ł$ is better than with a flat {\it a priori} distribution. We also discuss quantum-cosmological approach in which $ρ_Ł$ takes different values in different disconnected universes and argue that Weinberg's conjecture is not valid in this case as well. Finally, we extend our analysis to models of quintessence, with similar conclusions.

astro-ph↗