Large-Scale Structure with the Dark Energy Spectroscopic Instrument (DESI) ��

Kyle Dawson, University of Utah

On behalf of the DESI Collaboration

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14th Conference on the Intersections of

Particle and Nuclear Physics

Cosmic Discovery Space

Galaxies emitting in optical wavelengths and spatially resolved.

Galaxies and gas emission at redder wavelengths and spatially dense.

Hot dense state. Detectable relics include CMB, gravitational waves and neutrinos.

Astro. probes of Dark Matter

Modified

Gravity

Dark Energy

Inflation (Primordial structure)

Early Dark Energy

Inflation (Primordial Gravitational Waves)

New relativistic energy

(new particles, new phase transitions)

Acoustic Peak, Hubble constant

10^-3

Size (Mpc)

10²

10³

Size of a galaxy cluster

redshift

DESI/LSST

Neutrinos, hot dark matter, rate of structure growth

CMB-S4

BAO scale and larger

Cosmology with Surveys: A Staged Approach

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Imaging Spectroscopy Cosmic Microwave Background

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Stage-III Dark Energy Survey BOSS/eBOSS SPT/BICEP/Simons

(2013-2019) (2009-2019) (~2017 – )

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Stage-IV Rubin/LSST DESI CMB-S4

(~2024 – ) (2021 – ) (~2030 – )

Cosmology with Spectroscopic Surveys

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• Evolving distribution of matter in Universe
• Cosmic expansion and growth of structure​
• Derived Measurements: H(z), DM(z), fσ8(z)​
• Physics of dark energy​
• Composition of the Universe​
• Neutrino mass, Inflation, Laws of gravity

Expansion with Baryon Acoustic Oscillations

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• Friedmann Equation (includes all energy components):​

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• Baryon Acoustic Oscillations (BAO)
• Measure angular diameter distance and H(z)
• ​Provide relative distance estimates at many redshifts
• Constrain energy density as function of redshift

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DH

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DM

Growth History with Redshift Space Distortions

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• Linear growth equation:​ ​

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• Linear Growth Rate:​

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• Redshift Space Distortions (RSD) measure f sigma8

Samushia et al. 2013

Spectroscopy of the Cosmic Density Field

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Direct tracers Absorption in quasar spectra by​

Galaxies and quasars (z<2.1) foreground Lyman-alpha forest (z>2.1)​

Stage-III Dark Energy Surveys: BAO + RSD

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Stage-III Cosmological Model (eBOSS et al. 2021)

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• ∼1% precision estimates on the dark energy density, H₀, and amplitude of matter fluctuation regardless of cosmological model
• w_p(z=0.36) = -1.013 +/- 0.029 in w0waCDM, little degradation with w_a​
• 40X decrease in curvature/H0/sigma8/w0/neutrino mass posterior volume relative to Stage-II
• Planck+Pantheon+DES: additional 25X improvement → average 4X per parameter​​

Stage-III Cosmological Model (eBOSS et al. 2021)

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• ∼1% precision estimates on the dark energy density, H₀, and amplitude of matter fluctuation regardless of cosmological model
• w_p(z=0.36) = -1.013 +/- 0.029 in w0waCDM, little degradation with w_a​
• 40X decrease in curvature/H0/sigma8/w0/neutrino mass posterior volume relative to Stage-II
• Planck+Pantheon+DES: additional 25X improvement → average 4X per parameter​​

New Physics in Expansion History?​

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• BAO → insensitive to the strict cosmological priors in CMB-only estimates. ​
• BAO → insensitive to CMB anisotropies if using LCDM and BBN

Tension in Growth of Structure?

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• BAO + RSD → 3.5% precision on growth of structure ​
• CMB and DES: sigma8 tension, Omega_m tension, or no tension?

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Dark Energy Spectroscopic Instrument

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DESI survey covers over 14,000 deg² in 5 years!

• Located at 4-meter Mayall Telescope in Arizona
• Upgraded telescope for wide field spectroscopy
• Now dedicated to multi-object spectroscopy
• First Stage-IV Dark Energy Experiment
• Optimized for BAO measurements
• 10X improvement to w0wa posterior area compared

to Stage-II Type Ia supernovae measurements

• Comprehensive cosmology program
• Redshift Space Distortions
• Cross-correlations with other surveys
• More cosmology, galaxy evolution, and astrophysics

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DESI: Massively-multiplexed Spectroscopy

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DESI survey covers over 14,000 deg² in 5 years!

Focal plane assembly with 5000 fiber positioners

Mayall 4m telescope

10 spectrographs (360-980nm)

~1 minute to position fibers!

Uninterrupted Galaxy and Quasars from 0<z<3.5

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8 million Luminous Red Galaxies

(0.4 < z < 1.1)

16 million Emission Line Galaxies

(0.6 < z < 1.6)

3 million quasars + Ly-α forest (1 < z < 3.5)

13 million Bright Galaxies

(0.0 < z < 0.4)

DESI survey covers over 14,000 deg² in 5 years!

