The theory that the universe is expanding is widely accepted by astronomers. It's believed that this expansion happens at a constant rate, known as the Hubble constant, which is considered to be one of the fundamental quantities that describe our universe. However, this rate of expansion has been the subject of many studies, each presenting their own measurement of the Hubble constant.
One recent study offers another measurement of the Hubble constant. Astronomers from the H0LiCOW collaboration led by Sherry Suyu used telescopes in space and on the ground, including the NASA/ESA Hubble Space Telescope, to examine five galaxies – leading them to arrive at this independent measurement of the Hubble constant.
The five massive galaxies the study observed were located between the Earth and very distant quasars. These quasars are very luminous and the light they emit tends to bend around the huge masses of the galaxies due to strong gravitational lensing, which creates multiple background images of the quasar that usually smear into arcs.
"Our method is the most simple and direct way to measure the Hubble constant as it only uses geometry and General Relativity, no other assumptions,” said co-lead researcher Frédéric Courbin. The team measured the delays between the flickers of these different images of the quasars, which are directly related to the Hubble constant.
Using the time delays between the multiple images and computer models, the team arrived at an incredibly precise (at a 3.8 percent rate) measurement of the Hubble constant. “An accurate measurement of the Hubble constant is one of the most sought-after prizes in cosmological research today,” said researcher Vivien Bonvin.
The measurement Suyu and her team arrived at agrees with other measurements of the Hubble constant in the local universe that relied on Cepheid variable stars and supernovae as points of reference. These measurements, however, differ in value with those made by the ESA Planck satellite. This is to be expected because Planck measured the Hubble constant by observing the cosmic microwave background for the early universe.
“The expansion rate of the universe is now starting to be measured in different ways with such high precision that actual discrepancies may possibly point towards new physics beyond our current knowledge of the universe,” Suyu explained. The values acquired by groups of astronomers for the local universe — the region of the nearby universe that stretches about 1 billion light years in radius — seem to disagree with the accepted theoretical model of the universe.
This new measurement of the Hubble constant is challenging the way we understand the universe. It's not surprising, though, especially since there's still a lot we don't understand about our home. “The Hubble constant is crucial for modern astronomy as it can help to confirm or refute whether our picture of the universe — composed of dark energy, dark matter and normal matter — is actually correct, or if we are missing something fundamental,” Suyu further explained.