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11 pages, 278 KiB

Open AccessArticle

by Aroonkumar Beesham

Universe 2024, 10(11), 404; https://doi.org/10.3390/universe10110404 - 23 Oct 2024

Viewed by 1232

The idea of varying constants of nature is very old, and has commanded a lot of attention since first mooted. The variation in the gravitational parameter G and cosmological parameter

Λ

is still an active area of research. Since the idea of a [...] Read more.

The idea of varying constants of nature is very old, and has commanded a lot of attention since first mooted. The variation in the gravitational parameter G and cosmological parameter

Λ

is still an active area of research. Since the idea of a varying G was introduced by Dirac almost a century ago, there are even theories that have variable G such as the Brans–Dicke theory and the scale covariant theory. Both these theories also have a varying

Λ

in their full generalisations. A varying

Λ

was also introduced around the same time as that of varying G. It is interesting to note that a possible solution to the cosmological constant problem can be realised from a dynamic

Λ

. In this work, we focus on a varying

Λ

and G framework. In almost all studies in the simplest framework of variables

Λ

and G, it is found that one of them has to increase with time. However, observations and theoretical considerations indicate that both

Λ

and G should decrease with time. In this paper, we propose a solution to this problem, finding theories in which both

Λ

and G decrease with time. Full article

(This article belongs to the Special Issue Dark Energy and Dark Matter)

12 pages, 303 KiB

Open AccessArticle

Big Rip Scenario in Brans-Dicke Theory

by Sasmita Kumari Pradhan, Sunil Kumar Tripathy, Zashmir Naik, Dipanjali Behera and Mrutunjaya Bhuyan

Foundations 2022, 2(1), 128-139; https://doi.org/10.3390/foundations2010007 - 17 Jan 2022

Cited by 2 | Viewed by 4249

Abstract

In this work, we present a Big Rip scenario within the framework of the generalized Brans-Dicke (GBD) theory. In the GBD theory, we consider an evolving BD parameter along with a self-interacting potential. An anisotropic background is considered to have a more general view of the cosmic expansion. The GBD theory with a cosmological constant is presented as an effective cosmic fluid within general relativity which favours a phantom field dominated phase. The model parameters are constrained so that the model provides reasonable estimates of the Hubble parameter and other recent observational aspects at the present epoch. The dynamical aspects of the BD parameter and the BD scalar field have been analysed. It is found that the present model witnesses a finite time doomsday at a time of

t_{B R} ≃ 16.14 Gyr

, and for this scenario, the model requires a large negative value of the Brans-Dicke parameter. Full article

(This article belongs to the Special Issue Advances in Fundamental Physics)

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11 pages, 282 KiB

Open AccessFeature PaperArticle

Light-Like Shockwaves in Scalar-Tensor Theories

by Bence Racskó and László Árpád Gergely

Universe 2018, 4(3), 44; https://doi.org/10.3390/universe4030044 - 27 Feb 2018

Cited by 4 | Viewed by 2770

Abstract

Both electromagnetic shock-waves and gravitational waves propagate with the speed of light. If they carry significant energy-momentum, this will change the properties of the space-time they propagate through. This can be described in terms of the junction conditions between space-time regions separated by a singular, null hypersurface. We derived generic junction conditions for Brans-Dicke theory in the Jordan frame, exploring a formalism based on a transverse vector, rather than normal, which can be applied to any type of hypersurfaces. In the particular case of a non-null hypersurface we obtain a generalised Lanczos equation, in which the jump of the extrinsic curvature is sourced by both the distributional energy-momentum tensor and by the jump in the transverse derivative of the scalar. In the case of null hypersurfaces, the distributional source is decomposed into surface density, current and pressure. The latter, however, ought to vanish by virtue of the scalar junction condition. Full article

(This article belongs to the Special Issue BGL2017: 10th Bolyai-Gauss-Lobachevsky Conference on Non-Euclidean Geometry and Its Applications)

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