arXiv subjects

Olga Mena

Publications and source records attributed to Olga Mena.

At least 19 recordsLinked to original sources

arXiv2026

Late-Time Emergence of Dark Energy and Its Interaction with Dark Matter

We present an interacting scenario between dark energy (DE) and dark matter (DM), where DE has an emergent nature, that means, DE was absent in the early universe but it becomes effective only at late times. We consider two specific emergent DE models, one with no free parameters and the other featuring two parameters describing the speed and epoch of the transition. We constrain both scenarios using the cosmic microwave background (CMB) measurements from the Planck 2018 release, baryon acoustic oscillations from DESI DR2, and three different compilations of Type Ia supernovae (PantheonPlus, DES-Dovekie, and Union3). Our analysis indicates that current cosmological probes are not able to tightly constrain the speed of the transition. For both scenarios, the posterior distribution of the interaction parameter is shifted away from zero at more than 95\% CL whenever the CMB data are combined with any of these additional probes, with the preferred direction corresponding to a transfer of energy from DE to DM. While CMB alone yields a high value of $H_0$, in agreement with local determinations, this effect is reduced when DESI is added and disappears once supernova data are included. In contrast, the clustering parameter $S_8$ is consistently shifted toward lower values in the combined datasets, and it is correlated with the preference for a negative interaction. However, according to the $\Delta \chi^2_{\rm min}$ and Bayesian evidence, none of the interacting models is favored over $\Lambda$CDM or $w_0w_a$CDM, indicating that the interaction does not rescue these emergent DE models. Our results therefore highlight the limitations of these scenarios in addressing current cosmological tensions, while pointing to the crucial role of future data in further assessing their viability.

astro-ph.CO↗

arXiv2026

Probing the neutrino chemical potential with cosmological observations

The electron neutrino degeneracy parameter, $\xi_{\nu_\mathrm{e}} = \mu_{\nu_\mathrm{e}} / T$, is tightly constrained by Big Bang Nucleosynthesis (BBN), while the degeneracy parameters of the other neutrino species, $\xi_{\nu_\mathrm{x}}$, remain weakly constrained by cosmological observations alone. In this manuscript we shall compute up-to-date bounds on $\xi_{\nu_\mathrm{e}}$ and $\xi_{\nu_\mathrm{x}}$ assuming that either they are constant free-parameters along the cosmic history or that they are redshift dependent quantities. In the latter case we employ a model-independent reconstruction approach based on the Piecewise Cubic Hermite Interpolating Polynomial (PCHIP) formalism with four nodes, located at $z\simeq$ 10, 100, 1000 and $10^8$. We shall also consider two scenarios for neutrinos, specifically three degenerate neutrinos ($\xi_{\nu_\mathrm{e}}$ = $\xi_{\nu_\mathrm{x}}$) and the case in which we actually differentiate between $\xi_{\nu_\mathrm{e}}$ and $\xi_{\nu_\mathrm{x}}$. We perform a cosmological analysis combining CMB data from Planck, SPT, and ACT with BAO measurements from DESI, showing the impact of including BBN observables from either EMPRESS results, which allow for a non-zero chemical potential, or from LBT observations, compatible with the standard $\xi_\nu$ = 0 prediction. We explicitly show that the BBN data, via the change in neutron-to-proton interconversion rates, mostly constrain $\xi_{\nu_\mathrm{e}}$, parameter for which we observe a preferred non-zero positive value at $95\%$ C.L. in the non-degenerate neutrino case at the BBN period. Since the Hubble constant is correlated with $\xi_{\nu}$, through $N_{\rm eff}$, a larger value of $H_0$ is allowed within these models, making them really interesting scenarios where to test non-standard physics models.

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arXiv2025

Do low-redshift observations open the doors to an open universe?

