The universe’s most massive black holes fuel themselves by cooling gas around them, astronomers have found. Using data from NASA’s Chandra X-ray Observatory and the Very Large Telescope (VLT) in Chile, researchers have demonstrated how black hole outbursts trigger a self-sustaining feeding process.

The study, published in Nature Astronomy and led by Valeria Olivares from the University of Santiago de Chile, examined seven galaxy clusters. At the centres of these clusters lie enormous black holes, weighing millions to billions of times the Sun’s mass. These black holes feed on surrounding gas, releasing powerful jets that cool the gas and form filaments.

The research found that outbursts from black holes cool hot gas, forming narrow filaments of warm gas visible as glowing threads. Turbulence in the gas plays a key role in this cooling process. Some of the warm gas flows back into the black hole, fuelling more outbursts and continuing the cycle.

One key discovery was that the brightness of hot gas is linked to the brightness of warm gas in the clusters’ centres. When the hot gas shines brighter, the warm gas glows more intensely, confirming how black holes feed on surrounding gas.

Two galaxy clusters – Perseus and Centaurus – offer a striking visual of this phenomenon.

Perseus Cluster: The hot gas appears bluish-purple with solid pink filaments, while surrounding galaxies shine brightly.

Centaurus Cluster: The gas has a softer, diffused look, with filaments showing delicate, feathery textures.

Both clusters display central black holes surrounded by glowing filaments of gas, a visual representation of the self-sustaining feeding mechanism.

The study noted similarities between the gas filaments in galaxy clusters and the tails of “jellyfish galaxies,” where gas is stripped as galaxies move through their surroundings. This unexpected connection suggests a shared process across different cosmic phenomena.

The research brought together experts from Chile, the US, Australia, Canada, and Italy, leveraging advanced tools like the VLT’s MUSE (Multi Unit Spectroscopic Explorer) instrument to create 3D views of the universe. NASA’s Chandra programme, managed from Alabama, provided the X-ray data critical to this discovery.

Using the largest gravitational wave detector ever made, we have confirmed earlier reports that the fabric of the universe is constantly vibrating. This background rumble is likely caused by collisions between the enormous black holes that reside in the hearts of galaxies.

The results from our detector – an array of rapidly spinning neutron stars spread across the galaxy – show this “gravitational wave background” may be louder than previously thought. We have also made the most detailed maps yet of gravitational waves across the sky, and found an intriguing “hot spot” of activity in the Southern Hemisphere.

Our research is published today in three papers in the Monthly Notices of the Royal Astronomical Society.

Ripples in space and time

Gravitational waves are ripples in the fabric of space and time. They are created when incredibly dense and massive objects orbit or collide with each other.

The densest and most massive objects in the universe are black holes, the remnants of dead stars. One of the only ways to study black holes is by searching for the gravitational waves they emit when they move near each other.

Just like light, gravitational waves are emitted in a spectrum. The most massive black holes emit the slowest and most powerful waves – but to study them, we need a detector the size of our galaxy.

The high-frequency gravitational waves created by collisions between relatively small black holes can be picked up with Earth-based detectors, and they were first observed in 2015. However, evidence for the existence of the slower, more powerful waves wasn’t found until last year.

Several groups of astronomers around the world have assembled galactic-scale gravitational wave detectors by closely observing the behaviour of groups of particular kinds of stars. Our experiment, the MeerKAT Pulsar Timing Array, is the largest of these galactic-scale detectors.

Today we have announced further evidence for low-frequency gravitational waves, but with some intriguing differences from earlier results. In just a third of the time of other experiments, we’ve found a signal that hints at a more active universe than anticipated.

We have also been able to map the cosmic architecture left behind by merging galaxies more accurately than ever before.

Black holes, galaxies and pulsars

At the centre of most galaxies, scientists believe, lives a gargantuan object known as a supermassive black hole. Despite their enormous mass – billions of times the mass of our Sun – these cosmic giants are difficult to study.

Astronomers have known about supermassive black holes for decades, but only directly observed one for the first time in 2019.

