arXiv subjects

Alexander Vilenkin

Publications and source records attributed to Alexander Vilenkin.

At least 145 records · Page 8Linked to original sources

arXiv1997

Recycling universe

If the effective cosmological constant is non-zero, our observable universe may enter a stage of exponential expansion. In such case, regions of it may tunnel back to the false vacuum of an inflaton scalar field, and inflation with a high expansion rate may resume in those regions. An ``ideal'' eternal observer would then witness an infinite succession of cycles from false vacuum to true, and back. Within each cycle, the entire history of a hot universe would be replayed. If there were several minima of the inflaton potential, our ideal observer would visit each one of these minima with a frequency which depends on the shape of the potential. We generalize the formalism of stochastic inflation to analyze the global structure of the universe when this `recycling' process is taken into account.

astro-ph↗

arXiv1997

Cosmic Necklaces and Ultrahigh Energy Cosmic Rays

Cosmic necklaces are hybrid topological defects consisting of monopoles and strings, with two strings attached to each monopole. We argue that the cosmological evolution of necklaces may significantly differ from that of cosmic strings. The typical velocity of necklaces can be much smaller than the speed of light, and the characteristic scale of the network much smaller than the horizon. We estimate the flux of high-energy protons produced by monopole annihilation in the decaying closed loops. For some reasonable values of the parameters it is comparable to the observed flux of ultrahigh-energy cosmic rays.

astro-ph↗

arXiv1997

Topological defects and open inflation

Topological defects can naturally be formed soon after bubble nucleation in the open inflation scenario. The defects are not completely diluted away by the subsequent period of inflation in the bubble interior and can produce observable large-scale microwave background anisotropies. Superheavy strings and monopoles attached to the strings can act as gravitational lenses with angular separation between the images of up to an arc minute.

astro-ph↗

arXiv1997

Cosmic String Formation from Correlated Fields

We simulate the formation of cosmic strings at the zeros of a complex Gaussian field with a power spectrum $P(k) \propto k^n$, specifically addressing the issue of the fraction of length in infinite strings. We make two improvements over previous simulations: we include a non-zero random background field in our box to simulate the effect of long-wavelength modes, and we examine the effects of smoothing the field on small scales. The inclusion of the background field significantly reduces the fraction of length in infinite strings for $n < -2$. Our results are consistent with the possibility that infinite strings disappear at some $n = n_c$ in the range $-3 \le n_c < -2.2$, although we cannot rule out $n_c = -3$, in which case infinite strings would disappear only at the point where the mean string density goes to zero. We present an analytic argument which suggests the latter case. Smoothing on small scales eliminates closed loops on the order of the lattice cell size and leads to a ``lattice-free" estimate of the infinite string fraction. As expected, this fraction depends on the type of window function used for smoothing.

hep-ph↗

arXiv1996

Singularities in Inflationary Cosmology: A Review

We review here some recent results that show that inflationary cosmological models must contain initial singularities. We also present a new singularity theorem. The question of the initial singularity re-emerges in inflationary cosmology because inflation is known to be generically future-eternal. It is natural to ask, therefore, if inflationary models can be continued into the infinite past in a non-singular way. The results that we discuss show that the answer to the question is ``no.'' This means that we cannot use inflation as a way of avoiding the question of the birth of the Universe. We also argue that our new theorem suggests - in a sense that we explain in the paper - that the Universe cannot be infinitely old.

gr-qc↗

arXiv1996

Cosmic Defects

A (very) brief review of topological defects and their possible cosmic roles. I emphasize in particular that superheavy defects needed for structure formation can peacefully coexist with inflation, despite the claims to the contrary which are often made in the literature.

astro-ph↗

arXiv1996

Cosmic Strings and the String Dilaton

The existence of a dilaton (or moduli) with gravitational-strength coupling to matter imposes stringent constraints on the allowed energy scale of cosmic strings, $η$. In particular, superheavy gauge strings with $η\sim 10^{16} GeV$ are ruled out unless the dilaton mass $m_ϕ \gsim 100 TeV$, while the currently popular value $m_ϕ \sim 1 TeV$ imposes the bound $η\lsim 3 \times 10^{11} GeV$. Similar constraints are obtained for global topological defects. Some non-standard cosmological scenarios which can avoid these constraints are pointed out.

gr-qc↗

arXiv1996

Probability Distribution for $Ω$ in Open-universe Inflation

The problem of making predictions in eternally inflating universe that thermalizes by bubble nucleation is considered. A recently introduced regularization procedure is applied to find the probability distribution for the ensemble of thermalized bubbles. The resulting probabilities are shown to be independent of the choice of the time parametrization. This formalism is applied to models of open ``hybrid'' inflation with $Ω<1$. Depending on the parameters of the model, the probability distribution for $Ω$ is found to have a peak either very close to $Ω=1$, or at an intermediate value of $Ω$ in the range $0.03\lesssim Ω<1$.

astro-ph↗

arXiv1996

Uncertainties of predictions in models of eternal inflation

In a previous paper \cite{MakingPredictions}, a method of comparing the volumes of thermalized regions in eternally inflating universe was introduced. In this paper, we investigate the dependence of the results obtained through that method on the choice of the time variable and factor ordering in the diffusion equation that describes the evolution of eternally inflating universes. It is shown, both analytically and numerically, that the variation of the results due to factor ordering ambiguity inherent in the model is of the same order as their variation due to the choice of the time variable. Therefore, the results are, within their accuracy, free of the spurious dependence on the time parametrization.

