Dark Energy Spectroscopic Instrument

cosmology results from the Data Release 1

Otávio Alves

University of Michigan

Outline

1. DESI
2. The instrument
3. Baryon acoustic oscillations
4. Cosmology results
5. The future

Dark Energy Spectroscopic Instrument

• Kitt Peak National Observatory
• Nicholas Mayall telescope
• Tohono O’odham Nation
• 40M spectra

Dark Energy is our main target 🎯

Characterize the expansion
history in the Dark Energy era Baryon acoustic oscillations

Characterize growth of
large scale structure Redshift-space distortions

Dark Energy Task Force (2006, 2012)

• Stages I & II: Discovery phase & 2000s
• Stage III: BOSS/eBOSS, DES, +
• Stage IV: DESI, LSST, +

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gantt
  dateFormat  MM-YYYY
  axisFormat  %Y
  section Espectroscópico
  eBOSS: 07-2014, 6y
  DESI: desi, 03-2021, 5.5y
  +ext: desi-ext, after desi, 3y
  DESI-2: after desi-ext, 6y
  Spec-S5: 2037, 6y
  section Fotométrico
  DES:  08-2013, 6y
  LSST: 2025, 10y

The instrument

SDSS

DESI

5000 spectra in 30 minutes

at high resolution

Exposure times:

• DESI: 15 minutes
• eBOSS: 1 hour

Resulting in a 3D map of galaxies and quasars

Legacy surveys

Observations >70% completed

Baryon Acoustic Oscillations

Physical mechanism

• From Big Bang to \(z \approx 1100\), baryons and photons are strongly coupled.

• Acoustic waves propagate at:

\[ c_\text{s}(z) = \frac{c}{\sqrt{3\left(1+\frac{3}{4}\frac{\rho_\text{B}(z)}{\rho_\gamma(z)}\right)}} \]

Physical mechanism

• At \(z \approx 1100\), temperature is low enough to form HI. Acoustic oscillations stop.

• Acoustic modes can be observed in the distribution of matter (and photons). Fundamental mode is given by:

\[ r_\text{d} = \int_{z_\text{d}}^\infty \frac{c_\text{s}(z)}{H(z)} dz \]

BAO in the galaxy distribution

BAO as a standard ruler

The BAO measurements

Covariance matrices

theoretical covariances of power spectrum multipoles

• Based on Wadekar & Scoccimarro 2019.
• Trispectrum at tree-level using Kobayashi 2023.

Validated against mocks

Cosmology results

The BAO measurements

Combining BAO from multiple redshifts

Adding info on the sound horizon scale

Yields an \(H_0\) constraint

Neutrinos

Neutrino masses

Neutrino oscillations constrain squared mass differences.

Lowest total mass of the neutrino sector:

• Normal hierarchy: 0.059 eV
• Inverted hierarchy: 0.10 eV

CMB is sensitive to the sum of masses

angular diameter distance
to recombination neutrino effects in
CMB lensing

But thats’s degenerate with the Hubble constant and matter density.

Combining with DESI BAO

Combining with DESI BAO

Dark Energy

No \(w_0 w_a\) in DESI, CMB or SN alone

\(w(a) = w_0 + w_a(1-a)\)

DESI+SN+CMB \(\rightarrow\) hints of time-evolving \(w\)

DESI 2024 papers

• DESI 2024 I: First year data release
• DESI 2024 II: DR1 catalogs
• ✅ DESI 2024 III: BAO from Galaxies and Quasars at z < 2
• ✅ DESI 2024 IV: BAO from the Lyman-α Forest at z > 2
• DESI 2024 V: RSD from Galaxies and Quasars at z < 2
• ✅ DESI 2024 VI: Cosmological constraints from BAO measurements
• DESI 2024 VII: Cosmological constraints from RSD measurements
• + many supporting papers.

P.S.: Next BAO release is right around the corner.

Expectations for full-shape measurements

\(S_8\) “tension”

Neutrino masses: addition of broadband information,

Dark energy: impact on growth of structure,

+ other models beyond ΛCDM.

Future

There’s margin for further improvement

BGS
cosmic variance limited

LRG
new targets and easy redshifts

ELG
more targets than fibers

QSO
hard to identify new targets

DESI extension

• Same instrument
• +20% area
• +50% overlap with LSST
• +50% LRG targets

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gantt
  dateFormat  MM-YYYY
  axisFormat  %Y
  section Espectroscópico
  eBOSS: 07-2014, 6y
  DESI: desi, 03-2021, 5.5y
  +ext: desi-ext, after desi, 3y
  DESI-2: after desi-ext, 6y
  Spec-S5: 2037, 6y
  section Fotométrico
  DES:  08-2013, 6y
  LSST: 2025, 10y

DESI-2

• Focus at higher redshifts: \(2.2 < z < 5\)
  • More linear modes for primordial physics studies
  • Dark Energy in the matter-dominated era

• Instrument upgrades: Skipper CCDs for blue
• New tracers: Lyman-break galaxies (LBG) & Lyman-α emitters (LAE)

Early results from both DESI and LSST will shape future priorities

Together with a potential DESI upgrade, they will inform the design of a next-generation spectroscopic survey by telling us which potential science goals should be emphasized.

P5 2023 report

Spec-S5

• 2 telescopes: Mayall + Blanco

• Mirror upgrade: from 4m to 6m

• Instrument upgrade: 3 fibers per positioner = 26k fibers per telescope

• Science cases:

  • High redshift program
  • High number density at \(z < 2\)
  • Milky Way science (50M stellar spectra), stellar streams, dark matter

Summary

• Galaxy spectroscopic surveys entered a new era with DESI
• DR1 BAO measurements have interesting implications for:
  • Neutrinos, Dark Energy
• Full-shape measurements coming up soon
• Next BAO release not long after that
• Spectroscopic surveys = good data for a few decades
• DESI-2 will explore higher redshifts
• Spec-S5 will push constraints in primordial physics