Using Quasars to Measure the Cosmos –Key to Unraveling the Mystery of Dark Energy

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Being able to measure the universe accurately at great distances will have profound implications for scientists’ future understanding of dark energy and the ultimate fate of the cosmos.

Accurately estimating how far away an object in outer space is one of the toughest fundamental problems in astrophysics and is central to cosmology as it allows scientists to work out the age of the Universe and what it’s fundamental properties are. Because their new method uses quasars, some of the brightest objects known, scientists say they will be able to determine distances much further than achieved to date, paving the way to a better understanding of dark energy.

The image above shows quasar PG 0052+251, which is 1.4 thousand million light-years from Earth, at the core of a normal spiral galaxy. Astronomers are surprised to find host galaxies, such as this one, that appear undisturbed by the strong quasar radiation.

Quasars reside in a variety of galaxies, from normal to highly disturbed. When seen through ground-based telescopes, these compact, enigmatic light sources resemble stars, yet they are thousand of millions of light-years away and several hundred thousand million times brighter than normal stars.

Astronomers believe that a quasar turns on when a massive black hole at the nucleus of a galaxy feeds on gas and stars. As the matter falls into the black hole, intense radiation is emitted. Eventually, the black hole will stop emitting radiation once it consumes all nearby matter. Then it needs debris from a collision of galaxies or another process to provide more fuel.

Finding new ways to measure cosmic distances has a strong pedigree in cosmology. In the 1990s a way was discovered to use supernovae, the explosions of massive stars, to measure distances. From that finding stemmed the discovery of the acceleration of the Universe in 1998, which showed that 70% of the Universe was made up of dark energy.
Now, astronomers Darach Watson and colleagues at Copenhagen's Niels Bohr Institute have found a new way to measure Dr. have discovered a way to find accurate distances using quasars. Quasars are powered by the supermassive black holes at the hearts of galaxies, and are so luminous that they can vastly outshine all the stars in their host galaxy combined. Due to their extreme luminosity, scientists have been searching for ways to use them to measure distances since they were discovered in the 1960s. After more than four decades, those attempts have finally been successful.

“It was a discovery waiting to happen,” says Dr. Kelly Denney, a member of the team. It has been known for some time that the size of the gas cloud falling into the supermassive black hole is related to the luminosity of the quasar.

Watson and colleagues realised that new data on the sizes of gas clouds that were being measured for other reasons were now accurate enough to allow them to predict the luminosities of the quasars. Knowing how bright the quasars appeared from earth, they could easily determine how far away they were.

Quasars can be detected to very large distances, much farther than supernovae, which are currently the best measure of distance. “That’s what makes quasars so exciting,” says Watson, “seeing farther away means seeing farther back in time, and knowing how the universe expanded with time is the key to understanding dark energy.”

The Daily Galaxy via Niels Bohr Institute


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