When two galaxies merge, the black holes at their centres begin to spiral towards each other. In this process they send out slow, powerful gravitational waves that give us an opportunity to study them.

We do this using another group of exotic cosmic objects: pulsars. These are extremely dense stars made mainly of neutrons, which may be around the size of a city but twice as heavy as the Sun.

Pulsars spin hundreds of times a second. As they rotate, they act like lighthouses, hitting Earth with pulses of radiation from thousands of light years away. For some pulsars, we can predict when that pulse should hit us to within nanoseconds.

Our gravitational wave detectors make use of this fact. If we observe many pulsars over the same period of time, and we’re wrong about when the pulses hit us in a very specific way, we know a gravitational wave is stretching or squeezing the space between the Earth and the pulsars.

However, instead of seeing just one wave, we expect to see a cosmic ocean full of waves criss-crossing in all directions – the echoing ripples of all the galactic mergers in the history of the universe. We call this the gravitational wave background.

A surprisingly loud signal – and an intriguing ‘hot spot’

To detect the gravitational wave background, we used the MeerKAT radio telescope in South Africa. MeerKAT is one of the most sensitive radio telescopes in the world.

As part of the MeerKAT Pulsar Timing Array, it has been observing a group of 83 pulsars for about five years, precisely measuring when their pulses arrive at Earth. This led us to find a pattern associated with a gravitational wave background, only it’s a bit different from what other experiments have found.

The pattern, which represents how space and time between Earth and the pulsars is changed by gravitational waves passing between them, is more powerful than expected.

This might mean there are more supermassive black holes orbiting each other than we thought. If so, this raises more questions – because our existing theories suggest there should be fewer supermassive black holes than we seem to be seeing.

The size of our detector, and the sensitivity of the MeerKAT telescope, means we can assess the background with extreme precision. This allowed us to create the most detailed maps of the gravitational wave background to date. Mapping the background in this way is essential for understanding the cosmic architecture of our universe.

It may even lead us to the ultimate source of the gravitational wave signals we observe. While we think it’s likely the background emerges from the interactions of these colossal black holes, it could also stem from changes in the early, energetic universe following the Big Bang – or perhaps even more exotic events.

The maps we’ve created show an intriguing “hot spot” of gravitational wave activity in the Southern Hemisphere sky. This kind of irregularity supports the idea of a background created by supermassive black holes rather than other alternatives.

However, creating a galactic-sized detector is incredibly complex, and it’s too early to say if this is genuine or a statistical anomaly.

To confirm our findings, we are working to combine our new data with results from other international collaborations under the banner of the International Pulsar Timing Array.

This article is republished from The Conversation under a Creative Commons license. Read the original article here.

Washington:

The conventional wisdom among astronomers is that black holes – those exceptionally dense objects with gravity so powerful that not even light can escape – form in the violent explosion, called a supernova, of a massive dying star. But some, it turns out, may be born in a gentler fashion.

Researchers have identified a black hole that appears to have come into being through the collapse of the core of a large star in its death throes, but without the usual blast. It was observed gravitationally bound to two ordinary stars.

Black holes have previously been spotted orbiting with one other star or one other black hole in what are called binary systems. But this is the first known instance of a triple system with a black hole and two stars.

This system is located about 7,800 light-years from Earth in the constellation Cygnus. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

This black hole, called V404 Cygni, has been extensively studied since being confirmed in 1992. It previously was believed to be orbiting with only one other star, but data from the European Space Agency’s Gaia space observatory showed it instead has two companions.

The researchers said the black hole, with an estimated mass nine times greater than our sun, is in the process of eating one of its companions, a star about seven-tenths as massive as the sun. That star orbits the black hole every 6-1/2 days at a distance only about one-seventh of that separating Earth and the sun.

The black hole appears to be siphoning material off this star, which had puffed up in what is called a red giant phase as part of its natural aging process.

The researchers detected another star about 1.2 times as massive as the sun gravitationally bound to these two but rather far away, orbiting them every 70,000 years at a distance 3,500 times greater than that separating Earth and the sun.

