Researchers using the Hubble Space Telescope have observed a series of galaxies that contain both Cepheid variable stars we can see and have been home to Type Ia supernovae. These two kinds of objects are used to measure distances throughout our universe, and finding both kinds of objects together allows us to calibrate our measuring tools, so to speak.
Cepheids can be seen nearby and out to distances of a few hundred million light years. For the nearby ones, Gaia and other missions have measured the distances using parallax – a method based on simple triangulation that requires super accurate measurements.
Cepheids are fascinating stars that change in size and brightness over tens of days, and the periods of their repeating pulses are directly related to their luminosity. It is as if every 100-Watt bulb flashed every ten seconds, while every 40-Watt bulb flashed every 6.2 seconds. By measuring the rate of the pulses, we know how much light the stars give off, and by measuring how bright they appear, we can measure how far away they are.
Type Ia supernovae are exploding stars, and theoretically, all the stars are about the same mass and explode with the same energy, give or take some correction factors. This means that when we see the signature of a Type Ia supernova, we can measure the light of the explosion, and knowing its actual energy, calculate its distance.
Between Cepheids being used for everything within a few 100 million light-years and Type 1a supernovae being used for everything further away, we can measure a lot of the universe and how it is expanding. And in these newly published results, with their beautifully calibrated new data, they get a result that indicates the universe is expanding faster than theory and observations of the Cosmic Microwave Background say it should be. If true, this means there is something fundamentally wrong with our understanding of the details around the expansion of the universe.
NASA Goddard press release