Quantum tornado provides gateway to understanding black holes

Scientists have for the first time created a giant quantum vortex to mimic a black hole in superfluid helium that has allowed them to see in greater detail how analogue black holes behave and interact with their surroundings.

Research led by the University of Nottingham, in collaboration with King's College London and Newcastle University, have created a novel experimental platform: a quantum tornado.

They have created a giant swirling vortex within superfluid helium that is chilled to the lowest possible temperatures.

Through the observation of minute wave dynamics on the superfluid's surface, the research team has shown that these quantum tornados mimic gravitational conditions near rotating black holes.

The research has been published today in Nature.

Lead author of the paper, Dr Patrik Svancara from the School of Mathematical Sciences at the University of Nottingham explains: "Using superfluid helium has allowed us to study tiny surface waves in greater detail and accuracy than with our previous experiments in water. As the viscosity of superfluid helium is extremely small, we were able to meticulously investigate their interaction with the superfluid tornado and compare the findings with our own theoretical projections."

The team constructed a bespoke cryogenic system capable of containing several litres of superfluid helium at temperatures lower than -271 °C. At this temperature liquid helium acquires unusual quantum properties.

These properties typically hinder the formation of giant vortices in other quantum fluids like ultracold atomic gases or quantum fluids of light, this system demonstrates how the interface of superfluid helium acts as a stabilizing force for these objects.

Dr Svancara continues: "Superfluid helium contains tiny objects called quantum vortices, which tend to spread apart from each other. In our set-up, we've managed to confine tens of thousands of these quanta in a compact object resembling a small tornado, achieving a vortex flow with record-breaking strength in the realm of quantum fluids."

Researchers uncovered intriguing parallels between the vortex flow and the gravitational influence of black holes on the surrounding spacetime.

This achievement opens new avenues for simulations of finite-temperature quantum field theories within the complex realm of curved spacetimes.

Professor Silke Weinfurtner, leading the work in the Black Hole Laboratory where this experiment was developed, highlights the significance of this work: "When we first observed clear signatures of black hole physics in our initial analogue experiment back in 2017, it was a breakthrough moment for understanding some of the bizarre phenomena that are often challenging, if not impossible, to study otherwise. Now, with our more sophisticated experiment, we have taken this research to the next level, which could eventually lead us to predict how quantum fields behave in curved spacetimes around astrophysical black holes."

This groundbreaking research is funded by a £5 million grant from the Science Technology Facilities Council, distributed among teams at seven leading UK institutions, including the University of Nottingham, Newcastle University and King's College London.

The project has also been supported by both the UKRI Network grant on Quantum Simulators for Fundamental Physics and the Leverhulme Research Leaders Fellowship held by Professor Silke Weinfurtner.

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Bright galaxies put dark matter to the test

For the past year and a half, the James Webb Space Telescope has delivered astonishing images of distant galaxies formed not long after the Big Bang, giving scientists their first glimpses of the infant universe. Now, a group of astrophysicists has upped the ante: Find the tiniest, brightest galaxies near the beginning of time itself, or scientists will have to totally rethink their theories about dark matter.

The team, led by UCLA astrophysicists, ran simulations that track the formation of small galaxies after the Big Bang and included, for the first time, previously neglected interactions between gas and dark matter. They found that the galaxies created are very tiny, much brighter, and form more quickly than they do in typical simulations that don''t take these interactions into account, instead revealing much fainter galaxies.

Small galaxies, also called dwarf galaxies, are present throughout the universe, and are often thought to represent the earliest type of galaxy. Small galaxies are thus especially interesting to scientists studying the origins of the universe. But the small galaxies they find don't always match what they think they should find. Those closest to the Milky Way spin quicker or are not as dense as in simulations, indicating that the models might have omitted something, such as these gas-dark matter interactions.

The new research, published in The Astrophysical Journal Letters, improves the simulations by adding dark matter interactions with gas and finds that these faint galaxies may have been much brighter than expected early in the universe's history, when they were just beginning to form. The authors suggest scientists should try to find small galaxies that are much brighter than expected using telescopes like the Webb telescope. If they only find faint ones, then some of their ideas about dark matter might be wrong.

