How Much Do SN Ia Cosmological Constraints Depend on Our Knowledge of

Supernova Physics?David Jones

Moore Fellow, UC Santa Cruz

August 9, 2019

• A few of the progenitor controversies so far in Lijiang: • Single degenerate vs. double degenerate • Steady accretion vs. merger • Double vs. triple systems

The Progenitor Question Applied to SN Ia Cosmology

• A few of the progenitor controversies so far in Lijiang: • Single degenerate vs. double degenerate • Steady accretion vs. merger • Double vs. triple systems

• But for cosmology, we care about only one thing: what do these models say about cause of dispersion in SN Ia Hubble residuals? • Dispersion sets the ~ceiling for the size of unknown systematics that

could depend on SN Ia physics

The Progenitor Question Applied to SN Ia Cosmology

Foley+18

• Type Ia supernovae are “standardizable candles”

• Intrinsically brighter SN Ia have broader light curves and can measure distances to ~6% accuracy.

• Parameters measured from SALT2 model (spectral principal components) and standardized by the Tripp formula:

• Dispersion caused by scatter in these relations, uncertainty in the model used to determine parameters, missing components from the model, or selection effects

Measuring Cosmological Parameters from SN Ia

fig credit: supernova cosmology project

An Exciting Time for SN Ia CosmologyBetter than ever constraints on dark energy, but evolution of

D.E. with z still poorly constrained LSST coming soon, WFIRST on the horizon

Scolnic, Jones+18

An Exciting Time for SN Ia CosmologyOur best end-to-end test of the cosmological model is H0 and

we’re failing the test

each systematic as a fraction of

σwstat=0.032 in Scolnic+18

Calibration 0.49 0.56

SN Modeling 0.30 0.78

MW Extinction/Pec. Vel. 0.21 0.39

CC SN Contamination 0.33

each systematic as a fraction of σwstat=0.038 in

Jones+18

• Measuring H0 requires comparing SNe in star-forming galaxies to those in all galaxies

• Measuring w requires comparing SNe at low-z to those at high-z • Systematic uncertainties related to Ia physics affect how we:

1. Correct for relationships between SN Ia and their host galaxies 2. Correct for distance (Malmquist) biases

Systematic Uncertainties Related to SN Physics Becoming More Important

systematics on recent measurements of D.E. equation of state

• The Foundation Supernova Survey will observe up to 800 z < 0.1 SN Ia on the Pan-STARRS telescope (PIs: Foley, Scolnic, Rest) • mmag-level photometric calibration • well-tested reduction and analysis pipeline • 5 Cepheid calibrators and counting • untargeted survey, understand selection effects better

• First data release: Foley+18 • Host Galaxies: Jones+18 • Dark energy: Jones+19 • H0: Scolnic+in prep

Combined Hubble diagram from the PS1 telescope:

~1,400 SNe to date (including some CC SN

contaminants in the high-z sample)

Measuring More Accurate Cosmological Parameters by Building a Better Census of Nearby SNe Ia

Jones+19, arXiv: 1811.09286

• SN Ia progenitor properties must correlate with their host galaxy properties

• Causes systematic differences in SN Ia properties between low-z/high-z and Cepheid calibrator/Hubble flow samples

The Role of Supernova Host Galaxies

Kelly+10, Lampeitl+10, figure from Sullivan+10

Distances inferred from SN Ia appear to depend on their host galaxy mass (+/-

0.03 mag) and we don’t know why

7 8 9 10 11 12LOG host galaxy Mstellar (MO • )

-0.4

-0.2

0.0

0.2

0.4

mBco

rr - m

Bmod

(s, C

)s≥1s<1Mean residual

SNLS

0.0 0.2 0.4 0.6 0.8 1.0Relative number

High-MstellarLow-Mstellar

Hubb

le R

esid

ual

The Role of Supernova Host Galaxies

The Relationship Between SN Ia and their Host Galaxies

• We correct for host mass step, but what if the host mass dependence is tracing: • Metallicity: Hayden+13, Pan+14 • Star formation rate: Rigault+13,

Rigault+15, Jones+15 • Specific star formation rate:

Rigault+18 • U - V color: Roman+18 • Stellar ages: Rose+19 • Host galaxy dust: Scolnic+14

• What if the global host galaxy properties aren’t precise enough?