The reason the researchers suspect a gentle birthing process for the black hole is simple. The triple system would have broken apart, they said, if the star that became a black hole had exploded.

A black hole is thought to form when a large star exhausts the nuclear fuel at its core and collapses inward due to its own gravitational pull, triggering an immense explosion that blows off its outer layers into space. The resultant crushed core forms the black hole.

But some astronomers have proposed another path to black hole formation called “direct collapse” in which the star caves in after expending all its fuel but does not explode.

“We call these events a ‘failed supernova.’ Basically, the gravitational collapse just acts too quickly for the supernova to be able to trigger and you get an implosion instead – which sounds super dramatic and awesome but it’s ‘gentle’ in the sense that you don’t expel any matter,” said Massachusetts Institute of Technology astronomer Kevin Burdge, lead author of the study published in the journal Nature.

The researchers estimated that the members of this triple system first formed about 4 billion years ago as ordinary stars.

“The triple system could not have survived if the black hole was born with a natal kick, so this discovery tells us that at least some black holes form without a kick – implying a quiet implosion rather than an explosive supernova,” added Caltech astronomer and study co-author Kareem El-Badry.

This system will not have three members forever, considering that the black hole is consuming its closer neighbor. That suggests that some known binary systems with a black hole and an ordinary star originally may have formed as a triple system, only to have the black hole gobble up one of its partners.

“People have actually predicted that black hole binaries might form mostly through triple evolution, but there was never any direct evidence until now,” El-Badry said.
 

(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)

The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of our Milky Way black hole released in 2022, has captured a new view of the massive object at the centre of our Galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of Sagittarius A*. This image shows the polarised view of the Milky Way black hole. The lines overlaid on this image mark the orientation of polarisation, which is related to the magnetic field around the shadow of the black hole.
| Photo Credit: Reuters

Black holes are mysterious objects – there’s a lot we don’t know about them. One longstanding question has been whether rotating black holes, which are so powerful they drag space-time along with them, could be used as an energy source.

The physicist Roger Penrose suggested that, if an object fell into a rotating black hole in such a way that it split – with one part escaping – the part that left should effectively gain energy from the black hole.

So if we sent objects or light towards a rotating black hole, we may be able to get energy back. It’s difficult to directly prove all this, however. But we have recently published our second study, in Nature Communications, experimentally verifying a more general theory behind it. This theory concerns all rotating objects that can absorb matter or radiation, and a black hole is, in essence, just a very big and effective absorber.

The idea dates back to 1971 and the Soviet physicist Yakov Zel’dovich. Generalising Penrose’s idea, he predicted something very simple. If you take a cylinder that absorbs energy from waves, and you spin it, then it should actually spend its own energy to amplify some waves (boosting their energy).

This would apply to waves that possessed their own inherent rotation (known as angular momentum) in the same direction as the cylinder and had a low enough frequency with respect to the cylinder’s rotation rate.

Zel’dovich’s proposal in turn inspired Stephen Hawking’s famous idea that black holes should slowly radiate their energy away by amplifying photons from the quantum vacuum.

Tricky experiment

Despite the simplicity of the Zel’dovich effect and its key relation to fundamental physics, this effect had not been directly tested until recently.

Zel’dovich’s condition for amplification was general, but his description of a hypothetical system that could show such an effect was quite specific. It involved waves travelling in free space (at the speed of light) with a type of angular momentum known as OAM, short for orbital angular momentum, (meaning the light beams were twisted) and hitting a rapidly rotating cylinder.

But this suggested that the amplification effect would be tiny, because unless the cylinder could rotate at a speed comparable to that of light – a construction that would be mechanically impossible today – the OAM waves that could meet the condition would be spread over an area so large that the cylinder would be in (what Zel’dovich termed) a “non-wave zone” – it would barely interact with the waves at all.

Due to this, it was wrongly thought to be basically unobservable in experiments.

Hard proof

That is until we realised that the effect should also occur in sound waves, which travel much slower than the speed of light. Using sound waves with orbital angular momentum, in 2020 we showed Zel’dovich amplification for the first time in an experiment.

