The universe is constantly expanding, and the Hubble constant is scientists' estimate of the rate of that expansion. Knowing how fast the universe is expanding yields insight into its origin, age, evolution, and eventual fate. Scientists have estimated the Hubble constant using two independent methods, but there are significant discrepancies among the resulting values. One method uses data from studies of the radiation left over from the big bang, known as the cosmic microwave background, and the other uses measurements of objects in the modern universe to create a so-called cosmic distance ladder (see illustration). So, in the hope of finally resolving this disparity, scientists like Eberly's Hyungsuk Tak are devising novel methods of calculating the Hubble constant. But if they can't come to a consensus, we may need an entirely new physics to explain this cosmic conundrum.
This illustration and description from NASA, the European Space Agency, and the Space Telescope Science Institute lays out the three basic steps astronomers use to calculate the Hubble constant based on measurements in the modern universe: All the steps involve building a strong "cosmic distance ladder," by starting with measuring accurate distances to nearby galaxies and then moving to galaxies farther and farther away. This "ladder" is a series of measurements of different kinds of astronomical objects with an intrinsic brightness that researchers can use to calculate distances. Among the most reliable for shorter distances are Cepheid variables, stars that pulsate at predictable rates that indicate their intrinsic brightness. Astronomers recently used the Hubble Space Telescope to observe 70 Cepheid variables in the nearby Large Magellanic Cloud to make the most precise distance measurement to that galaxy. Astronomers compare the measurements of nearby Cepheids to those in galaxies farther away that also include another cosmic yardstick, exploding stars called Type Ia supernovas. These supernovas are much brighter than Cepheid variables. Astronomers use them as "milepost markers" to gauge the distance from Earth to far-flung galaxies. Each of these markers build upon the previous step in the "ladder." By extending the ladder using different kinds of reliable milepost markers, astronomers can reach very large distances in the universe. Astronomers compare these distance values to measurements of an entire galaxy's light, which increasingly reddens with distance, due to the uniform expansion of space. Astronomers can then calculate how fast the cosmos is expanding: the Hubble constant.