Free Astronomy Magazine September-October 2018

17 SEPTEMBER-OCTOBER 2018 SPACE CHRONICLES spond to their intrinsic brightness. By comparing their intrinsic brightness with their apparent brightness as seen from Earth, scientists can calcu- late their distances. Gaia further refined this yardstick by geometrically measuring the distance to 50 Cepheid variables in the Milky Way. These measurements were com- bined with precise measurements of their brightnesses from Hubble. This allowed the astronomers to more ac- curately calibrate the Cepheids and then use those seen outside the Milky Way as milepost markers. “When you use Cepheids, you need both distance and brightness,” ex- plained Riess. Hubble provided the information on brightness, and Gaia provided the parallax information needed to accurately determine the distances. Parallax is the apparent change in an object’s position due to a shift in the observer’s point of view. Ancient Greeks first used this technique to measure the distance from Earth to the Moon. “Hubble is really amazing as a gen- eral-purpose observatory, but Gaia is the new gold standard for calibrating distance. It is purpose-built for meas- uring parallax—this is what it was de- signed to do,” Stefano Casertano of Space Telescope Science Institute and a member of the SHOES Team added. “Gaia brings a new ability to recali- brate all past distance measures, and it seems to confirm our previous work. We get the same answer for the Hubble constant if we replace all previous calibrations of the distance ladder with just the Gaia parallaxes. It’s a crosscheck between two very powerful and precise observatories.” The goal of Riess’ team is to work with Gaia to cross the threshold of refining the Hubble constant to a value of only one percent by the early 2020s. Meanwhile, astrophysi- cists will likely continue to grapple with revisiting their ideas about the physics of the early universe. U sing two of the world’s most powerful space telescopes — NASA’s Hubble and ESA’s Gaia — astronomers have made the most precise measurements to date of the universe’s expansion rate. This is calculated by gauging the dis- tances between nearby galaxies using special types of stars called Cepheid variables as cosmic yardsticks. By comparing their intrinsic brightness as meas- ured by Hubble, with their apparent brightness as seen from Earth, scientists can calculate their distances. Gaia further refines this yardstick by geometri- cally measuring the distances to Cepheid variables within our Milky Way galaxy. This allowed astronomers to more precisely calibrate the distances to Cepheids that are seen in outside galaxies. [NASA, ESA, and A. Feild (STScI)] ! Using Hubble and newly released data from Gaia, Riess’ team meas- ured the present rate of expansion to be 73.5 kilometers (45.6 miles) per second per megaparsec. This means that for every 3.3 million light-years farther away a galaxy is from us, it appears to be moving 73.5 kilome- ters per second faster. However, the Planck results predict the universe should be expanding today at only 67.0 kilometers (41.6 miles) per sec- ond per megaparsec. As the teams’ measurements have become more and more precise, the chasm be- tween them has continued to widen, and is now about 4 times the size of their combined uncertainty. Over the years, Riess’ team has re- fined the Hubble constant value by streamlining and strengthening the “cosmic distance ladder,” used to measure precise distances to nearby and far-off galaxies. They compared those distances with the expansion of space, measured by the stretching of light from nearby galaxies. Using the apparent outward velocity at each distance, they then calculated the Hubble constant. To gauge the distances between nearby galaxies, his team used a spe- cial type of star as cosmic yardsticks or milepost markers. These pulsating stars, called Cepheid variables, brighten and dim at rates that corre-