Phenomenological Dark Energy Model Research Articles

A phenomenological approach to the dark energy models in the Finsler–Randers framework

This study extends two phenomenological models of dark energy within the framework of Finsler–Randers space–time, accommodating anisotropies. The models consider the cosmological constant Λ in two scenarios: one where Λ is proportional to the second time derivative of the scale factor ä, and another where it varies with the matter-energy density ρ. Earlier, such Λ-decaying cosmologies were proposed to address long-standing cosmological constant problems. However, following the discovery of late-time cosmic acceleration, the focus shifted to modeling dark energy. Since Λ is widely viewed as the most significant and suitable candidate for driving cosmic acceleration, it is worthwhile to revisit the phenomenological approach in this context. This work uses this approach to find solutions for the scale factor a(t) and other geometrical and physical parameters. Additionally, we analyze the evolution of density parameters Ωm, ΩΛ, and Ωκ, representing matter, dark energy, and curvature, from early to late times. The phenomenological approach is employed to solve the field equations, with model parameters constrained using recent observational data, yielding ranges consistent with observations. The solutions converge to the ΛCDM cosmology at early and late times. The added complexity introduced by Finsler–Randers geometry enhances accuracy compared to analogous solutions in Riemannian space–time.

Phenomenological dark energy model with hybrid dynamic cosmological constant

We investigate dark energy (DE) by associating it with vacuum energy or cosmological constant Λ which is taken to be dynamic in nature. Our approach is phenomenological and falls within the domain of variable-Λ cosmology. However, motivated by quantum theory of metastable vacuum decay, we proposed a new phenomenological decay law of Λ(t) where Λ(t) is a superposition of constant and variable components viz Λ(t) = ΛC + Λv which is indicated by the word ‘hybrid dynamic’ in the title. By taking a simplified two-fluid scenario with the Universe consisting of DE and another major component, we found the solutions for three particular phenomenological expressions and made a parametrization of the model in terms of dilution parameter (the dilution parameter has been defined in the text as the exponent of scale factor in the expression of density of the other major component, representing the dilution of the component with the expansion of Universe in the presence of dynamic DE). For pressureless dust and dynamic DE Universe, we found the present day matter density (Ω m0) and dilution parameter (u) to be Ω m0 = 0.29 ± 0.03, u = 2.90 ± 0.54 at 1σ by analysing 580 supernova from Union 2.1 catalogue. The physical features of the model in regard to scale factor evolution, deceleration parameter, cosmic age has also been studied and parallels have been drawn with ΛCDM model. The status of cosmological problems in the model has also been checked which showed that the model solves the cosmological constant problem but the coincidence problem still exists in the model.

Generalized Phenomenological Models of Dark Energy

It was first observed at the end of the last century that the universe is presently accelerating. Ever since, there have been several attempts to explain this observation theoretically. There are two possible approaches. The more conventional one is to modify the matter part of the Einstein field equations, and the second one is to modify the geometry part. We shall consider two phenomenological models based on the former, more conventional approach within the context of general relativity. The phenomenological models in this paper consider a Λ term firstly a function of a¨/a and secondly a function of ρ, where a and ρ are the scale factor and matter energy density, respectively. Constraining the free parameters of the models with the latest observational data gives satisfactory values of parameters as considered by us initially. Without any field theoretic interpretation, we explain the recent observations with a dynamical cosmological constant.

A Simple Phenomenological Emergent Dark Energy Model can Resolve the Hubble Tension

Abstract Motivated by the current status of cosmological observations and significant tensions in the estimated values of some key parameters assuming the standard ΛCDM model, we propose a simple but radical phenomenological emergent dark energy model where dark energy has no effective presence in the past and emerges at later times. Theoretically, in this phenomenological dark energy model with zero degrees of freedom (similar to a ΛCDM model), one can derive that the equation of state of dark energy increases from in the past to −1 in the future. We show that by setting a hard-cut 2σ lower bound prior for H 0 associated with a 97.72% probability from recent local observations, this model can satisfy different combinations of cosmological observations at low and high redshifts (SNe Ia, baryon acoustic oscillation (BAO), Lyα BAO, and cosmic microwave background (CMB)) substantially better than the concordance ΛCDM model with and Δ DIC ∼ −35.38. If there are no substantial systematics in SN Ia, BAO, or Planck CMB data, and assuming the reliability of current local H 0 measurements, there is a very high probability that with more precise measurements of the Hubble constant our proposed phenomenological model rules out the cosmological constant with decisive statistical significance and is a strong alternative to explain the combination of different cosmological observations. This simple phenomenologically emergent dark energy model can guide theoretically motivated dark energy model building activities.

$\Lambda$CDM-like models with future singularities

We consider new models of dark energy with finite time future singularities, by introducing the pressure density as a function of the scale factor. This approach gives acceptable phenomenological models of dark energy, that resemble that of the cosmological constant up to the present, which face future singularities at finite time and finite scale factor. Exact scalar field model representation was found for quintessence, Big Rip and type III singularity models. The simple form of the equation of state allows to establish a relationship between its current value, w0, and the time or redshift at which the singularity takes place. The effect on the growth of matter perturbations is analyzed.

Exacerbating the Cosmological Constant Problem with Interacting Dark Energy Models

Future cosmological surveys will probe the expansion history of the Universe and constrain phenomenological models of dark energy. Such models do not address the fine-tuning problem of the vacuum energy, i.e., the cosmological constant problem (CCP), but can make it spectacularly worse. We show that this is the case for "interacting dark energy" models in which the masses of the dark matter states depend on the dark energy sector. If realized in nature, these models have far-reaching implications for proposed solutions to the CCP that require the number of vacua to exceed the fine-tuning of the vacuum energy density. We show that current estimates of the number of flux vacua in string theory, N_{vac}∼O(10^{272 000}), are far too small to realize certain simple models of interacting dark energy and solve the cosmological constant problem anthropically. These models admit distinctive observational signatures that can be targeted by future gamma-ray observatories, hence making it possible to observationally rule out the anthropic solution to the cosmological constant problem in theories with a finite number of vacua.

Phenomenological models of dark energy interacting with dark matter

An interaction between dark matter and dark energy is usually introduced by a phenomenological modification of the matter conservation equations, while the Einstein equations are left unchanged. Starting from some general and fundamental considerations, in this work it is shown that a coupling in the dark sector is likely to introduce new terms also in the gravitational dynamics. Specifically in the cosmological background equations a bulk dissipative pressure, characterizing viscous effects and able to suppress structure formation at small scales, should appear from the dark coupling. At the level of the perturbations the analysis presented in this work reveals instead the difficulties in properly defining the dark sector interaction from a phenomenological perspective.

Generalized ghost dark energy in Brans–Dicke theory

It was argued that the vacuum energy of the Veneziano ghost field of QCD, in a time-dependent background, can be written in the general form, H + O(H2), where H is the Hubble parameter. Based on this, a phenomenological dark energy model whose energy density is of the form ρ = αH + βH2 was recently proposed to explain the dark energy dominated universe. In this paper, we investigate this generalized ghost dark energy model in the setup of Brans–Dicke cosmology. We study the cosmological implications of this model. In particular, we obtain the equation of state, the deceleration parameters, and a differential equation governing the evolution of this dark energy model. It is shown that the equation of state parameter of the generalized ghost dark energy can cross the phantom line (wD = −1) in some range of the parameters spaces.