After showing the effect existed in one system, we thought an electromagnetic version might not be so hard after all. We were able to remove the previous limitations by trapping the electromagnetic wave in a resonant circuit, rather than in free space. The oscillating waves in our single circuit didn’t have orbital angular momentum, but contained another type of angular momentum, termed “spin”.

With this circuit, we could funnel the oscillating magnetic part of the wave through a small area where we placed a rotating aluminium cylinder. We then measured how the power in the circuit changed with the cylinder rotation speed. If the cylinder was absorbing the field, it acted as a normal positive resistance in the circuit, draining the power. If it was amplifying the field, it acted as a negative resistance – as a power source.

We found that the amplification of the field by the cylinder was exactly as predicted by Zel’dovich’s condition – meaning we had proven the effect in electromagnetic waves for the first time.

In trying this experiment we also found something unexpected. The way this cylinder creates a negative resistance and amplifies the surrounding circuit when it spins fast enough is very similar to the way that wind turbines generate energy.

Inside a wind turbine is an induction generator, where an alternating current is sent in to create a rotating magnetic field around the rotor. And when the rotor blades spin faster than the surrounding rotating magnetic field, the current is amplified, and energy is generated.

While there is also other physics involved in modern induction generators, it is still astonishing that all the ingredients for proving the Zeldovich effect with electromagnetism were hiding in plain sight for so long.

This link we discovered to induction generators will enable us to optimise these electromagnetic experiments to test the Zel’dovich effect further, leaning on the many years of engineering that have gone in to making motor and generator technology better.

Perhaps the knowledge of this link to the Zel’dovich effect will also go the other way, providing engineers with a new physics perspective to harness for energy generation.

This experiment, showing the Zel’dovich effect is present in electromagnetism, also unlocks the potential to see the effect on a quantum level. Quantum theory tells us empty space is not empty – it has some fluctuations.

Any amplification effect should also be able to amplify such energy fluctuations into real photons – creating matter out of a quantum field. This would mean that a rotating cylinder, even in the absence of all other forces, would gradually slow down due to this process.

As for black holes, the implications are exciting. Perhaps in the future, harnessing the rotation of black holes could be used to power technology or spaceships.

Some have proposed conditions that would create a runaway energy generation effect termed a “black hole bomb”. With improvements to our experiment, we hope to also test this runaway amplification for the Zel’dovich effect.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

We know that all matter is composed of atoms, and atoms are made of protons and neutrons inside the nucleus and electrons outside. But unlike electrons, protons and neutrons are composite particles because they are further made up of quarks.

Quarks can’t exist in isolation. They can only be found in groups of two or three, if not more. Such clumps of quarks are called hadrons. Protons and neutrons are common examples. Physicists have mostly studied quarks based on the behaviour of hadrons, and are also interested in how quarks clump together.

When quarks clump

Two recent findings revealed new insights on this count. One, published on February 20, reported that three-quark clumps are more likely to form than two-quark clumps when a particular type of quark is more densely surrounded by some other particles. According to the international team of researchers that conducted this study, the finding rejects “conventional particle-physics models in which the consolidation of quarks is independent of the particle environment”.

Another study, published on March 15, reported observing clumps composed entirely of the heavier quarks. Protons and neutrons are clumps of lighter quarks and are thus more long-lived. Heavy-quark clumps are very short-lived and harder to study, requiring more sophisticated tools and computing power. Yet understanding them is important to complete our understanding of all quarks, and by extension how these elusive particles affect what we know about nuclear fusion and the fate of stars.

In fact, in the particular and unusual case of quark stars, understanding quarks could have a more direct impact.

The tension of every star

A star is a globe of matter that has found a way to strike a balance between two forces. The force of gravity — arising from the star’s mass — encourages the star to collapse under its own weight and implode. The nuclear force, expressed in the explosive energy released by fusion reactions at its core, pushes the star to blow up and outwards. In a star, the two forces are equally matched and it shines in the sky.

