DESI first year cosmological results

24 Mai 2024 - LPCA Seminar

 

Corentin Ravoux - Researcher at LPCA

on behalf of the DESI collaboration

Thanks to our sponsors and 72
participating institutions

1. Cosmological context

2. The Dark Energy Spectroscopic Instrument

3. The Lyman-α forest

5. Other Lyman-α forest studies

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

4. DESI first year BAO cosmological results

1. Cosmological context

2. The Dark Energy Spectroscopic Instrument

3. The Lyman-α forest

5. Other Lyman-α forest studies

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

4. DESI first year BAO cosmological results

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Cosmological context

How did the Universe evolve to its current state?
What are the fundamental constituents of our Universe?
How is matter distributed in the Universe?

The Story of Our Universe - NASA

You are here

Global content of our Universe

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

  • Evolution of the Universe described by the ΛCDM model

 

  • Composition today:
    • Only 5 % of its content is known
    • Properties of components constrained by observations

What is the nature of dark energy
and dark matter?

adapted from Dijkstra et al. 2017

Large scale structures

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

  • Initial density perturbations in the
    primordial Universe.
  • Perturbations grows to form the
    cosmic web: halos, filaments, walls and voids
  • Large-scale distribution of matter characterized by the linear power spectrum

Walther et al. 2021

Chabanier et al. 2019

How can we probe the cosmic web?
What can we learn from it?

Baryon acoustic oscillations

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

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                                                                                                                                                         DESI first year results - LPCA Seminar

Baryon acoustic oscillations

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

  • 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)

1. Cosmological context

2. The Dark Energy Spectroscopic Instrument

3. The Lyman-α forest

5. Other Lyman-α forest studies

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

4. DESI first year BAO cosmological results

DESI instrument

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

  • 4 m telescope at Kitt Peak Observatory

DESI collaboration 2022

DESI focal plane

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

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

Silber et al. 2022

DESI spectrographs

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

  • 10 Spectrograph modules containing 3 CCD each

CCD Image

Spectrum

Flux

Wavelength

DESI collaboration 2022

Targets of the cosmological survey

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

  • 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

DESI in a nutshell

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

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

Survey progress

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

  • Full coverage: 14,000 deg²
  • Y1 data set used in the results presented here

Y1 completion

DESI collaboration 2024

Survey progress

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

  • Full coverage: 14,000 deg²
  • Y1 data set used in the results presented here
  • Y3 data set secured

Y3 completion

DESI collaboration 2024

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

DESI science goals:

 

- Galaxy and quasar clustering

- Lyman-α forest

- Clusters and cross-correlations

- Galaxy and quasar physics

- Milky way Survey

- Transients and low-z

1. Cosmological context

2. The Dark Energy Spectroscopic Instrument

3. The Lyman-α forest

5. Other Lyman-α forest studies

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

4. DESI first year BAO cosmological results

The Lyman-α forest

Ravoux et al. 2023

  • Lines in quasar spectra at                                      caused by absorbers in the intergalactic medium (IGM) at
\lambda_{\mathrm{obs}} = \left(1 + z_{\mathrm{abs}}\right) \lambda_{\mathrm{Ly\alpha}}
z_{\mathrm{abs}}

DESI quasar z = 3.42

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Lyman-α physics

\tau_\alpha(r)=\int n_{\mathrm{HI}}(r) \sigma_\alpha(r) d r
  • Optical depth (degree of transparency of the medium):

Density of neutral hydrogen

Cross section of Lyman-α transition

\lambda_{\alpha} = 121.567\ \mathrm{nm}
  • Fraction of transmitted flux:
\mathrm{F}(\mathrm{r})=\exp \left(-\tau_\alpha(r)\right)

F

Lyman-α forest = non-linear tracer of neutral hydrogen in the intergalactic medium

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Contaminants

  • Near the quasar:
    • Intrinsic continuum
    • Broad absorption line quasars (BAL)
  • Along the line-of-sight:
    • Metal absorptions (C, Si, O, N...)
    • Damped Lyman-α systems (DLA)
  • Near the telescope:
    • Atmospheric absorption and emission
    • Instrument noise
    • Spectrograph resolution