gr-qc↗

arXiv1995

Quantum Cosmology and the Constants of Nature

In models where the constants of Nature can take more than one set of values, the cosmological wave function $ψ$ describes an ensemble of universes with different values of the constants. The probability distribution for the constants can be determined with the aid of the `principle of mediocrity' which asserts that we are a `typical' civilization in this ensemble. I discuss the implications of this approach for inflationary scenarios, the origin of density fluctuations, and the cosmological constant.

gr-qc↗

arXiv1995

Predictions from Quantum Cosmology

After reviewing the general ideas of quantum cosmology (Wheeler-DeWitt equation, boundary conditions, interpretation of $ψ$), I discuss how these ideas can be tested observationally. Observational predictions differ for different choices of boundary conditions. With tunneling boundary conditions, $ψ$ favors initial states that lead to inflation, while with Hartle-Hawking boundary conditions it does not. This difficulty of the Hartle-Hawking wave function becomes particularly severe if the role of `inflatons' is played by the moduli fields of superstring theories. In models where the constants of Nature can take more than one set of values, $ψ$ can also determine the probability distribution for the constants. This can be done with the aid of the `principle of mediocrity' which asserts that we are a `typical' civilization in the ensemble of universes described by $ψ$. The resulting distribution favors inflation with a very flat potential, thermalization and baryogenesis at electroweak scale, a non-negligible cosmological constant, and density fluctuations seeded either by topological defects, or by quantum fluctuations in models like hybrid inflation (as long as these features are consistent with the allowed values of the constants).

gr-qc↗

arXiv1995

Making Predictions in Eternally Inflating Universe

Eternally inflating universes can contain large thermalized regions with different values of the constants of Nature and with different density fluctuation spectra. To find the probability for a `typical' observer to detect a certain set of constants, or a certain fluctuation spectrum, one needs to compare the volumes occupied by different types of regions. If the volumes are taken on an equal-time hypersurface, the results of such a comparison are extremely sensitive to the choice of the time variable t. Here, I propose a method of comparing the volumes which is rather insensitive to the choice of t. The method is then applied to evaluate the relative probability of different minima of the inflaton potential and the probability distribution for the density fluctuation spectra.

gr-qc↗

arXiv1995

String Theory and Inflation

String theory abounds with light scalar fields (the dilaton and various moduli) which create a host of observational problems, and notably some serious cosmological difficulties similar to the ones associated with the Polonyi field in the earliest versions of spontaneously broken supergravity. We show that all these problems are naturally avoided if a recently introduced mechanism for fixing the vacuum expectation values of the dilaton and/or moduli is at work. We study both the classical evolution and the quantum fluctuations of such scalar fields during a primordial inflationary era and find that the results are naturally compatible with observational facts. In this model, dilatons or moduli within a very wide range of masses (which includes the SUSY-breaking favored 1 TeV value and extends up to the Planck scale) qualify to define a novel type of essentially stable ultra-weakly interacting massive particles able to provide enough mass density to close the universe

hep-th↗

arXiv1995

Defect Production in Slow First Order Phase Transitions

We study the formation of vortices in a U(1) gauge theory following a first-order transition proceeding by bubble nucleation, in particular the effect of a low velocity of expansion of the bubble walls. To do this, we use a two-dimensional model in which bubbles are nucleated at random points in a plane and at random times and then expand at some velocity $v_{\rm b}<c$. Within each bubble, the phase angle is assigned one of three discrete values. When bubbles collide, magnetic `fluxons' appear: if the phases are different, a fluxon--anti-fluxon pair is formed. These fluxons are eventually trapped in three-bubble collisions when they may annihilate or form quantized vortices. We study in particular the effect of changing the bubble expansion speed on the vortex density and the extent of vortex--anti-vortex correlation.

hep-ph↗

arXiv1995

Phase equilibration in bubble collisions

In the context of an Abelian gauge symmetry, spontaneously broken at a first-order transition, we discuss the evolution of the phase difference between the Higgs fields in colliding bubbles. We show that the effect of dissipation, represented by a finite plasma conductivity, is to cause the phases to equlibrate on a time-scale, determined by the conductivity, which can be much smaller than the bubble radii at the time of collision. Currents induced during the phase equilibration generate a magnetic flux, which is determined by the initial phase difference. In a three-bubble collision, the fluxes produced by each pair of bubbles combine, and a vortex can be formed. We find that, under most conditions, the probability of trapping magnetic flux to form a vortex is correctly given by the ``geodesic rule''.

hep-ph↗

arXiv1995

Density of strings formed at a second-order cosmological phase transition

We discuss the problem of estimating the characteristic length scale $ξ_{\rm s}$, and hence the initial density, of a system of cosmic strings formed at a continuous, second-order phase transition in the early universe. In particular, we examine the roles of the correlation length (or string width) $ξ_{\rm c}$ and the Ginzburg length $ξ_{\rm G}$ which defines the ``fuzziness'' of long strings. We argue that strings acquire a clear identity only once $ξ_{\rm s}$ exceeds both $ξ_{\rm c}$ and $ξ_{\rm G}$, and estimate its magnitude at that time.

hep-ph↗

arXiv1994

Predictions from Quantum Cosmology

The world view suggested by quantum cosmology is that inflating universes with all possible values of the fundamental constants are spontaneously created out of nothing. I explore the consequences of the assumption that we are a `typical' civilization living in this metauniverse. The conclusions include inflation with an extremely flat potential and low thermalization temperature, structure formation by topological defects, and an appreciable cosmological constant.

gr-qc↗

arXiv1994

Detection of Particles Under Potential Barrier

We introduce a model detector which registers the passage of a particle through the detector location, without substantially perturbing the particle wave function. (The exact time of passage is not determined in such measurements.) We then show that our detector can operate in a classically forbidden region and register particles passing through a certain point under a potential barrier. We show that it should be possible to observe the particle's track under the barrier.

hep-th↗