Cosmic Magnifying Glass Reveals Supernova Explosion 4 Billion Light Years Away

supernova explosion
Image showing a supernova explosion four billion light years away. W. M. KECK OBSERVATORY

Scientists have observed a supernova explosion from 4 billion light years away in an extreme case of gravitational lensing—where distant objects are magnified by galaxies bending the light emitted from them.

In a study published in the journal Science, researchers announced an extreme case of gravitational lensing. In it, they were able to take four images of a specific type of supernova—Type Ia. These exploding stars are well understood and have, over the past 20 years, been used to study the expansion of the universe.

But in this case, the supernova was magnified by more than 50 times, and produced four separate images. This provides scientists the opportunity to measure exactly how much the light was distorted by space. By using these measurements—along with future observations of other supernovas—scientists will be able to work out how fast the universe is expanding with incredible accuracy.

Gravitational lensing allows scientists to study the distant universe. Galaxies bend the light that travels through them, creating a curve in spacetime. This is one of the foundations of Einstein's theory of general relativity. The resulting curve creates a lens that deflects the path of the light passing by, creating a cosmic magnifying glass that scientists can use to study distant objects.

However, this only really works when the object of interest is sitting directly behind a galaxy—the pair must be in near-perfect alignment.

The technique is often used to study objects such as exploding stars, known as supernovas, and black holes.

A zoomed-in view of the gravitationally lensed supernova. Joel Johansson]

Ariel Goobar, professor in experimental particle astrophysics at Stockholm University, Sweden, and lead author of the new study, tells Newsweek in an email interview: "These are the kinds of well-calibrated explosions that have been used to accurately map the expansion history of the universe. It was thanks to observations of Type Ia supernovae that it was discovered that the expansion of the universe accelerates, a Nobel Prize-winning realization in 2011, and often attributed to the existence of a mysterious dominant component in the fabrics of the universe, 'dark energy.'"

But studying a Type Ia supernova comes with problems. Because it is a fleeting event, if and when it lines up with a galaxy, scientists must work fast to study it. In September, Goobar and his colleagues detected a supernova that had come into alignment with a foreground galaxy. They set up several telescopes, including the Hubble Space Telescope, to make detailed measurements of the explosion.

gravitational lensing
Image showing how gravitational lensing was used to magnify the supernova. ALMA (ESO/NRAO/NAOJ), L. Calçada (ESO), Y. Hezaveh et al., edited and modified by Joel Johansson

What they got was four highly magnified images of the light being returned at different times. Because scientists understand Type Ia supernova so well—the way they explode and how bright they are—the team was able to use the images to look at differences in the curvature of spacetime.

"What these observations show is that the supernova light is split into four separate images, corresponding to four different paths of the light around the lensing galaxy," Goobar says. "This presents us with an exciting possibility. Since the light beams travel through different paths, corresponding to different lengths, time differences in their arrival could arise.

"The time differences between the images are sensitive to both the mass and properties of the lens, but also to the overall expansion rate of the universe—the more the universe has expanded since the light started its journey, the longer the light paths. That also leads to longer differences between the arrival times, since light travels at a constant speed."

Should the scientists be able to produce similar observations of Type Ia supernova, they could be used to work out how fast the universe is expanding with unparalleled accuracy.

"In the mid-1960s the Norwegian astronomer Sjur Refsdal proposed this technique to measure the expansion rate of the universe, and we hope to be able to apply the technique to iPTF16geu [the supernova studied]. However, even if the observations of this particular supernova are insufficient for a precise measurement, we are very optimistic about being able to detect many new, lensed, Type Ia supernovae.

"The reason for our confidence is the easiness by which iPTF16geu was found. We are currently upgrading the camera used for this detection and, by the end of the year will be able to scan the sky looking for new gravitationally lensed supernovae over 10 times more efficiently than in the past. This will allow us to study gravity, how matter clumps in galaxies and the expansion of the universe."