To calculate the Hubble constant, astronomers need to know how far away things are. In the nearby cosmos, scientists measure distances using stars called Cepheid variables that periodically change in brightness. There\u2019s a well-known relationship between how fast one of these stars swings from brightest to faintest and how much energy it radiates. That relation, which was discovered in the early 20th century, allows astronomers to calculate the star\u2019s intrinsic brightness, and by comparing that to how bright it appears, they can calculate its distance.<\/p>\n

Using these variable stars, scientists can measure the distances to galaxies up to about 100 million light-years from us. But to see a bit farther away, and a bit further back in time, they use a brighter mile marker \u2014\u00a0a specific type of stellar explosion called a type Ia supernova. Astronomers can also calculate the intrinsic brightness of these \u201cstandard candles,\u201d which allows them to measure distances to galaxies billions of light-years away.<\/p>\n

Over the past two decades, these observations have helped astronomers pin a value on how fast the nearby universe is expanding: roughly 73 kilometers per second per megaparsec, which means that as you look further away, for each megaparsec (or 3.26 million light-years) of distance, space is flying away 73 kilometers per second faster.<\/p>\n

But that value clashes with one derived from another ruler embedded in the infant universe.<\/p>\n