Unanchored Type Ia Supernovae

• Unanchored Type Ia supernovae are defined by using relative brightness–redshift relations without relying on external distance calibrators.
• They enable a cosmology‐agnostic reconstruction of the Hubble diagram through methods like Gaussian Process regression and adjusted light-curve standardization.
• This approach also highlights progenitor diversity, including sub- and super-luminous events that challenge the canonical Chandrasekhar-mass model.

Unanchored Type Ia supernovae denote, in the strict cosmological sense, Type Ia supernovae used only through their relative brightness–redshift relation, without tying the sample to an external absolute distance scale such as Cepheids, TRGB, BAO, or CMB, so that the Hubble diagram shape is retained while its overall normalization remains degenerate with $H_0$ or the absolute magnitude (Colaço, 8 Mar 2025). In a distinct but related usage, some progenitor-focused studies apply the term to peculiar under- and over-luminous SNe Ia that are not tied to the canonical Chandrasekhar-mass picture or to the standard Phillips-relation phenomenology (Kalita et al., 2018). Both usages concern the loss of an external anchor, but they operate at different levels: one in cosmological distance calibration, the other in progenitor physics and explosion diversity.

1. Definition and conceptual scope

Type Ia supernovae are “the thermonuclear disruption of a carbon–oxygen (C–O) white dwarf (WD) star in a binary system,” and their use as cosmological distance indicators rests on empirical standardization of light-curve width and color rather than on exact intrinsic uniformity (Blondin, 2024). In cosmology, an “anchored” SN Ia sample has an absolute-magnitude calibration tied to a local distance scale, whereas an “unanchored” sample is standardized internally but lacks an external absolute calibration and therefore constrains only relative distances (Blondin, 2024).

The two main usages can be separated as follows.

Usage	Core meaning	Representative source
Cosmological	Relative-distance SN Ia Hubble diagram without an external absolute scale	(Colaço, 8 Mar 2025)
Progenitor-diversity	Peculiar Ia events not tied to the canonical $M_{\rm Ch}$ picture or standard-candle behavior	(Kalita et al., 2018)

For cosmological analyses, the standard distance-modulus relation is

$$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$

so any inference based only on relative SN magnitudes is intrinsically degenerate with the absolute magnitude $M_B$ and hence with $H_0$ (Colaço, 8 Mar 2025). This is why unanchored SNe Ia are powerful probes of the shape of the expansion history but cannot determine the absolute distance scale from SN data alone (Howell, 2010).

2. Relative-distance formalism and internal standardization

The observational basis of unanchored SN Ia cosmology is the standardizable-candle framework. Empirically, “More luminous SNe Ia decline more slowly past maximum light … This is known as the width–luminosity relation,” and standardization is commonly written in the generic form

$$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$

with a residual dispersion $\sim 0.1–0.15$ mag after correction (Blondin, 2024). In the classical stretch construction, one measures a width parameter $s$ by rescaling the rest-frame light-curve time axis and then builds a standardized light-curve template without fixing an absolute luminosity zero-point (Coelho et al., 2014).

This internal standardization is exactly the regime in which an unanchored sample operates. The absolute magnitude is not supplied externally; instead, only relative moduli are used, and the nuisance combination of $M$ and $H_0$ is floated or marginalized (Howell, 2010). In pedagogical form, this is why SN-only Hubble diagrams constrain the relative luminosity distance as a function of redshift rather than $M_{\rm Ch}$0 itself (Coelho et al., 2014).

A compact way to express the same idea is to define the unanchored luminosity distance

$M_{\rm Ch}$1

which removes any need to specify either $M_{\rm Ch}$2 or $M_{\rm Ch}$3 at the reconstruction stage (Colaço, 8 Mar 2025). In the Pantheon$M_{\rm Ch}$4-based implementation, the corrected apparent magnitude is related to $M_{\rm Ch}$5 through

$M_{\rm Ch}$6

so that

$M_{\rm Ch}$7

with $M_{\rm Ch}$8 the intercept of the SN Hubble diagram determined from SN data alone (Colaço, 8 Mar 2025). The Pantheon$M_{\rm Ch}$9 compilation used in this framework contains 1701 light curves of 1550 SNe Ia over $$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$0, and the associated covariance matrix includes both statistical and systematic errors, with the intercept uncertainty fully correlated across the sample (Colaço, 8 Mar 2025).

To obtain a continuous, model-independent $$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$1, Gaussian Process regression is applied with zero mean and a squared-exponential kernel,

$$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$2

where the hyperparameters $$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$3 are fixed by maximizing the GP marginal likelihood (Colaço, 8 Mar 2025). The reconstruction is tight at low redshift where SNe are abundant and broadens at high redshift where the data are sparse (Colaço, 8 Mar 2025). This yields a continuous relative-distance ladder without assuming $$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$4CDM or any explicit parameterization of $$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$5.