The detection of a significant deviation from a zero curvature would have profound consequences for inflationary theories and fundamental physics. Relative to high-redshift Planck's CMB measurements, indicating a $\sim 2\sigma$ evidence for a closed universe, low-redshift observations of BAO and SN Ia have the advantages of weak dependence on early universe physics, independently observational systematics, and strong redshift dependence of distances in constraining the cosmic curvature. Using the integrated observations from DESI BAO and SN Ia, we find an unexpected $2\sigma$ evidence for an open universe, regardless of the SN Ia sample employed. When considering DESI, SN Ia and the acoustic scale $\theta_\star$ data, the preference for an open universe exceeds the $3\sigma$ level, reaching $5\sigma$ for the case of DESY5 Supernovae data. Therefore, low-redshift observations favor an open universe, and this preference persists even when alternative high-redshift priors are adopted. Our results point to the existence of an additional tension between high- and low-redshift observations, present also in non-flat models beyond the minimal $\Lambda$CDM scheme, thereby challenging the standard inflationary predictions.

astro-ph.CO↗

arXiv2025

Dark energy and neutrinos along the cosmic expansion history

Recent cosmological measurements are hinting that dark energy may evolve, with its equation of state, $w_\mathrm{DE}$, even showing oscillatory patterns. In this work, we employ a model-independent approach to jointly reconstruct $w_\mathrm{DE}$ and the sum of neutrino masses, $\sum m_\nu$, adopting the PCHIP method with seven fixed nodes in which we allow the two parameters to vary. We employ CMB, Baryon Acoustic Oscillations and Supernovae Ia data to constrain the values of $w_\mathrm{DE}$ and $\sum m_\nu$ at each node. We conduct three different analyses in which we reconstruct $w_\mathrm{DE}$: one with fixed $\sum m_\nu=0.06~\mathrm{eV}$; one in which we allow $\sum m_\nu$ to vary, and one in which we also reconstruct $\sum m_\nu$ using the PCHIP method. We find the dark energy equation of state to be consistent with the cosmological constant scenario, except when including DESI data and allowing for phantom crossing, where we find a $95\%$ CL deviation from $w_\mathrm{DE}=-1$ around $z\sim1.2$. For neutrino masses, we obtain looser constraints when focusing on phantom dark energy, that show further early and late relaxation when reconstructing the mass via the PCHIP method.

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arXiv2025

Dark Energy Is Not That Into You: Variable Couplings after DESI DR2 BAO

In interacting dark energy (DE) and dark matter (DM) scenarios, the interaction function typically includes a coupling parameter $\xi$ that quantifies the strength of energy exchange between the dark sectors. While $\xi$ is often assumed to be constant, there is no fundamental reason to exclude a time-dependent coupling, which could provide a more general and realistic description of dark sector dynamics. In this work, we study two widely used interacting models involving pressureless DM and DE, where the coupling parameter is allowed to vary with the scale factor $a$. Specifically, we consider two parametrizations: $\xi(a) = \xi_0 + \xi_a (1-a)$ and $\xi(a) = \xi_0 \left(1 + \frac{1-a}{a^2 + (1-a)^2} \right)$, and constrain them using the latest cosmological observations, including Planck 2018 CMB data, DESI DR2 BAO measurements, and multiple Type Ia supernovae samples. Our results show that one scenario yields evidence for a non-zero interaction at more than 95\% confidence level, while the remaining cases indicate at most mild or inconclusive signs of interaction. These findings highlight the potential of variable coupling models and the importance of continued investigation into the nature of the dark sectors.

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arXiv2025

Neutrino mass tension or suppressed growth rate of matter perturbations?

Assuming a minimal $\Lambda$CDM cosmology with three massive neutrinos, the joint analysis of Planck cosmic microwave background data, DESI baryon acoustic oscillations, and distance moduli measurements of Type Ia supernovae from the Pantheon+ sample sets an upper bound on the total neutrino mass, $\sum m_\nu \lesssim 0.06$-$0.07$ eV, that lies barely above the lower limit from oscillation experiments. These constraints are mainly driven by mild differences in the inferred values of the matter density parameter across different probes that can be alleviated by introducing additional background-level degrees of freedom (e.g., by dynamical dark energy models). However, in this work we explore an alternative possibility. Since both $\Omega_\mathrm{m}$ and massive neutrinos critically influence the growth of cosmic structures, we test whether the neutrino mass tension may originate from the way matter clusters, rather than from a breakdown of the $\Lambda$CDM expansion history. To this end, we introduce the growth index $\gamma$, which characterizes the rate at which matter perturbations grow. Deviations from the standard $\Lambda$CDM value ($\gamma \simeq 0.55$) can capture a broad class of models, including non-minimal dark sector physics and modified gravity. We show that allowing $\gamma$ to vary significantly relaxes the neutrino mass bounds to $\sum m_\nu \lesssim 0.13$-$0.2$ eV, removing any tension with terrestrial constraints without altering the inferred value of $\Omega_\mathrm{m}$. However, this comes at the cost of departing from standard growth predictions: to have $\sum m_\nu \gtrsim 0.06$ eV one needs $\gamma > 0.55$, and we find a consistent preference for $\gamma > 0.55$ at the level of $\sim 2\sigma$. This preference increases to $\sim 2.5$-$3\sigma$ when a physically motivated prior $\sum m_\nu \ge 0.06$ eV from oscillation experiments is imposed.

