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Vivian Poulin

Publications and source records attributed to Vivian Poulin.

At least 19 recordsLinked to original sources

Peering Beyond the Veil of Last Scattering: A View of the Universe with CMB Spectral Distortions

The frequency spectrum of the cosmic microwave background is the most precise blackbody ever measured in nature, with deviations constrained at the level of almost one part per million from the COBE satellite. Nevertheless, departures away from a perfect blackbody are present in standard $Λ$CDM cosmology, lurking just beneath the surface of our current observational bounds. These spectral distortions provide invaluable information on our thermal history in both the post- and pre-recombination epochs, allowing us to peer beyond last scattering and into the primordial Universe. Here, we present an overview of the underlying physics responsible for generating CMB spectral distortions at all epochs. As an illustration of this rich physics, we review a comprehensive set of mechanisms capable of generating distortions both within and beyond the standard $Λ$CDM paradigm. We also discuss the information that can be gleaned by going beyond the monopole (sky-averaged) spectrum and exploiting the spatial information present in anisotropic spectral distortions. To supplement our discussion of the diverse science of spectral distortions, we provide an overview of the upcoming and proposed experimental landscape. We highlight that the combination of the TMS, COSMO, and BISOU experiments will provide the first discovery of a monopole $y$-type distortion within the coming decade. From space, the proposed FOSSIL experiment is forecasted to improve upon the original COBE/FIRAS measurement by roughly three orders of magnitude in sensitivity, bringing with it the detection of the $Λ$CDM $μ$-type distortion sourced by the dissipation of small scale acoustic modes in the pre-recombination plasma. With transformational measurements on the horizon, CMB spectral distortions offer a uniquely sensitive probe of the thermal history of the Universe at redshifts $z \lesssim 2 \times 10^6$.

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Phantom-Crossing Dark Energy and the $Ω_m$ Tug-of-War

Recent analyses combining data from the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), and Type Ia supernovae (SN) have revealed a tentative observational preference for phantom crossing in the dark energy equation of state $w$. We argue that this preference is a natural consequence of the $Ω_m$ tensions that arise when these datasets are individually fit to $Λ$CDM, specifically because of the ordering $Ω_m^\mathrm{BAO} < Ω_m^\mathrm{CMB} < Ω_m^\mathrm{SN}$. We show both theoretically and empirically that models with phantom crossing can shift all of these inferred $Ω_m$ values toward mutual alignment. In contrast, quintessence theories restricted to $w \geq -1$ can alleviate the tensions with SN data but only at the cost of exacerbating the BAO-CMB discrepancy. We therefore conclude that it is the BAO and CMB measurements - not the SN data - that drive the preference for phantom crossing over quintessence in joint analyses. Moreover, we point out that SN data exhibit greater tensions with the other datasets when fit to phantom-crossing models than when fit to quintessence, causing the preference for phantom crossing to be weaker in joint CMB+BAO+SN analyses than in analyses of CMB+BAO data alone.

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Boosting the optical depth to Thomson scattering with primordial black hole evaporation at high redshift

BAO and CMB data are somewhat discrepant when interpreted in the context of $Λ$cdm, discrepancies that show up as a `matter density deficit' and as a `CMB lensing excess'. One possible resolution is an increased optical depth to scattering off of free electrons in the post-recombination universe, $τ$, a possibility raised by Sailer et al. 2025 and Jhaveri et al. 2025. Since Planck measurements of the low-$\ell$ polarization `reionization bump' already constrain $τ$ from standard stellar-driven reionization at $z<10$, we investigate additional optical depth sourced by transient or partial reionization at higher redshift from exotic processes. For specificity, we explore the impact of Hawking radiation from a monochromatic spectrum of primordial black holes, retaining the high-$\ell$ $TT/TE/EE$ data that constrain such histories and varying the reionization redshift jointly. We find that the CMB data do not significantly prefer these additional signals: the boost is at most $Δτ\simeq 0.008$, well short of the $Δτ\simeq 0.03$ that would completely eliminate the moderate discrepancy. The matter density deficit and the lensing excess are not significantly eased: we explain why, tracing it to compensation from the reionization redshift and the residual PBH signal at $\ell > 30$.