Dark matter is a type of hypothetical matter that does not interact with electromagnetism or light. Thus, it is impossible to observe using optics, electricity or magnetism. But dark matter does interact with gravity, and its presence has been inferred from the gravitational effects it has on ordinary matter -- the stuff that makes up all the observable universe. Even though 84% of the matter in the universe is thought to be made of dark matter, it has never been detected directly.

All galaxies are surrounded by a vast halo of dark matter, and scientists think that dark matter was essential to their formation. The "standard cosmological model" astrophysicists use to understand galaxy formation describes how clumps of dark matter in the very early universe drew in ordinary matter through gravity, causing the formation of stars and creating the galaxies we see today. Because most dark matter particles -- called cold dark matter -- are thought to move much slower than the speed of light, this process of accumulation would have occurred gradually.

But over 13 billion years ago, prior to the formation of the first galaxies, ordinary matter, consisting of hydrogen and helium gas from the Big Bang, and dark matter were moving relative to one another. The gas streamed at supersonic velocities past dense thickets of more slowly moving dark matter that should have pulled it in to form galaxies.

"Indeed, in models that do not take streaming into account, this is exactly what happens," said Claire Williams, a UCLA doctoral student and the paper's first author. "Gas is attracted to the gravitational pull of dark matter, forms clumps and knots so dense that hydrogen fusion can occur, and thus forms stars like our sun."

But Williams and co-authors on the Supersonic Project team, a group of astrophysicists from the United States, Italy and Japan led by UCLA physics and astronomy professor Smadar Naoz, found if they added the streaming effect of different velocities between dark and ordinary matter to the simulations, the gas landed far away from the dark matter and was prevented from forming stars right away. When the accumulated gas fell back into the galaxy millions of years later, a massive burst of star formation occurred all at once. Because these galaxies had many more young, hot, luminous stars than ordinary small galaxies for a time, they shone much brighter.

"While the streaming suppressed star formation in the smallest galaxies, it also boosted star formation in dwarf galaxies, causing them to outshine the non-streaming patches of the universe," Williams said. "We predict that the Webb telescope will be able to find regions of the universe where galaxies will be brighter, heightened by this velocity. The fact that they should be so bright might make it easier for the telescope to discover these small galaxies, which are typically extremely hard to detect only 375 million years after the Big Bang."

Because dark matter is impossible to study directly, searching for bright patches of galaxies in the early universe could offer an effective test for theories about dark matter, which has been fruitless so far.

"The discovery of patches of small, bright galaxies in the early universe would confirm that we are on the right track with the cold dark matter model because only the velocity between two kinds of matter can produce the type of galaxy we're looking for," said Naoz, the Howard and Astrid Preston Professor of Astrophysics. "If dark matter does not behave like standard cold dark matter and the streaming effect isn't present, then these bright dwarf galaxies won't be found and we need to go back to the drawing board."

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Radio signal reveals supernova origin

In the latest issue of the journal Nature, astronomers from Stockholm University reveal the origin of a thermonuclear supernova explosion. Strong emission lines of helium and the first detection of such a supernova in radio waves show that the exploding white dwarf star had a helium-rich companion.

Supernovae of Type Ia are important for astronomers since they are used to measure the expansion of the Universe. However, the origin of these explosions has remained an open question. While it is established that the explosion is that of a compact white dwarf star somehow accreting too much matter from a companion star, the exact process and the nature of the progenitor is not known. The new discovery of supernova SN 2020eyj established that the companion star was a helium star that had lost much of its material just prior to the explosion of the white dwarf.

"Once we saw the signatures of strong interaction with the material from the companion we tried to also detect it in radio emission," explains Erik Kool, post-doc at the Department of Astronomy at Stockholm university and lead author of the paper. "The detection in radio is the first one of a Type Ia supernova -- something astronomers have tried to do for decades."

Supernova 2020eyj was discovered by the Zwicky Transient Facility camera on Palomar mountain, where the Oskar Klein Centre at Stockholm University are members.