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arXiv2025

Probing the cold nature of dark matter

A pressureless dark matter component fits well with several cosmological observations. However, there are indications that cold dark matter may encounter challenges in explaining observations at small scales, particularly at galactic scales. Observational data suggest that dark matter models incorporating a pressure component could provide solutions to these small-scale problems. In this work, we investigate the possibility that present-day dark matter may result from a decaying non-cold dark matter sector transitioning into the dark energy sector. As the sensitivity of astronomical surveys rapidly increases, we explore an interacting scenario between dark energy and non-cold dark matter, where dark energy has a constant equation of state ($w_{\rm de}$), and dark matter, being non-cold, also has a constant (non-zero) equation of state ($w_{\rm dm}$). Considering the phantom and quintessence nature of dark energy, characterized by its equation of state, we separately analyze interacting phantom and interacting quintessence scenarios. We constrain these scenarios using Cosmic Microwave Background (CMB) measurements and their combination with external probes, such as DESI-BAO and PantheonPlus. From our analyses, we find that a very mild preference for non-cold dark matter cannot be excluded based on the employed datasets. Additionally, for some datasets, there is a pronounced preference for the presence of an interaction at more than 95\% confidence level (CL). Moreover, when the dark energy equation of state lies in the phantom regime, the $S_8$ tension can be alleviated. This study suggests that cosmological models incorporating a non-cold dark matter component should be considered as viable scenarios with novel phenomenological implications, as reflected in the present work.

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arXiv2025

Scale and redshift dependent limits on cosmic neutrino properties

Cosmological neutrino mass and abundance measurements are reaching unprecedented precision. Testing their stability versus redshift and scale is a crucial issue, as it can serve as a guide for optimizing ongoing and future searches. Here, we perform such analyses, considering a number of redshift, scale, and redshift-and-scale nodes. Concerning the $k$-space analysis of $\sum m_\nu$, CMB observations are crucial, as they lead the neutrino mass constraints. Interestingly, some data combinations suggest a non-zero value for the neutrino mass with $2\sigma$ significance. The most constraining bound we find is $\sum m_\nu<0.54$ eV at $95\%$ CL in the $[10^{-3}, 10^{-2}]$ $h$/Mpc $k$-bin, a limit that barely depends on the data combination. Regarding the redshift- and scale-dependent neutrino mass constraints, high redshifts ($z>100$) and scales in the range $[10^{-3}, 10^{-1}]$ $h$/Mpc provide the best constraints. The least constraining bounds are obtained at very low redshifts $[0,0.5]$ and also at very small scales ($k>0.1\, h$/Mpc), due to the absence of observations. Highly relevant is the case of the $[100, 1100]$, $[10^{-2}, 10^{-1}]$ $h$/Mpc redshift-scale bin, where a $2$-$3\sigma$ evidence for a non-zero neutrino mass is obtained for all data combinations. The bound from CMB alone at $68\%$ CL is $0.63^{+0.20}_{-0.24}$ eV, and the one for the full dataset is $0.56^{+0.20}_{-0.23}$ eV, clearly suggesting a non-zero neutrino mass at these scales, possibly related to a deviation of the ISW amplitude in this redshift range. Concerning the analysis of $N_{\rm eff}$ in the $k$-space, at intermediate scales ranging from $k=10^{-3}$ $h$/Mpc to $k=10^{-1}$ $h$/Mpc, accurate CMB data provide very strong bounds, the most robust one being $N_{\rm eff}=3.09\pm 0.14$, comparable to the standard expected value without a $k$-bin analysis. [abridged]

astro-ph.CO↗

arXiv2024

Origin of cosmological neutrino mass bounds: background $\textit{versus}$ perturbations

