Free Astronomy Magazine July-August 2019

33 JULY-AUGUST 2019 SPACE CHRONICLES These measurements of the early universe come from the European Space Agency’s Planck satellite. This discrepancy has been identified in scientific pa- pers over the last several years, but it has been un- clear whether differences in measurement tech- niques are to blame, or whether the difference could result from unlucky measurements. The latest Hubble data lower the possibility that the discrepancy is only a fluke to 1 in 100,000. This is a significant gain from an earlier esti- mate, less than a year ago, of a chance of 1 in 3,000. These most precise Hubble measurements to date bolster the idea that new physics may be needed to explain the mismatch. “The Hubble tension be- tween the early and late universe may be the most exciting development in cosmology in decades,” said lead researcher and Nobel laureate Adam Riess of the Space Tele- scope Science Institute (STScI) and Johns Hopkins University, in Baltimore, Maryland. “This mis- match has been growing and has now reached a point that is really impos- sible to dismiss as a fluke. This disparity could not plausibly occur just by chance.” Scientists use a “cosmic distance ladder” to determine how far away things are in the universe. This method de- cise, their calculation of the Hubble constant has remained at odds with the expected value derived from observa- tions of the early universe’s ex- pansion. Those measurements were made by Planck, which maps the cosmic microwave background, a relic afterglow from 380,000 years after the big bang. The measurements have been thoroughly vetted, so as- tronomers cannot currently dismiss the gap between the two results as due to an error in any single measurement or method. Both values have been tested multiple ways. “This is not just two experi- ments disagreeing,” Riess explained. “We are measuring something fun- damentally different. One is a meas- urement of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and on measurements of how fast it ought to be expanding. If these values don’t agree, there becomes a very strong likelihood that we’re missing something in the cosmological model that connects the two eras.” Astronomers have been using Cepheid variables as cosmic yard- sticks to gauge nearby intergalactic distances for more than a century. But trying to harvest a bunch of these stars was so time-consuming as to be nearly unachievable. So, the team employed a clever new method, called DASH (Drift And Shift), using Hubble as a “point-and- shoot” camera to snap quick images of the extremely bright pulsating stars, which eliminates the time-con- suming need for precise pointing. “When Hubble uses precise pointing by locking onto guide stars, it can only observe one Cepheid per each 90-minute Hubble orbit around Earth. So, it would be very costly for pends on making accurate measure- ments of distances to nearby galax- ies and then moving to galaxies farther and farther away, using their stars as milepost markers. As- tronomers use these values, along with other measurements of the galaxies’ light that reddens as it passes through a stretching uni- verse, to calculate how fast the cos- mos expands with time, a value known as the Hubble constant. Riess and his SH0ES (Supernovae H0 for the Equation of State) team have been on a quest since 2005 to refine those distance measurements with Hubble and fine-tune the Hubble constant. In this new study, astronomers used Hubble to observe 70 pulsating stars called Cepheid variables in the Large Magellanic Cloud. The observations helped the astronomers “rebuild” the distance ladder by improving the comparison between those Cepheids and their more distant cousins in the galactic hosts of su- pernovas. Riess’s team reduced the uncertainty in their Hubble constant value to 1.9% from an earlier esti- mate of 2.2%. As the team’s meas- urements have become more pre- M easurements of today's expansion rate do not match the rate that was expected based on how the Uni- verse appeared shortly after the Big Bang over 13 billion years ago. Using new data from the NASA/ESA Hubble Space Telescope, astronomers have significantly lowered the possibility that this discrepancy is a fluke. [NASA/ESA]