NANOGrav has
been able to rule out some effects other than gravitational waves, such as
interference from the matter in our own solar system or certain errors in the
data collection. These newest findings set up direct detection of gravitational
waves as the possible next major step for NANOGrav and other members of the
International Pulsar Timing Array (IPTA), a collaboration of researchers using
the world’s largest radio telescopes.
Dustin
Madison, a postdoctoral researcher at WVU, comments “We can't yet say with confidence
that what we're seeing is gravitational waves, but if it is, the
"signal" makes a lot of sense given what we think we know about
supermassive black holes. This was
always how this was going to play out...enticing hints of a signal before we
would be able to definitively claim a detection. We're on the right track to make that definitive
assessment in just a couple of years.”
Looking to the future, he thinks researchers will then be able to
characterize the signal and learn more from it for years and years to come.
Gravitational waves are ripples in space-time caused by the movements of incredibly massive objects, such as black holes orbiting each other or neutron stars colliding. Astronomers cannot observe these waves with a telescope like they do stars and galaxies. Instead, they measure the effects passing gravitational waves have, namely tiny changes to the precise position of objects - including the position of the Earth itself. Gravitational waves were first detected in 2015 by NSF’s Laser Interferometer Gravitational-Wave Observatory (LIGO) by a team including other researchers at WVU. Like light from distant objects, gravitational waves are a cosmic messenger signal – one that holds great potential for understanding “dark” objects, like black holes.
NANOGrav chose to study the signals from pulsars because they serve as detectable, dependable Galactic clocks. These small, dense stars spin rapidly, sending pulses of radio waves at precise intervals toward Earth. Pulsars are in fact commonly referred to as the universe’s timekeepers, and this unique trait has made them useful for astronomical study.
This signal is incredibly enticing. It could be that our orchestra is tuning up, hinting that we're about to hear the grand symphony of waves from supermassive black holes that we expect pervades the Universe. Sarah Burke-Spolaor
But
gravitational waves can interrupt this observed regularity, as the ripples
cause space-time to undergo tiny amounts of stretching and shrinking. Those
ripples result in extremely small deviations in the expected times for pulsar
signals arriving on Earth. Such deviations indicate that the position of the
Earth has shifted slightly. By studying the timing of the regular signals from
many pulsars scattered over the sky at the same time, known as a “pulsar timing
array,” NANOGrav works to detect minute changes in the Earth’s position due to
gravitational waves stretching and shrinking space-time.
WVU Professor and NANOGrav member Sarah Burke-Spolaor explains “This signal is incredibly enticing. It could be that our orchestra is tuning up, hinting that we're about to hear the grand symphony of waves from supermassive black holes that we expect pervades the Universe,” Burke-Spolaor reflects. She adds, “If this signal is indeed gravitational waves, future study will offer unique insights into how the biggest black holes and galaxies in our universe form and evolve”.
“NANOGrav has been building to the first detection of low frequency gravitational waves for over a decade and today’s announcement shows that they are on track to achieving this goal,” said Pedro Marronetti, NSF Program Director for gravitational physics. “The insights that we will gain on cosmology and galaxy formation are truly unparalleled.”
NANOGrav is a collaboration of U.S. and Canadian astrophysicists and a National Science Foundation Physics Frontiers Center (PFC). Maura McLaughlin, WVU Professor and Co-Director of the NANOGrav PFC, added "We are so grateful for the support of the NANOGrav PFC, that's allowed us to dramatically increase both the number of pulsars being timed and the number of participants working on NANOGrav research over the past six years". WVU has played a significant role in the PFC; 12 of the 63 authors on this paper are WVU faculty, postdocs, and students. And low-frequency gravitational wave detection is one of the main aims of the Center for Gravitational Waves and Cosmology, formed in 2015 along with the award of the PFC. As, Duncan Lorimer, WVU Professor and Eberly College Associate Dean for Research, notes “The long-term institutional support provided by the College and University has played a critical role in NANOGrav’s success since its inception in 2007”.
NANOGrav
created their pulsar timing array by studying 47 of the most stably rotating
“millisecond pulsars” with both the GBT and the Arecibo Observatory in Puerto
Rico as reported in the January 2021 Astrophysical Journal Supplements. Not all
pulsars can be used to detect the signals that NANOGrav seeks – only the most
stably rotating and longest-studied pulsars will do. These pulsars spin
hundreds of times a second, with incredible stability, which is necessary to
obtain the precision required to detect and study gravitational waves.
Of the 47 pulsars studied, 45 had sufficiently long datasets of at least three years to use for the analysis. Researchers studying the data uncovered a spectral signature, a low-frequency noise feature, that is the same across multiple pulsars. The timing changes NANOGrav studies are so small that the evidence is not apparent when studying any individual pulsar, but in aggregate, they add up to a significant signature.
Potential Next Steps
To confirm direct detection of a signature from gravitational waves, NANOGrav’s researchers will have to find a distinctive pattern in the signals between individual pulsars. At this point, the sensitivity of the experiment is not currently good enough for such a pattern to be distinguishable. Boosting the signal requires NANOGrav to expand its dataset to include more pulsars studied for even longer lengths of time, which will increase the array’s sensitivity. In addition, by pooling NANOGrav's data together with those from other pulsar timing array experiments, a joint effort by the IPTA may reveal such a pattern. Students and faculty at WVU are important contributors to this effort, and in fact 24 WVU students have traveled to IPTA partner countries to undertake research abroad as part of NSF-funded programs led by WVU.
At the same
time, NANOGrav is developing techniques to ensure the detected signal could not
be from another source. They are producing computer simulations that help test
whether the detected noise could be caused by effects other than gravitational
waves, in order to avoid a false detection.
Publications referenced in this article
Gravitational Wave Search: https://iopscience.iop.org/article/10.3847/2041-8213/abd401
Narrowband Dataset: https://iopscience.iop.org/article/10.3847/1538-4365/abc6a0
Wideband Dataset: https://iopscience.iop.org/article/10.3847/1538-4365/abc6a1
For more information about NANOGrav, please visit the website at http://nanograv.org.