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Register a new userObservational Constraints on Undulant Cosmologies
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In an undulant universe, cosmic expansion is characterized by alternating periods of acceleration and deceleration. We examine cosmologies in which the dark-energy equation of state varies periodically with the number of e-foldings of the scale factor of the universe, and use observations to constrain the frequency of oscillation. We find a tension between a forceful response to the cosmic coincidence problem and the standard treatment of structure formation.
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In a class of models designed to solve the cosmological constant problem by coupling scalar or tensor classical fields to the space-time curvature, the universal scale factor grows as a power law in the age, $a propto t^alpha$, regardless of the matter content or cosmological epoch. We investigate constraints on such power-law cosmologies from the present age of the Universe, the magnitude-redshift relation, and from primordial nucleosynthesis. Constraints from the current age of the Universe and from the high-redshift supernovae data require large $alpha$ ($approx 1$), while consistency with the inferred primordial abundances of deuterium and helium-4 forces $alpha$ to lie in a very narrow range around a lower value ($approx 0.55$). Inconsistency between these independent cosmological constraints suggests that such power-law cosmologies are not viable.
We present new observational constraints on the elastic scattering of dark matter with electrons for dark matter masses between 10 keV and 1 TeV. We consider scenarios in which the momentum-transfer cross section has a power-law dependence on the relative particle velocity, with a power-law index $n in {-4,-2,0,2,4,6}$. We search for evidence of dark matter scattering through its suppression of structure formation. Measurements of the cosmic microwave background temperature, polarization, and lensing anisotropy from textit{Planck} 2018 data and of the Milky Way satellite abundance measurements from the Dark Energy Survey and Pan-STARRS1 show no evidence of interactions. We use these data sets to obtain upper limits on the scattering cross section, comparing them with exclusion bounds from electronic recoil data in direct detection experiments. Our results provide the strongest bounds available for dark matter--electron scattering derived from the distribution of matter in the Universe, extending down to sub-MeV dark matter masses, where current direct detection experiments lose sensitivity.
We use data from Supernovae (SNIa) Pantheon sample, from Baryonic Acoustic Oscillations (BAO), and from cosmic chronometers measurements of the Hubble parameter (CC), alongside arguments from Big Bang Nucleosynthesis (BBN), in order to extract constraints on Myrzakulov $F(R,T)$ gravity. This is a connection-based theory belonging to the Riemann-Cartan subclass, that uses a specific but non-special connection, which then leads to extra degrees of freedom. Our analysis shows that both considered models lead to $sim 1 sigma$ compatibility in all cases. For the involved dimensionless parameter we find that it is constrained to an interval around zero, however the corresponding contours are slightly shifted towards positive values. Furthermore, we use the obtained parameter chains so to reconstruct the corresponding Hubble function, as well as the dark-energy equation-of-state parameter, as a function of redshift. As we show, Model 1 is very close to $Lambda$CDM scenario, while Model 2 resembles it at low redshifts, however at earlier times deviations are allowed. Finally, applying the AIC, BIC and the combined DIC criteria, we deduce that both models present a very efficient fitting behavior, and are statistically equivalent with $Lambda$CDM cosmology, despite the fact that Model 2 does not contain the latter as a limit.
Ghost-free bimetric gravity is a theory of two interacting spin-2 fields, one massless and one massive, in addition to the standard matter particles and fields, thereby generalizing Einsteins theory of general relativity. To parameterize the theory, we use five observables with specific physical interpretations. We present, for the first time, observational constraints on these parameters that: (i) apply to the full theory, (ii) are consistent with a working screening mechanism (i.e., restoring general relativity locally), (iii) exhibit a continuous, real-valued background cosmology (without the Higuchi ghost). For the cosmological constraints, we use data sets from the cosmic microwave background, baryon acoustic oscillations, and type Ia supernovae. Bimetric cosmology provides a good fit to data even for large values of the mixing angle between the massless and massive gravitons. Interestingly, the best-fit model is a self-accelerating solution where the accelerated expansion is due to the dynamical massive spin-2 field, without a cosmological constant. Due to the screening mechanism, the models are consistent with local tests of gravity such as solar system tests and gravitational lensing by galaxies. We also comment on the possibility of alleviating the Hubble tension with this theory.
We derive new constraints set by SNIa experiments (`gold data sample of Riess et al.), X-ray galaxy cluster data (Allen et al. Chandra measurements of the X-ray gas mass fraction in 26 clusters), large scale structure (Sloan Digital Sky Survey spectrum) and cosmic microwave background (WMAP) on the quartessence Chaplygin model. We consider both adiabatic perturbations and intrinsic non-adiabatic perturbations such that the effective sound speed vanishes (Silent Chaplygin). We show that for the adiabatic case, only models with equation of state parameter $ |alpha |lesssim 10^{-2}$ are allowed: this means that the allowed models are very close to LambdaCDM. In the Silent case, however, the results are consistent with observations in a much broader range, -0.3<alpha<0.7.
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