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Abstract We present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Lyman-αforest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI), to be released in the DESI Data Release 1. DESI BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1 <z< 4.2. To mitigate confirmation bias, a blind analysis was implemented to measure the BAO scales. DESI BAO data alone are consistent with the standard flat ΛCDM cosmological model with a matter density Ωm=0.295±0.015. Paired with a baryon density prior from Big Bang Nucleosynthesis and the robustly measured acoustic angular scale from the cosmic microwave background (CMB), DESI requiresH0=(68.52±0.62) km s-1Mpc-1. In conjunction with CMB anisotropies fromPlanckand CMB lensing data fromPlanckand ACT, we find Ωm=0.307± 0.005 andH0=(67.97±0.38) km s-1Mpc-1. Extending the baseline model with a constant dark energy equation of state parameterw, DESI BAO alone requirew=-0.99+0.15-0.13. In models with a time-varying dark energy equation of state parametrised byw0andwa, combinations of DESI with CMB or with type Ia supernovae (SN Ia) individually preferw0> -1 andwa< 0. This preference is 2.6σfor the DESI+CMB combination, and persists or grows when SN Ia are added in, giving results discrepant with the ΛCDM model at the 2.5σ, 3.5σor 3.9σlevels for the addition of the Pantheon+, Union3, or DES-SN5YR supernova datasets respectively. For the flat ΛCDM model with the sum of neutrino mass ∑mνfree, combining the DESI and CMB data yields an upper limit ∑mν< 0.072 (0.113) eV at 95% confidence for a ∑mν> 0 (∑mν> 0.059) eV prior. These neutrino-mass constraints are substantially relaxed if the background dynamics are allowed to deviate from flat ΛCDM.
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This content will become publicly available on December 16, 2025
A bound on thermal $y$-distortion of the cosmic neutrino background
We consider the possibility that the cosmic neutrino background might have a nonthermal spectrum, and investigate its effect on cosmological parameters relative to standard $$\Lambda$$-Cold Dark Matter ($$\Lambda$$CDM) cosmology. As a specific model, we consider a thermal $$y$$-distortion, which alters the distribution function of the neutrino background by depleting the population of low-energy neutrinos and enhancing the high-energy tail. We constrain the thermal $$y$$-parameter of the cosmic neutrino background using Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillation (BAO) measurements, and place a $$95\%$$-confidence upper bound of $$y \leq 0.043$$. The $$y$$-parameter increases the number of effective relativistic degrees of freedom, reducing the sound horizon radius and increasing the best-fit value for the Hubble constant $$H_0$$. We obtain an upper bound on the Hubble constant of $$H_0 = 71.12\ \mathrm{km/s/Mpc}$$ at $$95\%$$ confidence, substantially reducing the tension between CMB/BAO constraints and direct measurement of the expansion rate from Type-Ia supernovae. Including a spectral distortion also allows for a higher value of the spectral index of scalar fluctuations, with a best-fit of $$n_{\mathrm{S}} = 0.9720 \pm 0.0063$$, and a $$95\%$$-confidence upper bound of $$n_{\mathrm{S}} \leq 0.9842$$.
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- Award ID(s):
- 2310363
- PAR ID:
- 10560316
- Publisher / Repository:
- Physical Review D
- Date Published:
- Journal Name:
- Physical review D
- ISSN:
- 2470-0029
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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