Quasars, The Brightest In Universe, Can Be Used As Cosmic Standard Candles To Measure Distances, Study Says

Researchers from Center for Astrophysics, Harvard & Smithsonian Institution have explained how quasars can be used as new ‘standard candles’ to help determine cosmic distances and other parameters.

Representational image (Source: Getty)

New Delhi: Quasars, similar to stars, are many light years away from Earth. They are the brightest objects in the universe, so much so that their luminosity drowns out the light from other stars in the same galaxy. A new study has now shown that quasars can be used to determine cosmological parameters.

Researchers from the Center for Astrophysics, Harvard, and Smithsonian Institution recently conducted a study in which they explain that quasars can be used as new "standard candles" to help determine cosmic distances and other cosmological parameters. 

The study has been published in the Astronomy & Astrophysics journal. Even for the quasars at very large distances from Earth, scientists found a close relationship between the luminosities of X-ray and ultraviolet radiations emitted by them, which could be used to obtain information about the cosmos.

What was previously used to measure cosmological distances?

Earlier, distances between galaxies were measured using Cepheid Stars. 

Cepheid Stars are a class of variable stars which exhibit periodic variations in their actual luminosity, that has a direct relation with the apparent luminosity. The apparent luminosity refers to the perceived brightness of the star, and it is used to estimate cosmological distances. 

The actual luminosity of a cosmological object is known as the intrinsic luminosity, and this determines the time period during which the luminosity of the Cepheid Star varies. 

This phenomenon was discovered by Harvard astronomer Henrietta Swan Leavitt.

American astronomer Edwin Hubble observed that the distances and velocities of galaxies are correlated, and published his observations in 1929. The distances were determined by comparing the calculated values of intrinsic luminosity with the observed values of apparent luminosity. 

However, it is possible to study only the Cepheid stars in nearby galaxies, in this way. That is why astronomers started to use supernovae, the explosive death of massive stars, as cosmic candles.

Astronomers compared the observed brightness or the apparent luminosity with the intrinsic luminosity, in order to determine the distance of the galaxy from the Earth. Then, they compared the galaxy's velocity to the distance, yielding the ‘Hubble relation’.

Type Ia supernovae show cosmic uniformity, which is why astronomers rely upon them to determine a galaxy's distance. Because of their cosmic uniformity and having the same intrinsic brightness, Type Ia supernovae are thought to be "standard candles". Supernovae being far away from Earth is a hindrance while calculating the distances. The most distant Type Ia supernova to be used as a cosmic candle till date belongs to an epoch about 3 billion years after the Big Bang. 

How can quasars be used as cosmic candles?

The team of researchers from CFA have proposed that quasars can be used as the new standard candles. The most distant quasars ever located belong to an epoch that is about 700 million years after the Big Bang. The range of redshifts of a standard candle, or the range of longer wavelengths of its spectrum, is increased, when quasars are used as cosmic candles. Also, hundreds of thousands of quasars have been discovered over the past few years.

The researchers found that there is a tight correlation between the X-ray and ultraviolet emission in quasars. The ultraviolet radiation is emitted from the hot disk of accreting material surrounding the supermassive blackhole at the heart of a quasar. The accretion disk is surrounded by a hot gas, in which electrons move at speeds close to the speed of light (3×10^8 metre per second). When UV photons and the high-speed electrons come in contact, the energy of the electrons increases, and they turn into X-rays. The scientists obtained UV results from the Sloan Digital Sky Survey DR14, and compared them with the analysed measurements of 2332 distant quasars in the new Chandra Source Catalog, and of 273 quasars in the Chandra COSMOS Legacy survey. 

The team observed that distant quasars, even those dating back to more than 85 per cent of the age of the universe, exhibited the correlation between UV and X-ray luminosity. This implies that the distance of each quasar can be determined using the relation between UV and X-ray emission of the quasar. Then, cosmological models can be tested using these distances. The results are yet to be confirmed, the scientists note. If quasars prove to be a valid source of testing cosmological models, they can serve as a new method to measure properties of the evolving universe, the scientists explain in the study.