Spinning Neutron Stars Reveal New Insights Into Elusive Continuous Gravitational Waves

Continuous Gravitational Waves

Artist’s impression of continuous gravitational waves generated by a spinning asymmetric neutron star. Credit: Mark Myers, Ozgrav-Swinburne University

Five years on from the first discovery of gravitational waves, an international team of scientists, including from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), are continuing the hunt for new discoveries and insights into the Universe. Using the super-sensitive, kilometer-sized LIGO detectors in the United States, and the Virgo detector in Europe, the team have witnessed the explosive collisions of black holes and neutron stars. Recent studies, however, have been looking for something quite different: the elusive signal from a solitary, rapidly-spinning neutron star.

Take a star similar in size to the Sun, squash it down to a ball about twenty kilometers across — roughly the distance from Melbourne airport to the city center — and you’d get a neutron star: the densest object in the known Universe. Now set your neutron star spinning at hundreds of revolutions per second and listen carefully. If your neutron star isn’t perfectly spherical, it will wobble about a bit, and you’ll hear a faint “humming” sound. Scientists call this a continuous gravitational wave.

So far, these humming neutron stars have proved elusive. As OzGrav postdoctoral researcher Karl Wette from the Australian National University explains: “Imagine you’re out in the Australian bush listening to the wildlife. The gravitational waves from black hole and neutron star collisions we’ve observed so far are like squawking cockatoos — loud and boisterous, they’re pretty easy to spot!

A continuous gravitational wave, however, is like the faint, constant buzz of a faraway bee, which is much more difficult to detect. So we’ve got to use a few different strategies. Sometimes we hone in on a particular direction — for example, a flowering bush where bees are likely to congregate. Other times, we close our eyes and listen keenly to all the sounds we can hear, and try to pick out any buzzing sounds in the background. So far, we haven’t had any luck, but we’ll keep trying! Once we do hear a continuous gravitational wave, we’ll be able to peer deep into the heart of a neutron star and unravel its mysteries, which is an exciting prospect.”

A recent collaborative study with OzGrav has taken a closer look at the remnants of exploded stars, called supernovae. OzGrav PhD student Lucy Strang from the University of Melbourne explains: “Our search targets fifteen young supernova remnants containing young neutron stars. We use three different pipelines: one optimized for sensitivity, one that can handle a rapidly evolving signal, and one optimized for one likely astrophysical scenario. This is the first LIGO study covering all three of these scenarios, maximizing our chance of a continuous wave detection.

“Continuous gravitational waves are proving very difficult to detect, but the same properties that make them elusive make them appealing targets. The exact form of the signal (i.e. its frequency, how rapidly the frequency changes, how loud it is, etc.) is dependent on what neutron stars are made of. So far, the structure of neutron stars is an open question that draws in all kinds of physicists. Even without a detection, a search allows us to peek behind the curtain at the unknown physics of neutron stars. When we do detect continuous waves, we’ll open the curtain and shine a spotlight on new physics. Until then, we can use the information we do have to refine our understanding and improve our search methods.”

OzGrav Associate Investigator Lilli Sun from the Australian National University says: “Young neutron stars in supernova remnants are promising targets to look for those tiny continuous gravitational waves, because they haven’t spent a long enough time to relax and smooth out the asymmetries introduced at their birth. In our endeavor to search for continuous waves from these young neutron stars in our third observing run, we take into consideration, for the first time, the possibilities that the interior configuration and structure of the star can result in signals emitted at two different harmonics. Although no signal has been detected in O3, we set interesting constraints on the neutron star properties. If such a signal can be detected in future observations when the detectors are more sensitive, it will shed light on the fascinating structure of a neutron star.”

OzGrav postdoctoral researcher Carl Blair from the University of Western Australia says: “Gravitational waves are being used to probe the most exotic objects in the Universe. Neutron stars — composed of matter collapsed in on itself like a giant atomic nuclei — have to be one of the most exotic. We don’t know that much about neutron stars because they’re so small and strange. Are they hard or soft? And when they spin fast as they collapse, do they wobble away that energy in the form of gravitational waves? While there is no evidence yet for continuous gravitational waves from neutron stars, limits have been placed on how wobbly a neutron star is from the fact that we haven’t measured gravitational waves from them yet.”