• If we use the wrong “step”, cosmological parameters could be biased

Roman+18

Hubb

le R

esid

ual

Jones+15

Resid

ual S

tep

Size

caveat: hard to be sure what is SN physics versus what is a model training uncertainty

The Relationship Between SN Ia and their Host Galaxies

• Already with Foundation DR1 and previous low-z data, we can double the low-z sample size when looking for local and global effects

• We looked at global/local host mass, host u-g color, and sSFR

• Is “local” information more fundamental? Found a complicated relationship between SN Ia distances and local/global effects but no strong evidence that local effects are larger (see also Roman+18, Rose+19)

Exploring Local and Global Host Galaxy Biases

sample SED fits from the Cepheid calibrator set

wavelength

mag

Jones+18b; arXiv: 1805.05911

Exploring Local and Global Host Galaxy Biasesto be clear: just because there are systematic uncertainties

doesn’t mean we expect large biases in our existing measurements

Rose+19, but didn’t take into account SH0ES mass

correction

• To what degree is SN Ia color variation correlated or uncorrelated with luminosity?

• Multiple models consistent with observed data, but the model with more uncorrelated color variation prefers a higher SALT2 β, which affects distances.

• See Mandel+17: intrinsic color component (low β) and MW-like reddening component (high β)

Understanding the SN Ia Dispersion Model

Jones+18a

Foley+18, Jones+19

bluer colors, similar to high-

z sample

systematics related to SN

selection effects are

greatly reduced

Foundation Reduces SN Ia Dispersion Model Systematics

Next Steps: A Re-Trained Model and Expansive Testing Framework

figure from R. Kessler

BYOSED:UnderstandingTypeIaSupernovaDistanceBiasesbySimula>ngSpectralVaria>ons

Wavelength(Angstrom)

Flux

Simulated

LogHostM

ass

FiJedSALT2X1ParameterBYOSED Manuscript Draft - v1.0 7/28/19

BYOSED: Understanding Type Ia Supernova Distance

Biases by Simulating Spectral Variations

August 8, 2019

Abstract

Type Ia Supernovae (SNIa) are one of the central pillars of the ⇤CDM “concordancecosmology.” The accelerating expansion of the universe was discovered with a sample ofa few dozen SNIa extending to z ⇠ 0.8, and over the last 20 years this sample has grownto include > 1000 SNIa reaching to z ⇠ 2.3. In the next decade, transient searchesfrom the Large Synoptic Survey Telescope (LSST) and the Wide Field Infrared SurveyTelescope (WFIRST) will accelerate this growth, increasing the SNIa sample to severalhundred thousand. As this enormous volume of SN Ia reduces statistical uncertaintiesfor cosmological measurements, other methods are needed to limit systematic uncer-tainties. We therefore provide an important tool for the next era of SNIa cosmology,byosed, which enables flexible testing of possible effects on Type Ia distance measure-ments using any baseline SED model in an open source Python framework. [JP] Smallsummary of "results".

1 IntroductionType Ia Supernovae (SNIa) are not standard candles. That is, they do not all reach thesame absolute magnitude at peak brightness. Rather, they are standardizable candles.Cosmological constraints depend upon SNIa luminosity distance measurements (Riesset al., 1998; Perlmutter et al., 1999), obtainable by fitting the observed light curveswith a model that adapts in (at least) shape and color. With careful training, the lightcurve fitter can use measurements of those shape and color parameters to correct allthe observed SNIa to a common absolute magnitude (e.g. Guy et al., 2007).

Distance uncertainties are known to be introduced by (at least) intrinsic variations inlight curve shape and color, along with the process of training the light curve fitting code(Mosher et al., 2014). What’s more, systematic biases can be introduced by assumptionsabout the relationships between shape, color, and luminosity (Scolnic et al., 2018). Inthe coming era of LSST and WFIRST, statistical uncertainties in SNIa cosmology willdiminish as the SNIa sample grows into the hundreds of thousands (Zhan & Tyson,2018; Hounsell et al., 2018). At that point, these systematic and distance uncertaintieswill be the limiting contribution to the error budget for cosmological measurements.

In addition to the known causes of uncertainties in SNIa distance measurementsthere have been many other effects proposed that may impact either SNIa luminosities

Justin Roberts-Pierel | University of South Carolina | jr23@email.sc.edu 1 / 7

Roberts-Pierel+in prep, see also Matt Siebert’s talk this afternoon

• Biggest Physics-related Uncertainties in SN Cosmology: 1. Understanding correlation(s) between SN Ia color and

luminosity 2. What is the most precise way to model the relationship

between SN Ia and their host galaxies?

• We can reduce the impact of uncertainties from SN Ia physics by matching low-z sample properties to high-z sample or Cepheid calibrator sample to Hubble flow sample properties, but it might not be enough in the 2020s • Alternatively, correcting for observed dependences of SNe Ia on

host galaxy properties but the observed dependences could be incomplete

• New data coming soon - DES, Foundation, Sirah, ZTF • Larger samples and easier-to-understand selection effects

• New analyses coming soon: new SALT model, new simulation and testing framework, new NIR constraints

Conclusions

Reminder: An Exciting Time for SN Ia CosmologyOur best end-to-end test of the cosmological model is H0 and

we’re failing the test

Verde+19