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The road towards precision measurements of $H_0$ with bright sirens in the Einstein Telescope era

Gravitational-wave standard sirens provide an independent probe of cosmic expansion since their luminosity distances are inferred directly from the gravitational-wave signal and, for bright sirens (BSs), the source redshifts are obtained through the identification of electromagnetic counterparts. In this work, we forecast the constraining power of future bright siren catalogues on the Hubble constant using simulated binary-neutron-star mergers detected by the Einstein Telescope with associated electromagnetic counterparts. We construct mock catalogues with different numbers of events and redshift distributions, with GW170817 as a reference BS, and analyse the resulting constraints within a flat $Λ$CDM cosmology. We find that, when bright sirens are used as a standalone probe, most of the information on $H_0$ is provided by low-redshift events, with the improvement in precision saturating for sources above $z \sim 1$; beyond that redshift, the constraining power of BSs is increasingly limited by the degeneracy with $Ω_m$, which prevents further gains in precision on $H_0$. In this case, approximately $90$ low-redshift BSs are required to reach $σ_{H_0} \sim 1\,{\rm km\,s^{-1}\,Mpc^{-1}}$, while about $45$ are sufficient for $σ_{H_0} \sim 2\,{\rm km\,s^{-1}\,Mpc^{-1}}$. When external BAO information is included to reduce the $H_0 - Ω_m$ degeneracy, intermediate-redshift sirens become more informative and the required number of events decreases substantially, to roughly $20$ and $15$ BSs for $σ_{H_0} \sim 1\,{\rm km\,s^{-1}\,Mpc^{-1}}$ and $σ_{H_0} \sim 2\,{\rm km\,s^{-1}\,Mpc^{-1}}$, respectively. These results highlight the importance of both electromagnetic counterpart identification and complementary background probes in making BSs a competitive, distance-ladder-independent test of the Hubble tension.

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What could an emerging Big Bang Nucleosynthesis discrepancy be hinting at?

The latest measurement of the primordial deuterium abundance is in $\sim 2σ$ tension with several state-of-the-art predictions of standard Big Bang nucleosynthesis (BBN), when using the baryon density inferred from the $Λ$CDM model fit to cosmic microwave background (CMB) data. This tension increases to $\sim 3σ$ for models attempting to solve the Hubble tension, such as early dark energy (EDE), which generally predict a larger baryon density than in $Λ$CDM. We test whether this discrepancy could be pointing to a non-standard expansion history during BBN. We compute light-element abundances with PRIMAT and compare $Λ$CDM, a $ΔN_{\rm eff}$ extension, and a very early dark energy (vEDE) component. For vEDE, we sample $ΔH/H$, the fractional increase of the expansion rate while deuterium burning is freezing out and helium-4 fusion is mostly over. The Bayesian analysis using BBN plus the CMB baryon-density constraint in the EDE cosmology gives $ΔH/H = 0.087^{+0.036}_{-0.037}$ during the deuterium burning epoch, i.e. at a temperature $T_{\rm D}\simeq0.03\,{\rm MeV}$, and no residual tension. The vEDE component preserves the observed deuterium abundance at the larger CMB baryon density while only mildly affecting helium-4. By contrast, extra radiation raises the helium-4 abundance too efficiently and does not reconcile the baryon density determinations. Together with inflation, dark energy, and EDE, our results hint at the presence of another light scalar field in cosmology.

