Shining bright through the ages

Type Ia supernova distances are in play in a rather contentious part of astronomy: measurements of the Hubble constant. The value of the Hubble constant is measured in one of two ways: using the cosmic microwave background (CMB), and using Type Ia supernovae and variable stars called Cepheids to measure the distances and velocities of far-off galaxies. The value measured with supernovae is significantly larger than the value measured with the CMB, however — and it suggests the presence of “dark energy”, a mysterious energy that’s accelerating the expansion of the universe.

Recently, it’s been hypothesized that the supernova-based measurement is biased by an overlooked relation between peak Type Ia supernova brightnesses and the age of their host galaxies. Accounting for this brightness–age relation, if it holds up, could eliminate the need for dark energy and relieve the discrepancy between the two measurements of the Hubble constant. But a new study led by Benjamin Rose (Space Telescope Science Institute) now refutes this proposed relation.

Light curve of the supernova SN2003ic in three different bands. SN2003ic was one of the 10 supernovae that didn’t pass cosmological quality cuts. Excluding this supernova alone causes the significance of the brightness–age relation to drop below the necessary threshold. CREDITS: Rose et al. 2020

Rose and collaborators started their analysis by examining the sample of 34 Type Ia supernovae that was used to claim the possible brightness–age relation. The authors found that 10 supernovae in the sample fail at least one of the quality cuts typically used for cosmological studies. These include the supernova not being observed prior to its peak brightness, and an overall lack of observations.

Rose and collaborators also argue that the prior study didn’t correctly account for the error on Type Ia supernova distances. Once these errors are accounted for and quality cuts are made to the supernova sample, the brightness–age relation appears negligible to measurements of the Hubble constant.

Rose and collaborators also attempted to determine a brightness–age relation using a larger, robust sample of 254 Type Ia supernovae. They found no relation significant enough to suggest that the supernova distances had been misestimated — so there should be no changes to the supernova-based measurement of the Hubble constant.

While this particular relation may not have borne out, Rose and collaborators agree that the properties of Type Ia supernovae must be constrained as much as possible for reliable distance measurements to be made. For now, however, it looks like dark energy may be here to stay!

Source: “Shining Bright Through the Ages” AASNOVA, 6 July 2020

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