Unusual, colliding neutron stars may unlock mysteries of universe expansion

9 Jul 2020

Artist’s impression of PSR J1913+1102. Image: Arecibo Observatory/University of Central Florida – William Gonzalez and Andy Torres

A newly discovered, unusual pulsar could allow astronomers to more accurately measure how fast our universe is expanding.

An international research team has come across something truly strange in the universe, involving one of its many cosmic ‘lighthouses’.

In a paper published to Nature, the team led by researchers from the University of East Anglia described the discovery of a pulsar dubbed PSR J1913+1102.

Pulsars are magnetised, spinning neutron stars that emit highly focused radio waves from their magnetic poles, observed as a pulsing signal not unlike a lighthouse. This latest pulsar is part of a binary system, which means it is locked in a fiercely tight orbit with another neutron star.

It’s estimated that the two neutron stars will collide in approximately 470m years and release incredible amounts of energy in the form of gravitational waves and light.

However, this pulsar is unusual in that the masses of the two neutron stars are very different. This asymmetric system gives scientists confidence that similar double neutron star mergers will provide vital clues about unsolved mysteries in astrophysics.

Furthermore, it could help us more accurately determine the rate of expansion of the universe, referred to as the Hubble constant.

Many more out there

Lead researcher Dr Robert Ferdman said that when gravitational waves were detected in 2017 following the collision of two neutron stars, it helped further confirm Albert Einstein’s theories. However, what was unexpected was the amount of matter ejected from the merger and its brightness.

“Most theories about this event assumed that neutron stars locked in binary systems are very similar in mass,” Ferdman said.

“Our new discovery changes these assumptions. Because one neutron star is significantly larger, its gravitational influence will distort the shape of its companion star – stripping away large amounts of matter just before they actually merge, and potentially disrupting it altogether.

“This ‘tidal disruption’ ejects a larger amount of hot material than expected for equal-mass binary systems, resulting in a more powerful emission.”

The disruption of the lighter neutron star would also enhance the brightness of the material ejected by the merger, allowing them to be observed with conventional telescopes. According to Ferdman, this could allow for a completely independent measurement of the Hubble constant and, as the two current main methods of measuring it are at odds with each other, this could be a “crucial way to break the deadlock” and understand how the universe evolved.

Co-author Dr Paulo Freire from the Max Planck Institute for Radio Astronomy in Germany said that this disruption could also enable astrophysicists to learn about the “exotic matter” that makes up these extremely dense objects.

“This matter is still a major mystery,” Freire said. “It’s so dense that scientists still don’t know what it is actually made of. These densities are far beyond what we can reproduce in Earth-based laboratories.”

Ferdman added that, most importantly, the discovery shows that there are many more of these systems out there.

Colm Gorey was a senior journalist with Silicon Republic