arXiv subjects

F. R. Klinkhamer

Publications and source records attributed to F. R. Klinkhamer.

At least 37 records · Page 2Linked to original sources

arXiv2020

On the emergence of an expanding universe from a Lorentzian matrix model

We present evidence that recent numerical results from the reduced classical equations of the Lorentzian IIB matrix model can be interpreted as corresponding to the emergence of an expanding universe. In addition, we propose an effective metric to describe the emerging (3+1)-dimensional spacetime. This metric gives, at all times, finite values for the Ricci and Kretschmann curvature scalars. With these results, we are able to give a heuristic discussion of the origin of the Universe in the context of the IIB matrix model.

gr-qc↗

arXiv2020

Another model for the regularized big bang

We propose a gravitational model with a Brans-Dicke-type scalar field having, in the would-be action, a "wrong-sign" kinetic term and a quartic interaction term. In a cosmological context, we obtain, depending on the boundary conditions, either the Friedmann solution or a kink-bounce solution. The expanding-universe Friedmann solution has a big bang curvature singularity, whereas the kink-bounce solution has a nonsingular bouncing behavior of the cosmic scale factor. The bounce occurs precisely at the moment when the scalar field of the kink-type configuration goes through zero, making for a vanishing effective gravitational coupling.

gr-qc↗

arXiv2020

Nonsingular bouncing cosmology from general relativity: Scalar metric perturbations

We derive the equations of motion for scalar metric perturbations in a particular nonsingular bouncing cosmology, where the big bang singularity is replaced by a spacetime defect with a degenerate metric. The adiabatic perturbation solution is obtained for nonrelativistic hydrodynamic matter. We get the same result by working with conformal coordinates. This last method is also valid for vector and tensor metric perturbations, and selected results are presented. We, finally, discuss several new effects from the linear perturbations of this nonsingular bouncing cosmology, such as across-bounce information transfer and the possible imprint on cosmological perturbations from a new phase responsible for the effective spacetime defect.

gr-qc↗

arXiv2020

More on the regularized big bang singularity

The big bang singularity of the expanding-universe Friedmann solution of the Einstein gravitational field equation can be regularized by the introduction of a degenerate metric and a nonzero length scale $b$. The result is a nonsingular bounce of the cosmic scale factor with a contracting prebounce phase and an expanding postbounce phase. The corresponding maximum values of the curvature and the energy density occur at the moment of the bounce and are proportional to powers of $1/b$. This article presents a detailed calculation of the dynamics of such a nonsingular bounce. In addition, a comparison is made between this nonsingular bounce and the bounces of loop quantum cosmology and string cosmology.

gr-qc↗

arXiv2019

Instability of the big bang coordinate singularity in a Milne-like universe

We present a simplified dynamic-vacuum-energy model for a time-symmetric Milne-like universe. The big bang singularity in this simplified model, like the one in a previous model, is just a coordinate singularity with finite curvature and energy density. We then calculate the dynamic behavior of scalar metric perturbations and find that these perturbations destabilize the big bang singularity.

gr-qc↗

arXiv2019

Vacuum energy decay from a q-bubble

We consider a finite-size spherical bubble with a nonequilibrium value of the $q$-field, where the bubble is immersed in an infinite vacuum with the constant equilibrium value $q_{0}$ for the $q$-field (this $q_{0}$ has already cancelled an initial cosmological constant). Numerical results are presented for the time evolution of such a $q$-bubble with gravity turned off and with gravity turned on. For small enough bubbles and a $q$-field energy scale sufficiently below the gravitational energy scale $E_\text{Planck}$, the vacuum energy of the $q$-bubble is found to disperse completely. For large enough bubbles and a finite value of $E_\text{Planck}$, the vacuum energy of the $q$-bubble disperses only partially and there occurs gravitational collapse near the bubble center.

gr-qc↗

arXiv2019

Nonsingular bouncing cosmology from general relativity

We investigate a particular type of classical nonsingular bouncing cosmology, which results from general relativity if we allow for degenerate metrics. The simplest model has a matter content with a constant equation-of-state parameter and we get the modified Hubble diagrams for both the luminosity distance and the angular diameter distance. Based on these results, we present a Gedankenexperiment to determine the length scale of the spacetime defect which has replaced the big bang singularity. A possibly more realistic model has an equation-of-state parameter which is different before and after the bounce. This last model also provides an upper bound on the defect length scale.

gr-qc↗

arXiv2019

Model for a time-symmetric Milne-like universe without big bang curvature singularity

We present a physics model for a time-symmetric Milne-like universe. The model is based on the $q$-theory approach to the cosmological constant problem, supplemented by an assumed vacuum-matter energy exchange possibly due to quantum-dissipative effects. Without fine-tuning of the initial vacuum energy density, we obtain a harmless big bang singularity (with finite values of the Ricci and Kretschmann curvature scalars) and attractor behavior towards Minkowski spacetime. The time-symmetric spacetime manifold of our model may provide the proper setting for a possible CPT-symmetric universe.