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Lyman-α clustering

Quasar

Lyman-α forest

Observer

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Lyman-α clustering

Quasar

Lyman-α forest

\delta_F = \frac{F}{\langle F \rangle} - 1

= Fraction of transmitted flux

F

Observer

Correlation

\xi_\alpha(\vec{r})=<\delta_F(\vec{x}) \delta_F(\vec{x}+\vec{r})>_x

3D auto correlation

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Lyman-α clustering

Quasar

Lyman-α forest

\delta_F = \frac{F}{\langle F \rangle} - 1

= Fraction of transmitted flux

F

Observer

Correlation

\xi_{\alpha q}(\vec{r})=<\delta_F(\vec{x}) \delta_{\mathrm{qso}}(\vec{x}+\vec{r})>_x

3D cross correlation with quasars

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Lyman-α BAO Y1

3D auto correlation

3D cross correlation with quasars

Separation (Mpc/h)

Separation (Mpc/h)

BAO scale

BAO scale

Measurement of the BAO scale with the Y1 data of DESI

DESI collaboration 2024

1. Cosmological context

2. The Dark Energy Spectroscopic Instrument

3. The Lyman-α forest

4. DESI first year BAO cosmological results

5. Other Lyman-α forest studies

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

BAO Y1 signal on galaxies

BAO scale

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

DESI collaboration 2024

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

All BAO signals from DESI Y1

DESI collaboration 2024

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

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 2024

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

A word on blinding

  • Blinded analysis to avoid confirmation bias:
    • For galaxies:  shift of all redshifts based on a random ΛCDM model
    • For Lyman-α forest: unknown shift of the BAO peak
  • Unblinding after passing an extending list of tests

Line-of-sight deviation from fiducial cosmology

Transverse deviation from fiducial cosmology

Example for Lyman-α forest:

DESI collaboration 2024

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Hubble constant

ΛCDM model with baryon information from BBN

DESI BAO

Supernovae

In tension with late time measurements (Supernovae)

H_0=(68.52 \pm 0.62) \mathrm{km} \mathrm{s}^{-1} \mathrm{Mpc}^{-1}

DESI + BBN

DESI collaboration 2024

CMB angular acoustic scale

\theta_{*}

Sound horizon scale

r_{d}

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Curvature

ΛCDM model with free spatial curvature (K)

 

\Omega_{\mathrm{K}}=0.0024 \pm 0.0016

DESI + CMB

In favor of a flat Universe

DESI collaboration 2024

Dark energy

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

 

w=-0.99_{-0.13}^{+0.15}
w=-0.997 \pm 0.025

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Assuming constant EoS, compatible with a cosmological constant

DESI + CMB + SN

DESI

  • wCDM model:

DESI collaboration 2024

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

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                                                                                                                                                         DESI first year results - LPCA Seminar

Dark energy

DESI + CMB + Pantheon+:    2.5σ

DESI + CMB + Union3:          3.5σ

DESI + CMB + DESY5:            2.5σ

First hint of varying dark energy equation of state

ΛCDM tension:

DESI collaboration 2024

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Neutrino mass

  • Neutrino mass impacts cosmology
  • CMB degeneracies broken by BAO, through Hubble constant measurement
\sum m_\nu<0.072 \mathrm{eV}
\sum m_\nu<0.195 \mathrm{eV}

DESI + CMB

DESI + CMB with (           )

Strong constraint on neutrino mass

w_0,w_a

DESI collaboration 2024

1. Cosmological context

2. The Dark Energy Spectroscopic Instrument

3. The Lyman-α forest

5. Other Lyman-α forest studies

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

4. DESI first year BAO cosmological results

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Lyman-α clustering

Quasar

Lyman-α forest

Observer

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Lyman-α clustering

Correlation in Fourier space

Quasar

Lyman-α forest

P_{\mathrm{1D}} = \langle \left| \delta_F(k) \right|^2 \rangle_{\mathrm{forests}}
\delta_F = \frac{F}{\langle F \rangle} - 1

= Fraction of transmitted flux

F

Observer

One dimensional power spectrum

Ravoux et al. 2023

  • Correlations along individual lines-of-sight in Fourier Space
  • Sensitive to small-scale
    matter clustering and IGM thermal state
  • Unique probe to constrain neutrino masses and dark matter properties (WDM, FDM...)