3. Anchoring an unanchored SN sample with absolute distances

The principal recent realization of unanchored SN Ia cosmology is the combination of reconstructed $$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$6 with absolute angular-diameter distances from strong-lensing time delays (Colaço, 8 Mar 2025). The essential link is the cosmic distance duality relation,

$$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$7

which is rewritten as

$$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$8

This isolates the role of the anchor: SNe supply the relative luminosity distance, while an independent probe supplies an absolute $$\mu(z)=m_B(z)-M_B=5\log_{10}\!\left(\frac{D_L(z)}{\mathrm{Mpc}}\right)+25,$$9 at the same redshift (Colaço, 8 Mar 2025).

For a time-delay strong-lensing system with lens redshift $M_B$0 and source redshift $M_B$1, the time-delay distance is defined as

$M_B$2

and, for a two-image SIS lens,

$M_B$3

In an FRW universe, $M_B$4, so $M_B$5 (Colaço, 8 Mar 2025).

The specific implementation uses 12 two-image SIS systems from Balmès & Corasaniti (2013) and 7 high-precision H0LiCOW systems from Wong et al. (2019). After requiring that both lens and source redshifts lie within the SN redshift range, 15 of 19 lenses are retained, spanning $M_B$6 and $M_B$7 (Colaço, 8 Mar 2025). Under spatial flatness,

$M_B$8

and with $M_B$9, one obtains a dimensionless SN-based prediction

$H_0$0

that depends only on the GP-reconstructed $H_0$1 at $H_0$2 and $H_0$3 (Colaço, 8 Mar 2025).

The comparison to lensing distances is performed through

$H_0$4

with an MCMC analysis using a flat prior $H_0$5 km/s/Mpc and an additional intrinsic scatter $H_0$6 added in quadrature to account for lens-environment and modeling systematics (Colaço, 8 Mar 2025). The resulting determination is

$H_0$7

at 68% confidence level (Colaço, 8 Mar 2025). In the paper’s comparison, this agrees with late-universe local measurements such as SH0ES and is in mild tension with Planck 2018 under flat $H_0$8CDM (Colaço, 8 Mar 2025).

In this construction, “model-independent” means not assuming any specific form for $H_0$9, no $$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$0CDM prior, and no BAO or CMB sound-horizon calibration; the remaining assumptions are the validity of CDDR, spatial flatness, and lens modeling (Colaço, 8 Mar 2025).

4. Distinct astrophysical usage: peculiar and non-canonical SNe Ia

A separate literature uses “unanchored” to describe SNe Ia whose progenitors or observables are not anchored to the canonical Chandrasekhar-mass scenario. This usage is rooted in the fact that SNe Ia show multiple progenitor channels—single-degenerate near-$$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$1, double-degenerate mergers and collisions, and double detonations in sub-$$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$2 WDs—and that several lines of evidence point towards the existence of multiple progenitor channels in order to explain the observed diversity (Blondin, 2024).

One version of this usage concerns under- and over-luminous SNe Ia that do not line up with the canonical $$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$3 picture. In higher-order Starobinsky-$$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$4 gravity, a single fixed gravity model can yield a limiting white-dwarf mass of $$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$5 for central density $$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$6 g/cc and $$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$7 for $$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$8 g/cc, thereby offering a unified framework for under- and over-luminous events (Kalita et al., 2018). In a broader synthesis of magnetized and modified-gravity white dwarfs, super-Chandrasekhar limiting masses of $$m_{B}^{\rm corr} = m_{B} + \alpha x_1 - \beta c + \ldots$$9 for magnetic models and $\sim 0.1–0.15$0 for modified-gravity models are proposed, alongside sub-Chandrasekhar limits down to $\sim 0.1–0.15$1 (Mukhopadhyay, 2015). Those papers explicitly question the uniqueness of the Chandrasekhar limit and argue for a possible second standard candle (Mukhopadhyay, 2015).

A different realization appears in failed-detonation models. In the failed GCD scenario, the deflagration begins in a Chandrasekhar-mass C–O WD, but no detonation is triggered; the WD remains partially bound, ejecta masses can be as low as $\sim 0.1–0.15$2, and the mass-weighted ejecta velocities lie between 3,730 and 5,229 km s$\sim 0.1–0.15$3, substantially below normal SNe Ia (IV et al., 2012). These objects are predicted to appear as sub-luminous low-velocity SNe Ia and are explicitly described as a concrete physical realization of an “unanchored” Ia class related to SN 2002cx/SN 2008ha-like events (IV et al., 2012).

The same theme appears in galactic chemical evolution. In dwarf spheroidal galaxies, the subclasses of sub-Chandrasekhar double detonations and SN Iax-like Chandrasekhar-mass carbon deflagrations are described as rarer than normal SNe Ia and chemically negligible in the solar neighborhood, yet potentially very important in metal-poor systems with stochastic star formation (Kobayashi et al., 2015). Their distinct Mn yields lead to opposite $\sim 0.1–0.15$4 trends—low for sub-Chandrasekhar events and high for SN Iax—so a mixture of the two is favored by the available dSph observations (Kobayashi et al., 2015).

