A distant supernova detected through a strange gravitational lensing quirk has been used to measure the expansion of the universe. The result adds an unexpected twist to a longstanding tension.
Gravitational lensing occurs when light from a distant object is bent and distorted by the gravity of a relatively nearby, massive object. This can result in multiple images of the distant object appearing around the near object, similar to the patterns you might see when looking through a distorted lens, such as the bottom of a glass of water. Because light from the background object takes a different path to form each image, those images may appear before us at different times.
patrick kelly at the University of Minnesota and colleagues used this strange effect to calculate the Hubble constant, a measure of the expansion rate of the universe. They did it with light from the Refsdal supernova, which is gravitationally reflected from a nearby galaxy cluster. It was first discovered in 2014, and a new image of the supernova appeared in 2015, allowing researchers to use the lag time between images to calculate the rate at which the universe is expanding away from Earth.
There are two main ways to measure the Hubble constant. The first, called the cosmic distance ladder, relies on measurements of relatively close objects to determine how fast they are moving away from Earth. The second uses observations of the cosmic microwave background (CMB), which is a relic of light left over from the Big Bang, so the measurements must be extrapolated forward in time using cosmologists’ best models of the universe.
The two methods have been at odds for decades, in what is called the Hubble stress: the distance ladder results in a Hubble constant of 73 kilometers per second per megaparsec (km/sec/mpc), and the CMB method gives a value of approximately 67 km. /sec/mpc. Researchers have long hoped that independent methods could help resolve this tension, but have yet to succeed. This new measurement using the Refsdal supernova gives a value of around 67 km/sec/mcf, in agreement with the CMB method despite being based on observations of a single object like the distance scale method.
The new result doesn’t rule out the higher value, but it does mean that the models used to study gravitationally lensed objects are up for grabs. “If the value of the Hubble constant turns out to be 73, as indicated by local measurements at this time, then there must be something faulty in our understanding of galaxy cluster lensing, and these models are routinely used to study the distant universe. says Kelly.
The researchers are following other lensed supernovae now to see if they can get more measurements with this method, and other teams are also hard at work on other independent ways to measure the Hubble constant. If they don’t find a way to make the measurements agree with each other, we may need entirely new models of exotic physics to explain what’s really going on.