arXiv subjects

Dan N. Vollick

Publications and source records attributed to Dan N. Vollick.

29 records · Page 2Linked to original sources

arXiv2003

1/R Curvature Corrections as the Source of the Cosmological Acceleration

Corrections to Einstein's equations that become important at small curvatures are considered. The field equations are derived using a Palatini variation in which the connection and metric are varied independently. In contrast to the Einstein-Hilbert variation, which yields fourth order equations, the Palatini approach produces second order equations in the metric. The Lagrangian $L(R)=R-α^2/R$ is examined and it is shown that it leads to equations whose solutions approach a de Sitter universe at late times. Thus, the inclusion of 1/R curvature terms in the gravitational action offers an alternative explanation for the cosmological acceleration.

astro-ph↗

arXiv2003

Holography in Closed Universes

The holographic bound states that the entropy in a region cannot exceed one quarter of the area (in Planck units) of the bounding surface. A version of the holographic principle that can be applied to cosmological spacetimes has recently been given by Fischler and Susskind. This version can be shown to fail in closed spacetimes and they concluded that the holographic bound may rule out such universes. In this paper I give a modified definition of the holographic bound that holds in a large class of closed universes. Fischler and Susskind also showed that the dominant energy condition follows from the holographic principle applied to cosmological spacetimes with $a(t)=t^p$. Here I show that the dominant energy condition can be violated by cosmologies satisfying the holographic principle with more general scale factors.

hep-th↗

arXiv2002

Liouville Field Theory and Dark Matter

One of the most intriguing unsolved problems in modern cosmology involves the nature of the observed dark matter in the universe. Various hypothetical particles have been postulated as the source of the dark matter, but as of yet none of these particles have been observed. In this paper I present an alternative view. The observations indicate the presence of a new force that is much weaker than gravity on small scales and stronger than gravity on large scales. This new force is described by a Liouville field that consists of a scalar field with an exponential potential. The solutions have a Newtonian behavior on small scales and produce forces that are weak compared to gravity. On large scales the field has a logarithmic behavior and will therefore produce flat rotation curves. The Liouville field may therefore be able to account for the observations that are usually taken to imply the existence of dark matter.

hep-th↗

arXiv2001

Anisotropic Born-Infeld Cosmologies

Anisotropic cosmological spacetimes are constructed from spherically symmetric solutions to Einstein's equations coupled to nonlinear electrodynamics and a positive cosmological constant. This is accomplished by finding solutions in which the roles of $r$ and $t$ are interchanged for all $r>0$ (i.e. $r$ becomes timelike and $t$ becomes spacelike). Constant time hypersurfaces have topology $R\times S^2$ and in all the spacetimes considered the radius of the two sphere vanishes as $t$ goes to zero. The scale factor of the other dimension diverges as $t$ goes to zero in some solutions and vanishes (or goes to a constant) in other solutions. At late times local observers would see the universe to be homogeneous and isotropic.

hep-th↗

arXiv2000

Spherically Symmetric Solutions and Dark Matter on the Brane

It has recently been suggested that our universe is a three-brane embedded in a higher dimensional spacetime. In this paper I examine static, spherically symmetric solutions that satisfy the effective Einstein field equations on a brane embedded in a five dimensional spacetime. The field equations involve a term depending on the five dimensional Weyl tensor, so that the solutions will not be Schwarzschild in general. This Weyl term is traceless so that any solution of $^{(4)}R=0$ is a possible four dimensional spacetime. Different solutions correspond to different five dimensional spacetimes and to different induced energy-momentum tensors on the brane. One interesting possibility is that the Weyl term could be responsible for the observed dark matter in the universe.

hep-th↗

arXiv2000

Negative Energies on the Brane

It has recently been proposed that our universe is a three-brane embedded in a higher dimensional spacetime. Here I show that black holes on the brane, black strings intersecting the brane, and gravitational waves propagating in the bulk induce an effective energy-momentum tensor on the brane that contains negative energy densities.

hep-th↗

arXiv2000

Cosmology on a Three-Brane

In this paper a general solution is found for a five dimensional orbifold spacetime that induces a $k=0$ cosmology on a three-brane. Expressions for the energy density and pressure on the brane in terms of the brane metric are derived. Given a metric on the brane it is possible to find five dimensional spacetimes that contain the brane. This calculation is carried out for an inflationary universe and for a metric that corresponds to a radiation dominated universe in standard cosmology. It is also shown that any $k=0$ cosmology can be embedded in a flat five dimensional orbifold spacetime and the equation of the three-brane surface is derived. For an inflationary universe it is shown that the surface is the usual hyperboloid representation of de Sitter space, although it is embedded in an orbifold spacetime.

hep-th↗

arXiv2000

Quantum Inequalities in Curved Two Dimensional Spacetimes

In quantum field theory there exist states for which the energy density is negative. It is important that these negative energy densities satisfy constraints, such as quantum inequalities, to minimize possible violations of causality, the second law of thermodynamics, and cosmic censorship. In this paper I show that conformally invariant scalar and Dirac fields satisfy quantum inequalities in two dimensional spacetimes with a conformal factor that depends on $x$ only or on $t$ only. These inequalities are then applied to two dimensional black hole and cosmological spacetimes. It is shown that the bound on the negative energies diverges to minus infinity as the event horizon or initial singularity is approached. Thus, neglecting back reaction, negative energies become unconstrained near the horizon or initial singularity. The results of this paper also support the hypothesis that the quantum interest conjecture applies only to deviations from the vacuum polarization energy, not to the total energy.

gr-qc↗

arXiv1998

Wormholes in String Theory

A wormhole is constructed by cutting and joining two spacetimes satisfying the low energy string equations with a dilaton field. In spacetimes described by the "string metric" the dilaton energy-momentum tensor need not satisfy the weak or dominant energy conditions. In the cases considered here the dilaton field violates these energy conditions and is the source of the exotic matter required to maintain the wormhole. There is also a surface stress-energy, that must be produced by additional matter, where the spacetimes are joined. It is shown that wormholes can be constructed for which this additional matter satisfies the weak and dominant energy conditions, so that it could be a form of "normal" matter. Charged dilaton wormholes with a coupling between the dilaton and the electromagnetic field that is more general than in string theory are also briefly discussed.

gr-qc↗

arXiv1998

Maintaining a Wormhole with a Scalar Field

It is well known that it takes matter that violates the averaged weak energy condition to hold the throat of a wormhole open. The production of such ``exotic'' matter is usually discussed within the context of quantum field theory. In this paper I show that it is possible to produce the exotic matter required to hold a wormhole open classically. This is accomplished by coupling a scalar field to matter that satisfies the weak energy condition. The energy-momentum tensor of the scalar field and the matter separately satisfy the weak energy condition, but there exists an interaction energy-momentum tensor that does not. It is this interaction energy-momentum tensor that allows the wormhole to be maintained.

gr-qc↗

arXiv1998

Negative Energy Density States for the Dirac Field in Flat Spacetime

Negative energy densities in the Dirac field produced by state vectors that are the superposition of two single particle electron states are examined. I show that for such states the energy density of the field is not bounded from below and that the quantum inequalities derived for scalar fields are satisfied. I also show that it is not possible to produce negative energy densities in a scalar field using state vectors that are arbitrary superpositions of single particle states.

gr-qc↗