How do scientists measure the expansion rate of the universe?
Quick Answer
They compare distances and redshifts of galaxies using methods such as Cepheid stars and Type Ia supernovae. The result is called the Hubble constant, and it is about 70 kilometers per second per megaparsec, though different methods disagree slightly.
The Full Story
The universe’s expansion rate is measured by building a cosmic distance ladder. Astronomers use Cepheid variable stars to calibrate nearby galaxy distances and then use Type Ia supernovae to reach much farther out. These measurements are combined with redshift to estimate the Hubble constant. A long-running puzzle is that different methods give slightly different values, a disagreement known as the Hubble tension.
Key Facts
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