Plus 10 million Milky Way stars

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First Year Sample

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• 6.3M Bright Galaxies (z<0.5)
• 2.6M stars

• 2.7M LRGs (0.4<z<1.1)
• 4.1M Emission line galaxies (0.6<z<1.6)
• 1.7M QSOs (0.9<z<2.1 tracers & z>2.1 with Lyα forest)

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First results on arxiv (8 papers):

Validation of Spectroscopic Samples

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First Year Map

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Primary Science Driver: Dark Energy with BAO

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• Stage-III BAO distance measurements:
• 0.70% precision at z < 1
• 1.19% precision at z > 1​
• DESI BAO distance science requirement:
• 0.28% precision at z<1.1
• 0.39% precision at 1.1<z<1.9
• <1% precision at z>1.9
• Expect to exceed requirements by ~1.4X in final measurements

Early BAO results: Correlation function monopole

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BOSS+eBOSS vs. 2 months of DESI

Beyond BAO: Redshift Space Distortions

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• Stage-III Spectroscopy
• 4.78% precision over the redshift interval 0 < z < 1.5
• 3.5% precision on sigma8, no tension with Planck
• DESI: 21 independent measurements to z<2.1 with median 5.2% precision

Stage-III

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DESI (forecast)

Beyond BAO: Redshift Space Distortions

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• Stage-III Spectroscopy
• 4.78% precision over the redshift interval 0 < z < 1.5
• 3.5% precision on sigma8, no tension with Planck
• DESI: 21 independent measurements to z<2.1 with median 5.2% precision

Stage-III

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DESI (forecast)

Can we do better here?

Beyond BAO: Redshift Space Distortions

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• Stage-III Spectroscopy
• 4.78% precision over the redshift interval 0 < z < 1.5
• 3.5% precision on sigma8, no tension with Planck
• DESI: 21 independent measurements to z<2.1 with median 5.2% precision

Stage-III

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DESI (forecast)

What about this redshift range?

Beyond RSD: Growth of Structure

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• Year 1 strategy
• Collaboration-wide demonstration of primary BAO+RSD measurements
• Measurements of primordial non-Gaussianity in power spectrum
• Year 3 strategy (in discussion now)
• Collaboration-wide effort to enhance growth measurements over all redshifts (explore growth of structure tension)
• Introduce higher order statistics into cosmology results

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Beyond RSD: Growth of Structure

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• Potential Year 3 Key Projects
• z<0.1 peculiar velocities with Tully Fisher + Fundamental plane + SNe Ia
• 0.1<z<1.6 bispectrum (fnl and ~30% improvement on RSD)
• galaxy-galaxy lensing to constrain RSD nuisance terms
• CMB-DESI cross-correlations to directly measure sigma8(z)
• 1D power spectrum in Lyman-alpha forest for sigma8(z>2)

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• Expect sub-percent precision on sigma8 with final RSD analysis

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• What improvement on sigma8 is possible relative to RSD only?

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Beyond DESI: High Density Galaxy Clustering

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• 0<z<1.5: covers matter-dominated to dark energy dominated regimes
• Redshift range where most Dark Energy models are best explored
• Limited by theory and computing
• nP=1 approximates optimal balance of number density at a fixed scale
• 10,000 galaxies/sqdeg → kmax=1 h/Mpc for z<1.5
• 180M galaxies over Rubin footprint “saturates” cosmological information

Dawson et al. 2022 Courtesy: Johannes Lange (Snowmass)

Beyond DESI: High Redshift Galaxy Clustering

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• z>2: matter-dominated regime with massive volume
• Experimentally limited with current facilities
• Spectroscopy of 10’s of millions of galaxies over the Rubin footprint would provide high precision BAO, RSD, neutrino mass, and inflationary constraints
• Targets for spectroscopy
• Plentiful (and faint) Lyman-break and Lyman-alpha emission galaxies
• Spectroscopy challenging

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Beyond DESI: Stage-V Roadmap

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• DESI will remain premier spectroscopic facility in late 2020’s
• z<1 galaxies easily measured to zfib<21.6
• z>2 Lyman Break galaxies: ~300/sqdeg
• z>2 Lyman-alpha emission galaxies: >1000/sqdeg w/new imaging
• >20M new galaxies overlapping a 10,000 sqdeg Rubin footprint
• Sub-percent BAO precision, percent level RSD precision possible at z>2
• Immediate tests of concordance cosmology
• Pilot new programs during Stage-V construction

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Stage-III: BOSS+eBOSS

2005 2010 2015 2020 2025 2030 2035 2040

Stage-IV:

DESI

Stage-V

extended

Summary

Credit: Clara Delabrouille

• Check arxiv for first major release (1pt statistics)
• DESI likely to exceed BAO science requirement
• Demonstrate RSD precision in ~1.5yrs
• Explore sigma8 improvements with 3yr sample
• Snowmass: motivate massive Stage-V expansion over all redshifts

Most important: Thank you to the firefighters and the NOIRLab staff

Thanks to our sponsors and �69 Participating Institutions!