Tag Archives: standard candle

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.

A new standard candle?

Standard candles have played a hugely important role in establishing the cosmological distance ladder. It’s easy to see why: the more distant something is the dimmer it appears, according to the inverse-square law. So if we know how bright something really is then, by measuring how bright it appears to be, we can determine its distance.

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 and Type Ia supernovae are perhaps the most well-known standard candles, and the study of these objects have transformed our understanding of the universe. But they (and the several other standard candles used in astronomy) are not without problems. One of the main difficulties is that we can’t see them over very large distances. Even Type Ia supernovae cannot be used to make reliable distance measurements beyond a redshift of about 1.7. So one of the most interesting astronomical results of 2011, at least in my opinion, was the surprising discovery of a standard candle that can work over truly cosmological distance scales: active galactic nuclei (AGNs).

Artist's impression of an accretion disc and torus around a black hole

An artist's impression of an accretion disc and torus around an AGN
Credit: NASA/CXC/M.Weiss

An AGN is one of the brightest objects in the universe and so can be seen over extreme distances. The power source for an AGN’s extreme luminosity is the supermassive black hole that lies at its centre. An accretion disc – a collection of matter that forms as matter spirals into a dense object – surrounds an AGN’s supermassive black hole. (See chapter 4 of New Eyes on the Universe for an explanation of accretion discs.) Further away from the black hole, at least with type-1 AGNs, lies a dense area of dust and gas known as the broad-line region. The region gets its name because the black hole’s gravitational influence whips the dust and gas around at high speed, and the Doppler effect causes emission lines to be broadened.

And how does this rather chaotic set-up generate a standard candle? Well, the broad-line region emits light because its gas has been ionised. The ionisation occurs because high-energy photons are emitted by the accretion disc and subsequently hit the region. The key point here is an accretion disc is a variable object: sometimes it ‘flares’. This makes it possible to compare the time at which the accretion disc emits light and the broad-line region re-emits light, and that time delay gives the radius of the broad-line region. What four astronomers – Darach Watson, Kelly Denney, Marianne Vestergaard and Tamara Davis – have found is that there’s a relationship between the size of the radius and the central luminosity of the AGN. They checked the relationship on a sample of 38 AGNs at a known distance and it seems that, although there is scatter in the data, the technique will work as a distance indicator. (You can read their paper at arxiv.)

The AGN standard candle is not as accurate as the Cepheid or supernova candles. But since AGNs can be seen over tremendous distances, and since they can be studied over long periods of time, it seems certain that the technique will become of increasing importance. In particular, a standard candle that lets astronomers measure distances directly up to a redshift of about 4 will provide a valuable tool for probing the nature of dark energy.