DEBASS: Dark Energy All-Sky SN Ia Survey

• Dark Energy Bedrock All-Sky Supernovae (DEBASS) is a program that collects a large, uniform low-redshift Type Ia supernova sample to serve as a precise anchor for dark energy studies.
• The initiative uses advanced photometric calibration and coordinated spectroscopic follow-up with DECam and instruments like WiFeS to achieve calibration precisions under 10 millimagnitudes.
• Forecasts project that DEBASS can enhance constraints on dark energy parameters by up to 60% in the Figure of Merit, significantly boosting the statistical power of cosmological analyses.

The Dark Energy Bedrock All-Sky Supernovae (DEBASS) program is a dedicated initiative to assemble the largest, most homogeneous, and precisely calibrated low-redshift ($z$) Type Ia supernova (SN Ia) sample in the southern hemisphere, specifically to serve as the foundational anchor for present and future cosmological studies of dark energy. Leveraging the Dark Energy Camera (DECam) and coordinated spectroscopic follow-up, DEBASS is designed to replace historic, disparate low-$z$ samples—thereby sharply reducing calibration systematics, improving the Hubble diagram anchor, and enhancing the overall statistical power for measuring the cosmic expansion, isotropy, and structure growth.

1. Scientific Rationale and Program Design

The principal motivation for DEBASS is the persistent limitation in cosmological studies arising from the heterogeneity of historical low-$z$ SN Ia samples. Previous compilations relied on observations from multiple telescopes, bands, and calibration schemes, leading to cross-sample photometric offsets and increased systematic uncertainties, particularly at the vital low-$z$ end of the Hubble diagram. Such offsets can propagate into uncertainties on the absolute calibration ($\mathcal{M}$) and distance modulus ($\mu$) and, consequently, into the derived constraints on the dark energy equation-of-state parameters ($w_0$, $w_a$).

DEBASS addresses this by observing all low-$z$ SN Ia candidates and their host galaxies with DECam on the 4m Blanco Telescope and, for spectroscopy, with instruments such as WiFeS. This unified approach yields a data set with consistent photometry, astrometry, and spectral typing, comparable directly to high-$z$ samples from the Dark Energy Survey (DES) and other next-generation cosmological surveys. The program is integrated with the broader DES and DECam Local Volume Exploration Survey (DELVE) data pipelines for robust operational synergy and calibration transfer.

2. Observational Strategy and Data Acquisition

DEBASS targets all-sky supernova discovery down to $z \approx 0.08$, producing both time-series multi-band (typically $griz$) photometry and host-galaxy spectra for redshift confirmation and environmental studies. The program utilizes an adaptive cadence to optimize light-curve coverage near maximum light for precise standardization. To support absolute calibration, tertiary standard stars within DEBASS fields are cross-calibrated with DELVE, DES, and Pan-STARRS1 photometric systems, achieving internal agreement at the $\lesssim$10 millimagnitude level.

As of mid-2025, DEBASS has identified and confirmed more than 400 SNe Ia, with an early public data release (DR0.5) of 77 SNe located within the DES footprint. These initial data demonstrate the program's capability to deliver high signal-to-noise light curves and robust calibrations across the surveyed sky.

3. Calibration, Systematics Control, and Light-Curve Modeling

A core feature of DEBASS is the stringent attention to photometric calibration and systematic uncertainties. The program employs several independent calibration pathways (e.g., DES/DELVE versus the alternative Popovic et al. solution), with all yielding residual offsets of only a few millimagnitudes when cross-compared with external catalogs like Foundation. Such control is essential, since even small zeropoint shifts ($\sim$0.01 mag) can mimic cosmological signals at the level of evolving dark energy in current analyses.

Bias correction and validation are extensively studied via detailed end-to-end image and population simulations. These include the survey’s real cadence, triggering and detection efficiencies, and modeled distributions of SN Ia stretch ($x_1$), color, and host properties. The resulting bias-corrected Hubble residual (scatter relative to the cosmological fit) is 0.08–0.10 mag across subsets—an improvement not typically achieved by previous low-$z$ samples. This scatter is consistent with simulation-based expectations in at least 10% of cases, reflecting accurate recovery of underlying population and observing effects.

A consistent light-curve fitting procedure, based on the latest SALT3 or equivalent models, is applied to all SN light curves. The standardized distance modulus is computed with the Tripp estimator: $\mu = m_B + \alpha x_1 - \beta c - \mathcal{M}$ where $m_B$ is the rest-frame peak $B$-band magnitude, $x_1$ and $c$ are the fitted stretch and color parameters, $\alpha$, $\beta$ are global coefficients, and $\mathcal{M}$ encodes the absolute magnitude and Hubble constant.

