How fast is the universe expanding? This is far from clear – on the contrary, the value of this constant, named after the astronomer Erwin Hubble, is one of the greatest mysteries of cosmology. A new map of the early universe, created using data from the Atacama Cosmology Telescope (ACT) in Chile, now confirms earlier and similarly calculated estimates of the expansion speed of the universe. The problem: direct measurements of the speed at which galaxies are moving apart result in a 10 percent higher value for the Hubble constant. So something doesn’t fit together.

The results, published in two preprints, are based on data on microwave background radiation collected with the ACT in the Atacama Desert from 2013 to 2016. These microwaves are, in a sense, the afterglow of the Big Bang; they come from all directions of space, but not completely uniformly. The variations indicate that regions of the early universe differed minimally in temperature. Over the past two decades, cosmologists have used these tiny fluctuations to derive something about the structure and evolution of the universe – including the speed at which the universe is currently expanding.

Similarly, the European Space Agency’s Planck Telescope has mapped microwave background radiation from 2009 to 2013. This data also found a similar value for the Hubble constant. But direct measurements using the brightness of certain stars and supernova explosions are about ten percent higher. The hope of many astronomers that the difference would decrease with increasing precision of the measurements has thus not been fulfilled. Instead, the discrepancy now stands on even more valid data.

“For the first time, we have two data sets that have been determined independently and with sufficient accuracy to make a comparison,” Erminia Calabrese, a cosmologist from the University of Cardiff, told the journal Nature. For her as a member of the Planck team, it was a relief that the prediction for the Hubble constant of the two experiments was within 0.3 percent.

So the debate over the Hubble constant will continue until further notice. While some astronomers continue to believe that the values will converge once the measurement methods are perfected, others are not so sure.

Adam Riess, an American astronomer at Johns Hopkins University in Baltimore, Maryland, has led many of the work to determine the Hubble constant using the luminosity of stars. He considers, for example, that the standard model of cosmology may not be correct. This model is the most recognized theory of the evolution of the universe and assumes that a big bang occurred some 13.8 billion years ago.

However, if this basic assumption was wrong, it may be why the two methods produce different results.