Probing the expansion of the Universe

arnaud.de-mattia@cea.fr

hugo.simon@cea.fr

DPhP, Jan. 16th

Probing the expansion of the Universe

In 1929: more distant galaxies are moving away from us more rapidly

Edwin Hubble at Mount Wilson

Probing the expansion of the Universe

In 1929: more distant galaxies are moving away from us more rapidly

\(\mathrm{velocity} = c \times \mathrm{redshift}\; z\)

\(\mathrm{distance} \propto (\mathrm{apparent ~luminosity})^{-1}\)

\(\mathrm{slope} = H_0\)

Probing the expansion of the Universe

In 1998: the expansion of the Universe is accelerating!

This is
DARK
ENERGY!

\(\mathrm{magnitude} \propto\log(\mathrm{distance})\)

\(\mathrm{redshift}\; z\)

Probing the expansion of the Universe

In 2024: dark energy is not constant(?) in time!

galaxies

\(\mathrm{redshift}\; z\)

Spectroscopic redshift surveys

10 years = \(10 \times \)

DESI Y5 galaxy samples

Bright Galaxies: 14M (SDSS: 600k)

0 < z < 0.4

LRG: 8M (SDSS: 1M)

0.4 < z < 0.8

ELG: 16M (SDSS: 200k)

0.6 < z < 1.6

QSO: 3M (SDSS: 500k)

Lya \(1.8 < z\)

Tracers \(0.8 < z < 2.1\)

Y5 \(\sim 40\)M galaxy redshifts!

\(z = 0.4\)

\(z = 0.8\)

\(z = 0\)

\(z = 1.6\)

\(z = 2.0\)

\(z = 3.0\)

Mayall Telescope - DESI instrument

Mayall Telescope

focal plane 5000 fibers

wide-field corrector

6 lenses, FoV \(\sim 8~\mathrm{deg}^{2}\)

Kitt Peak, AZ

4 m mirror

Mayall Telescope

focal plane 5000 fibers

fiber view camera

ten 3-channel spectrographs

49 m, 10-cable fiber run

Kitt Peak, AZ

Focal plane: 5000 robotic positioners

86 cm

0.1 mm

robots w/ 2 rotation axes

position spectroscopic fibers

Spectroscopic pipeline

wavelength

fiber number

\(z = 2.1\) QSO

\(z = 0.9\) ELG

Ly\(\alpha\)

CIV

CIII

[OII] doublet at \(3727 \AA\) up to \(z = 1.6\)

[OII]

Ly\(\alpha\) at \(1216 \AA\) down to \(z = 2.0\)

Probing the expansion of the Universe = measuring how physical
distances change with time
 (cosmological redshift)

Standard ruler

SN1a are standard
candles

standard intrinsic
luminosity

+
flux measurement
=

distance at given \(z\)

standard scale
+
angle measurement
=

distance at given \(z\)

Standard ruler

Is there any standard length scale

in the distribution of galaxies?

Probing the expansion of the Universe = measuring how physical
distances change with time
 (cosmological redshift)

Baryon acoustic oscillations

In the early Universe, plasma of baryons and photons tightly coupled through Thomson diffusion

CAASTRO, https://www.youtube.com/watch?v=jpXuYc-wzk4. WiggleZ team, Swinburne University

DESI Y1 BAO measurements

(compressed) data

BAO peak

its position = "size of the bubble"

today

Big Bang

DESI Y1 BAO measurements

(compressed) data

BAO peak

high probability region (68%)

fractional matter density (the rest is ~ dark energy)

cosmological parameters

cosmological inference

Hubble constant \(\times\) sound horizon scale

DESI Y1 BAO measurements

DESI Y1 BAO measurements

DESI Y1 BAO measurements

DESI Y1 BAO measurements

DESI Y1 BAO measurements

add dynamical dark energy w/ 2 extra parameters: \(w_0, w_a\)

cosmological constant \(\Lambda\) (our best model so far)

\(3.9\sigma\) = probability of 0.01%

\underbrace{ \qquad\qquad }_{\mathrm{SN1a}}

Many other analyses possible!

In November, we released new measurements of how fast galaxies cluster together: test of general relativity on cosmic scales

DESI measurements

general relativity

Visite_Mines_2025

By Arnaud De Mattia

Visite_Mines_2025

  • 58