arXiv subjects

J. D. Cohn

Publications and source records attributed to J. D. Cohn.

45 records · Page 3Linked to original sources

arXiv1998

Where do all the supercurvature modes go?

In the hyperbolic slicing of de Sitter space appropriate for open universe models, a curvature scale is present and supercurvature fluctuations are possible. In some cases, the expansion of a scalar field in the Bunch-Davies vacuum includes supercurvature modes, as shown by Sasaki, Tanaka and Yamamoto. We express the normalizable vacuum supercurvature modes for a massless scalar field in terms of the basis modes for the spatially-flat slicing of de Sitter space.

gr-qc↗

arXiv1998

Update on Open Universe Inflationary Models

An overview of some new results in open inflation over the past year, including the calculation of gravity wave contributions to the Cosmic Microwave Background. Proceedings for COSMO-97, held in Ambleside, U.K., hence very short due to space limitations (the version for the web is more than 3 pages because archive numbers have been added for the references).

astro-ph↗

arXiv1997

Primordial Gravitational Waves From Open Inflation

We calculate the spectrum of gravitational waves generated during inflation in open $(Ω_0<1)$ inflationary models. In such models an initial epoch of old inflation solves the horizon and flatness problems, and during this first epoch of inflation the quantum state of the graviton field rapidly approaches the Bunch-Davies vacuum. Then old inflation ends by the nucleation of a single bubble, inside of which there is a shortened epoch of slow-roll inflation giving $Ω_0<1$ today. In this paper we re-express the Bunch-Davies vacuum for the graviton field in terms of the hyperbolic modes inside the bubble and propagate these modes forward in time into the present era. We derive the expression for the contribution from these gravity waves to the cosmic microwave background anisotropy including the effect of a finite energy difference across the bubble wall.

astro-ph↗

arXiv1996

Open universes from bubbles: an introduction and update

An introduction to models of open universes originating from bubbles, including a summary of recent theoretical results for the power spectrum. To appear in the proceedings of the XXXIth Moriond meeting, "Microwave Background Anisotropies."

astro-ph↗

arXiv1996

Open Universes from Finite Radius Bubbles

The interior of a vacuum bubble in de Sitter space may give an open universe with sufficient homogeneity to agree with observations. Here, previous work by Bucher, Goldhaber and Turok is extended to describe a thin bubble wall with nonzero radius and energy difference across the wall. The vacuum modes present before formation of the bubble propagate into the interior of open universe and the power spectrum of the resulting gauge invariant gravitational potential is calculated. It is found to become scale invariant on small scales, with onset at about the same scale as that found in the zero radius case. There is sensitivity to the radius and energy difference at large scales, but it is expected that they cannot be strongly constrained because of cosmic variance. As the prediction of a scale invariant spectrum is robust with respect to variation of these parameters at small scales, it is a generic feature of the contribution of these modes for these thin wall models.

astro-ph↗

arXiv1993

The Strength of String Nonperturbative Effects and Strong-Weak Coupling Duality

A strong-weak coupling duality symmetry of the string equations of motion has been suggested in the literature. This symmetry implies that vacua occur in pairs. Since the coupling constant is a dynamical variable in string theory, tunneling solutions between strong and weak coupling vacua may exist. Such solutions would naturally lead to nonperturbative effects with anomalous coupling dependence. A highly simplified example is given.

hep-th↗

arXiv1992

Quantizing Effective Strings

This TASI lecture covers the material in hep-th/9205026. It reviewed the theory of effective strings, with particular emphasis on the manner in which Lorentz invariance is represented. The quantum properties of an example of an effective string are derived from the underlying field theory. A comparison is made with what one would expect if one assumed that quantum effective strings were governed by fundamental string actions such as the Nambu-Goto or the Polyakov actions. It is shown that the requirements on dimensions for consistent quantizations of fundamental strings imply no contradictions for effective strings.

hep-th↗

arXiv1992

Lorentz invariance of effective strings

Starting from a Poincaré invariant field theory of a real scalar field with interactions governed by a double-well potential in 2+1 dimensions, the Lorentz representation induced on the collective coordinates describing low-energy excitations about an effective string background is derived. In this representation, Lorentz transformations are given in terms of an infinite series, in powers of derivatives along the worldsheet. Transformations that act on the direction transverse to the string worldsheet involve a universal dimension $-1$ term. As a consequence, Lorentz invariance holds in this theory of long effective strings due to cancellations in the action between irrelevant terms and the dimension two term that describes free massless scalar fields in two dimensions. (in plain tex, no macropackages necessary)

hep-th↗

arXiv1992

The Marinari-Parisi Model and Collective Field Theory

We derive the supersymmetric collective field theory for the Marinari-Parisi model. For a specific choice of the superpotential, to leading order we find a one parameter family of ground states which can be connected via instantons. At this level of analysis the instanton size implied by the underlying matrix model does not appear.

hep-th↗