Four Times a Supernova

I’ve been a bit pressed for time recently (to put it mildly) so am a bit late catching up on a wonderful observation (by Kelly et al.) reported in last week’s issue of Science. Here’s the abstract:

In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster’s gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.

And here’s a nice picture of the system which I ripped of from a nice report in Physics World:


Multiple images of background objects caused by gravitational lensing have been observed before, but the key thing about this particular “Einstein Cross” is that the background object is a type of exploding star called a supernova. That means that the light it emits will decay over time. That light reaches us via four different paths around the intervening galaxy cluster so monitoring the different evolution in the four images will yield direct measurements of the physical scale of the cluster and hopefully  answer a host of interesting cosmological questions.


One Response to “Four Times a Supernova”

  1. Phillip Helbig Says:

    In the old days, the idea was to have a model for the lens (galaxy, cluster, etc) and use the time delay to measure the Hubble constant. (The basic idea is simple: all but one of the quantities involved—angles, flux ratios, etc—are dimensionless, so there is no intrinsic scale. However, the time delay between two images provides a quantity with dimension, setting the overall physical scale, and hence can be used to measure the Hubble constant (assuming that the lens is at low enough redshift and/or that we know the other cosmological parameters, otherwise there is some additional uncertainty).) This has been done in the past with QSOs (also suggested by Refsdal, though his original idea was a supernova; QSOs had not yet been discovered when he first worked out the idea), but there is some uncertainty in the measured time delay. Since a supernova has a very well defined light curve, the time delay should be measurable very precisely, especially since there are 6 pairs of images for which time delays can be measured.

    These days, we know the Hubble constant quite well from other methods, so it might be more interesting to assume a Hubble constant and use the time delays to constrain the lens model.

    Anyone who has doubts that QSOs are at the cosmological distances corresponding to their redshifts has to come up with some other explanation for gravitational-lens systems and also has to explain why the statistics of gravitational lensing are compatible with standard cosmology while this would not fall naturally out of any alternative cosmology.

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