SCIENCE

Astrophysicist Wang Jianmin offers new way to measure universe

Chinese astrophysicist looks at light from massive black holes from a different perspective

PUBLISHED : Tuesday, 02 April, 2013, 12:00am
UPDATED : Tuesday, 02 April, 2013, 4:42am

Astrophysicist Wang Jianmin knew the odds as he scrutinised the radiation from the vicinity of black holes.

Black holes are so gravitationally hungry that not even photons can escape their grip at the speed of light, but the glare from their accompanying quasars can be so bright, so stable and so long-lived that astronomers hope one day to use them as candles to measure the size of the universe.

In a paper in the academic journal Physical Review Letters in February, Wang's team said they had found the match that could make them work like standard candles. If it works, they would push astronomy trillions of light years deeper into space.

Astronomers owe much to standard candles. Without variable stars, they would not have known that the universe was expanding. Without supernovas, they would not even have guessed that the expansion was picking up speed, and Saul Perlmutter, Brian Schmidt and Adam Riess would not have won the Nobel Prize in Physics in 2011.

But to the dismay of star watchers, even the brightest candles today cannot shine more than half way into the universe. Type 1a supernovas, for instance, reduce rapidly at some point to near extinction. Distance in astronomy is looking back in time. Because very few, if any, 1a supernovas took shape in the first few billion years of the universe, they become increasingly rare the farther back one looks.

That means new candles are needed to answer some big questions such as when the universe was born, where it ends and whether its expansion will eventually slow down.

Wang's team was not the first to propose using a black hole candle following the discovery of quasars more than four decades ago. But it was not until 2011 that a team led by Dr Darach Watson, in Denmark, came up with a way to measure distance using quasars.

Wang said his team's approach was different.

Watson treated different black holes indifferently. As a result, they could only come up with observational data with large scatters

"Watson treated different black holes indifferently. As a result, they could only come up with observational data with large scatters," he said. "Just like only a specific type of supernova could be used as a standard candle, only a specific type of black hole would qualify … we found it. We proved it [in theory]."

The candidate they found is called a Super-Eddington accreting massive black hole, or SEAMBH for short.

The SEAMBH is located at the centre of a galaxy, its mass so big and its force so powerful that photons in neighbouring space are pulled and squeezed together by a process known as accretion, forms a slim disk as bright as one trillion suns.

Said Wang: "What is cool about SEAMBHs is that they appear where the 1a supernovas disappear.

"They also live very much longer than supernovas, providing ample opportunities for repeated observation."

The biggest challenge facing Wang's method is accurately measuring the mass of the black hole, because luminosity increases with mass. It took scientists more than two decades to work out an estimate of the mass of the super massive black hole at the centre of the Milky Way, but the margin of error was still as high as 10 per cent.

The measurement of black hole mass in distant galaxies would be much more difficult, Wang said. Even though his team includes Dr Hagai Netzer from Israel's Tel-Aviv University, one of the world's top black hole mass experts, it could take them more than a decade to come up with a solution to the problem with the help of large, costly optical telescopes with small view of fields.

But Wang said he was optimistic because of his confidence in their method. He has won support from the academy's observatory in Yunnan for his long-term observations.

Wang's paper drew a mixed response from competitors. Watson, an associate professor at the University of Copenhagen, said Wang's approach was a nice idea but impractical.

"Our methods have many similarities," Watson wrote in response to emailed questions. "The main distinction is that the method proposed by Wang et al needs to acquire all the data we need, and in addition it needs the mass of the black hole in the quasar and a number of other pieces of information.

"To get the black hole masses they require significantly more telescope time, which is a limiting factor. In addition, their method has many times the uncertainty because of these extra factors. Overall, it is a novel method, and it builds on the idea in our paper, adding more theoretical work.

"From the point of view of cosmology, however, it is not a competitor with the measure we proposed because the additional complexity adds very large uncertainties and requires far more resources to use it."

Wang said he welcomed comment but disagreed with Watson that his team had built upon a two-year-old idea.

"Our methods are completely different. They examine broad-line regions. We look at accretion disks," he said. "We're proposing the first practical method."

Astronomers are following developments closely.

Dr Wang Jiancheng , a researcher with the CAS observatory in Yunnan, said he was more interested in Wang's approach, and that's why he let Wang use their telescope.

"At the end of the day the winner will not be a scientist, but science," he said.

 

 

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