Sunday, March 25, 2018

Feature: Every second counts to trace a gravitational wave

When a gravitational wave reaches Earth, every second counts. The data processing speed will have a crucial impact on how much astronomers can learn from these space-time ripples, says computer scientist Cao Junwei. "In an era of multi-messenger astronomy, we have to shorten the time as much as possible so as to trigger the alert quickly enough for follow-up observations," says Cao, who leads the Chinese team in the international Laser Interferometer Gravitational-wave Observatory (LIGO) Scientific Collaboration. Last October, scientists from LIGO Scientific Collaboration, together with astronomers across the world, declared they had detected for the first time a gravitational wave from the collision of binary neutron stars and corresponding electromagnetic signals.The discovery was achieved with high data processing speed. Just 1.7 seconds after the gravitational wave detection network received the signal, a gamma-ray burst was detected by the Fermi space telescope. LIGO and Fermi immediately triggered alerts around the astronomical community, bringing about 70 ground and space detectors into follow-up observations of electromagnetic signals with various wave lengths, which helped locate the source of the gravitational wave more precisely.

Cao joined the LIGO Lab at the Massachusetts Institute of Technology (MIT) as a computer scientist in 2004. On returning to China, he led a team from Tsinghua University's Research Institute of Information Technology (RIIT) in joining the LIGO Scientific Collaboration in 2009.

"We were the only Chinese group in the collaboration. None of us specialized in astrophysics, but we were accepted," says Cao, who is vice dean of RIIT.

In the first five years, the Tsinghua team mainly helped build the computing platform and analyze data. Then they began devoting most of their efforts to speeding up data processing.

Few understood the importance of speed at the beginning.

"We suggested, from the outset, that fast computing should serve multi-messenger astronomy, which would require follow-up observations as soon as a gravitational wave signal is confirmed," Cao says. "The faster, the better."

In 2015, LIGO first detected gravitational waves from the collision of binary black holes, which verified the general relativity theory that Albert Einstein established a century ago. But it took scientists months to vet, validate and interpret the discovery before it was publicly announced.

LIGO detectors collect more than 16,000 data samples a second. To confirm a signal is generated by gravitational waves, scientists remove "noise" from the data, and then compare the data patterns with templates of gravitational waves.

More than 1,000 scientists are working for the LIGO Scientific Collaboration, more than half of them on data analysis. The data quality categories are defined by multiple analysis groups: Compact Binary Coalescence (CBC), Burst, Continuous Waves, Stochastic, and others.

"Our team, just a small group in the collaboration, is now focusing on GPU acceleration for CBC search and exploring the application of machine learning to real-time data analysis," Cao says.

Their accomplishments include a set of new data processing pipelines, in cooperation with the University of Western Australia.

"The new pipelines help speed up data filtering, so we can finish comparing data patterns with tens of thousands of templates within a second," says Tsinghua Associate Professor Du Zhihui.

"Now, the time between the arrival of a signal and the confirmation of it as gravitational waves has been shortened from several minutes to dozens of seconds. Next, we hope to shrink the time to three to five seconds," Du says.

Scientists began to enhance LIGO's detectors since 2008. The Advanced LIGO finished its second run in August 2017, and is expected to start its third in the middle of this year. Scientists will further upgrade its detectors between the two runs to improve its sensitivity, which might greatly increase the odds of discovering gravitational waves.

"With a higher sensitivity, the number of signals that are detected may soar from a few a year to several a day. We will fall far behind if we don't accelerate data processing," Cao says.

He hopes their work in the LIGO Scientific Collaboration will contribute to China's own gravitational wave detection projects. "China will participate in international cooperation actively to foster talent and accumulate experience," he says.

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