Active Galactic Nuclei: A New Standard Candle

Active Galactic Nuclei: A New Standard Candle

The ability to test, in detail, the properties of the universe during its early epochs has huge implications for establishing the validity of the Christian faith.

A team of four astronomers has discovered an accurate luminosity distance-measuring tool using active galactic nuclei (AGNs). This tool extends precision cosmology into the first five billion years of cosmic history. For the first time astronomers will be able to definitively test alternate gravity theory models and determine whether or not dark energy behaves differently in early cosmic history. By extending precision cosmology to the earliest epochs in the universe’s history astronomers now will be able to produce a much more detailed cosmic creation model.

During my teenage years my father bought the family a .22-caliber rifle with a telescopic sight and then constructed a shooting range in the basement of our home. After some practice I was able to light a match from thirty feet away with that rifle. It was a different matter altogether, however, when I took the rifle outside and tried to duplicate the feat from a hundred feet away. I was not able to calibrate the telescopic sight with sufficient precision, nor could I hold the rifle steadily enough. Someone much more skilled in sharpshooting than I told me none of these difficulties would have mattered because neither my rifle nor my sight was powerful enough to accomplish the task regardless of skill.

Similarly, deficiencies exist in the tools astronomers developed for measuring distances and other cosmological properties. Although these devices can determine properties corresponding to look back times (light travel times) equal to 8 or 9 billion years, they prove inadequate for look back times equal to 12 to 13 billion light-years. For decades, astronomers have yearned to study the characteristics of the universe that existed 12 to 13 billion years ago, when the universe’s first stars and galaxies formed.

This early cosmic era is also the point at which models for the origin of the universe diverge. Alternate cosmological theories predict cosmic characteristics substantially different from those posited by the standard hot big bang inflationary creation model. The Bible uniquely predicted that standard model thousands of years before any astronomers even suggested the universe might manifest that model’s characteristics.1 Thus, the ability to test, in detail, the properties of the universe during its early epochs has huge implications for establishing the validity of the Christian faith.

Measuring Recession Velocity and Distance

Precision cosmology and the testing of competing cosmological models depend on astronomers’ capacity to determine, as accurately as possible, how rapidly the universe was expanding at different epochs throughout its history. This capacity requires thousands of measurements of both the distance and the recession velocity of galaxies (the rate at which a particular galaxy is moving away from Earth) over the widest possible range of galaxy distances.

Obtaining recession velocities is straightforward. Astronomers simply measure the degree by which a galaxy’s spectral lines are shifted toward the red end of the spectrum. Einstein’s theory of special relativity states that the faster a galaxy moves away from us relative to the velocity of light the greater the “redshift” of its spectral lines will be. Today, astronomers have instruments capable of determining a galaxy’s redshift and, hence, its recession velocity often to an accuracy of one part in a hundred thousand (to within five decimal point precision).

The real limitation to precision cosmology is determining how far away all the different galaxies are from Earth. Some advancement has been made in this area. New technology allows astronomers to link together radio telescopes separated from one another by thousands of miles. Astronomers can now measure direct geometrically determined distances (based on standard plane geometry theorems) to galaxies as far away as 163 million light-years.2

Using Standard Candles

Beyond that 163-million-light-year distance, astronomers depend upon “standard candles”—a particular class of astronomical bodies wherein all the bodies manifest the same intrinsic luminosity. With all the bodies being equally luminous, their apparent luminosities will depend on the inverse square distance law (a body X times more distant will be X2 times dimmer). Astronomers measure the apparent luminosity of one of those bodies. They then compare it to the luminosity of another, much closer body in the same class for which an accurate direct geometric distance measurement exists. From this comparison, researchers can deduce the actual distance to the body that is farther away.

To date, the brightest astronomical bodies that serve as standard candles are Cepheid variable stars and type Ia supernovae. Cepheids are up to 100,000 times more luminous than the Sun. Using Cepheids, astronomers have determined accurate distance measures to galaxies as far away as 150 million light-years (look back times = 150 million years).

A single type Ia supernova at maximum brightness shines with as much light as 5 billion ordinary stars like the Sun. Consequently, they can be seen at great distances. The most distant ones that can provide reasonably accurate distance measurements reside at distances corresponding to look back times equal to 9 billion years.

In the late 1990s, measurements on about 50 type Ia supernovae at distances ranging from 1.9 to 7.0 billion light-years led to the discovery of the accelerating expansion of the universe. These measurements also revealed that dark energy (the physical factor responsible for the acceleration) is the dominant component of the universe.3

Dark energy’s capacity to accelerate cosmic expansion is proportional to the space surface of the universe. In a continually expanding universe the space surface increases as the universe grows older. Therefore, the accelerating effect due to dark energy becomes greater and greater as the universe ages. Gravity has the opposite effect. When the universe was young, it had a smaller space surface and massive objects were jammed closely together along that surface. But, as the universe expanded, the massive objects spread farther and farther apart. The separating distances between massive bodies implies that as the universe grows older gravity will become progressively weaker in its capacity to slow down cosmic expansion. Thus, depending on the values of the cosmic mass density and the cosmic dark energy density, at some point in the history of the universe cosmic expansion should transition from slowing down to speeding up.

Today, astronomers have accumulated measurements on nearly 500 type Ia supernovae extending out to look back times equal to 9.5 billion years ago.4 However, reliable distances beyond a corresponding look back time of 9.5 billion years have been beyond the scope of current tools—until now.

