Gravitational lens gives us a third estimate of the Universe’s expansion

Anyone who has ever measured something twice, like the width of a doorway, and gotten two different answers knows how annoying it can be. Now imagine you're a physicist, and what you're measuring tells us something fundamental about the Universe. There are a number of examples like this—we can't seem to get measurements to agree on how long neutrons survive outside of atomic nuclei, for example.

But few of these are more fundamental to the Universe's behavior as disagreements over what's called the Hubble Constant, a measure of how quickly the Universe is expanding. We've measured it using information in the cosmic microwave background and gotten one value. And we've measured it using the apparent distance to objects in the present-day Universe and gotten a value that differs by about 10 percent. As far as anyone can tell, there's nothing wrong with either measurement, and there's no obvious way to get them to agree.

Now, researchers have managed to make a third, independent measure of the Universe's expansion by tracking the behavior of a gravitationally lensed supernova. When first discovered, the lens had created four images of the supernova. But sometime later, a fifth appeared, and that time delay is influenced by the Universe's expansion—and thus the Hubble constant.

Inconsistent constant

The Hubble constant is a measure of the Universe's expansion, as you can tell from its units, which are kilometers per second per Megaparsec. So, each second, every Megaparsec of the Universe expands by a certain number of kilometers. Another way to think of this is in terms of a relatively stationary object a Megaparsec away: each second, it gets a number of kilometers more distant.

How many kilometers? That's the problem here. Measurements of the Cosmic Microwave Background using the Planck satellite produced a value of 67 km/s Mpc. Those done by tracking distant supernovae produce a value of 73 km/s Mpc. We're not sure why those measurements should differ, or whether there's a technical problem with one of them we've not yet identified. But it's considered a significant unsolved issue.

The new work involves a third way to measure the distance that's independent of the other two. It relies on gravitational lensing, where the distortion in space-time caused by a massive object acts as a lens to magnify an object in the background. Since these aren't perfect optical-quality lenses, there are often some distortions and unevenness. This causes the light from the background object to take different paths to Earth, and thus a single object can appear in several different locations distributed around the lens.

At cosmological scales, those paths can also require the light to travel very different distances to get to Earth. And, since light travels at a finite speed, it means we can look at a single object as it was at different times. Last year, for example, researchers identified a single Hubble Space Telescope image that captured a supernova as it was at three different times after its explosion.

The new work focuses on a similar instance, a supernova first identified in 2014, and now called SN Refsdal, after the astronomer who first proposed using lensed explosions to perform measurements. When first detected, the distant SN Refsdal was lensed by a cluster of galaxies called MACS J1149.6+2223, which created four distinct images of it. But studies of the lens formed by MACS J1149.6+2223 quickly showed that it would create an additional image roughly a year later.

Those predictions turned out to be correct. Images taken in late 2015 identified the fifth image of the event created by the gravitational lens.