A part of the new cosmic microwave background radiation map of the Universe, representing a region of space 20 billion light-years across.Credit: ACT Collaboration
A new map of the early Universe has reinforced a long-standing astronomy riddle about the speed with which the cosmos is expanding. The data, collected with a telescope in the Atacama Desert in Chile, supports earlier estimates of the age, geometry, and evolution of the Universe. But the findings clash with measurements of how fast galaxies fly apart, and predict that the Universe should expand at a significantly slower rate than currently observed.
The Atacama Cosmology Telescope (ACT) mapped the cosmic microwave background (CMB), the radiation ‘glow’ of the Big Bang. The findings, based on data collected between 2013 and 2016, were published on July 15 in two preprints in the arXiv repository.
CMB radiation comes from all directions in space, but it is not perfectly uniform: its variations in the sky reveal that the different regions of the early Universe differ slightly in temperature, by less than 0.03 degrees Kelvin. Over the past two decades, cosmologists have used those tiny variations, along with an established theory they call the standard model, to calculate some of the key characteristics of the structure and evolution of the Universe, including its age and the density of matter.
Cosmologists also use the variations to predict the rate at which the Universe is currently expanding, a measurement known as the Hubble constant after the American astronomer Edwin Hubble. The European Space Agency’s Planck telescope mapped the entire CMB sky from 2009 to 2013 with unprecedented precision, and that data is considered the gold standard of CMB cosmology. The ACT data now claims Planck’s findings and produces a very similar value for the Hubble constant.
But none of the results match direct Hubble constant measurements made using different techniques, a discrepancy known as the Hubble constant stress. Astronomers who use the brightness of particular types of stars and supernova explosions, collectively called standard sails, to calculate the rate of expansion, find that galaxies are moving away from each other at a rate approximately 10% faster than what the CMB maps.
Many researchers hoped that as the techniques became more precise, the gap would narrow. Instead, reducing the error bars for each type of study has only made the inconsistency more significant.
The Atacama cosmology telescope.Credit: Debra Kellner
ACT is the first ground-based CMB experiment that could have challenged Planck’s results, says Erminia Calabrese, a cosmologist at Cardiff University, UK, who led the analysis of the data. The telescope’s design and location, relatively close to the equator, allows it to map more of the CMB sky than other terrestrial or balloon telescopes, which have generally been limited to smaller regions.
Mapping the sky on a large scale is crucial in calculating the key parameters of cosmic expansion, Calabrese says. Another strength of ACT was that it made accurate measurements of the polarization of CMB radiation, thanks to an update in 2013, says lead researcher Suzanne Staggs of Princeton University in New Jersey. The polarization data reveals how foreground galaxies affect the spread of the CMB and help make cosmological measurements more accurate.
“For the first time, we have two independently measured data sets with enough precision to make a comparison,” says Calabrese. Having also been a member of Planck’s team, he says it was a relief to discover that Hubble’s constant predictions from the two experiments matched within 0.3% of each other.
This agreement between ACT and Planck on the Hubble constant is “a truly important milestone,” says Paul Steinhardt, a theoretical physicist at Princeton University. “I am very impressed by the quality of the new data and its analysis,” he adds.
“It’s always nice to have independent controls, and I think this really provides it,” says Wendy Freedman, an astronomer at the University of Chicago at Illinois and a pioneer in standard candles. Adam Riess, an astronomer at Johns Hopkins University in Baltimore, Maryland, who has led much of the cutting edge work on standard candles, says the ACT data deal is “reassuring” and “a testament to the quality of the work of the experimenters. ” and beware “.
But the tension in the Hubble constant remains. New techniques developed by various teams, including one led by Freedman, could help solve it. Steinhardt believes that the measurements will eventually converge as experimenters refine their methods.
But Riess says that perhaps it is the standard model of cosmology that could be wrong. “My intuition is that something interesting is happening.”