gr-qc↗

arXiv2019

Asymmetric nonsingular bounce from a dynamic scalar field

We present a dynamical model for a time-asymmetric nonsingular bounce with a post-bounce change of the effective equation-of-state parameter. Specifically, we consider a scalar-field model with a time-reversal-noninvariant effective potential.

gr-qc↗

arXiv2019

Regularized big bang singularity

Following up on earlier work on the regularization of the singular Schwarzschild solution, we now apply the same procedure to the singular Friedmann solution. Specifically, we are able to remove the divergences of the big bang singularity, at the price of introducing a 3-dimensional spacetime defect with a vanishing determinant of the metric. This particular regularization also suggests the existence of a "pre-big-bang" phase.

gr-qc↗

arXiv2019

Tetrads and q-theory

As the microscopic structure of the deep relativistic quantum vacuum is unknown, a phenomenological approach ($q$-theory) has been proposed to describe the vacuum degrees of freedom and the dynamics of the vacuum energy after the Big Bang. The original $q$-theory was based on a four-form field strength from a three-form gauge potential. However, this realization of $q$-theory, just as others suggested so far, is rather artificial and does not take into account the fermionic nature of the vacuum. We now propose a more physical realization of the $q$-variable. In this approach, we assume that the vacuum has the properties of a plastic (malleable) fermionic crystalline medium. The new approach unites general relativity and fermionic microscopic (trans-Planckian) degrees of freedom, as the approach involves both the tetrad of standard gravity and the elasticity tetrad of the hypothetical vacuum crystal. This approach also allows for the description of possible topological phases of the quantum vacuum.

gr-qc↗

arXiv2019

Lensing and imaging by a stealth defect of spacetime

We obtain the geodesics for the simplest possible stealth defect which has a flat spacetime. We, then, discuss the lensing properties of such a defect, and the corresponding image formation. Similar lensing properties can be expected to hold for curved-spacetime stealth defects.

gr-qc↗

arXiv2019

On a soliton-type spacetime defect

We review the construction of a particular soliton-type solution of the classical Einstein and matter-field equations. This localized finite-energy static classical solution can be interpreted as a single spacetime defect embedded in Minkowski spacetime and may give rise to several new effects. For a Skyrme-type theory with small enough matter-field energy scale compared to the Planck energy scale and for a sufficiently small defect length scale, the existence of a globally regular solution requires a negative active gravitational mass, so that the defect repels a distant test particle ("antigravity"). There also exist "stealth defects" which have a vanishing asymptotic gravitational mass. These stealth defects are, however, not entirely invisible as they bring about a new type of gravitational lensing.

gr-qc↗

arXiv2018

More on cold dark matter from q-theory

We consider the rapidly-oscillating part of a $q$-field in a cosmological context and find that its energy density behaves in the same way as a cold-dark-matter component, namely proportional to the inverse cube of the cosmic scale factor.

gr-qc↗

arXiv2018

Charge extraction from a black hole

Following up on earlier work about across-horizon quantum scattering and information transfer outwards across the black-hole horizon, we show that it is also possible to extract electric charge from a static nonrotating black hole. This result can be extended to other types of charge with a gauge-boson coupling and to nonstatic rotating black holes.

physics.gen-ph↗

arXiv2018

Anomalous Lorentz and CPT violation

If there exists Lorentz and CPT violation in nature, then it is crucial to discover and understand the underlying mechanism. In this contribution, we discuss one such mechanism which relies on four-dimensional chiral gauge theories defined over a spacetime manifold with topology $\mathbb{R}^3 \times S^1$ and periodic spin structure for the compact dimension. It can be shown that the effective gauge-field action contains a local Chern-Simons-like term which violates Lorentz and CPT invariance. For arbitrary Abelian $U(1)$ gauge fields with trivial holonomies in the compact direction, this anomalous Lorentz and CPT violation has recently been established perturbatively with a Pauli-Villars-type regularization and nonperturbatively with a lattice regularization based on Ginsparg-Wilson fermions.

hep-th↗

arXiv2018

A stealth defect of spacetime

We discuss a special type of Skyrmion spacetime-defect solution, which has a positive energy density of the matter fields but a vanishing asymptotic gravitational mass. With a mass term for the matter field added to the action (corresponding to massive "pions" in the Skyrme model), this particular soliton-type solution has no long-range fields and can appropriately be called a "stealth defect."

gr-qc↗

arXiv2018

Antigravity from a spacetime defect

We argue that there may exist spacetime defects embedded in Minkowski spacetime, which have negative active gravitational mass. One such spacetime defect then repels a test particle, corresponding to what may be called "antigravity."

gr-qc↗