multiply-imaged quasar (idea)

In 1979, gravitational lensing became an observational science when the double quasar Q0957+561 was discovered by Dennis Walsh. A double quasar is a quasar whose image appears twice, because it goes through another heavenly body and is lensed. This lensing is caused by the force of gravity on light and by the spacetime curvature.

For an image of multiple quasars to be considered a lensed quasar and not merely two quasars that appear next to each other they must satisfy 5 basic criteria

• There are two or more point-like images of very similar optical color.
• Redshifts (or receding velocity) of both quasar images are identical or very similar.
• Spectra of the various images are identical or very similar to each other.
• There is a lens (most likely a galaxy) found between the images, with a measured redshift much smaller than the quasar redshift.
• If the quasar is intrinsically variable, the fluxes measured from the two (or more) images follow a very similar light curve, except for certain lags - the time delays - and an overall offset in brightness.

These can be used to gauge values for the Hubble constant and the cosmological constant. They also can help finding dark matter or MACHOs.