South Africa’s MeerKAT tracks cosmic ripples in spacetime
Diligent observing with the MeerKAT radio telescope in the Northern Karoo, of a network of rapidly spinning neutron stars over the last five years, has unveiled additional evidence for the presence of a low frequency gravitational wave background — an underlying stretching and squeezing of spacetime on which our planet Earth bobs.
Regular observations of 80 millisecond pulsars, (shown here as bright point sources emitting white radio beams) over the last 5 years with MeerKAT, have revealed evidence for a gravitational wave background. These ripples in spacetime are likely caused by a population of inspiraling supermassive black holes that reside at the heart of massive galaxies (depicted as pink spirals). Image credit: Carl Knox, OzGrav, Swinburne University of Technology and South African Radio Astronomy Observatory.
The research project, called the MeerKAT Pulsar Timing Array, uses neutron stars, or pulsars, to assemble a gravitational wave detector in the sky. Each of the observed pulsars, thousands of light years away, and studded across the Milky Way, acts as a detector element. It is the largest galactic detector of its kind leading to the most detailed gravitational wave maps across the Universe to date.
The findings of this international effort, with contributors from Australian, European and South African institutes, including the University of Cape Town and the South African Radio Astronomy Observatory, are presented in three studies published in Monthly Notices of the Royal Astronomical Society today. Together, these works offer new insights into the likely sources of the background spacetime ripples. The prime suspects are the inspiraling orbits of the Universe’s most massive black holes. Analysing the gravitational wave background using a pulsar timing array detector, can therefore ultimately teach us how massive black holes shaped the Universe’s history and reveal the cosmic tracks they left behind.
What is the Gravitational Wave Background? Gravitational waves are ripples in the fabric of spacetime caused by some of the Universe’s most powerful events, from black hole mergers to potentially even cosmic strings. The gravitational waves combine to form an ever present background hum across the cosmic landscape. Within this rumble live valuable clues about the hidden processes that shaped the Universe. “Studying the background lets us tune into the echoes of cosmic events across billions of years,” explains Matt Miles, a researcher at OzGrav and Swinburne University of Technology, and a lead author of the studies. “It reveals how galaxies, and the universe itself, have evolved over time.”
The MeerKAT Telescope: The MeerKAT telescope is a 64-dish radio telescope located in the heart of the Northern Cape and one of world’s most sensitive radio telescopes to routinely observe pulsars. It was developed and built as a key project of the South African Department of Science and Technology[1], and is operated by SARAO, a national facility of the NRF. Its sensitivity, resolution and scientific capabilities earned global admiration when an inaugural image of the Milky Way’s galactic centre was unveiled in July 2018. MeerKAT does not only produce high resolution images of the radio sky, it is also a hyper sensitive clock, allowing scientists that study highly variable sources to obtain data tagged with exceptionally precise timestamps. This is what pulsar scientists are after. Science Operations lead Sarah Buchner applauds the innovative design by the SARAO team, “We have achieved fantastic pulsar sensitivity and timing precision” with MeerKAT. She also wishes to thank the team who operate MeerKAT on a daily basis, and adds “it is deeply moving to see the exquisite results from the pulsar timing array project!”
Pulsars and the MeerKAT Pulsar Timing Array experiment: The MeerKAT Pulsar Timing Array is an international experiment that routinely uses the extraordinary capabilities of MeerKAT to observe radio pulsars. Over the last five years, starting in February 2019, the project has monitored around 80 millisecond pulsars once a fortnight. Millisecond pulsars are the fastest spinning neutron stars we know of, spinning around their own axis up to several hundred times a second (or alternatively, once in just a few milliseconds). Given that neutron stars are only a few kilometres across in size, this is equivalent to Table Mountain with the mass of the Sun, spinning like a kitchen blender!
