Free Astronomy Magazine September-October 2018

16 SEPTEMBER-OCTOBER 2018 SPACE CHRONICLES can measure today. However, those predictions don’t seem to match the new measurements of our nearby contemporary universe. “With the addition of this new Gaia and Hubble Space Telescope data, we now have a serious tension with the Cosmic Microwave Background data,” said Planck teammember and lead analyst George Efstathiou of the Kavli Institute for Cosmology in Cambridge, England, who was not involved with the new work. “The tension seems to have grown into a full-blown incompatibility be- tween our views of the early and late time universe,” said team leader and Nobel Laureate Adam Riess of the Space Telescope Science Institute and the Johns Hopkins University in Baltimore, Maryland. “At this point, clearly it’s not simply some gross error in any one measurement. It’s as though you predicted how tall a child would become from a growth chart and then found the adult he or she became greatly exceeded the predic- tion. We are very perplexed.” In 2005, Riess and members of the SHOES (Supernova H0 for the Equa- tion of State) Team set out to meas- ure the universe’s expansion rate with unprecedented accuracy. In the following years, by refining their techniques, this team shaved down the rate measurement’s uncertainty to unprecedented levels. Now, with the power of Hubble and Gaia com- bined, they have reduced that un- certainty to just 2.2 percent. Because the Hubble constant is needed to es- timate the age of the universe, the long-sought answer is one of the most important numbers in cosmol- ogy. It is named after astronomer Edwin Hubble, who nearly a century ago discovered that the universe was uniformly expanding in all di- rections — a finding that gave birth to modern cosmology. Galaxies appear to recede from Earth proportional to their distances, mean- ing that the farther away they are, the faster they appear to be moving away. This is a consequence of ex- panding space, and not a value of true space velocity. By measuring the value of the Hubble constant over time, astronomers can construct a pic- ture of our cosmic evolution, infer the make-up of the universe, and uncover clues concerning its ultimate fate. The two major methods of measur- ing this number give incompatible results. One method is direct, build- ing a cosmic “distance ladder” from measurements of stars in our local universe. The other method uses the CMB to measure the trajectory of the universe shortly after the Big Bang and then uses physics to describe the universe and extrapolate to the pres- ent expansion rate. Together, the measurements should provide an end-to-end test of our basic under- standing of the so-called “Standard Model” of the universe. However, the pieces don’t fit. Hubble and Gaia team up to fuel cosmic conundrum by NASA/ESA C ombining observations from NASA’s Hubble Space Telescope and the European Space Agency’s (ESA) Gaia space observa- tory, astronomers further refined the previous value for the Hubble con- stant, the rate at which the universe is expanding from the big bang 13.8 billion years ago. But as the measure- ments have become more precise, the team’s determination of the Hubble constant has become more and more at odds with the measurements from another space observatory, ESA’s Planck mission, which is coming up with a different predicted value for the Hubble constant. Planck mapped the primeval universe as it appeared only 360,000 years after the big bang. The entire sky is imprinted with the signature of the big bang encoded in microwaves. Planck measured the sizes of the rip- ples in this Cosmic Microwave Back- ground (CMB) that were produced by slight irregularities in the big bang fireball. The fine details of these rip- ples encode how much dark matter and normal matter there is, the tra- jectory of the universe at that time, and other cosmological parameters. These measurements, still being as- sessed, allow scientists to predict how the early universe would likely have evolved into the expansion rate we