The incredibly high-powered collision of two neutron stars reported on last month could force scientists to rethink what we think we know about the universe, Dr. Enrico Ramirez-Ruiz, a professor at the University of California at Santa Cruz’ astronomy and astrophysics department, said.
The star collision, detected by a LIGO network detector in Livingston, Louisiana on 25 April 2019 and christened GW190425, was thought to have taken place some 290-720 million light years away, but to have been strong enough to send shockwaves through space-time which could be detected here on Earth.
However, whereas the first-ever confirmed observation of gravitational waves in 2015 seemed to conform to astrophysicists’ understanding of neutron stars, the new findings don’t. At least not entirely.
“We have a dilemma,” Dr. Ramirez-Ruiz said, speaking to Quanta Magazine.
In a paper on the subject, together with colleagues from the University of Copenhagen and the Harvard & Smithsonian’s Center for Astrophysics, Ramirez-Ruiz et al. calculated that GW190425’s characteristics seem to challenge scientists’ understanding of the prevalence of these kinds of binary stars.
For starters, the neutron star pair’s mass, equivalent to about 3.4 times that of our Sun, seems too big compared to that of the 18 or so confirmed binary neutron star combos spotted in the Milky Way among the 2,500 or so known neutron stars.
On the basis of LIGO’s findings, it’s possible that such ‘hefty’ pairings of stars may be much more common than astronomers previously estimated, and that humanity simply hasn’t been able to detect their presence, or formation, due to the absence of omni-directional sensors like LIGO’s, but also due to a bias in previous radio surveys “against observing such systems if they are born from a far-merging channel”.
“Moreover, the comparable merger rate challenges our understanding of supernova explosions in massive stars as more massive new stars are born from heavier progenitors such that the relative formation rate of massive to normal binary neutron star systems should be at least suppressed by an order of magnitude,” Ramirez-Ruiz and his colleagues wrote.
Dr. Benn Farr, an assistant professor of physics at the University of Oregon who’s part of the LIGO collaboration, admitted to Quanta that there’s still just too much that scientists don’t know about how binary stars behave. “We know a great deal about stellar formation and evolution, but a lot of the physics related to producing compact binaries,” such as the exchange of mass, or how they become close enough to merge, “is still very poorly understood”.
Ultimately, Ramirez-Ruiz believes that the disparity between current theory, which estimates that less than one in ten neutron stars in the universe are large enough to create massive neutron star pairs like GW190425, and the recently discovered evidence to the contrary, is “a call to action”.
“We have a population of pulsar stars that we see, and all of the binary population models are aimed to explain that population. All of the sudden LIGO says, well, that population is not representative of the population of double neutron stars. So we have to rethink the paradigm of assembly and how these things are made,” he emphasised.
In any case, noting that this new understanding of neutron stars means that scientists will “have to go back to the drawing board”, including as far as their computer models of galactic behaviour are concerned, the academic said that for him, this process would be “very exciting”.
