Is Dark Energy Just an Illusion? Neutron Stars Will Tell Us

Simulated Merger Neutron Star Binary
SISSA scientists provide the first simulation of neutron star collisions in extensions of general relativity relevant for cosmology, offering a new approach to test gravity.

A huge amount of mysterious dark energy is necessary to explain cosmological phenomena, such as the accelerated expansion of the Universe, with Einstein’s theory. But what if dark energy was just an illusion and general relativity itself had to be modified? A new SISSA study, published in Physical Review Letters, offers a new approach to answer this question. Thanks to huge computational and mathematical effort, scientists produced the first simulation ever of merging binary neutron stars in theories beyond general relativity that reproduce a dark- energy like behavior on cosmological scales. This allows the comparison of Einstein’s theory and modified versions of it, and, with sufficiently accurate data, may solve the dark energy mystery.

For about 100 years now, general relativity has been very successful at describing gravity on a variety of regimes, passing all experimental tests on Earth and the solar system. However, to explain cosmological observations such as the observed accelerated expansion of the Universe, we need to introduce dark components, such as dark matter and dark energy, which still remain a mystery.

Simulated Merger of a Neutron Star Binary

Simulated merger of a neutron star binary. Credit: Miguel Bezares – GRAMS group SISSA

Enrico Barausse, astrophysicist at SISSA (Scuola Internazionale Superiore di Studi Avanzati) and principal investigator of the ERC grant GRAMS (GRavity from Astrophysical to Microscopic Scales) questions whether dark energy is real or, instead, it may be interpreted as a breakdown of our understanding of gravity. “The existence of dark energy could be just an illusion,” he says, “the accelerated expansion of the Universe might be caused by some yet unknown modifications of general relativity, a sort of ‘dark gravity’.”

The merger of neutron stars offers a unique situation to test this hypothesis because gravity around them is pushed to the extreme. “Neutron stars are the densest stars that exist, typically only 10 kilometers in radius, but with a mass between one or two times the mass of our Sun,” explains the scientist. “This makes gravity and the spacetime around them extreme, allowing for abundant production of gravitational waves when two of them collide. We can use the data acquired during such events to study the workings of gravity and test Einstein’s theory in a new window.”

Simulated Merger of Neutron Star Binary

Simulated merger of a neutron star binary. Credit: Miguel Bezares – GRAMS group SISSA

In this study, published in Physical Review Letters, SISSA scientists in collaboration with physicists from Universitat de les Illes Balears in Palma de Mallorca, produced the first simulation of merging binary neutron stars in theories of modified gravity relevant for cosmology: “This type of simulations is extremely challenging,” clarifies Miguel Bezares, first author of the paper, “because of the highly non-linear nature of the problem. It requires a huge computational effort –months of run in supercomputers – that was made possible also by the agreement between SISSA and CINECA consortium as well as novel mathematical formulations that we developed. These represented major roadblocks for many years till our first simulation.”

Thanks to these simulations, researchers are finally able to compare general relativity and modified gravity. “Surprisingly, we found that the ‘dark gravity’ hypothesis is equally good as general relativity at explaining the data acquired by the LIGO and Virgo interferometers during past binary neutron star collisions. Indeed, the differences between the two theories in these systems are quite subtle, but they may be detectable by next-generation gravitational interferometers, such as the Einstein telescope in Europe and Cosmic Explorer in USA. This opens the exciting possibility of using gravitational waves to discriminate between dark energy and ‘dark gravity’,” Barausse concludes.

Reference: “No Evidence of Kinetic Screening in Simulations of Merging Binary Neutron Stars beyond General Relativity” by Miguel Bezares, Ricard Aguilera-Miret, Lotte ter Haar, Marco Crisostomi, Carlos Palenzuela and Enrico Barausse, 1 March 2022, Physical Review Letters.
DOI: 10.1103/PhysRevLett.128.091103

6 Comments on "Is Dark Energy Just an Illusion? Neutron Stars Will Tell Us"

  1. Take a 10 kilometre sphere place it in xyz space and call it red neutron star sphere 1
    Place a space gap of 10 kilometres between it and
    Take a 10 kilometre sphere place it in x1y1z1 space and call it red neutron star sphere 2

    After a length of time 1

    Sphere 1 becomes red yellow green and light blue as shown in diagram panel 2
    Place a space gap of 10 kilometres now reduced to 5 kilometres
    Sphere 2 becomes red yellow green and light blue as shown in diagram panel 2

    After a length of time 2
    Sphere 1 merges with sphere 2 red yellow green light blue

