The universe is constantly expanding, and that expansion is accelerating, but we aren’t sure exactly how quickly. Two sets of measurements to estimate the rate of expansion conflict with one another, which may be a sign that our basic understanding of the cosmos is wrong. What’s more, two new attempts by astronomers to solve this problem have complicated things further.
The rate at which the universe’s expansion accelerates is described by a number called the Hubble constant. There are two main sets of data that we use to estimate this key number: measurements of the cosmic microwave background (CMB), which is a relic of the first light to shine through the cosmos, and local measurements, which use observations of supernovae and other relatively nearby objects to determine how fast cosmic expansion is carrying them away from us.
Analyses of these two sets of observations have consistently provided clashing results, with the CMB data, which comes from the Planck satellite, indicating that the universe’s expansion is accelerating about 9 per cent slower than the rate suggested by the supernova data.
Simone Aiola at the Flatiron Institute in New York and his colleagues used the Atacama Cosmology Telescope in Chile to take new, more precise observations of the CMB. They used those measurements to calculate the Hubble constant and found a value that agrees with the Planck satellite data.
“This tension seems to be real, because the CMB now has two sets of measurements that conflict with local measurements,” says Aiola. “If this value had been in between the Planck and local measurements, it would have really increased the mess.”
As it is, this strengthens the argument that the CMB value of the Hubble constant is correct, indicating that either the local measurements were somehow wrong or that the two values are actually different due to physics that we don’t yet understand.
“It would be very exciting if the tension is real,” says Antonella Palmese at the Fermi National Accelerator Laboratory in Illinois. “The hope is that some new physics will be needed to describe the universe and we are just starting to see that now as our measurements are becoming more precise.”
Palmese and her colleagues have used measurements of gravitational waves – ripples that stretch and squeeze space-time because of the movements of massive objects – to calculate an independent value of the Hubble constant. They used data on three mergers of massive objects from the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and the Virgo detector in Italy.
Their value is in between the two others, but the measurements aren’t precise, so it is compatible with both. This first attempt demonstrates that gravitational waves could be a major help in solving this mystery, and Palmese says that after LIGO and Virgo’s next observing run – currently delayed by the coronavirus pandemic – we should have enough data to make a more definitive measurement of the Hubble constant.
This could also help identify any problems with the other local measurements. “It should be very close to the local measurements, but it doesn’t rely on the same type of observation, so if there is some kind of problem with the local measurements, we should be able to find it with gravitational waves,” says Palmese.
If the observations remain in tension, it means that there must be some sort of physical mechanism that changed the Hubble constant between when the light in the CMB was emitted and now. There are many guesses as to what that new physics might be, from unexpected properties of dark matter and dark energy to the possibility that we may just live in a strange corner of the universe, but we need more measurements before we can say for sure that it is time for a brand new theory.
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