"The Nordic Optical telescope on La Palma was fundamental for following up this supernova," says Professor Jesper Sollerman at the Department of Astronomy and co-author of the paper. "As were spectra from the large Keck telescope on Hawai'i that immediately revealed the very unusual helium-dominated material around the exploded star."

"This is clearly a very unusual Type Ia supernova, but still related to the ones we use to measure the expansion of the universe," adds Joel Johansson from the Department of Physics.

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Confirmed: Atmospheric helium levels are rising

Scientists at Scripps Institution of Oceanography at UC San Diego used an unprecedented technique to detect that levels of helium are rising in the atmosphere, resolving an issue that has lingered among atmospheric chemists for decades.

The atmospheric abundance of the 4-helium (4He) isotope is rising because 4He is released during the burning and extraction of fossil fuels. The researchers report that it is increasing at a very small but, for the first time, clearly measurable rate. The 4He isotope itself does not add to the greenhouse effect that is making the planet warmer, but measures of it could serve as indirect markers of fossil-fuel use.

The National Science Foundation-supported study appears today in the journal Nature Geoscience.

"The main motivation was to resolve a longstanding controversy in the science community about atmospheric helium concentrations," said study lead author Benni Birner, a former graduate student and now postdoctoral researcher at Scripps Institution of Oceanography at UC San Diego.

The isotope 4He is produced by radioactive decay in the Earth's crust and accumulates in the same reservoirs as fossil fuels, in particular those of natural gas. During the extraction and combustion of fossil fuels, 4He is coincidentally released, which creates another means to evaluate the scale of industrial activity.

The study's breakthrough is in the technique the Scripps Oceanography team used to measure how much helium is in the atmosphere. Birner and Scripps geoscientists Jeff Severinghaus, Bill Paplawsky, and Ralph Keeling created a precise method to compare the 4He isotope to levels of the common atmospheric gas nitrogen. Because nitrogen levels in the atmosphere are constant, an increase in He/N2 is indicative of the rate of 4He buildup in the atmosphere.

Study co-author and Scripps Oceanography geochemist Ralph Keeling, overseer of the famed carbon dioxide measurement known as the Keeling Curve, describes the study as a "masterpiece of fundamental geochemistry." Though helium is relatively easy for scientists to detect in air samples, present at levels of five parts per million of air, no one had done the work to measure it carefully enough to observe an atmospheric increase, he said.

The study also provides a foundation for scientists to better understand the valuable 3-helium (3He) isotope, which has uses for nuclear fusion, cryogenics, and other applications. Proposals to acquire the scarce gas from the moon are an indication of the lengths to which manufacturers will go to harvest it.

According to previous work by other researchers, the 4He isotope exists in the atmosphere in what appears to be an unvarying ratio with 3He. The atmospheric rise of 4He isotope measured at Scripps therefore implies that the 3He isotope must be rising at a comparable rate as 4He. The research by Birner's team raises several questions about the accuracy of scientists' previous assumptions about how 3He is produced and in what quantity.

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Ancient helium leaking from core offers clues to Earth's formation

Helium-3, a rare isotope of helium gas, is leaking out of Earth's core, a new study reports. Because almost all helium-3 is from the Big Bang, the gas leak adds evidence that Earth formed inside a solar nebula, which has long been debated.

Helium-3 has been measured at Earth's surface in relatively small quantities. But scientists did not know how much was leaking from the Earth's core, as opposed to its middle layers, called the mantle.

The new study pins down the core as a major source of helium-3 for the Earth. Some natural processes can generate helium-3, such as the radioactive decay of tritium, but helium-3 is made primarily in solar nebulae -- massive, spinning clouds of gas and dust like the one that gave rise to our Solar System. Because helium is one of the earliest elements produced in the universe, most helium-3 can be traced back to the Big Bang.

As a planet grows, it accumulates material from its surroundings, so its composition reflects the environment in which it formed. To get high concentrations of helium-3 deep in the core, Earth would have had to form inside a thriving solar nebula, not on its fringes or during its waning phase.

