The brightest standard candles.
Type Ia supernovae and Cepheid variables remain the most precise distance probes available to extragalactic astronomy. CELESTE extends their reach and sharpens their precision simultaneously.
Imagery · ESO / S. Brunier
Key Science Topic
Dark energy &
Dark energy &
the Hubble Tension.
Sharpening the cosmic distance ladder that anchors both the local Hubble constant and the dark-energy equation of state.
The local universe and the early universe report two different expansion rates. The Cepheid-anchored distance ladder yields H₀ ≈ 73 km s⁻¹ Mpc⁻¹; cosmic microwave background measurements give 67. The gap, now in excess of 5σ, refuses to close — and may signal new physics beyond the standard cosmological model.
CELESTE directly attacks this tension. Diffraction-limited visible imaging on giant ground apertures sharpens every rung of the cosmic distance ladder — calibrating Cepheids and Type Ia supernovae with unprecedented precision in galaxies out to 100 Mpc. The combined improvement is expected to drive the local-universe H₀ uncertainty below one percent.
New standardisation techniques such as Twins Embedding use rest-frame visible spectroscopy to pair supernovae that match in their spectroscopic signatures, dramatically reducing residual scatter in inferred distances. CELESTE plus Keck delivers the high-throughput visible spectroscopy this approach requires for the high-redshift end of the sample.
The Brief
i.
Science Objectives
Measure the local Hubble constant to sub-percent precision, resolving the present 5σ tension between local and CMB measurements of cosmic expansion. Separately, constrain the dark-energy equation of state by enhancing the Roman and Rubin LSST supernova programs.
ii.
Observables & Measurements
Diffraction-limited photometry of Cepheid variables in galaxies out to 100 Mpc, removing crowding bias. Rest-frame visible spectra of Type Ia supernovae out to redshift 1.7 for Twins Embedding standardisation.
iii.
Key Functional Requirements
Angular resolution ≤ 15 mas at 500 nm to resolve individual Cepheids in crowded fields. Stable photometric calibration to 0.5% over multi-orbit campaigns. Three-octave spectroscopic coverage (0.5 – 2.0 μm) for supernova follow-up.
iv.
CELESTE Uniqueness
CELESTE provides four times the angular resolution of Hubble in the visible, eliminating the crowding systematic that currently dominates Cepheid-distance uncertainties. For supernova spectroscopy, the diffraction-limited PSF dramatically reduces sky background within the extraction aperture, enabling high-SNR spectra at redshifts inaccessible to current facilities.