Before we look at the Supernova (the cross) and the Cepheids (the red circles) in the Spiral Galaxy of UGC 9391 it’s worth describing what a Cepheid actually is and why it’s important.
A Cepheid variable is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude.
A strong direct relationship between a Cepheid variable’s luminosity and pulsation period established Cepheids as important indicators of cosmic benchmarks for scaling galactic and extragalactic distances. This robust characteristic of classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt after studying thousands of variable stars in the Magellanic Clouds.
Essentially the Cepheids give us an understanding around how far away they are and the speed at which they are moving (away from us). This helps us understand the current rate of expansion of the Universe which appears to be speeding up and moving up to 7% faster than first expected.
What can this galaxy tell us about the expansion rate of the universe? Perhaps a lot because UGC 9391, featured, not only contains Cepheid variable stars (red circles) but also a recent Type Ia supernova (blue X). Both types of objects have standard brightnesses, with Cepheids typically being seen relatively nearby, while supernovas are seen much farther away. Therefore, this spiral is important because it allows a calibration between the near and distant parts of our universe.
Unexpectedly, a recent analysis of new Hubble data from UGC 9391 and several similar galaxies has bolstered previous indications that Cepheids and supernovas are expanding with the universe slightly faster than expected from expansion measurements of the early universe. Given the multiple successes of early universe concordance cosmology, astrophysicists are now vigorously speculating about possible reasons for this discrepancy.
Candidate explanations range from the sensational, such as the inclusion of unusual cosmological components types such as phantom energy and dark radiation, to the mundane, including statistical flukes and underestimated sources of systematic errors. Numerous future observations are being planned to help resolve the conundrum.