Active Galactic Nuclei

A team of four astronomers (three at the Niels Bohr Institute in Denmark and one at the University of Queensland in Australia) has found a novel way to transform very bright non-standard candles into standard candles. Their work establishes the reliability of a standard candle that would allow astronomers to perform precision cosmology measurements corresponding to look back times as far back as 13 billion years ago (when the universe was just 0.75 billion years old).5

Active galactic nuclei are the most luminous of the persistently shining objects in the universe. An active galactic nucleus (AGN) is a very compact region in the center of a galaxy wherein a supergiant black hole is sucking in copious amounts of gas and dust. As that gas and dust approaches the event horizon of the black hole about 10 percent of the gas and dust’s mass is converted into energy. (By comparison the conversion of mass into energy in the Sun’s nuclear furnace is 0.7 percent.) AGNs are so very bright thanks to their extremely efficient conversion of mass into energy and the fact that the conversion occurs within a tiny volume surrounding the black hole.

The AGN family includes quasars, Seyfert galaxies (see figure 1), blazars, starburst galaxies, and radio loud galaxies. Because of their enormous brightness, AGNs can be used to probe the features of the most distant regions of the universe, regions that correspond to the earliest times of cosmic history. 

Figure 1. NGC 1275, a Seyfert galaxy
NGC 1275 is 237 million light-years away. The central cores of Seyfert galaxies form a subclass of active galactic nuclei.
Image credit: NASA/ESA/Hubble Space Telescope (AURA/STScI)

The intrinsic luminosities of AGNs manifest a wide range of values. The four-astronomer team noted, however, that a tight correlation exists “between the luminosity of an AGN and the radius of its broad-line region.”6 The physical explanation for the correlation lies in the supermassive black hole that resides in the heart of every AGN. High-velocity gas clouds surrounding that black hole produce broad emission lines in the spectrum of the AGN. The larger the diameter of this region, the greater the intrinsic continuum luminosity of the AGN. Therefore, an accurate measurement of the diameter of the broad-line emitting region yields a precise value of the AGN’s intrinsic luminosity. As noted already, the inverse square law for the dimming of light allows one to translate a measure of intrinsic luminosity into a determination of the actual distance to the light-emitting source.

There is one critical proviso. This new distance-measuring technique requires that at least one AGN’s distance be determined either by a direct geometric distance measuring technique or by a well-established standard candle technique such as exploiting Cepheid variable stars or type Ia supernova in the AGN. Currently, only the AGN NGC 3227 and NGC 4051 have direct distance estimates. However, the three AGN NGC 3227, NGC 4051, and NGC 4151 are all close enough to obtain reliable Cepheid-derived distances with the Hubble Space Telescope or with instruments of comparable capability.

Researchers’ Predictions

The four-astronomer team estimates that in 10 years or less their AGN distance measuring technique, using only existing astronomical instruments, will provide an independent determination of the universe’s age and of cosmic expansion rates throughout the past 11.5 billion years. With more time and better equipment, the technique should provide precise measures of cosmic expansion rates throughout the past 13 billion years.

These results establish that AGNs will give astronomers a detailed, accurate picture of the universe’s early expansion history. This forthcoming insight will also give cosmologists a better measure of the universe’s geometry, a more accurate date for the age of the universe, superior insight into the nature of dark energy, a much more detailed history of the universe, and a more exact picture of the cosmic creation event.

This superior history and picture will allow deeper and more rigorous tests of the big bang creation model. Moreover, it could provide even more evidence for the Bible’s description of cosmic creation and history. We at Reasons To Believe predict that the biblical big bang creation model and design of the universe for humanity’s benefit will pass these future measurement tests with flying colors.

  1. Hugh Ross, A Matter of Days (Colorado Springs: NavPress, 2004), 139–48.
  2. J. A. Braatz et al., “The Megamaser Cosmology Project. II. The Angular-Diameter Distance to UGC 3789,”Astrophysical Journal 718 (August 1, 2010): 657–65; Hugh Ross, “Results from Two Research Teams Improve Direct Distance Measurements and Strengthen the Creation Model, Part 2 (of 2),”Today’s New Reason to Believe, posted September 6, 2010,
  3. Adam G. Riess et al., “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant,”Astronomical Journal 116 (September 1998): 1009–38; S. Perlmutter et al., “Measurements of Ω and Λ from 42 High-Redshift Supernovae,”Astrophysical Journal 517 (June 1, 1999): 565–86.
  4. M. Sullivan et al., “SNLS3: Constraints on Dark Energy Combining the Supernova Legacy Survey Three-Year Data with Other Probes,”Astrophysical Journal 737 (August 20, 2011): id. 102; N. Suzuki et al., “The Hubble Space Telescope Cluster Supernova Survey: V. Improving the Dark Energy Constraints Above z>1 and Building an Early-Type-Hosted Supernova Sample,” (May 2011): eprint arXiv:1105.3470v1; Steven A. Rodney et al., “Type Ia Supernovae at z>1.5 from the HST Multi-Cycle Treasury Surveys,” American Astronomical Society, AAS Meeting #218, #219.01, Bulletin of the American Astronomical Society 43 (May 2011); Richard Kessler et al., “First-Year Sloan Digital Sky Survey-II Supernova Results: Hubble Diagram and Cosmological Parameters,”Astrophysical Journal Supplement 185 (November 2009): 32–84.
  5. D. Watson et al., “A New Cosmological Distance Measure Using Active Galactic Nuclei,”Astrophysical Journal Letters 740 (October 20, 2011): id. L49.
  6. D. Watson et al., page 1 of id. L49.