The cosmological upper bound on the total neutrino mass is the dominant limit on this fundamental parameter. Recent observations-soon to be improved-have strongly tightened it, approaching the lower limit set by oscillation data. Understanding its physical origin, robustness, and model-independence becomes pressing. Here, we explicitly separate for the first time the two distinct cosmological neutrino-mass effects: the impact on background evolution, related to the energy in neutrino masses; and the "kinematic" impact on perturbations, related to neutrino free-streaming. We scrutinize how they affect CMB anisotropies, introducing two effective masses enclosing $\textit{background}$ ($\sum m_\nu^\mathrm{Backg.}$) and $\textit{perturbations}$ ($\sum m_\nu^\mathrm{Pert.}$) effects. We analyze CMB data, finding that the neutrino-mass bound is mostly a background measurement, i.e., how the neutrino energy density evolves with time. The bound on the "kinematic" variable $\sum m_\nu^\mathrm{Pert.}$ is largely relaxed, $\sum m_\nu^\mathrm{Pert.} < 0.8\,\mathrm{eV}$. This work thus adds clarity to the physical origin of the cosmological neutrino-mass bound, which is mostly a measurement of the neutrino equation of state, providing also hints to evade such a bound.

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arXiv2024

Redshift Space Distortions corner interacting Dark Energy

Despite the fact that the $Λ$CDM model has been highly successful over the last few decades in providing an accurate fit to a broad range of cosmological and astrophysical observations, different intriguing tensions and anomalies emerged at various statistical levels. Given the fact that the dark energy and the dark matter sectors remain unexplored, the answer to some of the tensions may rely on modifications of these two dark sectors. This manuscript explores the important role of the growth of structure in constraining non-standard cosmologies. In particular, we focus on the interacting dark energy (IDE) scenario, where dark matter and dark energy interact non-gravitationally. We aim to place constraints on the phenomenological parameters of these alternative models, by considering different datasets related to a number of cosmological measurements, to achieve a complementary analysis. A special emphasis is devoted to redshift space distortion measurements (RSD), whose role in constraining beyond the standard paradigm models has not been recently highlighted. These observations indeed have a strong constraining power, rendering all parameters to their $Λ$CDM canonical values, and therefore leaving little room for the IDE models explored here.

astro-ph.CO↗

arXiv2024

Earth tomography with supernova neutrinos at future neutrino detectors

Earth neutrino tomography is a realistic possibility with current and future neutrino detectors, complementary to geophysics methods. The two main approaches are based on either partial absorption of the neutrino flux as it propagates through the Earth (at energies about a few TeV) or on coherent Earth matter effects affecting the neutrino oscillations pattern (at energies below a few tens of GeV). In this work, we consider the latter approach focusing on supernova neutrinos with tens of MeV. Whereas at GeV energies, Earth matter effects are driven by the atmospheric mass-squared difference, at energies below $\sim 100$ MeV, it is the solar mass-squared difference what controls them. Unlike solar neutrinos, which suffer from significant weakening of the contribution to the oscillatory effect from remote structures due to the neutrino energy reconstruction capabilities of detectors, supernova neutrinos can have higher energies and thus, can better probe the Earth's interior. We shall revisit this possibility, using the most recent neutrino oscillation parameters and up-to-date supernova neutrino spectra. The capabilities of future neutrino detectors, such as DUNE, Hyper-Kamiokande and JUNO are presented, including the impact of the energy resolution and other factors. Assuming a supernova burst at 10~kpc, we show that the average Earth's core density could be determined within $\lesssim 10\%$ at $1σ$ confidence level, being Hyper-Kamiokande, with its largest mass, the most promising detector to achieve this goal.

