Credit: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)
THREE STEPS TO HUBBLE CONSTANT (ARTIST’S ILLUSTRATION).
This illustration shows the three steps astronomers used to measure the universe's
expansion rate to an unprecedented accuracy, reducing the total uncertainty to 2.4
percent.
Astronomers made the measurements by streamlining and strengthening the construction of the cosmic distance ladder, which is used to measure accurate distances to galaxies near and far from Earth.
Beginning at left, astronomers use Hubble to measure the distances to a class of
pulsating stars called Cepheid Variables, employing a basic tool of geometry called
parallax. This is the same technique that surveyors use to measure distances on Earth. Once astronomers calibrate the Cepheids' true brightness, they can use them as cosmic
yardsticks to measure distances to galaxies much farther away than they can with the
parallax technique. The rate at which Cepheids pulsate provides an additional fine-tuning to the true brightness, with slower pulses for brighter Cepheids. The
astronomers compare the calibrated true brightness values with the stars' apparent
brightness, as seen from Earth, to determine accurate distances.
Once the Cepheids are calibrated, astronomers move beyond our Milky Way to nearby galaxies (shown at center). They look for galaxies that contain Cepheid stars and another reliable yardstick, Type Ia supernovae, exploding stars that flare with the
same amount of brightness. The astronomers use the Cepheids to measure the true brightness of the supernovae in each host galaxy. From these measurements, the
astronomers determine the galaxies’ distances.
They then look for supernovae in galaxies located even farther away from Earth. Unlike
Cepheids, Type Ia supernovae are brilliant enough to be seen from relatively longer
distances. The astronomers compare the true and apparent brightness of distant
supernovae to measure out to the distance where the expansion of the universe can be
seen (shown at right). They compare those distance measurements with how the light from
the supernovae is stretched to longer wavelengths by the expansion of space. They use these two values to calculate how fast the universe expands with time, called the Hubble
constant.