Cosmology & Photometric Redshifts
What is the universe made of, and why do the two measurements of its expansion disagree?
Why it matters
The Hubble constant measured from the local distance ladder and the value inferred from the cosmic microwave background disagree by more than their stated uncertainties. Gravitational-wave events with identified optical counterparts offer a third route that shares no calibration with either, which is why the counterpart searches under theme 01 are also a cosmology program.
Redshifts are the other half. A photometric redshift is only as good as the spectral features the filter set can resolve, and medium bands resolve the 4000 Å break and the brighter emission lines that broadband surveys blur together.
What 7DS contributes
At R = 30–70 across 0.4–0.9 µm, 7DT straddles the boundary between broadband photometry and low-resolution spectroscopy, capturing the 4000 Å break and prominent emission lines at a resolution that lifts much of the color–redshift degeneracy inherent to broadband surveys. Forecasts give σ_NMAD = 0.003–0.007 at 19 < m625 < 22 for the five-year stacked WTS, with strong gains when combined with SPHEREx all-sky data.
What the program aims to deliver
- Cosmological parameters from several independent methods, including standard sirens
- Photometric redshifts precise enough to map large-scale structure out to redshift 1
These are program targets over the seven years of the survey, not results in hand. What has been observed so far is on the status page, and published work is listed under publications.