Alexander Vilenkin

Cosmic Strings and Domain Walls

Phase transitions in the early universe can give rise to microscopic topological defects: vacuum domain walls, strings, walls bounded by strings, and monopoles connected by strings. This article reviews the formation, physical properties and the cosmological evolution of various defects. A particular attention is paid to strings and their cosmological consequences, including the string scenario of galaxy formation and possible observational effects of strings.

Ultrahigh Energy Cosmic Rays without Greisen-Zatsepin-Kuzmin Cutoff

We study the decays of ultraheavy (m_X \geq 10^{13}GeV) and quasistable (lifetime \tau_X much larger than the age of the Universe t_0) particles as the source of Ultra High Energy Cosmic Rays (UHE CR). These particles are assumed to constitute a tiny fraction \xi_X of CDM in the Universe, with \xi_X being the same in the halo of our Galaxy and in the intergalactic space. The elementary-particle and cosmological scenarios for these particles are briefly outlined. The UHE CR fluxes produced at the decays of X- particles are calculated. The dominant contribution is given by fluxes of photons and nucleons from the halo of our Galaxy and thus they do not exibit the GZK cutoff. The extragalactic components of UHE CR are suppressed by the smaller extragalactic density of X-particles and hence the cascade limit is relaxed. We discuss the spectrum of produced Extensive Air Showers (EAS) and a signal from Virgo cluster as signatures of this model.

Formation and evolution of cosmic D strings

We study the formation of D and F cosmic strings in D-brane annihilation after brane inflation. We show that D string formation by quantum de Sitter fluctuations is severely suppressed, due to suppression of RR field fluctuations in compact dimensions. We discuss the resonant mechanism of production of D and F strings, which are formed as magnetic and electric flux tubes of the two orthogonal gauge fields living on the world-volume of the unstable brane. We outline the subsequent cosmological evolution of the D−F string network. We also compare the nature of these strings with the ordinary cosmic strings and point out some differences and similarities.

Inflationary Spacetimes Are Incomplete in Past Directions

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.

Eternal inflation and the initial singularity

It is shown that a physically reasonable spacetime that is eternally inflating to the future must possess an initial singularity.

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.

Gravitational Field of Vacuum Domain Walls

Abstract An exact solution of Einsteins equations is found describing the gravitational field of a plane vacuum domain wall. The metric has event horizons, has no true geometric singularities, and is locally flat everywhere except on the wall itself.

Gravitational wave bursts from cosmic strings

Cusps of cosmic strings emit strong beams of high-frequency gravitational waves (GW). As a consequence of these beams, the stochastic ensemble of gravitational waves generated by a cosmological network of oscillating loops is strongly non-Gaussian, and includes occasional sharp bursts that stand above the rms GW background. These bursts might be detectable by the planned GW detectors LIGO/VIRGO and LISA for string tensions as small as G&mgr; approximately 10(-13). The GW bursts discussed here might be accompanied by gamma ray bursts.

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. {copyright} {ital 1997} {ital The American Physical Society}

On likely values of the cosmological constant

We discuss models in which the smallness of the effective vacuum energy density