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

DESI EDR FFT measurement

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

Lyman-α clustering

Quasar

Lyman-α forest

\delta_F = \frac{F}{\langle F \rangle} - 1

= Fraction of transmitted flux

F

Observer

Interpolation

\delta_{F\mathrm{map}}

3D tomographic map of Lyman-α absorption

Tomographic map

Corentin Ravoux                                                                                                                                                         DESI first year results - LPCA Seminar

  • Gives a 3D map tracing matter at redshift z > 2

 

  • Applications:
    • Identification of proto-cluster candidates
    • Cross-correlations with cosmic voids, tracer of velocity flows in the cosmic web

Ravoux et al. 2020

Ravoux et al. 2022

eBOSS DR16 measurement

Conclusion

  • Hint of varying dark energy from DESI Y1 BAO data
  • Strong constraints on ΛCDM and neutrino mass
  • Lyman-α forest yields a lot of applications

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}

Latest constraints with eBOSS

  • Neutrino mass  (P1D +CMB):
\sum m_{\nu} < 0.11~\mathrm{eV}
m_{\mathrm{X}} > 5.3~\mathrm{keV}
  • Warm dark matter model:

Palanque-Delabrouille et al. 2020 (JCAP)

  • Other constraints: Fuzzy dark matter, sterile neutrinos, running of the primordial power spectrum

at 95% C.L.

at 95% C.L.

Forecasts for DESI

\sigma\left(\sum m_{\nu}\right) = 0.03~\mathrm{eV}
  • Constraints on WDM improved by a factor 1.6
  • IGM thermal parameters by a factor 2.
  • Neutrino mass, in association with BAO and CMB:

 Valluri et al. 2022

Credits to A. Font-Ribera

Preliminary

Wiener filter map on eBOSS data

Lyman-α forest

Quasar

Reconstructed Lyman-α  contrast

Void

Ravoux et al. 2020

Applications

  • Identification of proto-cluster candidates:
    • Densest regions of the map with density threshold
    • Criteria on the number of crossed lines-of-sight

 

  • High redshift cosmic voids:
    • 3D spherical void finder
    • Void can be used to probe the dynamic of the cosmic web

Ravoux et al. 2020

Ravoux et al. 2020

Lyman-α / Void cross correlation

  • Distance between void centers and Lyman-α flux contrast:

eBOSS measurement

Ravoux et al. 2022

3D cross correlation with voids

Multipole expansion

  • Use of a spherical decomposition to measure deformation of voids
  • Effect of velocities seen in eBOSS data

 

Hexadecapole

r x Monopole

Quadrupole

View of velocity
flow around voids at z > 2

\beta = \frac{b_{\eta} f}{b}= 0.52 \pm 0.05

Ravoux et al. 2022

Growth rate forecasts

  • RSD very effective for high-z
  • Peculiar velocities for low-z

 

  • Constraint improvement with combination of methods

Julian's plot

Methods

  • Growth rate measurement methods with peculiar velocities:

Density/momentum power spectra

Density-velocity comparison

Forward modeling

Maximum likelihood

\langle \delta_{\mathrm{g}}(k) \delta_{\mathrm{g}}(k) \rangle
\langle \delta_{\mathrm{g}}(k) \mathbf{v}(k) \rangle
\langle \mathbf{v}(k) \mathbf{v}(k) \rangle
(\delta_{\mathrm{g}}, \mathbf{v})
\nabla \cdot \mathbf{v}_{\mathrm{pred}} \propto-a H f \delta
\mathbf{v}_{\mathrm{measured}}

Covariance matrix

\begin{aligned} \mathcal{L}(p) \propto & \exp\left[-\frac{1}{2} \right. \end{aligned}

Velocities from SNIa (ZTF, LSST)

Density field from galaxy surveys (DESI, Euclid)

Covariance matrice computed from theory and coordinates

\delta
\delta
v
v
\Bigg]
-1
T
\left(C_{gg}\right)
\left(C_{gv}\right)
\left(C_{vv}\right)
\left(C_{gv}\right)