hep-ph↗

arXiv2024

Neutrino cosmology after DESI: tightest mass upper limits, preference for the normal ordering, and tension with terrestrial observations

The recent DESI Baryon Acoustic Oscillation measurements have led to tight upper limits on the neutrino mass sum, potentially in tension with oscillation constraints requiring $\sum m_{\nu} \gtrsim 0.06\,{\text{eV}}$. Under the physically motivated assumption of positive $\sum m_{\nu}$, we study the extent to which these limits are tightened by adding other available cosmological probes, and robustly quantify the preference for the normal mass ordering over the inverted one, as well as the tension between cosmological and terrestrial data. Combining DESI data with Cosmic Microwave Background measurements and several late-time background probes, the tightest $2\sigma$ limit we find without including a local $H_0$ prior is $\sum m_{\nu}<0.05\,{\text{eV}}$. This leads to a strong preference for the normal ordering, with Bayes factor relative to the inverted one of $46.5$. Depending on the dataset combination and tension metric adopted, we quantify the tension between cosmological and terrestrial observations as ranging between $2.5\sigma$ and $5\sigma$. These results are strenghtened when allowing for a time-varying dark energy component with equation of state lying in the physically motivated non-phantom regime, $w(z) \geq -1$, highlighting an interesting synergy between the nature of dark energy and laboratory probes of the mass ordering. If these tensions persist and cannot be attributed to systematics, either or both standard neutrino (particle) physics or the underlying cosmological model will have to be questioned.

astro-ph.CO↗

arXiv2024

Shedding light on the $Δm^2_{21}$ tension with supernova neutrinos

One long-standing tension in the determination of neutrino parameters is the mismatched value of the solar mass square difference, $Δm_{21}^2$, measured by different experiments: the reactor antineutrino experiment KamLAND finds a best fit larger than the one obtained with solar neutrino data. Even if the current tension is mild ($\sim 1.5σ$), it is timely to explore if independent measurements could help in either closing or reassessing this issue. In this regard, we explore how a future supernova burst in our galaxy could be used to determine $Δm_{21}^2$ at the future Hyper-Kamiokande detector, and how this could contribute to the current situation. We study Earth matter effects for different models of supernova neutrino spectra and supernova orientations. We find that, if supernova neutrino data prefers the KamLAND best fit for $Δm_{21}^2$, an uncertainty similar to the current KamLAND one could be achieved. On the contrary, if it prefers the solar neutrino data best fit, the current tension with KamLAND results could grow to a significance larger than $5σ$. Furthermore, supernova neutrinos could significantly contribute to reducing the uncertainty on $\sin^2θ_{12}$.

hep-ph↗

arXiv2024

A double take on early and interacting dark energy from JWST

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 $Λ$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 $Ω_m$ 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 $σ_8$, 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 $Λ$CDM picture.

astro-ph.CO↗

arXiv2024

Updating neutrino mass constraints with Background measurements

Low-redshift probes, such as Baryon Acoustic Oscillations (BAO) and Supernovae Ia luminosity distances, have been shown to be crucial for improving the bounds on the total neutrino mass from cosmological observations, due to their ability to break degeneracies among the different parameters. Here, we expand background observations to include $H(z)$ measurements from cosmic chronometers, distance moduli from Gamma Ray Bursts (GRBs), and angular diameter distances from galaxy clusters. For the very first time, we find neutrino mass limits below the minimal expectations from neutrino oscillation probes, suggesting non-standard neutrino and/or cosmological scenarios. The tightening of the neutrino mass bound is due to the slightly higher value of the Hubble constant $H_0$ preferred by the former three background probes, and also due to the improved errors on $H_0$ and the matter mass-energy density $\Omega_{\rm m}$. All values of $H_0$ are however in agreement at the $1-2\sigma$ level. Interestingly, it is not only the combination of the three background probes that is responsible for the $\sum m_\nu <0.06$~eV limits, but also each of them independently. The tightest bound we find here is $\sum m_\nu<0.043$~eV at $2\sigma$ after combining Cosmic Microwave Background Planck data with DESI BAO, Supernovae Ia, GRBs, cosmic chronometers, and galaxy clusters, showing a clear tension between neutrino oscillation results and cosmological analyses. In general, removing either one of the two background probes still provides a limit $\sum m_\nu \lesssim 0.06$~eV, reassuring the enormous potential of these low-redshift observations in constraining the neutrino mass.

