https://www.um.edu.mt/library/oar/handle/123456789/404 2026-09-30T14:24:29Z https://www.um.edu.mt/library/oar/handle/123456789/149558 Title: Endowing Λ with a dynamic nature : constraints in a spatially curved universe Authors: Farrugia, Christine R.; Sultana, Joseph; Mifsud, Jurgen Abstract: In this study, we consider three dark energy models in which Λ is not constant, but has a dynamic nature that depends on the Hubble parameter 𝐻 and/or its time derivative˙𝐻. We analyze the generalized running vacuum model, for which Λ⁡(𝐻) =𝐴 +𝐵⁢𝐻2 +𝐶⁢˙𝐻, along with the two models obtained by setting 𝐵 or 𝐶 equal to zero. A null value for 𝐶 yields the classical running vacuum model (RVM), while 𝐵 =0 corresponds to what we term the generalized running vacuum subcase, or GRVS. Our main aim is to investigate whether these models can accommodate nonzero spatial curvature. To this end, we carry out a Markov chain Monte Carlo analysis using data for the observables associated with type-Ia supernovae, cosmic chronometers, the cosmic microwave background, and baryon acoustic oscillations, as well as two values for the Hubble constant. Then we include data relating to the growth of large-scale structure (LSS) and repeat the procedure. Our results indicate that taking LSS observations into account helps to tighten constraints and determine a definite sign for the model parameters. In the case of the RVM and GRVS, the addition of growth data result in dynamical vacuum energy being preferred to a cosmological constant at a little over 1⁢𝜎. This happens in both the flat and nonflat scenarios—there are only a few exceptions—but comes at the cost of an extra parameter, which can degrade the performance of the models (as assessed by model selection criteria). Of special relevance is the fact that the inclusion of LSS data appear to increase compatibility with a flat geometry. It also brings the constraints on the Hubble constant closer to the range of values established by Planck. 2020-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149557 Title: Spatial curvature in f(R) gravity Authors: Farrugia, Christine R.; Sultana, Joseph; Mifsud, Jurgen Abstract: In this work, we consider four 𝑓⁡(𝑅) gravity models—the Hu-Sawicki, Starobinsky, Exponential and Tsujikawa models—and use a range of cosmological data, together with Markov Chain Monte Carlo sampling techniques, to constrain the associated model parameters. Our main aim is to compare the results we get when Ω𝑘,0 is treated as a free parameter with their counterparts in a spatially flat scenario. The bounds we obtain for Ω𝑘,0 in the former case are compatible with a flat geometry. It appears, however, that a higher value of the Hubble constant 𝐻0 allows for more curvature. Indeed, upon including in our analysis a Gaussian likelihood constructed from the local measurement of 𝐻0, we find that the results favor an open universe at a little over 1⁢𝜎. This is perhaps not statistically significant, but it underlines the important implications of the Hubble tension for the assumptions commonly made about spatial curvature. We note that the late-time deviation of the Hubble parameter from its Λ⁢CDM equivalent is comparable across all four models, especially in the nonflat case. When Ω𝑘,0 =0, the Hu-Sawicki model admits a smaller mean value for Ωcdm,0⁢ℎ2, which increases the said deviation at redshifts higher than unity. We also study the effect of a change in scale by evaluating the growth rate at two different wave numbers 𝑘†. Any changes are, on the whole, negligible, although a smaller 𝑘† does result in a slightly larger average value for the deviation parameter 𝑏. 2021-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149556 Title: Gravitational analog of the canonical acoustic black hole in Einstein–scalar–Gauss–Bonnet theory Authors: Cañate, Pedro; Sultana, Joseph; Kazanas, Demosthenes Abstract: In this work, in the context of modified gravity, a curved spacetime analogous to the ‘canonical acoustic black hole (CABH)’ is constructed. The source is a self-interacting scalar field which is non-minimally coupled to gravity through the Gauss–Bonnet invariant. The scalar-Gauss–Bonnet coupling function is characterized by three positive parameters: σ with units of (length), μ with units of (length)4, and a dimensionless parameter s, thus defining a three-parameter model for which the line element of CABH is a solution. The spacetime is equipped with spherical and static symmetry and has a single horizon determined in Schwarzschild coordinates by the region r = μ1/4. The solution admits a photon sphere at r = (3μ)1/4, and it is shown that in the region (3μ)1/4 ⩽ r < ∞ the scalar field satisfies the null, weak, and strong energy conditions. Nonetheless, the model with s = 1 has major physical relevance since for this case the scalar field is well defined in the entire region r ⩾ μ1/4, while for s ≠ 1 the scalar field blows up on the horizon. 2021-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149555 Title: Constraining f(R) models with cosmic chronometers and the HII galaxy Hubble diagram Authors: Sultana, Joseph; Yennapureddy, Manoj K.; Melia, Fulvio; Kazanas, Demosthenes Abstract: We consider several well-known f(R) cosmological models and constrain their parameters, namely the deviation parameter b and the cosmological parameters Ωm and h. We first obtain analytical approximations for the Hubble rate H(z) and the luminosity distance dL(z) in terms of these parameters, and then test these against the observational expansion rate derived from cosmic chronometers (CCs) and the distance modulus in the H ii galaxy Hubble diagram, obtained in a model-independent way using Gaussian processes. We first optimize the models based solely on the CCs and then repeat this process with a joint analysis using both the CCs and H ii galaxies. 2022-01-01T00:00:00Z