Cosmology is like an orchestra where different sections suddenly start playing out of sync. Measurements of the universe's expansion rate (the Hubble constant) from the early and late universe give different numbers. Other observations hint at a "negative" mass for neutrinos neutron stars — a physical nonsense. And data on dark energy dark energy seem to suggest it changes over time.
The authors of a new study have found a way to restore harmony without adding new instruments. They propose that the parameter describing when the universe became transparent to light (the optical depth to reionization, τ) is slightly higher — around 0.11. It's as if the conductor nudged the strings' entrance a bit, and the whole orchestra fell into place. It turns out that this value of τ simultaneously resolves all three tensions within the standard ΛCDM model expansion of the universe.
The resulting picture is surprisingly coherent. Predictions for distances in the late universe match data from the DESI spectroscopic instrument dark energy. The properties of dark energy stop "evolving" and agree with a simple cosmological constant. To test this hypothesis, new precise measurements of the cosmic microwave background polarization cosmic dust are needed — for example, with the future LiteBIRD satellite.
as written by the authors arXiv:2606.30903
The measurement of the sum of neutrino masses is among the primary promises of precision cosmology, achievable by combining complementary early- and late-Universe probes. However, these datasets currently exhibit mild-to-strong disagreements within $$\Lambda$$CDM and its simplest extensions, giving rise to multiple tensions, including the Hubble tension, the preference for "negative" neutrino mass, and indications of evolving dark energy. It has recently been shown that these tensions can be alleviated by adopting a higher value of the optical depth to reionization parameter, $$\tau$$, when large-scale cosmic microwave background (CMB) polarization data are ignored. We extend this proposal and show that an especially high prior on $$\tau = 0.11 \pm 0.006$$ simultaneously addresses all three of these tensions, significantly reducing the need for new physics beyond $$\Lambda$$CDM. We determine the "concordance" value of $$\tau$$ by requiring physical neutrino mass and consistency of the Hubble constant, $$H_0$$, inferred from the CMB and that preferred by the Dark Energy Spectroscopic Instrument (DESI) baryon acoustic oscillation (BAO) and full-shape measurements. Within this high-$$\tau$$ Universe, we obtain the first $$2\sigma$$ detection of a positive neutrino mass, $$\Sigma m_{ u} = 0.10^{+0.04}_{-0.05}$$~eV at 68\% C.L., while restoring cosmological concordance between datasets within $$\Lambda$$CDM. In particular, low-redshift distance predictions are consistent with DESI BAO observations and the inferred dark-energy equation-of-state parameters are consistent with a cosmological constant, both with and without supernovae data. The concordance power of our $$\tau$$ prior further motivates new measurements of $$\tau$$, e.g., through large angular scale CMB polarization observations with the \textit{LiteBIRD}, CLASS, or proposed PICO experiments. (Abridged)
Neutrinos are so light and elusive that every second, trillions of these particles, emitted by the Sun, pass through your body without you noticing.