The new research adds further clues to the mystery surrounding Earth's formation, lending additional evidence to the theory that our planet formed inside the solar nebula.

The study was published in the AGU journal Geochemistry, Geophysics, Geosystems, which publishes research on the chemistry, physics, geology and biology of Earth and planetary processes.

About 2,000 grams of helium-3 leak out of the Earth every year, "about enough to fill a balloon the size of your desk," said lead study author Peter Olson, a geophysicist at the University of New Mexico. "It's a wonder of nature, and a clue for the history of the Earth, that there's still a significant amount of this isotope in the interior of the Earth."

The researchers modeled helium during two key stages of Earth's history: early formation, when the planet was accumulating helium, and following the formation of the Moon, after which helium was lost. Evidence suggests an object one-third the size of the Earth hit the planet early in its history, around 4 billion years ago and that impact would have re-melted the Earth's crust, allowing much of the helium to escape. The gas continues escaping to this day.

Using the modern helium-3 leak rate along with models of helium isotope behavior, the researchers estimated there are between 10 teragrams (1013 grams) to a petagram (1015 grams) of helium-3 in the core -- a vast quantity that Olson said points to Earth's formation inside the solar nebula, where high concentrations of the gas would have allowed it to build up deep in the planet.

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Astronomers discover a new type of star covered in helium burning ashes

A team of German astronomers, led by Professor Klaus Werner of the University of Tübingen, have discovered a strange new type of star covered in the by-product of helium burning. It is possible that the stars might have been formed by a rare stellar merger event.The fascinating results are published in Monthly Notices of the Royal Astronomical Society.

While normal stars have surfaces composed of hydrogen and helium, the stars discovered by Werner and his colleagues have their surfaces covered with carbon and oxygen, the ashes of helium burning -- an exotic composition for a star. The situation becomes more puzzling as the new stars have temperatures and radii that indicate they are still burning helium in their cores -- a property typically seen in more evolved stars than those observed by Werner and his team in this study.

Published alongside the work of Professor Werner and his team, a second paper from a group of astronomers from the University of La Plata and the Max Planck Institute for Astrophysics offers a possible explanation for their formation. "We believe the stars discovered by our German colleagues might have formed in a very rare kind of stellar merger event between two white dwarf stars," says Dr Miller Bertolami of the Institute for Astrophysics of La Plata, lead author of the second paper. White dwarfs are the remnants of larger stars that have exhausted their nuclear fuel, and are typically very small and dense.

Stellar mergers are known to happen between white dwarfs in close binary systems due to the shrinking of the orbit caused by the emission of gravitational waves. "Usually, white dwarf mergers do not lead to the formation of stars enriched in carbon and oxygen," explains Miller Bertolami, "but we believe that, for binary systems formed with very specific masses, a carbon- and oxygen-rich white dwarf might be disrupted and end up on top of a helium-rich one, leading to the formation of these stars."

Yet no current stellar evolutionary models can fully explain the newly discovered stars. The team need refined models in order to assess whether these mergers can actually happen. These models could not only help the team to better understand these stars, but could also provide a deeper insight into the late evolution of binary systems and how their stars exchange mass as they evolve. Until astronomers develop more refined models for the evolution of binary stars, the origin of the helium covered stars will be up for debate.

"Normally we expect stars with these surface compositions to have already finished burning helium in their cores, and to be on their way to becoming white dwarfs. These new stars are a severe challenge to our understanding of stellar evolution." explains Professor Werner.

From Science Daily

Understanding the power of our Sun

 Stars produce their energy through nuclear fusion by converting hydrogen into helium -- a process known to researchers as "hydrogen burning." There are two ways of carrying out this fusion reaction: one, the so-called pp cycle (proton-proton reaction) or the other, the Bethe Weizsäcker cycle (also known as the CNO cycle, derived from the elements carbon (C), nitrogen (N) and oxygen (O)).