In addition, recent studies announced by the international research team — including the U.S./international LIGO Scientific Collaboration, European Virgo Collaboration and Japanese KAGRA Collaboration — have focussed on pulsars. These are neutron stars that act as cosmic lighthouses, beaming out copious energy in the form of radio waves. Pulsars are like giant spinning magnets, except they’re billions of times stronger than the ones stuck to your fridge. So strong, in fact, that the magnetic field distorts the shape of the neutron star, and may lead to a tell-tale hum of continuous gravitational waves. While the recent studies did not pick up anything, they found tight constraints on how loud the “hum” could be, which, in some cases, are starting to challenge theoretical predictions.

OzGrav PhD student Deeksha Beniwal from the University of Adelaide says: “Gravitational-wave observation from O3 run of LIGO and Virgo detectors has allowed us to set realistic constraints on signals expected from young pulsars. O3 observations also provide an opportunity to test out different pipelines — such as different search methods for continuous wave signals — in realistic environments.”

OzGrav postdoctoral researcher Meg Millhouse from the University of Melbourne says: “Continuous gravitational waves from neutron stars are much smaller than the gravitational waves LIGO and Virgo have seen so far. This means we need different techniques to detect them. And, because these are long-lasting signals, we need to look at lots of data which can be very difficult computationally. The recent LIGO-Virgo papers published showcase a wide range of these clever approaches to detect continuous gravitational waves. Even though there were no detections in the most recent data analyzed, we’re in a good position to keep searching and possibly make a detection when LIGO collects more data.”

Scientists estimate that there are billions of neutron stars in the Milky Way with a faint murmur of continuous gravitational waves. Further studies have therefore taken an “ears wide open” approach, combing through the LIGO and Virgo data for any hint of a signal. The results so far suggest that these murmurings are extremely quiet and out of the detectors’ “ear” range. However, as detector technology becomes more advanced and sensitive, the first-ever detection of continuous gravitational waves could soon become a reality.

8 Comments on "Spinning Neutron Stars Reveal New Insights Into Elusive Continuous Gravitational Waves"

  1. Babu G. Ranganathan | May 27, 2021 at 9:32 am | Reply

    Babu G. Ranganathan*
    (B.A. Bible/Biology)

    JUST BECAUSE SCIENCE CAN EXPLAIN how an airplane works doesn’t mean that no one designed or made the airplane. And just because science can explain how life or the universe works doesn’t mean there was no Designer and Maker behind them.

    Natural laws may explain how the order in the universe works and operates, but mere undirected natural laws cannot explain the origin of that order. Once you have a complete and living cell then the genetic code and biological machinery exist to direct the formation of more cells from raw materials such as amino acids and other chemicals, but how could life or the cell have naturally originated when no directing code and mechanisms existed in nature? Read my Internet article: HOW FORENSIC SCIENCE REFUTES ATHEISM.

    WHAT IS SCIENCE? Science simply is knowledge based on observation. No human observed the universe coming by chance or by design, by creation or by evolution. These are positions of faith. The issue is which faith the scientific evidence best supports.

    SCIENCE SHOWS THAT THE UNIVERSE CANNOT BE ETERNAL because it could not have sustained itself eternally due to the law of entropy (increasing and irreversible net energy decay, even in an open system). Even a hypothetical oscillating universe could not continue to oscillate eternally! Einstein’s General Theory of Relativity shows that space, matter, and time all are physical and all had a beginning. Space even produces particles because it’s actually something, not nothing. What about the Higgs boson (the so-called “God Particle”)? The Higgs boson, even if it existed, would not have created mass from nothing, but rather it would have converted energy into mass. Einstein showed that all matter is some form of energy. Even time had a beginning! Time is not eternal.

    The law of entropy doesn’t allow the universe to be eternal. If the universe were eternal, everything, including time (which modern science has shown is as physical as mass and space), would have become totally entropied by now and the entire universe would have ended in a uniform heat death a long, long time ago. The fact that this hasn’t happened already is powerful evidence for a beginning to the universe.

    Popular atheistic scientist Stephen Hawking admits that the universe had a beginning and came from nothing but he believes that nothing became something by a natural process yet to be discovered. That’s not rational thinking at all, and it also would be making the effect greater than its cause to say that nothing created something. The beginning had to be of supernatural origin because science teaches us from the First Law of Thermodynamics that natural laws and processes do not have the ability to bring something into existence from nothing.

    The supernatural origin of the universe cannot be proved by science but science points to a supernatural intelligence and power for the origin and order of the universe. Where did God come from? Obviously, unlike the universe, God’s nature doesn’t require a beginning.

