Authors: T. de Jaeger, L. Galbany, A. G. Riess, B. E. Stahl, B. J. Shappee, A. V. Filippenko, W. Zheng
First Author’s Institution: Institute for Astronomy, University of Hawaii
Status: Submitted to MNRAS
Never Ask the Universe Its Age
The Hubble constant (H0) is arguably the most influential parameter in cosmology, as it helps determine the age and expansion rate of the Universe. Many people have measured the Hubble constant, with the two most precise methods (where precision = smallest errors) being the cosmic microwave background (CMB) and supernovae. However, their measured values disagree in such a way that there is about a 1 in 3.5 million probability this disagreement was caused by chance. Understanding the root of this tension is currently one of the most hotly pursued topics in astronomy right now; there is a ton of motivation to measure the Hubble constant using other, independent methods, so as to shed light on potential issues in either the CMB or supernovae techniques.
The conventional supernovae measurement of H0 utilizes a type of exploding star called type Ia supernovae (see this astrobite for a comprehensive review of the different types of supernovae). Type Ia supernovae have long been established as excellent probes of measuring distances because they have a very well-defined relationship between their peak brightness and rate at which the brightness of the supernova declines.
Type II Supernovae
Instead of type Ia supernovae, this paper uses type II supernovae. Type II supernovae, also known as core-collapse supernovae, are explosions of massive stars (M>8 M☉). While type Ia supernovae are caused by the thermonuclear explosions of white dwarf stars, type II supernovae occur when a massive star collapses under its own gravity (see Figure 1 for a schematic of the lifecycle of a type II supernova). Astronomers can distinguish between these two types of supernovae because type II supernovae have hydrogen in their emission spectra, whereas type Ia supernovae do not.
While Type Ia supernovae are brighter, they are also less common than type II supernovae, so developing a method to measure H0 that utilizes type II supernovae would be highly beneficial (see Figure 2). While type II supernovae do not follow the peak luminosity-decline rate relation like type Ia supernovae, they do have a well-defined relation between their luminosity, photospheric expansion velocity, and color during the plateau phase of their explosion, where intrinsically brighter Sne II have higher velocities and are bluer. The photospheric expansion velocity is the expansion velocity of the supernova over time after it explodes.
Measuring the Hubble Constant with type II supernovae
To calibrate the intrinsic brightness (i.e. how bright a star would be without the effects of distance making it appear fainter) of these supernovae, the authors utilized the distance ladder approach. In this method, the authors measured the distance to a galaxy containing a type II supernova using another distance indicator, like the Cepheid P-L relation, the tip of the red giant branch, or water megamasers. Once they know the distance, they can measure the apparent luminosity of the supernova to determine the intrinsic luminosity of the supernova, and from there, use this calibration to measure the distances to supernovae much further away.
The authors used 13 supernovae to calibrate the intrinsic brightness of type II supernovae, which is shown in Figure 3. Then, with a calibration in hand, the authors measured distances to 89 other supernovae (using data compiled from eight different supernovae surveys!), measuring a Hubble constant of 75.4 +/- 3.8 km/s/Mpc. This value is statistically consistent with the type Ia supernovae measurement of 73.04 +/- 1.04 km/s/Mpc, but in about a 2.2-sigma tension with the CMB value of H0 of 67.4 +/- 0.5 km/s/Mpc. The authors hope to continue this study by increasing the number of SNe II in their sample and reducing their systematic errors for future studies.
Astrobite edited by Aldo Panfichi
Featured image credit: WIRED, edited by Abby Lee