Nonstandard self-interactions can alter the evolution of cosmological neutrinos, mainly by damping free streaming, which should leave traces in cosmological observables. Although overall effects are opposite to those produced by neutrino mass and a larger $N_{\mathrm{eff}}$, they cannot be totally canceled by these last. We harness cosmological data that includes cosmic microwave background from Planck 2018, BAO measurements, local $H_0$, Ly-$\alpha$, and SNIa, to constrain massive neutrino self-interactions with a very light scalar mediator. We find that the effective coupling constant, at the 95% C.L., should be $g_{\mathrm{eff}}<1.94\times {10}^{-7}$ for only Planck 2018 data and $1.97\times {10}^{-7}$ when $\mathrm{Planck}+\mathrm{BAO}$ are considered. This bound relaxes to $2.27\times {10}^{-7}$ ($2.3\times {10}^{-7}$) for $H_0$ ($H_0+\mathrm{SNe}+\mathrm{Ly}$-$\alpha$) data. Using the $\mathrm{Planck}+\mathrm{BAO}$ dataset, the $H_0$ tension lowers from $4.3\sigma$ (for $\mathrm{\Lambda }\mathrm{CDM}$) to $3.2\sigma$. The Akaike information criterion penalizes the self-interacting model due to its larger parameter space for Planck or $\mathrm{Planck}+\mathrm{BAO}$ data, but favors the interacting model when we use local $H_0$ measurements. A somewhat larger value for $H_0$ is preferred when we include the whole data pool, which comes accompanied with a larger value of $N_{\mathrm{eff}}$ and a more constricted bound on $\mathrm{\Sigma }{m}_{\nu }$.

In this work, we study the previously unexplored resonant region of neutrino self-interactions. Current disagreement on late and early time observations of the Universe expansion could be solved with new physics acting before the recombination era. Nonstandard neutrino self-interactions are among the most appealing candidates to solve this issue since they could be testable in the near (or midterm) future. We use linear cosmological datasets to test neutrino self-interactions for a sample of fixed scalar mediator masses in the range ${10}^{-2}\mathrm{eV}\le m_{\phi }\le {10}^2\mathrm{eV}$. The resonant behavior produces observable effects at lower couplings than those reported in the literature for heavy and light mediators. We observe that in the best case scenario, using the $\mathrm{Planck}+\text{BAO}$ dataset, the tension with local measurements of $H_0$ eases from $4.9\sigma$ (for $\mathrm{\Lambda }\mathrm{CDM}$) down to $2.8\sigma$. Albeit, this is driven mainly by the addition of extra radiation, with $\mathrm{\Delta }{N}_{\mathrm{eff}}\sim 0.5$. The joint dataset which includes Planck, BAO, and $H_0$ prefers a nonzero interaction from $2.3\sigma$ to $3.9\sigma$ significance in the range $0.5\mathrm{eV}\le m_{\phi }\le 10\mathrm{eV}$. Although, this last result is obtained with data that are still in tension. These results add the last piece in the parameter space of neutrino self-interactions at the linear perturbation regime.

The main aim of this paper is to present the multiscalar field components as candidates for the dark energy of the Universe and their observational constraints. We start with the canonical quintessence and phantom fields with quadratic potentials and show that a more complex model should satisfy current cosmological observations. Then, we present some implications for a combination of two fields, called quintom models. We consider two types of models: one as the sum of the quintessence and phantom potentials, and one including an interacting term between fields. We find that adding one extra degree of freedom, via the interacting term, enriches the dynamics considerably and could lead to an improvement in the fit of $-2\ln \mathrm{\Delta }{\mathcal{L}}_{\max }=5.19$ compared to $\mathrm{\Lambda }\mathrm{CDM}$. The resultant effective equation of state is now able to cross the phantom divide line and in several cases presents an oscillatory or discontinuous behavior, depending on the interaction value. The parameter constraints of the scalar field models (quintessence, phantom, quintom, and interacting quintom) are performed using cosmic chronometer, type Ia supernovae, and baryon acoustic oscillation data, and the log-Bayes factors are computed to compare the performance of the models. We show that single scalar fields may face serious trouble and hence more complex models, i.e., multiple fields are necessary.

The main aim of this work is to explore the possibility that cold dark matter (CDM) and early dark energy (EDE) can be described by canonical scalar fields that are coupled at the level of its conservation equations. The formalism covers dynamical aspects at the background and linear perturbation levels for an arbitrary coupling function, followed by an example of it. We emphasize the impact of this model on the matter power spectrum and the cosmic microwave background (CMB) spectra, with or without direct interaction. Our findings indicate that the presence of a scalar field can partially counteract the known effects of the other, opening the possibility to avoid some undesired aspects, such as the increase in ${\mathrm{\Omega }}_m$ that usually is needed in the case of a purely EDE scalar field scenario, in order to fit the CMB spectra. This opens up the possibility to analyzing whether the interaction can help to ameliorate the cosmological tensions.