The neutron star mergers expel a heavy element-enriched fireball that can be observed as a kilonova. The geometry of the kilonova is an important diagnosis of the merger and is dictated by the properties of ultra-dense matter and the energetics of a black hole’s collapse.

But we still don’t know much about this violent phenomenon. When a kilonova was detected in 2017 at a distance of 140 million light-years, scientists were able to collect detailed data for the first time.

The data from this massive explosion is still being interpreted by scientists around the world, including Albert Sneppen and Darach Watson of the University of Copenhagen, who made a surprising discovery.

Astronomers suggest that the collision of neutron stars causes an explosion that, contrary to what was believed until recently, has the shape of a perfect sphere. While it’s currently unclear how this is feasible, the finding could provide a new way to understand basic physics and calculate the age of the Universe.

Albert Snappen, a Ph.D. student at the Niels Bohr Institute and first author of the study published in the journal Nature said: “You have two super-compact stars that orbit each other 100 times per second before collapsing. Our intuition, and all previous models, say that the explosion cloud created by the collision should have a flattened and rather asymmetrical shape.”

That is why scientists are surprised that this is not the case for the 2017 kilonova. It is completely symmetrical and has a shape that almost approaches a perfect sphere.

Darach Watson, an associate professor at the Niels Bohr Institute and second author of the study, said: “Nobody expected the explosion to look like this. It makes no sense that it is spherical, like a ball. But our calculations clearly show that it is. This likely means that the theories and simulations of kilonovae we’ve considered over the past 25 years are missing important physics.”

Illustration of spherical explosion
Illustration of spherical explosion made by Albert Snappen.

But the main puzzle is how the kilonova can be spherical. Scientists speculated that unexpected physics must be involved.

The explosion becomes spherical as a huge amount of energy shoots out from the center, smoothing out an otherwise asymmetrical shape. Therefore, the spherical shape indicates a surprising amount of energy in the core of the collision.

The neutron stars briefly combine into a single hypermassive neutron star during the collision before collapsing into a black hole. The researchers wonder if there is a significant part of the secret hidden in this collapse:

If the star falls into a black hole, the energy of the hypermassive neutron star’s immense magnetic field may be released, creating a “magnetic bomb.” The distribution of materials in the explosion may become more convex due to the release of magnetic energy. If so, creating the black hole could take a lot of energy.

However, another feature of the scientists’ discovery must be adequately explained by this explanation. All the elements produced are heavier than iron, but previous theories suggest that the extremely heavy elements, such as gold or uranium, should form in different locations in the kilonova than the smaller elements, such as strontium or krypton, and should be dispersed in different directions. emitted . However, only the lighter elements, evenly distributed throughout space, are detected by scientists.

Therefore, scientists believe that the enigmatic elementary particles, neutrinos, about which much is still unknown, also play a key role in the phenomenon.

Albert Snape said: “An alternative idea is that in the milliseconds that the hypermassive neutron star is alive, it emits very powerfully, possibly including a huge number of neutrinos. Neutrinos can cause neutrons to convert into protons and electrons, creating lighter elements in general. This idea also has shortcomings, but we believe neutrinos play an even more important role than we thought.”

“The shape of the explosion is also interesting for a completely different reason: there is a lot of discussion among astrophysicists about how fast the universe is expanding. The speed tells us, among other things, how old the universe is.”

“And the two existing methods of measuring it differ by about a billion years. Here we may have a third method that can complement and be tested against the other measurements.”

“The so-called “cosmic distance ladder” is used today to measure how fast the universe is growing. This is done simply by calculating the distance between different objects in the universe, which act as rungs on the ladder.”

Darach Watson continues: “If they’re bright and mostly spherical, and if we know how far away they are, we can use kilonovae as a new way to measure distance independently — a new kind of cosmic ruler.”

“Knowing what the shape is is crucial here, because if you have an object that is not spherical, it will radiate differently depending on your viewing angle. A spherical explosion provides much greater precision in the measurement.”

Magazine reference:

  1. Sneppen, A., Watson, D., Bauswein, A., et al. Spherical symmetry in the kilonova AT2017gfo/GW170817. Nature 614, 436-439 (2023). DOI: 10.1038/s41586-022-05616-x