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New research suggests innovative method to analyse the densest star systems in the Universe

New research suggests innovative method to analyse the densest star systems in the Universe
Artist’s illustration of supernova remnant Credit: Pixabay

In a recently published study, a team of researchers led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash university suggests an innovative method to analyse gravitational waves from neutron star mergers, where two stars are distinguished by type (rather than mass), depending on how fast they’re spinning.


Neutron stars are extremely dense stellar objects that form when giant stars explode and die—in the explosion, their cores collapse, and the protons and electrons melt into each other to form a remnant neutron star.

In 2017, the merging of two neutron stars, called GW170817, was first observed by the LIGO and Virgo gravitational-wave detectors. This merger is well-known because scientists were also able to see light produced from it: high-energy gamma rays, visible light, and microwaves. Since then, an average of three scientific studies on GW170817 have been published every day.

In January this year, the LIGO and Virgo collaborations announced a second neutron star merger event called GW190425. Although no light was detected, this event is particularly intriguing because the two merging neutron stars are significantly heavier than GW170817, as well as previously known double neutron stars in the Milky Way.

Scientists use gravitational-wave signals—ripples in the fabric of space and time—to detect pairs of neutron stars and measure their masses. The heavier neutron star of the pair is called the ‘primary’; the lighter one is ‘secondary’.

The recycled-slow labelling scheme of a binary neutron star system

A binary neutron star system usually starts with two ordinary stars, each around ten to twenty times more massive than the Sun. When these massive stars age and run out of ‘fuel’, their lives end in supernova explosions that leave behind compact remnants, or neutron stars. Each remnant neutron star weighs around 1.4 times the mass of the Sun, but has a diameter of only 25 kilometres.

The first-born neutron star usually goes through a ‘recycling’ process: it accumulates matter from its paired star and begins spinning faster. The second-born neutron star doesn’t accumulate matter; its spin speed also slows down rapidly. By the time the two neutron stars merge—millions to billions of years later—it’s predicted that the recycled neutron star may still be spinning rapidly, whereas the other non-recycled neutron star will probably be spinning slowly.

Another way a binary neutron star system might form is through continuously changing interactions in dense stellar clusters. In this scenario, two unrelated neutron stars, on their own or in other separate star systems, meet each other, pair up and eventually merge like a happy couple due to their gravitational waves. However, current modelling of stellar clusters suggests that this scenario is ineffective in merging the neutron stars.

OzGrav postdoctoral researcher and lead author of the study Xingjiang Zhu says: ‘The motivation for proposing the recycled-slow labelling scheme of a binary neutron star system is two-fold. First, it’s a generic feature expected for neutron star mergers. Second, it might be inadequate to label two

A mini fractal universe may lie inside charged black holes (if they exist)

Black holes are perhaps the strangest, least-understood objects in our universe. With so much potential — being linked to everything from wormholes to new baby universes — they have sucked in physicists for decades. 

But as strange as these known objects are, even stranger types of black holes could be dreamed up. In one upside-down, hypothetical version of the universe, a bizarre type of black hole could exist that is stranger than an M.C. Escher sketch. Now, a team of researchers has plunged into the mathematical heart of so-called charged black holes and found a slew of surprises, including an inferno of space-time and an exotic fractal landscape … and potentially more.

Related: 9 ideas about black holes that will blow your mind

Welcome to a holographic superconductor

There are all sorts of potential, hypothetical black holes: ones with or without electric charge, ones spinning or stationary, ones surrounded by matter or those floating in empty space. Some of these hypothetical black holes are known for certain to exist in our universe; for example, the rotating black hole surrounded by infalling matter is a pretty common presence. We’ve even taken a picture of one.

But some other kinds of black holes are purely theoretical. Even so, physicists are still interested in exploring them — by diving into their mathematical foundations, we can realize new relationships and implications of our physical theories, which can have real-world consequences. 

One such theoretical black hole is an electrically charged black hole surrounded by a certain kind of space known as anti-de Sitter. Without getting into too much of the nitty-gritty, this kind of space has constant negative geometric curvature, like a horse saddle, which we know is not a good description of our universe. (A cosmos with anti-de Sitter space, all else being the same, would have a negative cosmological constant, which means that any matter would tend to condense into a black hole, versus the known accelerating expansion that is flinging the universe apart. 

This horse-saddle space doesn’t exist in our universe, But that’s okay: It turns out that these exotic black holes still have surprisingly intricate structures worth exploring.

