The background hum of space could reveal hidden black holes

A visualization of a supercomputer simulation of merging
black holes sending out gravitational waves. Credit:
NASA/C. Henze

Deep space is not as silent as we have been led to believe.
Every few minutes a pair of black holes smash into each
other. These cataclysms release ripples in the fabric of
spacetime known as gravitational waves. Now Monash University
scientists have developed a way to listen in on these events.
The gravitational waves from black hole mergers imprint a
distinctive whooping sound in the data collected by
gravitational-wave detectors. The new technique is expected
to reveal the presence of thousands of previously hidden
black holes by teasing out their faint whoops from a sea of
static.


Last year, in one of the biggest astronomical discoveries of
the 21st century, LIGO Scientific Collaboration (LSC) and Virgo
Collaboration researchers measured from a pair of merging
neutron stars.

Drs Eric Thrane and Rory Smith, from the ARC Centre of
Excellence for Gravitational Wave Discovery (OzGrav) and Monash
University, were part of the team involved in last year’s
discovery and were also part of the team involved in the
detection of first gravitational-wave discovery in 2015, when
ripples in the fabric of space time generated by the collision
of two in the distant Universe were
first witnessed, confirming Albert Einstein’s 1915 general
theory of relativity.

To date, there have been six confirmed, or gold plated,
gravitational-wave events announced by the LIGO and Virgo
Collaborations. However there are, according to Dr Thrane, more
than 100,000 gravitational wave events every year too faint for
LIGO and Virgo to unambiguously detect. The gravitational waves
from these mergers combine to create a gravitational-wave
background. While the individual events that contribute to it
cannot be resolved individually, researchers have sought for
years to detect this quiet gravitational-wave hum.

In a
landmark paper
in the US journal, Physical Review X,
the two researchers have developed a new, more sensitive way of
searching for the gravitational-wave background.

“Measuring the gravitational-wave background will allow us to
study populations of black holes at vast distances. Someday,
the technique may enable us to see gravitational waves from the
Big Bang, hidden behind gravitational waves from black holes
and neutron stars,” Dr Thrane said.

The researchers developed computer simulations of faint black
hole signals, collecting masses of data until they were
convinced that – within the simulated data – was faint, but
unambiguous evidence of . Dr Smith is optimistic that
the method will yield a detection when applied to real data.
According to Dr Smith, recent improvements in data analysis
will enable the detection of “what people had spent decades
looking for.” The new method is estimated to be one thousand
times more sensitive, which should bring the long-sought goal
within reach.

Importantly the researchers will have access to a new $4
million supercomputer, launched last month (March) at the
Swinburne University of Technology. The computer, called
OzSTAR, will be used by scientists to look for gravitational
waves in LIGO data.

According to OzGRav Director, Professor Matthew Bailes, the
supercomputer will allow OzGrav’s researchers to attempt these
kind of landmark discoveries.

“It is 125,000 times more powerful than the first supercomputer
I built at the institution in 1998.”

The OzStar computer differs from most of the more than 13,000
computers used by the LIGO community, according to Dr Smith,
including those at CalTech and MIT. OzStar employs graphical
processor units (GPUs), rather than more traditional central
processing units (CPUs). For some applications, GPUs are
hundreds of times faster. “By harnessing the power of GPUs,
OzStar has the potential to make big discoveries in
gravitational-wave astronomy,” Dr Smith said.

Explore further:
Machine
learning could help search for gravitational waves

More information: Optimal search for an astrophysical
gravitational-wave background, journals.aps.org/prx/accepted/ …
7164e1aab4bf827ea464

On Arxiv: arxiv.org/abs/1712.00688

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