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Decaying Dark Matter Halo Abundance from a Revised Spherical Collapse Model

We present a semi-analytical framework for the halo mass function (HMF) in decaying dark matter (DDM) cosmologies, in which dark matter decays into a massive daughter particle inheriting a velocity kick $v_k$ and a massless dark radiation component. Building on the Press-Schechter formalism, we encode the DDM physics through a spherical collapse model that explicitly tracks the decay-induced mass loss, yielding a modified, mass-dependent critical collapse threshold $δ_c(M_0)$ and a mapping $M_{\rm coll}(M_0)$ between the initial Lagrangian mass and the collapsed halo mass. The critical threshold exhibits a characteristic transition between two analytically tractable plateaus: a large-mass limit, where all daughter particles are retained by the halo, and a small-mass limit, where all daughters escape and the collapse is equivalent to that of a dark matter species decaying entirely into dark radiation, making $δ_c$ independent of $M_0$ and $v_k$. We provide semi-analytical results and fits for both limits and a fitting formula for the transition, whose single free parameter $M_1 \propto v_k^3\,\tildeΓ^{-1/2} t_{\rm ta}$ has a transparent physical interpretation: it is the mass scale at which the kick velocity equals the halo orbital velocity. We validate our predictions against a suite of N-body simulations at $z=0$ and $z\approx 1$, finding good agreement across models spanning mild to strong HMF suppression relative to $Λ$CDM. Residual deviations for the largest kick velocities at $z=0$ are observed. Via a halo-by-halo comparison between simulations, we trace the discrepancy to the definition of the halo mass when daughter orbits extend beyond the halo boundary. The resulting fitting functions for $δ_c(M_0,Γ,v_k)$ and $M_{\rm coll}(M_0)$ provide an efficient and accurate route to DDM constraints from current and forthcoming probes of the halo mass function.

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The $H_0$ World Cup. I. Summary of the baseline group stage results

The Hubble tension has reached a nominal significance above $7σ$, while new high-precision measurements of the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO) sharpen the test of proposed solutions. Using a common framework, we compare fourteen representative alternatives to the standard $Λ$ Cold Dark Matter ($Λ$CDM) model in light of up-to-date CMB, BAO and supernovae data to gauge their ability to resolve the tension. The models span late-time modifications, modified recombination, and exotic pre-recombination expansion histories driven by additional radiation or a localized dark energy injection. We evaluate each proposal with complementary frequentist and Bayesian measures of the residual calibration tension and of the improvement in the joint fit. Both approaches identify the same broad hierarchy. Early dark energy and early modified gravity models perform best, shifting the $H_0$ inference without local measurement priors toward $70\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ and reducing the residual discrepancy with SH0ES to approximately $2.5-3.6σ$, depending on the model and statistic, while receiving strong support over $Λ$CDM in the combined fit. Varying the electron mass at recombination yields an intermediate improvement, whereas the enhanced-radiation and late-time scenarios do not improve over $Λ$CDM. This Letter summarizes the group stage of the competition; in a companion paper (Paper II) we present the results of an exhaustive set of analyses and assess their robustness to variations in modeling assumptions and datasets.

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The $H_0$ world cup. II. A comprehensive competition between proposed Hubble tension solutions

Cosmology stands at a crossroads. The Hubble tension has reached a nominal significance above $7σ$, while analyses combining DESI BAO and Type Ia supernova data show emerging hints of departures from $Λ$CDM. Meanwhile, high-precision CMB measurements from ACT and SPT enable a timely and more stringent reassessment of proposed solutions to the tension. In this paper, we revisit the $H_0$ Olympics, a systematic contest comparing proposed alternatives to $Λ$CDM using common datasets, likelihoods, and statistical criteria. In this updated edition, the $H_0$ World Cup, we subject fourteen representative solutions to a common analysis of current CMB, BAO, and SN data. The contenders span four broad mechanisms: late-time modifications of the expansion history, modified recombination, additional pre-recombination radiation, and early non-radiative energy injection. Relative to the original analysis, the present competition includes models and mechanisms proposed in the intervening years and evaluates all contenders using both Bayesian and Frequentist tests of tension and model performance, letting the neutrino mass sum vary. We further test if late-time extensions through curvature or the Chevallier-Polarski-Linder (CPL) dark energy parametrization can aid the success of the models. Finally, we subject the leading contenders to dedicated robustness tests involving alternative CMB likelihoods and multipole cuts, supernova samples, large-scale-structure information, and big-bang nucleosynthesis constraints. This framework assesses both the ability of each mechanism to ease the Hubble tension and the robustness of our conclusions to datasets and analysis choices.

