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The universe is expanding – and we may finally know how fast



When two stars cross paths, their gravitational pull draws them closer until they are locked in a light and violent dance. The two stars eventually melt, a process that can take a billion years to become one.

Most of these star mergers tend to be quite similar, with both stars of the same masses. However, astronomers recently reported the merger of a rather odd pairing, an asymmetric binary star system.

The new discovery is detailed in a study published Wednesday in the journal Natureand can help astronomers resolve a disagreement about the rate of expansion of the universe.

Pulsars acts like a lighthouse in the cosmos. SA / JPL-Caltech

The stars in question are neutron stars. One of them is a pulse, a fast-rotating neutron star or the super-dense remains of a star that exploded in a supernova. These stars emit electromagnetic radiation in the form of light, narrow rays that sweep across the cosmos in a circular motion as the star itself spins.

If you observe the stars from a distance, it will look as if they are pulsing in flashes, which is how they got their name.

The team of astronomers behind the new discovery first discovered the asymmetric couple in 2014 with the Arecibo Observatory in Puerto Rico. The two stars are interconnected and about half a billion billion will collide together. A rare event that we will probably miss.

But they didn’t realize just how massive they are until two years later.

Robert Ferdman, a researcher at the University of East Anglia’s School of Physics, and lead author of the study, describes that reversed exactly how they came to the discovery.

“We had to keep track of these pulses for at least one and a half to two years before we got to grips with the masses,” says Ferdman. “When we did, we realized that we had two neutron stars that are actually completely different in mass.”

One of the stars is much larger than the other and measures 1.62 times the mass of the sun, while the other measures 1.27 solar masses. It may not seem like a big difference, but the usual mass ratio of two merged stars falls somewhere around 0.9, while this couple’s mass ratio is an incredible 0.78.

“It’s far beyond the norm,” says Ferdman.

In 2017, astronomers discovered the merger of two neutron stars for the first time in an event known as GW170817. When the stars collide, they produce a strong burst of gravitational waves or ripples in the space-time fabric.

This newly discovered asymmetric binary system will merge in about 470 million years, and the gravitational effects of the larger neutron star will distort the shape of its smaller companion by removing large amounts of matter before they merge. This, in turn, will release a greater amount of hot material than if the two stars were on equal playing fields, massively.

An artist’s impression of the newly discovered pulsar.Courtesy of Arecibo Observatory / University of Central Florida – William Gonzalez and Andy Torres.

“You see the mergers being made,” says Ferdman. “It is a precursor to what will happen to these types of systems that we discover.”

The research team behind the new study believes that there is a whole population of these joined mergers in the universe and that they can help solve an age issue of cosmic expansion.

Since the Big Bang gave birth to the universe some 13.7 billion years ago, the cosmos has continued to expand. The universe’s velocity of expansion is measured by Hubble Constant, which is calculated by comparing the galaxy’s distance to the apparent recession away from Earth.

To do so, the two main ways of determining the Hubble constant are either by looking at the light from nearby variable stars, or the cosmic microwave background, which is electromagnetic radiation, in different galaxies.

“These two methods are at odds with each other right now, they don’t quite agree with each other,” says Ferdman.

Therefore, the study suggests using the gravitational waves and the electromagnetic light emitted by these neutron star mergers to measure the Hubble constant instead.

“This is completely independent of how these two are measured, it would be a way to break the tie,” he adds. “Depending on how bright these things turn out to be, if we turn 15-20, we can get an accurate measurement of Hubble Constant to favor one method over the other for the expansion rate of the universe.”

The team is currently scouring the cosmos and looking for more of these odd neutron stars to use their explosive light as a guide.

Abstract: The discovery of a radioactively driven kilonova associated with the binary neutron star fusion GW170817 is still the only confirmed electromagnetic equivalent to a gravitational wave event1,2. However, observations of recent electromagnetic emissions are not in line with expectations from standard neutron star models. Although the large measured ejecta mass 3.4 could be explained by a precursor system that is asymmetrical in terms of the stellar component masses (i.e., with a mass ratio q of 0.7 to 0.8), the known galactic population of merging of double-neutron star systems (that is, those that will coincide within billions of years or less) has so far only consisted of almost equal mass (q> 0.9) binary6. The Pulsar PSR J1913 + 1102 is a dual system in a five-hour, low eccentricity (0.09) orbit, with an orbital separation of 1.8 solar radii7, and the two neutron stars are predicted to coincide during 470–11 +12 million years due to gravitational wave emissions. Here, we report that the masses of the pulsar and the companion neutron star, as measured by a dedicated pulsar timing campaign, are 1.62 ± 0.03 and 1.27 ± 0.03 solar masses, respectively. With a measured mass ratio of q = 0.78 ± 0.03, this is the most asymmetric merging system reported to date. On the basis of this discovery, our population synthesis analysis means that such asymmetric binaries represent between 2 and 30 percent (90 percent state) of the total pool of binaries. The coalescence of a member of this population experiences a possible explanation for the anomalous properties of GW170817, including the observed kilonova emission from that event.


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