    The disorder in the universe can be explained because of chance and random processes, but the order can be explained only because of intelligence and design.

    Gravity may explain how the order found in the precise and orderly courses of thousands of billions of stars is maintained, but gravity cannot explain the origin of that order.

    Some evolutionary astronomers believe that trillions of stars crashed into each other leaving surviving stars to find precise orderly orbits in space. Not only is this irrational, but if there was such a mass collision of stars then there would be a super mass residue of gas clouds in space to support this hypothesis. The present level of residue of gas clouds in space doesn’t support the magnitude of star deaths required for such a hypothesis. And, as already stated, the origin of stars cannot be explained by the Big Bang because of the reasons mentioned above. It’s one thing to say that stars may decay and die into random gas clouds, but it is totally different to say that gas clouds form into stars.

    Even the father of Chaos theory admitted that the “mechanisms” existing in the non-living world allow for only very rudimentary levels of order to arise spontaneously (by chance), but not the kind or level of order we find in the structures of DNA, RNA, and proteins. Yes, individual amino acids have been shown to come into existence by chance but not protein molecules which require that the various amino acids be in a precise sequence just like the letters found in a sentence.

    Some things don’t need experiment or scientific proof. In law there is a dictum called prima facie evidence. It means “evidence that speaks for itself.”

    An example of a true prima facie would be if you discovered an elaborate sand castle on the beach. You don’t have to experiment to know that it came by design and not by the chance forces of wind and water.

    If you discovered a romantic letter or message written in the sand, you don’t have to experiment to know that it was by design and not because a stick randomly carried by wind put it there. You naturally assume that an intelligent and rational being was responsible.

    It’s interesting that Carl Sagan would have acknowledged sequential radio signals in space as evidence of intelligent life sending them, but he wouldn’t acknowledge the sequential structure of molecules in DNA (the genetic code) as evidence of an intelligent Cause. Read my popular Internet article, HOW DID MY DNA MAKE ME.

    I encourage all to read my popular Internet articles:


    Visit my latest Internet site: THE SCIENCE SUPPORTING CREATION (This site answers many arguments, both old and new, that have been used by evolutionists to support their theory)


    *I have given successful lectures (with question and answer period afterwards) defending creation before evolutionist science faculty and students at various colleges and universities. I’ve been privileged to be recognized in the 24th edition of Marquis “Who’s Who in The East” for my writings on religion and science.

  2. BibhutibhusanPatel | May 27, 2021 at 10:08 am | Reply

    The basic ìdeàl shape of a spìnning neutron star is a Sphere.But often in real observation,these have eccentrìçity in axis of spin due to deformed shape in 3-D.Neutron stars are resulted from the supernòvas and sùch defòrmations aŕe seen fròm the beginìng.Now,sùch deformation in shape and wobble in spin geneŕates feeble gravitational wave.A humming-sound is even created due to friction of spinning ñeutron star with the
    surròùnding.Thanks,for such important observations and data ìn the artiçle.

  3. Aleksandr7364 | May 27, 2021 at 12:28 pm | Reply

    In one country, the standard kilogram is calculated by the reactive thrust of electromagnetic waves. This is the right direction. And gravitational waves, which do not exist, must be sought under your feet and above your head.

    • Torbjörn Larsson | May 28, 2021 at 1:24 pm | Reply

      The current International System of Units allo anyone to use a Kibble balance for realisation of a kilogram [ https://en.wikipedia.org/wiki/Kilogram ].

      If you for no good reason assume that the gravitational waves we see do not exist for you, why would you go looking? And would any of that be science (as this is a science site)?

      • Aleksandr7364 | May 30, 2021 at 11:52 am | Reply

        I’m serious: the waves that people have under their feet and the waves that are above their heads are different. As well as under and over cars or other objects. This is gravity – the reactive thrust of electromagnetic waves.
        If the waves that go up and press objects to the ground, neutralize, such as a halo – objects will become weightless.

  4. Torbjörn Larsson | May 28, 2021 at 1:17 pm | Reply

    ” We don’t know that much about neutron stars because they’re so small and strange. Are they hard or soft? ”

    It may be the latter:

    “Squishy Neutron Star … results show that neutron stars of different masses may have the same size”.

    [ https://www.quantamagazine.org/squishy-neutron-star-setback-dampens-hopes-of-exotic-matter-20210526/ ]

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