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Can Distance Duality Violation Save Late-time Solutions to the Hubble Tension?

The discrepancy between early- and late-Universe determinations of the Hubble constant may point to physics beyond $Λ$CDM or to unaccounted-for systematics. Numerous late-time modifications to the expansion history have been proposed to alleviate this discrepancy, with limited success. Recent works have shown that, when the sound-horizon and supernova calibrations are held fixed, any purely late-time resolution requires a violation of the cosmic distance duality relation (CDDR). Recasting the tension in the $r_d$-$M_B$ plane, we show explicitly that distance duality, together with BAO and uncalibrated supernova data and a fixed sound-horizon calibration, determines $H_0$ independently of the late-time expansion history. We then test the viability of the required CDDR violation by separately constraining reciprocity violation and photon number non-conservation, deriving a new constraint on reciprocity-violating distortions of angular-diameter distances from BAO and cosmic-chronometer data. Combining this result with existing photon-number-conservation constraints, we find that the level of distance-duality violation needed to resolve the tension is strongly disfavoured by current data. We therefore conclude that, for fixed sound-horizon and supernova calibrations, no modification confined to the late-time expansion history -- even one violating distance duality -- can resolve the Hubble tension, pointing instead toward early-Universe physics or unresolved local systematics.

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A sound horizon independent measurement of $H_0$ from BOSS, DESI and DES Y3

We present a sound horizon independent measurement of the Hubble parameter using a multiprobe large-scale structure analysis. Removing the dependency on the sound horizon with a rescaling procedure at the matter power spectrum level, we analyse the BOSS full-shape power spectrum and bispectrum (for the first time) using the effective field theory of large-scale structure up to one loop. We combine this analysis with the auto- and cross-angular power spectra from the DESI Legacy Imaging Survey DR9, the $3 \times 2$pt analysis from DES Y3, and the CMB gravitational lensing power spectrum from Planck PR3. Our baseline analysis, that does not rely on supernovae data, yields $h = 0.702^{+0.022}_{-0.024}$, $Ω_m = 0.310 \pm 0.013$, and $σ_8 = 0.799 \pm 0.020$, corresponding to $3-4 \%$ precision measurements. When adding supernovae data from Pantheon+, we obtain a $2.6 \%$ measurement of $h$, with $h = 0.686 \pm 0.018$. We further note that our EFTBOSS analysis indicates a slight deviation of the BAO scale parameter (at $1.8 σ$) from its $Λ$CDM value, caused by the small scales of the bispectrum. We finally use the sound horizon-free EFTBOSS analysis as a diagnosis for the presence of new physics, finding that our results are consistent with the recent hints of evolving dark energy.

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Interacting dark sector from intrinsic entropy couplings

We introduce a new class of interacting dark sector models that couple the intrinsic entropy of dark matter to scalar field dark energy. Using the Lagrangian formulation for relativistic perfect fluids, we construct consistent covariant actions that incorporate algebraic and derivative entropy couplings. These interactions leave the expansion history unchanged, rendering the background cosmology indistinguishable from $Λ$CDM or uncoupled quintessence. At the level of cosmological perturbations, the entropy couplings generate scale-dependent modifications to the dark matter Euler equation, while the continuity equation remains unaltered at linear order. The resulting interactions correspond to a pure-momentum exchange within the dark sector. We show that intrinsic entropy perturbations can carry primordial scale dependence, and non-minimal couplings can lead to a scale-dependent suppression or enhancement of structure growth. Finally, we demonstrate that these models are generically compatible with current Cosmic Microwave Background observations, while inducing distinctive signatures in large-scale structure. The framework provides a theoretically well-motivated and observationally viable extension to the standard cosmological model, opening new directions to explore novel interactions in the dark sector.

