Gravitational-Wave Scientists’ Brilliant New Method To Refine the Hubble Constant – The Expansion and Age of the Universe

Illustration Merging Neutron Stars

Artist’s illustration of a pair of merging neutron stars. Credit: Carl Knox, OzGrav-Swinburne University

A team of international scientists, led by the Galician Institute of High Energy Physics (IGFAE) and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), has proposed a simple and novel method to bring the accuracy of the Hubble constant measurements down to 2%, using a single observation of a pair of merging neutron stars.
The Universe is in continuous expansion. Because of this, distant objects such as galaxies move away from us. In fact, the further away they are, the faster they move. Scientists describe this expansion through a famous number known as the Hubble constant, which tells us how fast objects in the Universe recede from us depending on their distance to us. By measuring the Hubble constant in a precise way, we can also determine some of the most fundamental properties of the Universe, including its age.
For decades, scientists have measured Hubble’s constant with increasing accuracy, collecting electromagnetic signals emitted throughout the Universe but arriving at a challenging result: the two current best measurements give inconsistent results. Since 2015, scientists have tried to tackle this challenge with the science of gravitational waves: ripples in the fabric of space-time that travel at the speed of light. Gravitational waves are generated in the most violent cosmic events and provide a new channel of information about the Universe. They’re emitted during the collision of two neutron stars—the dense cores of collapsed stars–and can help scientists dig deeper into the Hubble constant mystery.
Unlike black holes, merging neutron stars produce both gravitational and electromagnetic waves, such as x-rays, radio waves and visible light. While gravitational waves can measure the distance between the neutron-star merger and Earth, electromagnetic waves can measure how fast its whole galaxy is moving away from Earth. This creates a new way to measure the Hubble constant. However, even with the help of gravitational waves, it’s still tricky to measure the distance to neutron-star mergers–that’s, in part, why current gravitational-wave based measurements of the Hubble constant have an uncertainty of ~16%, much larger than existing measurements using other traditional techniques.

In a recently published article in the prestigious journal The Astrophysical Journal Letters, a team of scientists led by ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and Monash University alumni Prof Juan Calderón Bustillo (now La Caixa Junior Leader and Marie Curie Fellow at the Galician Institute of High Energy Physics of the University of Santiago de Compostela, Spain), has proposed a simple and novel method to bring the accuracy of these measurements down to 2% using a single observation of a pair of merging neutron stars.

According to Prof Calderón Bustillo, it’s difficult to interpret how far away these mergers occur because “currently, we can’t say if the binary is very far away and facing Earth, or if it’s much closer, with the Earth in its orbital plane.” To decide between these two scenarios, the team proposed to study secondary, much weaker components of the gravitational-wave signals emitted by neutron-star mergers, known as higher modes.

“Just like an orchestra plays different instruments, neutron-star mergers emit gravitational waves through different modes,” explains Prof Calderón Bustillo. “When the merging neutron stars are facing you, you will only hear the loudest instrument. However, if you are close to the merger’s orbital plane, you should also hear the secondary ones. This allows us to determine the inclination of the neutron-star merger, and better measure the distance.”

However, the method is not completely new: “We know this works well for the case of very massive black hole mergers because our current detectors can record the merger instant when the higher modes are most prominent. But in the case of neutron stars, the pitch of the merger signal is so high that our detectors can’t record it. We can only record the earlier orbits,” says Prof Calderón Bustillo.

Future gravitational-wave detectors, like the proposed Australian project NEMO, will be able to access the actual merger stage of neutron stars. “When two neutron stars merge, the nuclear physics governing their matter can cause very rich signals that, if detected, could allow us to know exactly where the Earth sits with respect to the orbital plane of the merger,” says co-author and OzGrav Chief Investigator Dr. Paul Lasky, from Monash University. Dr. Lasky is also one of the leads on the NEMO project. “A detector like NEMO could detect these rich signals,” he adds.

In their study, the team performed computer simulations of neutron-star mergers that can reveal the effect of the nuclear physics of the stars on the gravitational waves. Studying these simulations, the team determined that a detector like NEMO could measure Hubble’s constant with a precision of 2%.

