Thermonuclear diversity and the Hubble tension

Homogeneity is the hallmark of standard candle-based cosmology investigations. Thermonuclear supernovae (Type-Ia, SNeIa) violate this essential requirement if they develop along multiple evolutionary pathways. In this work, the impact of thermonuclear diversity on cosmological parameter constraints is quantified using Pantheon+, one of the largest ensembles of SNeIa compiled to probe cosmology to date. Evidence of diversity is encoded in supernova light curves. Pantheon+ is shown to be diverse, with features indicative of multiple thermonuclear subclasses. Diversity driven systematic effects have been quantified on a supernova-by-supernova basis; event selections based on light curve derived metrics were subsequently used to characterize diversity dependent trends and limit their impact. A diversity mitigated estimate of the Hubble-Lemaître parameter, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>H</a:mi><a:mn>0</a:mn></a:msub><a:mo>=</a:mo><a:mn>67.9</a:mn><a:mo>±</a:mo><a:mn>0.8</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mrow><a:mi>km</a:mi><a:mtext> </a:mtext><a:msup><a:mi mathvariant="normal">s</a:mi><a:mrow><a:mo>−</a:mo><a:mn>1</a:mn></a:mrow></a:msup><a:mtext> </a:mtext><a:mrow><a:msup><a:mrow><a:mi>Mpc</a:mi></a:mrow><a:mrow><a:mo>−</a:mo><a:mn>1</a:mn></a:mrow></a:msup></a:mrow></a:mrow></a:math> (68% C.L.), was obtained by reanalyzing Pantheon+. The Hubble tension, an apparent disparity between early and late Universe determinations of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msub><d:mi>H</d:mi><d:mn>0</d:mn></d:msub></d:math>, is eased from <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mo>∼</f:mo><f:mn>5</f:mn><f:mi>σ</f:mi></f:math> to <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mo>&lt;</h:mo><h:mn>1</h:mn><h:mi>σ</h:mi></h:math> after accounting for the diverse thermonuclear scenarios that govern SNeIa. Diversity mitigated subsets of Pantheon+ also show a <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mo>∼</j:mo><j:mrow><j:mn>3.4</j:mn><j:mi>σ</j:mi></j:mrow></j:math> preference for a flat <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:msub><l:mi>w</l:mi><l:mn>0</l:mn></l:msub><l:msub><l:mi>w</l:mi><l:mi>a</l:mi></l:msub><l:mi>CDM</l:mi></l:math> cosmology with dark energy equation of state parameters <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mrow><n:mo stretchy="false">(</n:mo><n:msub><n:mi>w</n:mi><n:mn>0</n:mn></n:msub><n:mo>,</n:mo><n:msub><n:mi>w</n:mi><n:mi>a</n:mi></n:msub><n:mo stretchy="false">)</n:mo><n:mo>=</n:mo><n:mo stretchy="false">(</n:mo><n:mo>−</n:mo><n:mn>1.084</n:mn><n:mo>±</n:mo><n:mn>0.180</n:mn><n:mo>,</n:mo><n:mo>−</n:mo><n:mn>2.066</n:mn><n:mo>±</n:mo><n:mn>0.675</n:mn><n:mo stretchy="false">)</n:mo></n:mrow></n:math>. A strategy for precise SNeIa-derived cosmological inferences, dominated by statistical rather than systematic uncertainties, is also presented.