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Neutrino mass limits and decaying dark matter: background evolution versus perturbations

We revisit cosmological neutrino mass bounds when a fraction of dark matter is allowed to decay to massless dark radiation. By compensating the late-time increase in the matter density induced by neutrinos becoming non-relativistic, decaying dark matter (DDM) can render datasets solely sensitive to the background density effectively insensitive to neutrino masses. Using data from baryonic acoustic oscillations (BAO) and Type Ia supernovae together with a distance prior from the cosmic microwave background (CMB), we find that neutrino masses as large as ${\cal O}(1\,\mathrm{eV})$ are allowed without degrading the fit. Moreover, the combination of BAO data with the CMB distance prior yields a preference for a non-zero DDM fraction, and alleviates the need for dynamical dark energy with phantom crossing. However, the degeneracy introduced by DDM is decisively broken once perturbation observables are included. Incorporating the full $\textit{Planck}$ CMB likelihood, and in particular CMB lensing, restores strong constraints on the neutrino mass in the DDM scenario, $\sum m_ν\lesssim 0.079\,\mathrm{eV}$. In contrast, neutrino mass constraints in a smooth dark energy model described by the Chevallier-Polarski-Linder parametrization become merely $\sim 25\%$ stronger compared to background-only analyses. Our results highlight the essential role of structure-growth measurements in assessing extensions of the dark sector and to obtain robust cosmological neutrino mass bounds.

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In-depth analysis of the clustering of dark matter particles around primordial black holes. Part III: CMB constraints

In a mixed dark matter scenario in which primordial black holes (PBHs) would co-exist with thermally produced self-annihilating particles, one expects the former to be surrounded by extremely dense halos made of the latter, built up during radiation domination. Here, as a continuation of previous work, we derive observational limits on such a scenario from a full statistical analysis of cosmic microwave background (CMB) data. We quantify how a tiny fraction $\fbh$ of PBHs could restrict the parameter space available to thermal particle dark matter, limiting the $s$-wave annihilation cross section to values $\lesssim 10^{-30}\,{\rm cm^3/s}\,(\mchi/100\,{\rm GeV})\,(\fbh/10^{-6})^{-3}$ if PBHs are typically heavier than $\sim 10^{-10}\,\Msun$, which can also be turned into constraints on PBHs in this mass range. In contrast, asteroid mass or lighter PBHs could live in perfect peace with these particles. Finally, we shortly discuss the implications of the recent tentative interpretation of Subaru-HSC microlensing events as PBHs.

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Double the axions, half the tension: multi-field early dark energy eases the Hubble tension

We show that the strong constraints placed by Planck NPIPE Cosmic Microwave Background (CMB) data on axion-like early dark energy (EDE) are significantly alleviated in models with multiple fields. We find a $1.5σ$ residual tension with the Local Distance Network value of $H_0$ in a 2-field model, with no improvement beyond two fields, and a best-fit value of $H_0$ $\sim 1.4σ$ larger than in the 1-field case. The second field improves the fit to high-$\ell$ CMB data, where 1-field EDE is most strongly disfavored, and suggests modifications to the pre-recombination history over a wider redshift range.

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A frequentist view on the two-body decaying dark matter model

