There is a source of tension in cosmology and new observations have turned this puzzling situation into a full-blown problem. The rate of expansion of the universe is higher today than it was 13 billion years ago. This goes against what our theories suggest.
The latest measurements, as reported in The Astrophysical Journal, were conducted with the combined power of Hubble and the European Space Agency’s telescope Gaia. Astronomers used the two instruments to refine the cosmic ladder, which allowed them to estimate the expansion rate of the universe. The value they ended up with is 73.5 kilometers (45.6 miles) per second per megaparsec.
In other words, for every megaparsec (equivalent to 3.3 million light-years) a galaxy is away from us, it appears to be receding 73.5 kilometers per second faster. The measurement of the expansion rate made using the first light in the universe, the cosmic microwave background, put the value at 67 kilometers (41.6 miles) per second per megaparsec. This 9 percent difference is vast, in fact, its four times the combined uncertainty for the two telescopes. The expansion rate was assumed to either decrease or stay the same, not increase as the universe ages.
“The tension seems to have grown into a full-blown incompatibility between our views of the early and late time universe,” lead author and Nobel Laureate Adam Riess, of the Space Telescope Science Institute and Johns Hopkins University, said in a statement. “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 prediction. We are very perplexed.”
The work has been done using a particular class of stars known as the Cepheid variables. These stars pulsate at a rate that corresponds to their true luminosity, which helps scientists work out their distance. This relationship was discovered by indefatigable astronomer Henrietta Swan Leavitt. Gaia is amazing at measuring the distance of stars in our galaxy and Hubble has a keen eye for the apparent brightness of these objects. Together they increased the precision of Leavitt’s law and used it to measure the expansion rate with an uncertainty of just 2.2 percent.
“Hubble is really amazing as a general-purpose observatory, but Gaia is the new gold standard for calibrating distance. It is purpose-built for measuring parallax – this is what it was designed to do,” Stefano Casertano of the Space Telescope Science Institute added. “Gaia brings a new ability to recalibrate 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 team's next goal is to continue to reduce the uncertainty of the expansion rate to below 1 percent by the early 2020s. At the same time, researchers worldwide will continue their work to understand what the new expansion rate measurement means for our current model of cosmology. Unexpected properties of dark energy, unknown interactions of dark matter, and other intriguing ideas will be tested in this new paradigm.