    After a length of time 3
    Sphere 1 & 2 are merged red yellow green light blue
    What size is this sphere
    How much of the volume of this sphere adds up to the volume of two 10 kilometres spheres
    How much of the volume of this sphere is less than the volume of two 10 kilometre spheres
    Ie is sphere 1 & 2 the merged neutron stars = 12.5 kilometre diameter sphere approx
    Or
    Is sphere 1 & 2 the merged neutron stars = 8 kilometre diameter sphere + loss of rest
    Or
    Is sphere 1 & 2 the merged neutron star = 5 kilometre diameter sphere + loss of the rest

    But i would have thought in the space time continuum
    Take a 10 kilometre sphere place it in xyz space and make it rotate clockwise and call it red neutron star sphere 1c
    Place a space gap of 10 kilometres between it and
    Take a 10 kilometre sphere place it in x1y1z1 space and make it rotate clockwise and call it red neutron star sphere 2c
    or
    Take a 10 kilometre sphere place it in x2y2z2 space and make it rotate anticlockwise and call it red neutron star sphere 1ac
    or
    Take a 10 kilometre sphere place it in x3y3z3 space and make it rotate anticlockwise and call it red neutron star sphere 2ac
    That
    1c and 2c would merge clockwise
    1c and 2ac would splurge and not merge but dark matter would emerge red hot coals in interstellar space
    1ac and 2ac would merge anticlockwise
    1ac and 2c would splurge and not merge but dark matter would emerge red hot coals in interstellar space

  2. Bernie Court | March 6, 2022 at 7:49 am | Reply

    I can see the rationale behind the suspicion that some observed phenomena may be an illusion. Conduct a thought experiment. Observe the effect of a vigorous explosion in a vacuum. After the explosion the resulting debris gives objects of all different speeds. Those furthest away are travelling faster than those close in to the origin. They are not accelerating, they are just travelling faster than those close in. The acceleration was produced during the initial explosion and no longer exists.

    • I-may-be-wrong | March 7, 2022 at 4:30 pm | Reply

      I believe that the data recorded shows that the other objects are still accelerating away from us, and not just moving away from us at a constant speed which would be the case with your thought experiment

  3. Conduct a thought experiment mark ii
    Observe the effect of a vigourous explosion in 3d space
    After the explosion the resulting debris travels in orbit around the centre at the same speed experiment mark ii
    Those furthest away ie 2 radii take one turn to rotate 360 degrees
    Those nearest ie 1 radii take two turns to rotate 360 degrees
    They are not accelerating they are just travelling at the same speed those at 1 radii and those at 2 radii
    Take a ballerina doing a pirohuette
    When their arms are out they are at 2 radii and hence will do 1 pirohuette
    When their arms are in they are at 1 radii and hence will do 2 pirohuettes
    When the arms are in are they going faster no its an optical appearance illusion they do two turns which seems like they are going faster but in reality they are travelling at the same speed as when their arms are outstretched
    So whats the secret
    If you are a ballerina and doing a pirohuette with your arms in
    Make your footwork faster and then you may achieve 3 turns in the same time it takes the arms outstretched to do one and then one could say you are travelling faster
    Its a velocity illusion the acceleration is produced by quicker ballerina footwork on the spot when u conduct a physical real experiment
    Otherwise
    One 360 degree turn at 2 radii and two 360 degree turns at 1 radi = both objects are travelling at the same speed
    Beware of the old ballerina footwork twirl
    And now for my next faux pas

  4. LOL… DARK Energy… DARK Matter… is significant of a Comic Book approach to comprehending the structure/function of the Universal “Fabric”. Yuh know, like THE DARK KNIGHT? Hole-ley UNIVERSE, BATMAN.
    But… Go on ahead, embrace the CARTOON version and expound on the virtues of a FROZEN FRAME of REFERENCE. As Jean-luc Picard once commanded: ENGAGE! … the ERROR of an EGO-CENTRIC Universe. In deed: the notion of INFINITY must consent to HUMAN MORTALITY: Trim to fit. Hand tailored to the abysmally brief span of one Life.
    The UNIVERSE (MULTIVERSE/SINGULARITY) knows nothing of Humans or Mortality. It just gets-on with it.
    Yea, verily: Einstein is still DEAD. YET his Miscognition lives on. To WIT/To WOO: How can a self-contained “container” be contained? PARADOXICAL? No. Merely a feature of MYOPIC VISION, there, Magoo; despite a “SCIENTIFIC” Seal of Approval.
    When one takes a STAND, it is helpful(?) to know where one’s feet are planted. Albeit one might be (literally) STARSTRUCK with Wonderment.
    Can’t blame one for trying.
    🦇
    🦋

  5. … a good effort, though!

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