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arXiv2024

Neutrinos in Cosmology

Neutrinos are the least known particle in the Standard Model of elementary particle physics. They play a crucial role in cosmology, governing the universe's evolution and shaping the large-scale structures we observe today. In this chapter, we review crucial topics in neutrino cosmology, such as the neutrino decoupling process in the very early universe. We shall also revisit the current constraints on the number of effective relativistic degrees of freedom and the departures from its standard expectation of 3. Neutrino masses represent the very first departure from the Standard Model of elementary particle physics and may imply the existence of new unexplored mass generation mechanisms. Cosmology provides the tightest bound on the sum of neutrino masses, and we shall carefully present the nature of these constraints, both on the total mass of the neutrinos and on their precise spectrum. The ordering of the neutrino masses plays a major role in the design of future neutrino mass searches from laboratory experiments, such as neutrinoless double beta decay probes. Finally, we shall also present the futuristic perspectives for an eventual direct detection of cosmic, relic neutrinos.

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arXiv2024

How robust are the parameter constraints extending the $\Lambda$CDM model?

We present model-marginalized limits on the six standard $\Lambda$CDM cosmological parameters ($\Omega_{\rm c} h^2$, $\Omega_{\rm b} h^2$, $\theta_{\rm MC}$, $\tau_{\rm reio}$, $n_s$ and $A_s$), as well as on selected derived quantities ($H_0$, $\Omega_{\rm m}$, $\sigma_8$, $S_8$ and $r_{\rm drag}$), obtained by considering several extensions of the $\Lambda$CDM model and three independent Cosmic Microwave Background (CMB) experiments: the Planck satellite, the Atacama Cosmology Telescope, and South Pole Telescope. We also consider low redshift observations in the form of Baryon Acoustic Oscillation (BAO) data from the SDSS-IV eBOSS survey and Supernovae (SN) distance moduli measurements from the \textit{Pantheon-Plus} catalog. The marginalized errors are stable against the different minimal extensions of the $\Lambda$CDM model explored in this study. The largest impact on the parameter accuracy is produced by varying the effective number of relativistic degrees of freedom ($N_{\rm eff}$) or the lensing amplitude ($A_{\rm lens}$). Nevertheless, the marginalized errors on some \textit{derived} parameters such as $H_0$ or $\Omega_{\rm m}$ can be up to two orders of magnitude larger than in the canonical $\Lambda$CDM scenario when considering only CMB data. In these cases, low redshift measurements are crucial for restoring the stability of the marginalized cosmological errors computed here. Overall, our results underscore remarkable stability in the mean values and precision of the main cosmological parameters once both high and low redshift probes are fully accounted for. The marginalized values can be used in numerical analyses due to their robustness and slightly larger errors, providing a more realistic and conservative approach.

astro-ph.CO↗

arXiv2024

A Semiblind Reconstruction of the History of Effective Number of Neutrinos Using CMB Data

We explore the possibility of redshift-dependent deviations in the contribution of relativistic degrees of freedom to the radiation budget of the cosmos, conventionally parameterized by the effective number of neutrinos $N_{\rm eff}$, from the predictions of the standard model. We expand the deviations $\Delta N_{\rm eff}(z)$ in terms of top-hat functions and treat their amplitudes as the free parameters of the theory to be measured alongside the standard cosmological parameters by the Planck measurements of the cosmic microwave background (CMB) anisotropies and Baryonic Acoustic Oscillations, as well as performing forecasts for futuristic CMB surveys such as PICO and CMB-S4. We reconstruct the history of $\Delta N_{\rm eff}$ and find that with the current data the history is consistent with the standard scenario. Inclusion of the new degrees of freedom in the analysis increases $H_0$ to $68.71\pm 0.44$, slightly reducing the Hubble tension. With the smaller forecasted errors on the $\Delta N_{\rm eff}(z)$ parametrization modes from future CMB surveys, very accurate bounds are expected within the possible range of dark radiation models.

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