- Researchers use quasars to measure the universe’s expansion across the past 12 billion years.
- They examined the XMM-Newton archive and gathered X-ray data of more than 7,000 quasars.
Our Universe is thought to consist of 3 kinds of substance: ordinary matter, dark matter, and dark energy. As per the existing popular model, Universe is filled with dark matter (70%) and dark energy (25%), which is responsible for the current acceleration of the Universe’s expansion.
This model is built on a vast amount of data collected via several ground-based and space-based telescope over the years. The data (associated with cosmic microwave background, galaxy clusters, supernova explosion and the gravitational distortion on different galaxies caused by dark matter) can be used to examine the expansion of the Universe in recent epochs of cosmic history, up to 9 billion years ago.
Now researchers at University of Firenze, Italy and Durham University, UK, have suggested another kind of cosmic tracer — quasars — that measures the universe’s expansion across the past 12 billion years.
It has been theorized that large galaxies contain a supermassive black hole in their center. These supermassive black holes pull in matter from their surroundings at extremely fast rates. The matter forms a swirling disc as it falls onto the black hole. This disc radiates in ultraviolet light and visible light, heating up nearby electrons and producing X-ray. The cores of such galaxies are called quasars or active galactic nucleus.
However, some of these black holes are not engulfing any matter because they have already consumed everything around them and there is nothing left to eat anymore. The Milky Way’s supermassive black hole, for example, is all out of food. It does not have an active galactic nucleus, thus we do not appear as a quasar to other distant galaxies.
Using Quasar To Probe The Cosmic Expansion
In 2015, researchers realized that a relation between a quasar’s luminosity at X-ray and ultraviolet wavelengths can be used to determine the distance of these sources, as well as to investigate expansion history of the Universe.
The Type Ia supernova — a type of supernova that occurs in a binary system in which one of the stars is a white dwarf — generates explosions of anticipated brightness, which enables scientists to pinpoint the distance. In the late 1990s, scientists observed these supernovas and unveiled the accelerated expansion of the Universe over the past few billion years.
Artistic impression of active galaxies to measure the Universe’s expansion | Credit: ESA
Since we can observe quasars out to much greater distances from Earth than Type Ia supernova, we can use quasars to accurately analyze much earlier epochs in the cosmic history. In this study, researchers have shown how to do that and the outcomes are pretty interesting.
What Exactly They Did?
Researchers examined the XMM-Newton (X-ray space observatory launched in 1999) archive and gathered X-ray data of more than 7,000 quasars. This includes very distant quasars that are over 11 billion years old (which means light has traveled for more than 11 billion years before reaching us).
They then combined the X-ray data with ground-based ultraviolet observations, and complemented it with more nearby quasars and with some distant ones, captured through NASA’s Swift X-ray and Chandra observatories, respectively.
This huge sample allowed them to scrutinize the relationship between quasars’ ultraviolet and X-ray emission in close detail, and thus refine the method of estimating their distance.
The researchers identified two different sets of quasars: 30% of the sources emit lower amounts of X-rays characterized by higher energies, while 70% of them shine brightly in low-energy X-rays. They only considered those in which relationship between ultraviolet and X-ray emission seems clearer.
Eventually, they bring the sample down to approximately 1,600 quasars for best observations. They combined the quasar sample with Type Ia supernova samples and found similar outcomes in the overlapping epochs.
However, when they merged older quasars (that span more than 12 billion years of cosmic history) with Type-Ia supernovae, they found several inconsistencies between these two samples.
Analyzing Universe’s expansion by combining quasars and Type Ia supernovae | Courtesy of researchers
This indicates that the current standard model of the universe requires the addition of a few more parameters to reconcile data with theory. Dark energy with increasing density could be one of those parameters.
The model also helps astronomers who are studying Hubble constant (a measurement unit used to describe the universe expansion) and Planck’s observations of the cosmic microwave background. However, before solving any cosmic conundrum, the team will have to investigate several more models in great details.
The researchers plan to observe more quasars to further improve their results. ESA’s Euclid mission, planned to be launched in 2022, will provide more refined data and help researchers uncover the mysterious nature of dark energy.