Special Issue "The Dark Side of the Universe: Dark Energy, Dark Matter and Modified Gravity"

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (30 November 2018)

Special Issue Editors

Guest Editor
Dr. Tiberiu Harko

1. Department of Physics, Babes-Bolyai University, Kogalniceanu Street, Cluj-Napoca 400084, Romania
2. Department of Mathematics, University College London, Gower Street, London, WC1E 6BT, UK
3. School of Physics, Sun Yat-Sen University, Guangzhou 510275, China
E-Mail
Phone: 0040720 944 087
Interests: general relativity; modified theories of gravity; dark matter; cosmology; astrophysics and applied mathematics
Guest Editor
Dr. Francisco S. N. Lobo

Institute of Astrophysics and Space Sciences, Science Faculty of the University of Lisbon, Portugal
Website | E-Mail
Interests: modified gravity; dark energy; cosmology; dark matter; black holes; energy conditions; causal structure of spacetime

Special Issue Information

Dear Colleagues,

Modern astrophysical and cosmological models are plagued with two severe theoretical problems, namely, the dark energy and the dark matter enigmas. Relative to the latter, the dynamics of test particles around galaxies, as well as the corresponding mass discrepancy in galactic clusters, is explained by postulating the existence of a hypothetical form of matter, called dark matter. Relative to the dark energy problem, high precision observational data has confirmed with startling evidence that the Universe is undergoing a phase of accelerated expansion. This phase is one of the most important and challenging current problems in cosmology, and represents a new imbalance in the governing gravitational equations. Several candidates, responsible for this expansion, have been proposed in the literature, in particular, dark energy models and modified theories of gravity, amongst others. Outstanding questions are related to the nature of this so-called “dark energy” that is driving the acceleration of the universe, and whether it is due to the vacuum energy or a dynamical field. On the other hand, the late-time cosmic acceleration, as well as the elusive dark matter, may be due to modifications of General Relativity, which introduce new degrees of freedom to the gravitational sector itself.

This Special Issue will explore a plethora of viable dark matter, dark energy and modified gravity models, some of which consistently reproduce the inflationary epoch, and will be tested against large-scale structure and lensing, astrophysical and laboratory measurements, as well as laboratory and space-based Equivalence Principle experiments. These Solar System tests, large scale structure measurements and lensing data, amongst others, restrict the range of allowed modified gravity models and offer a window into understanding the perplexing nature of the cosmic acceleration and of gravity itself.

Dr. Tiberiu Harko
Dr. Francisco S. N. Lobo
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Galaxies is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 350 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • modified gravity
  • dark energy
  • dark matter, cosmology

Published Papers (4 papers)

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Research

Open AccessArticle Einstein-Gauss-Bonnet Gravity with Extra Dimensions
Received: 28 January 2019 / Revised: 25 February 2019 / Accepted: 12 March 2019 / Published: 19 March 2019
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Abstract
We consider a theory of modified gravity possessing d extra spatial dimensions with a maximally symmetric metric and a scale factor, whose (4+d)-dimensional gravitational action contains terms proportional to quadratic curvature scalars. Constructing the 4D effective field theory [...] Read more.
We consider a theory of modified gravity possessing d extra spatial dimensions with a maximally symmetric metric and a scale factor, whose ( 4 + d ) -dimensional gravitational action contains terms proportional to quadratic curvature scalars. Constructing the 4D effective field theory by dimensional reduction, we find that a special case of our action where the additional terms appear in the well-known Gauss-Bonnet combination is of special interest as it uniquely produces a Horndeski scalar-tensor theory in the 4D effective action. We further consider the possibility of achieving stabilised extra dimensions in this scenario, as a function of the number and curvature of extra dimensions, as well as the strength of the Gauss-Bonnet coupling. Further questions that remain to be answered such as the influence of matter-coupling are briefly discussed. Full article
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Open AccessArticle What if Newton’s Gravitational Constant Was Negative?
Received: 30 January 2019 / Revised: 7 March 2019 / Accepted: 11 March 2019 / Published: 18 March 2019
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Abstract
In this work, we seek a cosmological mechanism that may define the sign of the effective gravitational coupling constant, G. To this end, we consider general scalar-tensor gravity theories as they provide the field theory natural framework for the variation of the [...] Read more.
In this work, we seek a cosmological mechanism that may define the sign of the effective gravitational coupling constant, G. To this end, we consider general scalar-tensor gravity theories as they provide the field theory natural framework for the variation of the gravitational coupling. We find that models with a quadratic potential naturally stabilize the value of G into the positive branch of the evolution and further, that de Sitter inflation and a relaxation to General Relativity is easily attained. Full article
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Open AccessArticle Minimum Length Uncertainty Relations in the Presence of Dark Energy
Received: 13 December 2018 / Revised: 31 December 2018 / Accepted: 4 January 2019 / Published: 8 January 2019
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Abstract
We introduce a dark energy-modified minimum length uncertainty relation (DE-MLUR) or dark energy uncertainty principle (DE-UP) for short. The new relation is structurally similar to the MLUR introduced by Károlyházy (1968), and reproduced by Ng and van Dam (1994) using alternative arguments, but [...] Read more.
We introduce a dark energy-modified minimum length uncertainty relation (DE-MLUR) or dark energy uncertainty principle (DE-UP) for short. The new relation is structurally similar to the MLUR introduced by Károlyházy (1968), and reproduced by Ng and van Dam (1994) using alternative arguments, but with a number of important differences. These include a dependence on the de Sitter horizon, which may be expressed in terms of the cosmological constant as l dS 1 / Λ . Applying the DE-UP to both charged and neutral particles, we obtain estimates of two limiting mass scales, expressed in terms of the fundamental constants G , c , , Λ , e . Evaluated numerically, the charged particle limit corresponds to the order of magnitude value of the electron mass ( m e ), while the neutral particle limit is consistent with current experimental bounds on the mass of the electron neutrino ( m ν e ). Possible cosmological consequences of the DE-UP are considered and we note that these lead naturally to a holographic relation between the bulk and the boundary of the Universe. Low and high energy regimes in which dark energy effects may dominate canonical quantum behaviour are identified and the possibility of testing the model using near-future experiments is briefly discussed. Full article
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Open AccessArticle Relieving Tensions Related to the Dark Matter Interpretation of the Fermi-LAT Data
Received: 7 June 2018 / Revised: 17 August 2018 / Accepted: 20 August 2018 / Published: 29 August 2018
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Abstract
Recently, many studies indicate that the GeV gamma ray excess signal from the central Milky Way can be best explained by ∼40–50 GeV dark matter annihilating via the bb¯ channel. However, this model appears to be disfavored by the recent Fermi-LAT [...] Read more.
Recently, many studies indicate that the GeV gamma ray excess signal from the central Milky Way can be best explained by ∼40–50 GeV dark matter annihilating via the b b ¯ channel. However, this model appears to be disfavored by the recent Fermi-LAT data for dwarf spheroidal galaxies and the constraint from synchrotron radiation. In this article, we describe a consistent picture to relieve the tensions between the dark matter annihilation model and the observations. We show that a baryonic feedback process is the key to alleviate the tensions and the ∼40–50 GeV dark matter model is still the best one to account for the GeV gamma ray excess in the Milky Way. Full article
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