As they spin, pulsars emit beams of radio emission from their magnetic poles. These beams flash across the MeerKAT receivers trained in their direction, creating a steady string of incoming pulses. Recording these regular flashes allows researchers to use pulsars as metronomes or natural clocks. The MeerKAT Pulsar Timing Array systematically analyses the pulses of an ensemble of Southerly pulsars, aiming to detect minuscule errors in their clockwork, which is a tell-tale signature of passing gravitational waves. “To find evidence for a gravitational wave background, we first need to model the timing behaviour of each of the pulsars in our network very precisely,” explains Marisa Geyer, co-author and lecturer at the University of Cape Town and former commissioning scientist of MeerKAT. “Once we know the individual pulsars well, we can start analysing the combined behaviour of the group of pulsars. If we see pulsars in the same direction in the sky lose time in a connected way, we start suspecting that it is not the pulsars that are acting funny, but rather a gravitational wave background that has interfered”.
Key Discoveries:
- Quickest to a Gravitational Wave Signal
In just one-third of the time compared to other global experiments, the MeerKAT team is seeing signs of a gravitational wave background. Jaikhomba Singha, a postdoctoral fellow at UCT and a co-author of the paper says, “pulsar timing array experiments are long term in nature and searching for a gravitational wave background is a slow process. From past experience, we know that this may need 15 years of data. It is amazing to see that with MeerKAT evidence for the signal is possible even in a data-span of just 4.5 years.” The background, likely from merging supermassive black holes, is also a stronger signal than other published results. Matt explains, “the signal we’re seeing hints at a more interesting and active Universe than we were expecting. We know that supermassive black holes are out there merging, but now we’re starting to ask where and how many?” - Detailed Gravitational Wave Maps with Unexpected Hotspots
The team also used the pulsar timing array to construct a precise gravitational wave sky map. Kathrin Grunthal, a PhD researcher at the Max Planck Institute for Radio Astronomy and one of the lead authors, points out: “By looking for variations in the gravitational waves across the sky, we hunt for the fingerprint of the astrophysical processes behind the signal.” The researchers improve upon existing methods to identify a bright spot — an intriguing anomaly suggesting a directional bias in the gravitational wave signal. “The presence of a hotspot could suggest a distinct gravitational wave source, such as a pair of black holes billions of times the mass of our Sun,” explains Rowina Nathan, a researcher at Monash University and another lead author of the studies.
Future Directions and Broader Impact: These findings open up exciting questions about the formation of massive black holes and the Universe’s early history. Further monitoring with the MeerKAT array will refine these gravitational wave maps, potentially uncovering new cosmic phenomena. The research also has broad implications, offering data that may aid scientists in exploring the origins and evolution of supermassive black holes, the formation of galaxy structures, and even hints of early events in the Universe. Later this decade this research will expand even further, by using the international SKA-Mid telescope currently under construction in the Karoo and of which MeerKAT will form a part.
Atiqur Rahman, a PhD student at UCT, working on a MeerKAT Pulsar Timing Array project, says “It is a wonderful time to work in the field of gravitational waves using pulsar timing experiments. While PTA experiments are seeing increasing evidence for a gravitational wave background, the source of this is not fully known. In my project we are searching for the presence of unconventional polarisation modes of the gravitational wave signals, to understand their origins better.”
[1] Now the Department of Science, Technology and Innovation
For Media Consideration:
Links to papers
Matt’s contact details (based in Melbourne, Australia):
Email: matthewmiles@swin.edu.au / matt.t.miles22@gmail.com
South African co-authors:
Marisa Geyer: Pulsar researcher and lecturer at UCT, chair of the African Pulsar Timing and former commissioning member of the MeerKAT telescope. MPTA and APT member.
Email: marisa.geyer@uct.ac.za
APT Website: https://africanpulsartiming.github.io/
MPTA Website: https://mpta-gw.github.io/
Sarah Buchner: Science Operations lead at SARAO and MeerKAT commissioning scientist.
Email: sbuchner@sarao.ac.za
Fernando Camilo: Chief Scientist at the South African Radio Astronomy Observatory
Email: fernando@sarao.ac.za
Jaikhomba Singha: Post-doctoral fellow at UCT, member of MPTA, African Pulsar Timing group, InPTA and IPTA
Email: jaikhomba.singha@uct.ac.za
Acronyms
APT – African Pulsar Timing (group)
InPTA – Indian Pulsar Timing Array
IPTA – International Pulsar Timing Array
MPTA – MeerKAT Pulsar Timing Array
NRF – National Research Foundation
PTA – Pulsar Timing Array
SARAO – South African Radio Astronomy Observatory
UCT – University of Cape Town
