The Nature of Dark Energy

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Cosmology".

Deadline for manuscript submissions: 31 July 2024 | Viewed by 1211

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1. Observatório Nacional, Rua General José Cristino 77, São Cristóvão, Rio de Janeiro 20921-400, RJ, Brasil
2. Facultad de Ciencias, Departamento de Matemáticas, Universidad El Bosque, Ak. 9 # 131 A - 2, Bogotá, Colombia
Interests: gravitation; cosmology; particle astrophysics

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Department of Mathematics, Presidency University, 86/1 College Street, Kolkata 700073, India
Interests: theoretical and observational cosmology; dark energy; modified gravity theories; matter creation; massive neutrinos
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Special Issue Information

Dear Colleagues,

At present, dark energy remains one of the great mysteries of the universe that continues to challenge both theoretical and observational cosmologists, from the point of view of more fundamental physics, with key questions arising, e.g., what it is made of? It is well known that the standard model of cosmology has been very successful in explaining different characteristics of the evolution and dynamics of the universe with respect to its very good fit to different types of observational data.  However, it is also well known that this simple but successful model does not answer fundamental theoretical questions related precisely to the fundamental physics of the dark sector, such as: the fine-tuning problem of the cosmological constant and the energy problem of the cosmological and quantum vacuum. From an observational point of view, it is well known that tensions and anomalies are present in the several observational data sets, which have been pointed out in the last decade, such as the rate of expansion and growth of structure in the universe, and also the measurement of the anisotropies of the cosmic background radiation. These highly significant statistical inconsistencies and theoretical problems give us hints of systematic problems in the observational data or entirely new physics beyond the standard model of cosmology and particle physics,  that are totally related to the fundamental character or nature of dark energy.

From a theoretical point of view, we know that the standard model of cosmology is based on three basic assumptions, namely: the validity of general relativity as theory of gravitation, a component of dark energy represented by the cosmological constant and the cosmological principle, which establishes a scale of homogeneity and isotropy in the large-scale structure of the universe. Testing these theoretical foundations is essential and this is how a plethora of cosmological models have emerged, including: modified gravity models, different types of dark energy parameterizations, including interaction models (DM/DE), holographic models of the universe where gravity can be an emergent property and not a fundamental force, among others.

From an observational point of view, it is possible to examine the standard model of particle physics and beyond, making an injection of energy in the early Universe in order to resolve or at least alleviate the aforementioned cosmological tensions, introducing new relativistic degrees of freedom in the form of right-hand and sterile neutrinos, dark bosons, dark radiation and gravitational waves that bring new physics such as: phase transitions, domain walls, inflation fields, primordial black holes, graviton stochastic background, among others. Aligned with this search, it is also important to mention the importance of multi-messenger astronomy and of observational projects that will begin to collect data in the near future the next generation of experiments like LSST, CMB-S4, CORE, e-LISA, EINSTEIN TELESCOPE, DECIGO, EUCLID, LSST, SKA, DUNE, KATRIN, XENON100, among others, will achieve unprecedented precision, which will provide a dramatic leap forward in our understanding of the fundamental nature of space and time, dark energy, baryogenesis/leptogenesis, CP- Problem, among many other loose things.

Finally, it is important to point towards new statistical inference techniques such as machine learning, which has the advantage of, on the one hand, being an independent model and, on the other, being able to handle large amounts of data and high precision in signal processing, which is important to rule out systematics.

Prof. Dr. Alexander Bonilla
Dr. Supriya Pan
Guest Editors

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Keywords

  • modified gravity
  • dark energy parameterizations
  • holographic models and emergent gravity
  • inhomogeneous and anisotropic models
  • injection of energy in the early universe
  • dark bosons, dark radiation and dark matter models
  • machine learning and inference methods
  • new observational facilities

Published Papers (2 papers)

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Research

15 pages, 1443 KiB  
Article
Phantom Scalar Field Cosmologies Constrained by Early Cosmic Measurements
by José Antonio Nájera and Celia Escamilla-Rivera
Universe 2024, 10(6), 232; https://doi.org/10.3390/universe10060232 - 23 May 2024
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Abstract
In this work, we explore new constraints on phantom scalar field cosmologies with a scalar field employing early-time catalogs related to CMB measurements, along with the local standard observables, like Supernovae Type Ia (SNIa), H(z) measurements (Cosmick clocks), and Baryon [...] Read more.
In this work, we explore new constraints on phantom scalar field cosmologies with a scalar field employing early-time catalogs related to CMB measurements, along with the local standard observables, like Supernovae Type Ia (SNIa), H(z) measurements (Cosmick clocks), and Baryon Acoustic Oscillation (BAO) baselines. In particular, we studied a tracker phantom field with hyperbolic polar coordinates that have been proposed in the literature. The main goal is to obtain precise cosmological constraints for H0 and σ8, in comparison to other constructions that present tension in early cosmological parameters. Our results show that phantom scalar field cosmologies have a reduced statistical tension on H0 that it is less than 3σ using model-independent CMB catalogs as SPT-3G+WMAP9 and ACTPol DR-4+WMAP9 baselines. This suggests that these models, using a different phantom potential, might address the Hubble constant problem and reduce the systematics involved. Full article
(This article belongs to the Special Issue The Nature of Dark Energy)
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10 pages, 410 KiB  
Article
Cosmological Test of an Ultraviolet Origin of Dark Energy
by Hans Christiansen, Bence Takács and Steen H. Hansen
Universe 2024, 10(5), 193; https://doi.org/10.3390/universe10050193 - 25 Apr 2024
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Abstract
The accelerated expansion of the Universe is impressively well described by a cosmological constant. However, the observed value of the cosmological constant is much smaller than expected based on quantum field theories. Recent efforts to achieve consistency in these theories have proposed a [...] Read more.
The accelerated expansion of the Universe is impressively well described by a cosmological constant. However, the observed value of the cosmological constant is much smaller than expected based on quantum field theories. Recent efforts to achieve consistency in these theories have proposed a relationship between Dark Energy and the most compact objects, such as black holes (BHs). However, experimental tests are very challenging to devise and perform. In this article, we present a testable model with no cosmological constant in which the accelerated expansion can be driven by black holes. The model couples the expansion of the Universe (the Friedmann equation) with the mass function of cosmological halos (using the Press–Schechter formalism). Through the observed link between halo masses and BH masses, one thus gets a coupling between the expansion rate of the Universe and the BHs. We compare the predictions of this simple BH model with SN1a data and find poor agreement with observations. Our method is sufficiently general to allow us to also test a fundamentally different model, also without a cosmological constant, where the accelerated expansion is driven by a new force proportional to the internal velocity dispersion of galaxies. Surprisingly enough, this model cannot be excluded using the SN1a data. Full article
(This article belongs to the Special Issue The Nature of Dark Energy)
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