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Forconi, M., Giare, W., Mena, O., Ruchika, Di Valentino, E., Melchiorri, A., et al. (2024). A double take on early and interacting dark energy from JWST. J. Cosmol. Astropart. Phys., 05(5), 097–37pp.
Abstract: The very first light captured by the James Webb Space Telescope (JWST) revealed a population of galaxies at very high redshifts more massive than expected in the canonical Lambda CDM model of structure formation. Barring, among others, a systematic origin of the issue, in this paper, we test alternative cosmological perturbation histories. We argue that models with a larger matter component ohm m and/or a larger scalar spectral index n s can substantially improve the fit to JWST measurements. In this regard, phenomenological extensions related to the dark energy sector of the theory are appealing alternatives, with Early Dark Energy emerging as an excellent candidate to explain (at least in part) the unexpected JWST preference for larger stellar mass densities. Conversely, Interacting Dark Energy models, despite producing higher values of matter clustering parameters such as sigma 8 , are generally disfavored by JWST measurements. This is due to the energy -momentum flow from the dark matter to the dark energy sector, implying a smaller matter energy density. Upcoming observations may either strengthen the evidence or falsify some of these appealing phenomenological alternatives to the simplest Lambda CDM picture.
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Dimitriou, A., Figueroa, D. G., & Zaldivar, B. (2024). Fast likelihood-free reconstruction of gravitational wave backgrounds. J. Cosmol. Astropart. Phys., 09(9), 032–51pp.
Abstract: based) techniques for reconstructing the spectral shape of a gravitational wave background (GWB). We focus on the reconstruction of an arbitrarily shaped signal (approximated by a piecewise power-law in many frequency bins) by the LISA detector, but the method can be easily extended to either template-dependent signals, or to other detectors, as long as a characterisation of the instrumental noise is available. As proof of the technique, we quantify the ability of LISA to reconstruct signals of arbitrary spectral shape (blind reconstruction), considering a diversity of frequency profiles, and including astrophysical backgrounds in some cases. As a teaser of how the method can reconstruct signals characterised by a parameter-dependent template (template reconstruction), we present a dedicated study for power-law signals. While our technique has several advantages with respect to traditional MCMC methods, we validate it with the latter for concrete cases. This work opens the door for both fast and accurate Bayesian parameter estimation of GWBs, with essentially no computational overhead during the inference step. Our set of tools are integrated into the package GWBackFinder, which is publicly available in GitHub.
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Figueroa, D. G., & Loayza, N. (2025). Geometric reheating of the Universe. J. Cosmol. Astropart. Phys., 03(3), 073–44pp.
Abstract: We study the post-inflationary energy transfer from the inflaton (phi) into a scalar field (chi) non-minimally coupled to gravity through xi R|chi|2, considering models with inflaton potential Vinf proportional to |phi| p around phi = 0. This corresponds to the paradigm of geometric preheating, which we extend to its non-linear regime via lattice simulations. Considering alpha-attractor T-mo del potentials as a proxy, we study the viability of proper reheating for p = 2, 4, 6, determining whether radiation domination (RD) due to energetic dominance of chi over phi, can be achieved. For large inflationary scales Lambda, reheating is frustrated for p = 2, it can be partially achieved for p = 4, and it becomes very efficient for p = 6. Efficient reheating can be however blocked if chi sustains self-interactions (unless these are extremely feeble), or if Lambda is low enough, so that inflaton fragmentation brings the universe rapidly into RD. Whenever RD is achieved, either due to reheating (into chi) or to inflaton fragmentation, we characterize the energy and time scales of the problem, as a function of Lambda and xi.
Keywords: physics of the early universe; inflation
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Mangano, G., Miele, G., Pastor, S., Pisanti, O., & Sarikas, S. (2012). Updated BBN bounds on the cosmological lepton asymmetry for non-zero theta(13). Phys. Lett. B, 708(1-2), 1–5.
Abstract: We discuss the bounds on the cosmological lepton number from Big Bang Nucleosynthesis (BBN), in light of recent evidences for a large value of the neutrino mixing angle theta(13), sin(2) theta(13) greater than or similar to 0.01 at 2 sigma. The largest asymmetries for electron and mu, tau neutrinos compatible with He-4 and H-2 primordial yields are computed versus the neutrino mass hierarchy and mixing angles. The flavour oscillation dynamics is traced till the beginning of BBN and neutrino distributions after decoupling are numerically computed. The latter contains in general, non-thermal distortion due to the onset of flavour oscillations driven by solar squared mass difference in the temperature range where neutrino scatterings become inefficient to enforce thermodynamical equilibrium. Depending on the value of theta(13), this translates into a larger value for the effective number of neutrinos, N-eff. Upper bounds on this parameter are discussed for both neutrino mass hierarchies. Values for N-eff which are large enough to be detectable by the Planck experiment are found only for the (presently disfavoured) range sin(2) theta(13) <= 0.01.
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