Decaying dark matter (DDM) has emerged as an interesting framework to extend the $Λ$-cold-dark-matter (LCDM) model, as many particle physics models predict that dark matter may not be stable over cosmic time and can impact structure formation. In particular, a model in which DDM decays at a rate $Γ$ and imprints a velocity kick $v$ onto its decay products leads to a low amplitude of fluctuations, as quantified by the parameter $S_8$, in better agreement with that measured by some weak lensing surveys. Bayesian analyses have provided mixed conclusions regarding its viability, with a reconstructed clustering amplitude only slightly below the standard LCDM value. In this paper, we contrast previous results with a frequentist analysis of Planck and SDSS BAO data. We find that the $68\%$ confidence level region corresponds to a decay half-life of $6.93^{+7.88}_{-2.85}$Gyr and a velocity kick of $1250^{+1450}_{-1000}$~km/s. These $1σ$ constraints strongly differ from their Bayesian counterparts, indicating the presence of volume effect in the Bayesian analysis. Moreover, we find that under the DDM model, the frequentist analysis predicts lower values of $S_8$, in agreement with those found by KiDS-1000 and DES-Y3 at $\sim 1.5σ$. We further show that previously derived KiDS-1000 constraints that appeared to exclude the best-fit model from Planck data were driven by priors on the primordial amplitude $A_s$ and spectral index $n_s$. When those are removed from the analysis, KiDS-1000 constraints on the DDM parameters are fully relaxed. It is only when applying Planck-informed priors on $A_s$ and $n_s$ to the KiDS-1000 analysis that one can constrain the model. We note that without such priors, the scales best measured by KiDS-1000 do not exactly match the $S_8$ kernel, so $S_8$ constraints should not be applied directly to a model in place of the full likelihood.

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The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged]

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A primordial origin to cosmic tensions: towards reconciling $H_0$ and $S_8$ with early dark energy and scale-dependent primordial non-Gaussianities

The Hubble ($H_0$) tension between direct measurements of the expansion rate and the prediction of the $Λ$CDM cosmological model calibrated on the Cosmic Microwave Background (CMB), is a strong motivation to explore alternative cosmological models. A popular class of such models includes an additional component of dark energy relevant in the early Universe, which solves the Hubble tension. These Early Dark Energy (EDE) models however typically overpredict the value of the $S_8$ parameter. Here, we show how combining EDE with scale-dependent primordial non-Gaussianities (sPNG) can in principle both increase $H_0$ and decrease $S_8$ at once, even conceivably allowing to solve the potential $S_8$ tension between measurements of weak gravitational lensing and the $Λ$CDM expectation. Such sPNG are related to non-trivial inflationary physics, and observationally affect the non-linear regime of structure formation while leaving the linear regime mostly untouched. Depending on the amplitude of the sPNG, such models can either yield back the $Λ$CDM expectation for the power spectrum in the non-linear regime, and hence an $S_8$ parameter compatible with $Λ$CDM, or can even reconcile the value of $S_8$ from old weak-lensing measurements with the CMB, while solving the Hubble tension in all cases. In such models, both tensions would then be entirely related to pre-CMB physics of the early Universe.

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Impact of ACT DR6 and DESI DR2 for Early Dark Energy and the Hubble tension

The data release six of the Atacama Cosmology Telescope (ACT DR6) and the second data release from the Dark Energy Spectroscopic Instrument (DESI DR2) recently became available. In light of these data, we update constraints on the Early Dark Energy (EDE) resolution to the Hubble tension. While ACT DR6 does not favor EDE over the core cosmological model $Λ$CDM, it allows for a significantly larger maximum contribution of EDE, $f_{\rm EDE}$, in the pre-recombination era than the latest analysis of {\it Planck} NPIPE despite increased precision at small angular scales. Moreover, EDE rises the value of $H_0r_s$, improving consistency between CMB and DESI DR2 data. We find a residual tension with SH0ES of $\sim 2 σ$ for the combination of {\it Planck} at $\ell <1000$ + ACT DR6 + lensing + Pantheon-plus + DESI DR2, a significant decrease from $3.7 σ$ for analyses that use NPIPE and SDSS BAO data. A profile likelihood analysis reveals significant prior-volume effects in Bayesian analyses which do not include SH$0$ES, with confidence intervals of $f_{\rm EDE}=0.09\pm 0.03$ and $H_0= 71.0\pm1.1$ km/s/Mpc. When including DESI data, the EDE model with $H_0=73$ km/s/Mpc provides a better fit than the $Λ$CDM model with $H_0=68.4$ km/s/Mpc. The inclusion of SH$0$ES data rises the preference well above $5σ$, with $Δχ^2=-35.4$. Our work demonstrates that after ACT DR6 and DESI DR2, EDE remains a potential resolution to the Hubble tension.

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