DESI DR2 cosmological results

7 April 2025 - group meeting

 

Corentin Ravoux

Baryon acoustic oscillations

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Eisenstein et al. 2007

  • Sound wave created by an overdensity of baryons and dark matter in the primordial plasma

Baryon Acoustic Oscillations (BAO):

  • Sound waves in the primordial Universe ...  

Baryon acoustic oscillations

Baryon Acoustic Oscillations (BAO):

  • Sound waves in the primordial Universe ...
  • ... imprint a characteristic scale in the density distribution
  • At recombination (z~1100)
    • Baryon/photon decouples
    • Sound waves froze at sound horizon scale:

 

 

  • Effect of all overdensities in the primordial plasma:
    • Statistical BAO signal in the matter distribution
r_{\mathrm{d}} \simeq 150 \mathrm{Mpc}

Eisenstein et al. 2007

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Baryon acoustic oscillations

  • BAO = Standard ruler
    • Measurement of distance at a given redshift
    • Expansion history from different redshift measurements (encoded in                 and         )
D_{\mathrm{M}}(z)
H(z)

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

DESI instrument

  • 4 m telescope at Kitt Peak Observatory

DESI collaboration 2022

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

DESI focal plane

  • 5 μm positioning in few sec. for 5000 targets
  • All fibers connected to the spectrographs

Silber et al. 2022

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

DESI spectrographs

  • 10 Spectrograph modules containing 3 CCD each

CCD Image

Spectrum

Flux

Wavelength

DESI collaboration 2022

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Targets of the cosmological survey

  • BGS: Bright Galaxy Survey
  • LRG: Luminous Red Galaxy
  • ELG: Emission Line Galaxy
  • QSO: Quasar
    • z > 2.1: with a Lyman-α forest
    • z < 2.1: as tracers

40 million redshifts in 5 years

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

DESI in a nutshell

Chaussidon et al. 2022

Galaxies

Stars

Photometry

Target Selection (BGS, LRG, ELG, QSO)

Observing (5000 spectra simultaneously)

Catalog of spectra and redshifts

Zhou et al. 2022

Silber et al. 2022

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Survey progress

  • Full coverage: 14,000 deg²
  • Now public !

DR1 completion

DESI collaboration 2024

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Survey progress

  • Full coverage: 14,000 deg²
  • DR2 data set used in the results presented here

DR2 completion

DESI collaboration 2024

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

BAO DR2 signal

  • 3D auto-correlation of galaxies for BGS, LRG, ELG and QSO as tracers
  • BAO scale measured over different redshifts

DESI collaboration 2025

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Cosmological constraints

  • Overall BAO scale:

 

 

  • BAO anisotropy:
\frac{D_{\mathrm{V}}(z)}{r_{\mathrm{d}}} = \frac{1}{r_{\mathrm{d}}}\left(\frac{z D_{\mathrm{M}}^2(z)c}{H(z)}\right)^{1 / 3}
\frac{c}{D_{\mathrm{M}}(z) H(z)}

DESI collaboration 2025

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Hubble constant

ΛCDM model with baryon information from BBN

In tension with late time measurements (Supernovae)

H_0=(68.51 \pm 0.58) \mathrm{km} \mathrm{s}^{-1} \mathrm{Mpc}^{-1}

DESI + BBN

CMB angular acoustic scale

\theta_{*}

Sound horizon scale

r_{d}

DESI collaboration 2025

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Curvature

ΛCDM model with free spatial curvature (K)

 

\Omega_{\mathrm{K}}=0.0023 \pm 0.0011

DESI + CMB

In favor of a flat Universe

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Dark energy

w = P/\rho
  • Dark Energy equation of state (-1 for ΛCDM):

 

w=-0.916 \pm 0.078
w=-0.995 \pm 0.023

Assuming constant EoS, compatible with a cosmological constant

DESI + CMB + SN (Pantheon +)

DESI

  • wCDM model:

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Dark energy

  • Varying equation of state (CPL):

 

 

  • Can mimic wide range of viable cosmological models (phase transition, scalar field, modified gravity)
  • ΛCDM:
w(a) = w_0 + (1 - a) w_a
w_0 = -1 \ \ \ w_a = 0

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Dark energy

DESI + CMB + Pantheon+:    2.8σ

DESI + CMB + Union3:          3.8σ

DESI + CMB + DESY5:            4.2σ

DESI + CMB:                            3.1σ

Strong hint of varying dark energy equation of state

ΛCDM tension:

DESI collaboration 2025

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Neutrino mass

  • Neutrino mass impacts cosmology
  • CMB degeneracies broken by BAO, through Hubble constant measurement
\sum m_\nu<0.064 \mathrm{eV}
\sum m_\nu<0.129 \mathrm{eV}

DESI + CMB

DESI + CMB + DESY5 with     (           )

Strong constraint on neutrino mass

w_0,w_a

DESI collaboration 2025

Corentin Ravoux                                                                                                                                                                          Group meeting - April 2025

Conclusion

  • Hint of varying dark energy from DESI DR2 BAO data
  • Strong constraints on ΛCDM and neutrino mass
  • H0 still in tension with SNe Ia

BAO for different species

Interplay between different species in the primordial plasma and after recombination

Cosmological probes: RSD

\mathbf{v_{\parallel}}
\mathbf{v_{\parallel}}
  • Redshifts precisely measured by spectroscopic surveys:
    • Universe expansion + peculiar velocity
    • Position shifted by the line-of-sight peculiar velocity

 

  • Redshift Space Distortions (RSD): Distortion of cosmological observable due to peculiar velocities

 

(1 + z_{\mathrm{obs}}) = (1+z_{\mathrm{cosmo}})(1 + z_{\mathrm{peculiar}})

Neutrinos in cosmology

P1D range

Baur et al. 2016 (JCAP)

P1D range

CMB range

  • Matter power spectrum impacted by:
    • Sum of neutrino masses
    • Dark matter model (e.g. warm dark matter)

Unique probe to constrain neutrino masses and dark matter properties

\sum m_{\nu}