Co-author of the study Prof Tim Dietrich, from the University of Potsdam, says: “We found that fine details describing the way neutrons behave inside the star produce subtle signatures in the gravitational waves that can greatly help to determine the expansion rate of the Universe. It is fascinating to see how effects at the tiniest nuclear scale can infer what happens at the largest possible cosmological one.”

Samson Leong, undergraduate student at The Chinese University of Hong Kong and co-author of the study points out “one of the most exciting things about our result is that we obtained such a great improvement while considering a rather conservative scenario. While NEMO will indeed be sensitive to the emission of neutron-star mergers, more evolved detectors like Einstein Telescope or Cosmic Explorer will be even more sensitive, therefore allowing us to measure the expansion of the Universe with even better accuracy!”.

One of the most outstanding implications of this study is that it could determine if the Universe is expanding uniformly in space as currently hypothesized. “Previous methods to achieve this level of accuracy rely on combining many observations, assuming that the Hubble constant is the same in all directions and throughout the history of the Universe,” says Calderón Bustillo. “In our case, each individual event would yield a very accurate estimate of “its own Hubble constant,” allowing us to test if this is actually a constant or if it varies throughout space and time.”

Reference: “Mapping the Universe Expansion: Enabling Percent-level Measurements of the Hubble Constant with a Single Binary Neutron-star Merger Detection” by Juan Calderón Bustillo, Samson H. W. Leong, Tim Dietrich and Paul D. Lasky, 30 April 2021, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/abf502

12 Comments on "Gravitational-Wave Scientists’ Brilliant New Method To Refine the Hubble Constant – The Expansion and Age of the Universe"

  1. Aleksandr7364 | May 5, 2021 at 11:28 am | Reply

    Gravitational waves do not exist because gravity is the reactive thrust of ordinary electromagnetic waves:
    The universe is expanding because with each galaxy born, the mass of the center of the universe becomes smaller, and the smaller body attracts with less force.
    For the same reason, both galaxies and planetary systems around stars are expanding.

    • Torbjörn Larsson | May 9, 2021 at 7:10 am | Reply

      Are you seriously trying to say that what we don’t see what we de facto see?

      I could stop there but as the rest of that comment is pseudoscience, and I would guess that the link is self promotion of you own variety of it, it behooves me to point that out. [See my response to plb4333 for a description of the science, which can be further referenced to be the science at hand by encyclopedias such as Wikipedia.] Let us keep it to the observed facts and known science.

  2. Bibhutibhusanpatel | May 6, 2021 at 2:36 am | Reply

    Sìngle Binary Neùtron-Star Merger Detection Method ùsìng bòth gŕavitatiònaĺ wave and electròmagnetic wavee with mòst success mèasured Hubbĺe Cònstant to be ìncèased by 16%.Thanks tò thè Aùthers fòŕ theìr good wòŕks.Ofcòùŕse there is a tŕick,when consìďered total gaĺaxý and then thìs còmparied with other galaxiès increased in value ŕedùce to null and the value of Hubble Cònstant règaìn to same unìqùe previòùs òne.

  3. There is no expansion, only movements within. It would look like a diffused pattern with very little variations, when seen from afar. Space is infinite, so how would the term ‘expanding’ be used? Infinity has no size containment where shrinking or expanding come into play…

    • Torbjörn Larsson | May 9, 2021 at 7:06 am | Reply

      No, it is definitely expansion – you are describing a common myth. And if space is infinite, which it may well be, how would there be increasingly large “movements” as we look further away?

      Here is a bice description of that, and related myths (such as that we sit at a ‘center’ of the expanding universe):

      “Don’t Believe These 5 Myths About The Big Bang”

      “The Universe we know today, filled with stars and galaxies across the great cosmic abyss, hasn’t been around forever. Despite the fact that there are approximately 2 trillion galaxies visible to us spanning distances of tens of billions of light-years, there’s a limit to how far away we can look. That isn’t because the Universe is finite — in fact, it may well be infinite after all — but because it had a beginning that occurred a finite amount of time ago: the Big Bang.

      The fact that we can look at our Universe today, see it expanding and cooling, and infer our cosmic origins is one of the most profound scientific achievements of the 20th century. The Universe began from a hot, dense, matter-and-radiation filled state some 13.8 billion years ago, and has been expanding, cooling, and gravitating ever since. But the Big Bang itself doesn’t work the way most people think. Here are the top 5 myths that people believe about the Big Bang.