Related: The 18 biggest unsolved mysteries in physics

One of the reasons it’s worth exploring is that charged black holes share a lot of similarities with rotating black holes, which certainly do exist in our universe, but charged black holes are mathematically simpler to grapple with. So by studying charged black holes we can gain some insights into real-world rotating black holes. 

Also, physicists have found that when these black holes become relatively cool, they build up a “haze” of quantum fields around their surfaces. This haze sticks to the surface, pulled inward by the never-tiring gravity of the black hole itself, but pushed outward by the electric repulsion of the same black hole. A haze of quantum fields operating in stability on a surface is also known as a superconductor. Superconductors have real-world applications (namely, they can transmit electric current with

Physicists Calculate Upper Limit For Speed Of Sound In The Universe

KEY POINTS

  • Physicists tested sound as it travels through different materials
  • Sound can almost reach its upper limit when traveling in solid atomic hydrogen
  • The finding is vital in different fields of studies like materials science and condensed matter physics

Sound waves can travel to up to 36 kilometers or more than 22 miles per second when traveling through solids or liquids, a new study by a team of physicists revealed. The physicists said that their calculation could be the first known variables representing the threshold of sound waves.    

Before this new finding, the speed of sound was measured based on Albert Einstein’s theory of special relativity that identified sound waves threshold similar to that of the speed of light (300,000 kilometers or over 186,000 miles per second).

In a study, published in the journal Science Advances, the physicists said to calculate for the threshold of the speed of sound, they factored in the two dimensionless fundamental constants. These constants are the fine structure constant and the proton-to-electron mass ratio. 

The physicists explained that these two fundamental constants have already been used in calculations needed to understand the Universe. For instance, the dimensionless fundamental constants are also the basis for calculations of nuclear reactions, proton decay, and nucleosynthesis in stars. The balance between the fundamental constants could also point to the habitable zone where possible life forms could start outside Earth. 

With identifying the upper limit of sound, their finding also became significant in other fields of studies. Setting a known upper threshold of sound is particularly crucial to studies that test the limits of matter such as materials science and condensed matter physics.      

“We believe the findings of this study could have further scientific applications by helping us to find and understand limits of different properties such as viscosity and thermal conductivity relevant for high-temperature superconductivity, quark-gluon plasma and even black hole physics,” Kostya Trachenko, professor at the Queen Mary University of London, said in a press release. 

Another major finding of the study is the possibility that sound travels the fastest in solid atomic hydrogen. To come up with this result, the physicists tested sound as it travels through different materials. 

By performing sophisticated quantum mechanical calculations, the team of physicists found that sound can almost reach its upper limit when traveling in solid atomic hydrogen. The speed of sound came nearly twice as fast as the speed of sound in diamond given that diamond is already the hardest known material in the world.   

Photo by Soundtrap on Unsplash Photo by Soundtrap on Unsplash Photo: Photo by Soundtrap on Unsplash

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When Did The Universe Get Its First Quantum Fields?

No matter how we look at the Universe — at low temperatures or ultra-high energies, from our own backyard to the most distant recesses of the observable cosmos — we find that the same laws of physics apply. The fundamental constants remain the same; gravitation appears to behave the same; the quantum transitions and relativistic effects are identical. At all points in time, at least for the parts of the Universe we can observe, General Relativity (governing gravity) and Quantum Field Theory (governing the other known forces) appear to apply in the exact same form we find them appearing here on Earth. But has it always been this way? Is there a time where the Universe didn’t have the same quantum fields in it, or perhaps no quantum fields at all? That’s what Patreon supporter Chris Shaw wants to know, asking:

“When did the first quantum fields form in the universe? Have they been there since the Big Bang or even from the inflationary period before?”

Perhaps surprisingly, quantum fields are there even under conditions where you might not expect them. Here’s what we know so far.

When we think about fields, most of us conceive of them the same way scientists did back in the 1800s: when you have some type of source — like an electric charge or a permanent magnet — it creates a field around it at every point in space. That field exists whether or not there are other particles there to be affected by it, but you can detect the presence of that field (as well as what it affects and how) by observing what happens to charges of various types that interact with that field.

Iron filings, which themselves can get magnetized, respond to magnetic fields by aligning along the direction of a field. Electric charges, in the presence of an electric field (or in motion in the presence of a magnetic field), will experience a force that accelerates them dependent on the strength of the field.

Even gravitation, whether in Einstein’s or Newton’s conception, can be visualized as a field: where matter or energy of any form will respond to the cumulative gravitational effects at its location in space, determining its future trajectory.