Scientific Method / Science & Exploration

Paging scientist Ringwald: Could quasars become standard candles?

Light fluctuation patterns from black holes may shed light on cosmic expansion history.

Composite optical and X-ray image of quasar 3C 186, one of the most distant yet observed. Two such quasars on opposite sides of the sky could close a loophole in interpreting quantum entanglement experiments.

In the quest to characterize the history and evolution of the Universe, cosmologists seek to identify standard candles—astronomical objects for which the intrinsic brightness is known, or can be easily determined. For very large distances, the problem is finding standard candles that are sufficiently bright. The discovery that white dwarf supernovas are standard candles led directly to the measurement of cosmic acceleration, not to mention the 2011 Nobel Prize in physics. However, supernovas are each one-time events that occur at random time intervals in a particular galaxy. Large surveys are still required to keep watch for them.

Quasars, on the other hand, are numerous and persistent sources of light. They're extremely bright jets of light beaming from supermassive black holes at the centers of galaxies, from an era when such black holes were gorging themselves with matter. While their light emission isn't predictable in the same way that type Ia supernovas are, researchers may have discovered some patterns that would allow them to be used as standard candles. Based on a sample of 14 quasars in the MACHO (MAssive Compact Halo Object) survey, De-Chang Dai, Glenn Starkman, Branislav Stojkovic, Dejan Stojkovic, and Amanda Weltman developed two possible methods for obtaining the distance to a quasar from its spectrum. The techniques appear promising, but eventually a larger sample size—and a reasonable physical explanation for the patterns the researchers found—will be needed for quasars to truly be considered standard candles.

Many galaxies at cosmologically significant distances contain quasars. Unlike white dwarf supernovas, which peak at the point of explosion and fade slowly after that, a quasar's output of light is persistent. It can be observed over very long periods of time. Quasars are also very bright: their name came from the fact they appeared to be intense star-like points of light, even over distances of billions of light-years.

Looking for patterns

Quasars may be persistent, but they are not stable in their light output and vary a lot in the wavelengths of light they produce. As a result, they had been largely dismissed as possible standard candles. Some studies prior to the current paper hinted they might have patterns in their fluctuations, but the cause (assuming the effect is real) is not understood.

The researchers' first method involved identifying short-term patterns in 13 quasars from the MACHO sample. The timing of the light fluctuations received at Earth was shifted from what the quasars emitted, due to the expansion of the Universe—an effect related to cosmic redshift that stretches wavelengths. The authors corrected the timing using the known distances found through other techniques, so the light variations matched how they would appear when emitted by the quasars. They plotted the variation in ultraviolet flux—the rate of energy output per unit area—for more than six years. During that period, the quasars fluctuated in brightness substantially and on different timescales from each other, but with noticeable rises and falls in flux.

The researchers then fitted straight lines to periods of flux increase and decrease longer than 90 days. They found the slopes of these lines to be nearly the same. While the amount and duration of the change in flux differed, the rate of change in flux was similar. (This is analogous to comparing several cars with different top speeds, but the same acceleration capabilities). The data points were scattered, but still showed a clear trend: the quasars all seemed to vary in their light output at a certain rate, independent of their distance from Earth.

Finally, the researchers fitted straight lines to the flux variations from the fourteenth quasar in the sample, but without correcting for redshift. These lines had the nearly same slope, but a different value than that obtained using the corrected spectra from the 13 quasars. A simple formula comparing the slope values yielded a number remarkably close to the real distance value for the last quasar, a highly suggestive result.

The authors acknowledged the similar slope values could be coincidences, since their sample size is small and sufficiently scattered. However, if they are not coincidental, quasars could be used as standard candles using a simple technique. First, astronomers would identify variations in the flux longer than 90 days, and fit straight lines to those periods of increase or decrease. Next, they would determine the slopes of those straight lines to obtain the hopefully universal rates of change. Finally, a comparison to the distance-independent rate of change reveals the distance to the quasar—an additional way of measuring cosmologically significant distances.


The second method the researchers developed involves bootstrapping. They assumed the rise and fall patterns in the fluxes of two quasars corresponded to each other, and adjusted the shape of the variation of one quasar until it matched the other. This is less ad hoc than it sounds: they used a well-established method to fit parameters, called chi-squared fitting. The amount of adjustment was related to the relative distance between the quasars. This means that, unlike the first technique, the second method relies on knowing the distance to one quasar by some other means before the second quasar could be used as a standard candle.

If the similarity in flux variations is real, then quasars could be a new and very valuable way to measure the history of cosmic expansion. However, the small number of quasars in the MACHO sample means this study is preliminary—and no known physical mechanism explains why every quasar should have such a constant rate of change in its flux. On the other hand, that could simply be another aspect of our ignorance: astronomers lack a complete model for how quasars shine the way they do. It could be that improved theoretical modeling would show precisely the fluctuation patterns to make quasars into standard candles. Much hard work still needs to be done, both on the theoretical and observational side, but the potential benefit to cosmology would be phenomenal.

Physical Review Letters, 2012. DOI: 10.1103/PhysRevLett.108.231302  (About DOIs).

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