Quasars as standard clocks

There’s an interesting paper in the 6 June 2012 issue of Phys. Rev. Letters. The paper, entitled “Using quasars as standard clocks for measuring cosmological redshift” (and available as a preprint here on arXiv) describes how quasars might be used to probe the largest distances in the Universe.

The use of standard candles has of course been important in developing an understanding of the distance scale of the Universe. If you know how bright something really is, then by measuring how bright it appears you can determine its distance. In essence, you just have to employ the inverse-square law.

A standard candle appears dimmer the more distant it is

A standard candle appears dimmer the more distant it is
Credit: Karen Kwitter

Cepheid variables, for example, were used to probe the local Universe; type Ia supernovae have allowed astronomers to probe even deeper into the cosmos. However, to probe the largest distances with a standard candle we need the brightest sources. The most distant known supernova occurred at a redshift of 1.7; to get beyond that we need to use something like quasars (which have been identified at redshifts beyond 7). The trouble with quasars, however, is that they vary hugely in luminosity. They certainly aren’t a standard candle.

But could quasars be a standard clock?

Dejan Stojkovic and his colleagues have analysed the light curve data of 13 quasars, each of which was at a different redshift. They plotted a graph of quasar flux (in other words the actual energy emitted per unit time) against time. All values were transformed into the rest frame of the quasar, so in each case the light curve described what was happening when the radiation was emitted. When they laid the different light curves on top of one another they found that the curves matched. That leads to an intriguing thought: if quasar light curves are similar then you can use the redshift of one quasar to determine the redshift of an unknown quasar simply by recording how its brightness changes over time. Stojkovic and his colleagues tested two methods for doing this.

First, they identified straight-line segments in the light curves that were related to quasar redshift and then discarded the rest of the light curve. They then matched the slope of this straight line for a quasar with known redshift to the slope of a line from the light curve of an “unknown” quasar (whose redshift they of course knew). The method gave accurate values for the unknown resift.

Second, they employed a more statistical approach that matched several parts of the quasar light curves (rather than just a straight-line segment). Again, by fitting a “test quasar” light curve to an “unknown quasar” light curve, they were able to find the ratio of the redshifts with good accuracy.

Thus if Stojkovic and colleagues are correct then astronomers might be able to use quasars as standard clocks. It’s potentially a new technique for determining cosmic distances. It isn’t going to be instantly useful: they need to check the technique on more than just 13 quasars, and they need to develop algorithms to do the light-curve matching. It would also help if we knew why such a relationship exists: at present the authors have no theoretical explanation for the effect. But if the work stands up, astronomers will soon have a tool that lets them probe distances on a truly cosmological scale.