Shining Bright Through the Ages

Accurate distance measurements are critical to astronomy. A Type Ia supernova is one of the few objects that we can trust for making distance measurements since they have a fixed peak brightness. But can the brightness of such a supernova change significantly based on the properties of its host galaxy? And what does this mean for our understanding of dark energy?

Measurements of the Hubble constant via different methods over time. Type Ia supernovae are used in conjunction with Cepheid variable stars for the Cepheid method. CMB stands for Cosmic Microwave Background. TRGB stands for “tip of the red giant branch”, which refers to a certain set of stars. The discrepancy between measurements of the Hubble constant has grown with time despite increasing precision. Click to enlarge. [Freedman et al. 2019]

Lighthouses in the Distant Universe

A Type Ia supernovae is what’s known as a “standard candle” — we know what its brightness is at a particular distance, and when we observe these supernovae in distant galaxies, we can extrapolate to determine how far away those galaxies are. Like lighthouses, the fainter a Type Ia supernova is, the further away it is.

Accurate distance measurements form the backbone of astronomy, and Type Ia supernovae are especially valuable because they allow us to measure extremely large distances beyond the reach of other standard candles. However, the properties of Type Ia supernovae were and are determined empirically, so a mistaken assumption about a supernova can trickle down and cause miscalculations further down the road.

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. [Rose et al. 2020]

Sample Adjustments

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.

Hubble–Lemaître residuals versus host galaxy age for Type Ia supernovae in the Pantheon sample. The residuals denote the difference between predicted and measured values of the supernova brightness. The black points are individual supernovae and the blue points are bins of 25 supernovae. The red dashed line is the claimed brightness–age relation based on the sample of 34 Type Ia supernovae, and the dark line is the relation as determined by the Pantheon sample. [Rose et al. 2020]

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!

Citation

“Evidence for Cosmic Acceleration Is Robust to Observed Correlations between Type Ia Supernova Luminosity and Stellar Age,” B. M. Rose et al 2020 ApJL 896 L4. doi:10.3847/2041-8213/ab94ad