This astrophysical usage is therefore about progenitor and yield diversity rather than about the absolute calibration of the Hubble diagram. A plausible implication is that the same adjective marks different kinds of missing anchor: in one case the missing anchor is an absolute distance scale, in the other a unique Chandrasekhar-mass progenitor paradigm.

5. Systematics, evolution, and interpretive limits

Unanchored SN Ia cosmology removes the external absolute scale, but it does not remove astrophysical or observational systematics. Even when only relative distances are used, cosmological inferences assume that the standardized luminosity–shape–color relation is universal or at least well modeled across redshift (Blondin, 2024). The same review emphasizes that if the mix of progenitor channels evolves with cosmic time, the standardized absolute magnitudes could shift with redshift, mimicking or biasing cosmological signatures such as the apparent acceleration (Blondin, 2024).

Host-galaxy dependencies exemplify this limitation. In SNLS3, SNe in high- and low-mass hosts differ by $\sim 0.1–0.15$5 mag in corrected absolute magnitude, and not accounting for this would bias $\sim 0.1–0.15$6 by $\sim 0.1–0.15$7 (Howell, 2010). Dust, rest-frame $\sim 0.1–0.15$8-band calibration, K-corrections, and population drift are likewise described as leading systematics in SN Ia cosmology (Howell, 2010). A plausible implication is that an unanchored Hubble diagram is still only as robust as its treatment of redshift-dependent population and calibration effects.

Not all observed diversity is equally damaging. In the asymmetric delayed-detonation interpretation of normal SNe Ia, the HVG/LVG spectral dichotomy and nebular line shifts arise from viewing-angle effects associated with off-center ignition, and the paper concludes that “the spectral evolution diversity is no longer a concern in using SNe Ia as cosmological standard candles” (Maeda et al., 2010). This reduces one specific class of feared “unanchored” diversity by reinterpreting it as geometric rather than as evidence for multiple luminosity families (Maeda et al., 2010).

At low redshift, peculiar velocities are an additional concern because they perturb the Hubble relation. A field-level Bayesian Hierarchical Model based on a BORG analysis of the 2M++ spectroscopic galaxy catalogue shows that, in simulated local-universe analogues, the true $\sim 0.1–0.15$9 is recovered once non-linear peculiar velocities and their correlated uncertainties are modeled, while ignoring peculiar velocities shifts the inferred $s$0 only by $s$1 km s$s$2 Mpc$s$3 in the range $s$4 (Tsaprazi et al., 12 Feb 2025). The paper concludes that it is unlikely that the $s$5 tension originates in unaccounted-for non-linear velocity dynamics (Tsaprazi et al., 12 Feb 2025).

6. Cosmological significance and future development

The original SN Ia evidence for acceleration was, in a precise sense, largely unanchored. Because SNe Ia constrain the curvature of the Hubble diagram, they can distinguish acceleration from deceleration without fixing the absolute luminosity scale, and therefore constrain dimensionless expansion-history quantities such as $s$6 or the shape of $s$7 (Blondin, 2024). This is why SN-only analyses can measure the relative expansion history and constrain dark-energy parameterizations while treating $s$8 or the combination $s$9 as a nuisance parameter (Howell, 2010).

What unanchored SNe Ia cannot do by themselves is measure $M$0. A separate anchor is required, whether from local geometric distances, BAO, strong-lensing time delays, or gravitational waves as standard sirens (Colaço, 8 Mar 2025). The strong-lensing application illustrates the general logic: SNe supply a relative distance ladder, and an external absolute ruler or clock supplies the normalization (Colaço, 8 Mar 2025).

In the present literature, the unanchored-SN-plus-lensing determination yields $M$1 km/s/Mpc and therefore supports a higher late-universe value than Planck, while remaining less precise than SH0ES or Planck; the paper explicitly characterizes the method as “more a consistency check than a high-precision competitor” (Colaço, 8 Mar 2025). This suggests that unanchored SNe Ia are presently most valuable as cosmology-agnostic cross-checks: they isolate the relative-distance information carried by SNe Ia and test whether independent anchors lead to consistent absolute scales.

Future improvements are expected on both sides of the construction. More and better SNe at higher redshifts tighten the reconstruction of $M$2, while more well-modeled strong lenses and lensed supernovae improve the absolute distance anchor; Euclid, Roman, Rubin, and JWST are explicitly identified as surveys that can sharpen this cosmology-agnostic approach (Colaço, 8 Mar 2025). A plausible implication is that the long-term importance of unanchored SNe Ia will lie not only in $M$3 inference but also in diagnosing which parts of the SN Ia cosmology pipeline are empirical, which are geometric, and which remain vulnerable to population evolution.