A brand new approach might use quasars to immediately measure the speed of enlargement of the universe

One of the biggest challenges in measuring the expansion of the universe is the fact that many of the methods we use are model dependent. The best known example is the use of distant supernovae, where we compare the standard brightness of a Type Ia supernova to its apparent brightness to determine its distance. However, knowing the standard brightness depends on comparing it to the brightness of Cepheid variables, which in turn is determined by measuring the distances to neighboring stars via parallax. Each step of this cosmic distance ladder depends on the step before it.

Different methods of cosmic distance measurement. Photo credit: Wikipedia user Brews O’Hare

This does not mean that our distance measurements are necessarily wrong, but it does make them more prone to systematic error. The past few years have shown that our various cosmic expansion measures are not entirely in line. Either a systematic error has crept into our data, or something is wrong with our cosmological model. One way out of this mess would be to find new ways to measure cosmic expansion. Preferably methods that are not model-dependent. We have made some progress in this area. Methods using phenomena such as astronomical measles and gravitational waves have shown promise. In a recent study in Physical Review A, a method using quasars and gravitational lenses is presented.

A quasar can produce many images through gravitational lenses. Photo credit: NASA / ESA / D. Spieler (STScI)

Quasars are incredibly bright and distant objects. They are powered by active supermassive black holes in young galaxies. The light we see from quasars has traveled billions of years to reach us, and so it is shifted red by the expansion of the universe during that time. Instead of trying to measure the distance of these quasars, this new method examines quasars that are gravitationally captured by nearby galaxies.

When a galaxy is between us and a distant quasar, the galaxy’s gravitational mass bends the quasar light around the galaxy. As a result, we can often see multiple images of the quasar instead of a single image. Each of these images is the result of a different path of light as captured by the galaxy. Due to the gravitational lens, the light paths of these images can differ in distance by dozens of light years. The quasar images that we see can therefore differ by decades in age. Since the universe is constantly expanding, a younger quasar image is less red-shifted than an older one.

The idea behind this new work is wonderfully simple. Just compare the redshifts of the quasar images with lenses near a galaxy and you can determine how much the universe has expanded over a decade or century. By doing this with galaxies at different distances, you can determine not only the rate of cosmic expansion, but also whether that rate has changed over time.

The broadening of spectral lines. Photo credit: Swinburne University of Technology

In practice, it is very difficult to compare these redshifts. The redshift difference between two quasar images is extremely small, and because light has traveled so far, its spectral lines are blurred by the gas that passed it to reach us. It is an effect known as Doppler broadening. So we cannot directly compare the redshifts.

To solve this problem, the team suggests a method called intensity correlation patches. The method takes the Doppler broadened light of a quasar image and compresses it into an average line. Since two lens images have similar Doppler broadening when compressed, their main difference is the difference in redshift. The two images can then be compared based on their interference or spotting. It is an optical effect, similar to how two slightly different notes can interfere with each other to create a warbling tone.

In principle, this method could give us a detailed measure of the evolution of cosmic expansion and release the tension of modern cosmology. The next step is to create a ground-based telescope detector that can take such a measurement.

Reference: R. Merlin et al. “Intensity Correlation Spots as a Technique for Removing Doppler Broadening.” Physical Review A 103.4 (2021): L041701.

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