4. Early Results, Legacy Comparisons, and Quality Metrics

In its first data release, DEBASS achieves a robust median absolute deviation (RSD) in Hubble diagram residuals of $\sim$0.10 mag, with the intrinsic scatter ($\sigma_{\rm int}$, prior to bias correction) at about 0.12 mag. These results reflect only preliminary processing—further bias corrections are expected to reduce the scatter further, especially as the full simulation suite is tuned to the finalized selection functions and host-galaxy properties.

Comparison of DEBASS SN distances to those from the Foundation sample yields a median residual offset of $0.016 \pm 0.019$ mag (DES calibration) and $-0.015 \pm 0.019$ mag (Popovic et al. calibration), both statistically consistent with zero. Such millimagnitude-level offsets are well within the calibration tolerances required for current and next-generation dark energy analyses, validating DEBASS’s role as a replacement for historic low-$z$ samples.

The initial measurement of the host-galaxy mass step ($0.06 \pm 0.04$ mag, before bias correction) and the absence of significant selection effects on distance moduli in the simulations further strengthen the sample’s cosmological utility.

5. Forecasted Impact on Dark Energy Cosmology

Fisher matrix forecasts incorporating the full DEBASS low-$z$ sample ($n>400$) integrated with existing and future high-$z$ datasets, show substantial improvement in statistical precision for dark energy analyses:

• Uncertainties in the equation-of-state parameters are projected to improve by $\sim$30% for $w_0$ and $\sim$24% for $w_a$ due purely to the replacement of legacy low-$z$ anchors.
• The Dark Energy Task Force Figure of Merit (FoM), defined approximately as $\propto 1/\sqrt{\det[\mathrm{Cov}(w_0, w_a)]}$, is forecast to rise by up to 60%.
• A competitive constraint on the growth rate parameter, $f\sigma_8$, at the 25% level becomes possible.

These forecasts reflect the critical leverage that a calibrated low-$z$ anchor exerts on both the normalization and the shape of the Hubble diagram, and on the detangling of systematic versus cosmological effects, especially when testing for transitions or evolution in $w(z)$.

6. Integration with Broader Survey Infrastructure and Future Prospects

By applying the same photometric instrumentation and reduction pipelines as DES, and enabling seamless cross-matching with DELVE and Pan-STARRS1 standards, DEBASS provides a self-consistent data set that bridges local and distant SN measurements. This uniformity circumvents many of the problematic cross-calibration and selection issues that can generate systematic biases mimicking, e.g., time-varying dark energy signals—a major concern expressed in recent literature comparing DES and Pantheon+ results (Efstathiou, 13 Aug 2024, Notari et al., 18 Nov 2024).

With the current data and methodology framework, DEBASS forms the backbone of the low-$z$ supernova cosmology infrastructure for upcoming wide-field surveys (e.g., LSST, Roman). The scalable architecture and unified calibration approach set the stage for continued accumulation of hundreds to thousands of low-$z$ SNe Ia, enabling not only improved constraints on dark energy parameters but also robust cross-checks of isotropy, growth of structure, and reconciliation of $H_0$ measurements.

7. Summary Table: Key DEBASS Early Performance Metrics

Metric	Value/Range	Context
SNe Ia collected (2021–2025)	>400	Largest uniform low-$z$ SNe Ia sample in S. Hemisphere
Public early release	77 SNe (62 for cosmology)	In DES footprint, $0.005
Robust median Hubble scatter	$\sim$0.10 mag	Pre-bias-correction, $<$0.12 mag intrinsic
Calibration precision	$\leq$10 millimag	DEBASS, DES, and Pan-STARRS1 agreement
Median DEBASS–Foundation offset	$\pm$0.016 mag	Demonstrates consistency
Forecast $\Delta w_0$, $\Delta w_a$	+30%, +24% (statistical improvement)	With full DEBASS inclusion
FoM improvement	Up to 60%	vs. historic low-$z$ samples
$f\sigma_8$ constraint	$\sim$25%	Low-$z$ SNe leverage

The DEBASS program thus establishes a technical and statistical “bedrock” for supernova cosmology, providing an internally consistent low-$z$ anchor for precision measurements of dark energy and the cosmic expansion history (Acevedo et al., 14 Aug 2025, Sherman et al., 14 Aug 2025).