Free Astronomy Magazine July-August 2019

34 JULY-AUGUST 2019 SPACE CHRONICLES the telescope to observe each Cepheid,” explained team member Ste- fano Casertano, also of STScI and Johns Hopkins. “Instead, we searched for groups of Cepheids close enough to each other that we could move be- tween them with- out recalibrating the telescope point- ing. These Cepheids are so bright, we only need to ob- serve them for two seconds. This tech- nique is allowing us to observe a dozen Cepheids for the du- ration of one orbit. So, we stay on gyro- scope control and keep ‘DASHing’ around very fast.” The Hubble as- tronomers then combined their re- sult with another set of observa- tions, made by the Araucaria Proj- ect, a collaboration between as- tronomers from institutions in Chile, the U.S., and Europe. This group made distance measure- ments to the Large Magellanic Cloud by observing the dimming of light as one star passes in front of its partner in eclipsing binary-star systems. The combined measure- ments helped the SH0ES Team re- fine the Cepheids’ true brightness. With this more accurate result, the team could then “tighten the bolts” of the rest of the distance ladder that extends deeper into space. The new estimate of the Hubble constant is 74 kilometers (46 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 74 out space, start- ing the initial ex- pansion. Dark energy may also be the reason for the universe’s accelerated ex- pansion today. The new theory suggests that there was a third d a r k - e n e r g y episode not long after the big bang, which ex- panded the uni- verse faster than astronomers had predicted. The existence of this “early dark en- ergy” could ac- count for the tension between the two Hubble constant values, Riess said. An- other idea is that the universe con- tains a new subatomic particle that travels close to the speed of light. Such speedy particles are collec- tively called “dark radiation” and include previously known particles like neutrinos, which are created in nuclear reactions and radioactive decays. Yet another attractive possibility is that dark matter (an invisible form of matter not made up of protons, neutrons, and electrons) interacts more strongly with normal matter or radiation than previously as- sumed. But the true explanation is still a mystery. Riess doesn’t have an an- swer to this vexing problem, but his team will continue to use Hubble to reduce the uncertainties in the Hub- ble constant. Their goal is to de- crease the uncertainty to 1%, which should help astronomers identify the cause of the discrepancy. ! T his illustration shows the three basic steps astronomers use to calculate how fast the universe expands over time, a value called the Hubble con- stant. All the steps involve building a strong "cosmic distance ladder," by start- ing 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 re- searchers can use to calculate distances. [NASA, ESA, and A. Feild (STScI)] kilometers (46 miles) per second faster, as a result of the expansion of the universe. The number indicates that the uni- verse is expanding at a 9% faster rate than the prediction of 67 kilo- meters (41.6 miles) per second per megaparsec, which comes from Planck’s observations of the early universe, coupled with our present understanding of the universe. One explanation for the mismatch involves an unexpected appearance of dark energy in the young uni- verse, which is thought to now com- prise 70% of the universe’s con- tents. Proposed by astronomers at Johns Hopkins, the theory is dubbed “early dark energy,” and suggests that the universe evolved like a three-act play. Astronomers have already hypothe- sized that dark energy existed dur- ing the first seconds after the big bang and pushed matter through-