Quasars as Mileposts for Universal Expansion

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In the late 1950s, radio telescopes picked up some unusual radio wave emissions from mysterious distant objects. Today, we know them as “quasars” — short for “quasi-stellar radio sources” — regions around the black holes at the center of massive galaxies that give off a great deal of radio waves, as well as visible light.

Black holes provide the power source. As matter — gas and dust, primarily — wanders near a black hole, it doesn’t cross the event horizon and fall into the hole directly. Instead, it forms an accretion disk, and the light is the result of the energy produced as the black hole gobbles up that gas and dust.

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Quasars have long been a boon to astronomers: images of a double quasar helped confirm Einstein’s prediction of gravitational lensing in 1979. Now physicists have developed a new technique that uses light from quasars as cosmic mileposts to map the structure and expansion history of the universe — possibly shedding light on the nature of dark energy, the favored culprit for our accelerating universe.

Based on data from the Massive Compact Halo Objects (MACHO) project, the work appeared in Physical Review Letters this summer.

The key is the high redshift patterns in quasar light, a direct effect of the expansion of the universe, since such a shift is indicative of an object moving away from us through space. A redshift in quasar light was first proposed by an astronomer named John Bolton in the 1960s. His colleagues were skeptical of the redshift claim initially, but subsequent work on the optical spectra of such objects verified that there were, indeed, high redshifts in the emission lines.

The higher the redshift, the further away the object. So based on the redshift data, it’s clear that quasars are very far away, and hence date back to the earlier days of the cosmos. Physicists suspect that quasars were more common in the early universe than they are today, and that most galaxies — including our own Milky Way — have had their active “quasar moments,” even if they are pretty quiet today.

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So, how does this new technique exploit quasars? Case Western physicist Glen Starkman, one of the study’s authors, said that his collaborators found that variation patterns in the light from 14 quasars over several hundred days proved very consistent — Starkman described it as a “dimmer switch” — and from that they could infer the redshift of each quasar.

Once they knew the redshift, they could use that information to determine what size our universe must have been when that light was first emitted, compared to the size it is today. The next step is to increase the sample size, to make sure this consistent variation pattern isn’t just a statistical fluke.

Ultimately, the object is to calculate the redshifts of millions of quasars. That amount of data would enable them to map out the history of the universe, from those early quasar-filled days to the present, in great detail.

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Quasar light would be a nice complementary data point to the supernovae currently used as “standard candles” by astronomers. The redshifts from Type 1A supernovae, after all, led to the Nobel-Prize-worthy discovery that the expansion of our universe is accelerating.

But supernovae only tell us about the universe at a time when it was 2.7 times smaller than today. Quasars are older, and would bring us back to a time when the cosmos was one-eighth its current size. And supernovae are relatively rare events; quasars are more common.

“This could help us learn about how gravity has assembled structure in the universe,” Starkman told the Case Western blog. “And the rate of structure growth can help us determine whether dark energy or modified laws of gravity drive the accelerated expansion of the universe.”

Image: Artist’s rendering of ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun. Credit: M. Kornmesser, European Southern Observatory.

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