But once a star runs out of material to fuse, nuclear fusion weakens and gravity starts to gain the upper hand. Eventually, the star will ‘die’ and implode. Its fate in its afterlife depends on how large and massive it was when it lived, as a result forming a white dwarf, a neutron star or a black hole.

Scientists have estimated that if the Sun were 20-times more massive, it may collapse into a black hole when it dies. If it were only eight-times heavier, it could become a neutron star. But could there be stars that are too heavy to form a neutron star yet not too heavy to form a black hole, and thus form a quark star?

Enter ‘quark matter’

In neutron stars, the strength with which the core collapses will fuse all protons and electrons inside into neutrons, thus its name. Physicists understand neutron stars well — on paper. The problem is they can’t run any direct experiments on them in any laboratory on the earth. They also don’t know either the masses or the radii of most neutron stars in the universe. So astrophysicists are very interested in studying them.

The matter inside neutron stars is extremely dense. For example, two Suns’ worth of mass is packed into a sphere only 25 km wide. This creates an immense pressure that could be forcing the neutrons into a new state of matter.

An old open problem in physics asks whether this state could be quark matter — when there are no longer any neutrons, only quarks.

In December 2023, researchers from the University of Helsinki reported in the journal Nature Communications that the insides of most massive neutron stars have an 80-90% chance of being made of quark matter.

The research team combined astrophysical observations with theoretical ab initio (from scratch) calculations to develop a model that they ran using a supercomputer, and arrived at this result. However, these astrophysical observations were small in number, meaning the result is not so reliable. Astrophysicists need more observational data to understand quark matter and how exactly it forms.

The need for quarks

A popular way of calculating the bulk properties of any material is to use an equation of state — an equation that, when solved with data about some of a material’s physical properties, reveals the values of other properties. For neutron stars, this is the Tolman-Oppenheimer-Volkoff equation: it is very complex but it assigns a probability to the presence of quarks within neutron stars.

Physics has a rich tradition of giving quirky names to things physicists find. For example, quarks come in six ‘flavours’ — three are called charm and strange; quarks themselves have a property called colour charge; and so on. The name ‘quark’ itself is courtesy physicist Murray Gell-Mann, who named these particles after a line in James Joyce’s 1939 masterpiece, Finnegan’s Wake.

Protons are positively charged and therefore have a magnetic moment (a turning force exerted by a magnetic field) associated with them. But neutrons have a magnetic moment, too, yet they are neutrally charged. So physicists in the 1960s figured neutrons must be made of smaller particles that gave rise to the magnetic moment but whose electric charges cancel themselves out. Gell-Mann called them quarks and their existence was confirmed in the 1970s.

Setting quarks free

There are six types of quarks: up, down, top, bottom, strange, and charm. Each quark can have one of three types of colour charge. Then there are also antiquarks, their antimatter versions. A quark-antiquark clump is called a meson (they don’t annihilate each other because they are of different types, e.g. up + anti-down). Three-quark clumps are called baryons and they form the normal matter surrounding us.

Quarks are further held together by another set of particles called gluons. Because nuclear forces are very strong, quarks are always tightly bound to each other and are not free, even in the vacuum of empty space.

The nuclear force that holds quarks together is explained by a theory called quantum chromodynamics. It predicts that at sufficiently high (by all means extreme) energies, nuclear matter can become ‘deconfined’ to create a new phase of matter in which quarks don’t have to exist in clumps.

Physicists have been able to obtain evidence of deconfinement by smashing lead ions against each other at very high energies in machines like the Large Hadron Collider. In these experiments, a state of matter called a quark-gluon plasma exists for a brief moment; the ‘plasma’ means the quarks are independent. According to the Big Bang theory, the universe was filled with this plasma before the particles clumped and formed the first blobs of matter.

This clumping process may release energy or modify its surroundings in a way that astrophysicists can look for, and eventually discover a quark star. Until then, the possibility will live on as one of the many open problems of physics.

Qudsia Gani is an assistant professor in the Department of Physics, Government Degree College Pattan, Baramulla.