      1.) The Big Bang is the explosion that began our Universe.

      Every time we look out at a distant galaxy in the Universe and try to measure what its light is doing, we see the same pattern emerge: the farther away the galaxy is, the more significantly its light is systematically shifted to longer and longer wavelengths. This redshift that we observe for these objects follows a predictable pattern, with double the distance meaning that the light is shifted by twice as much.

      Distant objects, therefore, appear to be receding away from us. Just as a police car speeding away from you will sound lower-pitched the faster it moves away from you, the greater we measure an object’s distance to be from us, the greater the measured redshift of its light will be. It makes a lot of sense, then, to think that the more distant objects are moving away from us at faster speeds, and that we could trace every galaxy we see today back to a single point in the past: an enormous explosion.

      But this is a total misconception about what the Big Bang actually is. It isn’t that these galaxies are moving through the Universe itself, but rather that the fabric of space that makes up the Universe itself is expanding. Just as raisins appear to recede in proportion to their distance in a leavening ball of dough, the galaxies appear to recede from one another as the Universe expands. The raisins aren’t in motion relative to the dough; the action of the expanding dough itself simply appears to drive them apart.

      It wasn’t an initial explosion that causes galaxies to move away from one another, but rather the physics of the expanding Universe as governed by Einstein’s General Relativity that causes space (with galaxies contain within it) to expand. There was no explosion, just a rapid expansion that has been evolving based on the cumulative gravitational effects of everything contained within our Universe.”

      [ ]

  4. Adolfo Del Castillo | May 6, 2021 at 11:26 am | Reply

    The infinity of the universe taxes my understanding or rather my ability to understand. if we compound the issues to include increasing acceleration the further we go, the closer we come to the observation of an astrophysicist whose first name is NEAL; “THE UIVERSE IS NOT ONLY STRANGER THAN YOU THINK, IT IS STRANGER THAN WE CAN THINK”. ”All we can do is to keep trying.

  5. Francis Grant | May 6, 2021 at 12:19 pm | Reply

    How about giving us a rundown on the status of project NEMO. Have no idea what this is all about.
    Great article! I’m not clear on some of the discussion but more info will come out I’m sure.
    Frank Grant

    • Torbjörn Larsson | May 9, 2021 at 7:12 am | Reply

      Great comment! I lacked that rundown too and had to dig out a reference, see my own response to the article.

  6. Francisco Peña Castellanos | May 6, 2021 at 2:45 pm | Reply

    Very practical and interesting technology. I would like to try the Pearlaqua Micro System to purify the water in our house. May I have information about cost and way of shipping to Colombia.

  7. Torbjörn Larsson | May 9, 2021 at 6:58 am | Reply

    Preprint here: .

    As always the increasing number of proposed ways to measure universe expansion rate is exciting, and methods that promise to give numbers fast and/or precise numbers stands out. Unfortunately the expected event rate is < 0.1 yrs^-1, and the suggested NEMO detector is suggested "operational in the late 2020s and early 2030s" ["Neutron Star Extreme Matter Observatory: A kilohertz-band gravitational-wave detector in the global network", arxiv 2007.03128].

    Meanwhile we have seen promises for new types of expansion rate measurements that need no new detectors and may produce result within a few years ["Fast radio bursts could help solve the mystery of the universe’s expansion There’s a new player in cosmology’s biggest debate", ScienceNews].

    "Hagstotz expects there will sufficient FRBs with distance estimates in the next year or two to accurately determine the Hubble constant. "

  8. BibhutibhusanPatel | May 10, 2021 at 2:07 am | Reply

    Ìn ŕèal sense if ñeed to get some result is 1% than zero,in ìncrease in the expansion rate for measùred
    16% às ďèscrìbèd in the artìcĺe in nice way .Àgain thìs unit increase is mòdìfied by comparing t
    he same wìth sòmè stanďard lìke Type 1a sùpernova present nèar to the supermassìve black hòle at the centre òf òuŕ galaxy for study of two neutròn star merger event occuring lìke in the milkyway.

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