The pp cycle is the predominant energy source in our Sun, only about 1.6 per mil of its energy comes from the CNO cycle. However, the Standard Solar Model (SSM) predicts that the CNO cycle is probably the predominant reaction in much larger stars. As early as the 1930s, the cycle was theoretically predicted by the physicists Hans Bethe and Carl Friedrich von Weizsäcker and subsequently named after these two gentlemen. While the pp cycle could already be experimentally proven in 1992 at the GALLEX experiment, also in the Gran Sasso massif, the experimental proof of the CNO cycle has so far not been successful.

Both the pp cycle and the CNO cycle produce countless neutrinos -- very light and electrically neutral elementary particles. The fact that neutrinos hardly interact with other matter allows them to leave the interior of the sun at almost the speed of light and to transport the information about their origin to earth unhindered. Here the ghost particles have no more than to be captured. This is a rather complex undertaking, which is only possible in a few large-scale experiments worldwide, since neutrinos show up as small flashes of light in a huge tank full of a mixture of water, mineral oil and other substances, also called scintillator. The evaluation of the measured data is complex and resembles looking for a needle in a haystack.

Compared to all previous and ongoing solar neutrino experiments, Borexino is the first and only experiment worldwide that is able to measure these different components individually, in real time and with a high statistical power. This week, the Borexino research collaboration was able to announce a great success: In the scientific journal Nature, they present their results on the first experimental detection of CNO neutrinos -- a milestone in neutrino research.

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A stripped helium star solves the massive black hole mystery

Stellar black holes form when massive stars end their life in a dramatic collapse. Observations have shown that stellar black holes typically have masses of about ten times that of the Sun, in accordance with the theory of stellar evolution. Recently, a Chinese team of astronomers claimed to have discovered a black hole as massive as 70 solar masses, which, if confirmed, would severely challenge the current view of stellar evolution. The publication immediately triggered theoretical investigations as well as additional observations by other astrophysicists. Among those to take a closer look at the object was a team of astronomers from the Universities of Erlangen-Nürnberg and Potsdam. They discovered that it may not necessarily be a black hole at all, but possibly a massive neutron star or even an 'ordinary' star.

The putative black hole was detected indirectly from the motion of a bright companion star, orbiting an invisible compact object over a period of about 80 days. From new observations, a Belgian team showed that the original measurements were misinterpreted and that the mass of the black hole is, in fact, very uncertain. The most important question, namely how the observed binary system was created, remains unanswered. A crucial aspect is the mass of the visible companion, the hot star LS V+22 25. The more massive this star is, the more massive the black hole has to be to induce the observed motion of the bright star. The latter was considered to be a normal star, eight times more massive than the Sun.

A team of astronomers from Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) and the University of Potsdam had a closer look at the archival spectrum of LS V+22 25, taken by the Keck telescope at Mauna Kea, Hawaii. In particular, they were interested in studying the abundances of the chemical elements on the stellar surface. Interestingly, they detected deviations in the abundances of helium, carbon, nitrogen, and oxygen compared to the standard composition of a young massive star. The observed pattern on the surface showed ashes resulting from the nuclear fusion of hydrogen, a process that only happens deep in the core of young stars and would not be expected to be detected at its surface.

'At first glance, the spectrum did indeed look like one from a young massive star. However, several properties appeared rather suspicious. This motivated us to have a fresh look at the archival data', said Andreas Irrgang, the leading scientist of this study and a member of the Dr. Karl Remeis-Observatory in Bamberg, the Astronomical Institute of FAU.

The authors concluded that LS V+22 25 must have interacted with its compact companion in the past. During this episode of mass-transfer, the outer layers of the star were removed and now the stripped helium core is visible, enriched with the ashes from the burning of hydrogen.

However, stripped helium stars are much lighter than their normal counterparts. Combining their results with recent distance measurements from the Gaia space telescope, the authors determined a most likely stellar mass of only 1.1 (with an uncertainty of +/-0.5) times that of the Sun. This yields a minimum mass of only 2-3 solar masses for the compact companion, suggesting that it may not necessarily be a black hole at all, but possibly a massive neutron star or even an 'ordinary' star.

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