Black holes could come in 'perfect pairs' in an ever expanding Universe

Researchers from the University of Southampton, together with colleagues from the universities of Cambridge and Barcelona, have shown it's theoretically possible for black holes to exist in perfectly balanced pairs -- held in equilibrium by a cosmological force -- mimicking a single black hole.

Black holes are massive astronomical objects that have such a strong gravitational pull that nothing, not even light, can escape. They are incredibly dense. A black hole could pack the mass of the Earth into a space the size of a pea.

Conventional theories about black holes, based on Einstein's theory of General Relativity, typically explain how static or spinning black holes can exist on their own, isolated in space. Black holes in pairs would eventually be thwarted by gravity attracting and colliding them together.

However, this is true if one assumes the Universe is standing still. But what about one which is constantly moving? Could pairs of black holes exist in harmony in an ever expanding Universe, perhaps masquerading as one?

"The standard model of cosmology assumes that the Big Bang brought the Universe into existence and that, approximately 9.8 billion years ago, it became dominated by a mysterious force, coined 'dark energy', which accelerates the Universe at a constant rate," says Professor Oscar Dias of the University of Southampton.

Scientists refer to this mysterious force as a 'cosmological constant'. In a Universe explained by Einstein's theory with a cosmological constant, black holes are immersed in a cosmological accelerated background. This moves the theoretical goal posts over how black holes can interact and exist together.

Through complex numerical methods, the team behind this latest study show that two static (non-spinning) black holes can exist in equilibrium -- their gravitational attraction offset by the expansion associated with a cosmological constant. Even in the acceleration of an ever expanding Universe, the black holes remain locked at a fixed distance from one another. As hard as expansion may try to pull them apart, the gravitational attraction compensates.

"Viewed from a distance, a pair of black holes whose attraction is offset by cosmic expansion would look like a single black hole. It might be hard to detect whether it is a single black hole or a pair of them," comments Professor Dias.

Professor Jorge Santos of the University of Cambridge adds: "Our theory is proven for a pair of static black holes, but we believe it could be applied to spinning ones too. Also, it seems plausible that our solution could hold true for three or even four black holes, opening up a whole range of possibilities."

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New type of star gives clues to mysterious origin of magnetars

Magnetars are the strongest magnets in the Universe. These super-dense dead stars with ultra-strong magnetic fields can be found all over our galaxy but astronomers don't know exactly how they form. Now, using multiple telescopes around the world, including European Southern Observatory (ESO) facilities, researchers have uncovered a living star that is likely to become a magnetar. This finding marks the discovery of a new type of astronomical object -- massive magnetic helium stars -- and sheds light on the origin of magnetars.

Despite having been observed for over 100 years, the enigmatic nature of the star HD 45166 could not be easily explained by conventional models, and little was known about it beyond the fact that it is one of a pair of stars, is rich in helium and is a few times more massive than our Sun.

"This star became a bit of an obsession of mine," says Tomer Shenar, the lead author of a study on this object published today in Science and an astronomer at the University of Amsterdam, the Netherlands. "Tomer and I refer to HD 45166 as the 'zombie star'," says co-author and ESO astronomer Julia Bodensteiner, based in Germany. "This is not only because this star is so unique, but also because I jokingly said that it turns Tomer into a zombie."

Having studied similar helium-rich stars before, Shenar thought magnetic fields could crack the case. Indeed, magnetic fields are known to influence the behaviour of stars and could explain why traditional models failed to describe HD 45166, which is located about 3000 light-years away in the constellation Monoceros. "I remember having a Eureka moment while reading the literature: 'What if the star is magnetic?'," says Shenar, who is currently based at the Centre for Astrobiology in Madrid, Spain.

Shenar and his team set out to study the star using multiple facilities around the globe. The main observations were conducted in February 2022 using an instrument on the Canada-France-Hawaii Telescope that can detect and measure magnetic fields. The team also relied on key archive data taken with the Fiber-fed Extended Range Optical Spectrograph (FEROS) at ESO's La Silla Observatory in Chile.

Once the observations were in, Shenar asked co-author Gregg Wade, an expert on magnetic fields in stars at the Royal Military College of Canada, to examine the data. Wade's response confirmed Shenar's hunch: "Well my friend, whatever this thing is -- it is definitely magnetic."

Shenar's team had found that the star has an incredibly strong magnetic field, of 43,000 gauss, making HD 45166 the most magnetic massive star found to date. "The entire surface of the helium star has a magnetic field almost 100,000 times stronger than Earth's," explains co-author Pablo Marchant, an astronomer at KU Leuven's Institute of Astronomy in BelgiumThis observation marks the discovery of the very first massive magnetic helium star. "It is exciting to uncover a new type of astronomical object," says Shenar, "especially when it's been hiding in plain sight all along."

Moreover, it provides clues to the origin of magnetars, compact dead stars laced with magnetic fields at least a billion times stronger than the one in HD 45166. The team's calculations suggest that this star will end its life as a magnetar. As it collapses under its own gravity, its magnetic field will strengthen, and the star will eventually become a very compact core with a magnetic field of around 100 trillion gauss -- the most powerful type of magnet in the Universe.

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HETDEX reveals galaxy gold mine in first large survey

Astronomers have barely scratched the surface of mapping the nearly endless stars and galaxies of the heavens. Using supercomputers, researchers with The University of Texas at Austin have has now revealed the locations of more than 200,000 new astronomical objects. Their goal is to map even more and use that knowledge to predict the ultimate fate of the universe.

The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) has scanned the dark skies of the Davis Mountains in West Texas since 2017 with a keen eye towards capturing spectroscopic data on Lyman-alpha frequency light from neutral hydrogen emission in galaxies over 10 billion light years away. These galaxies emit a signature wavelength of light from hydrogen that signals to astronomers the intense creation of new stars.

The HETDEX collaboration involves a large team including astronomers, engineers, technicians, and graduate students from six academic institutions in the United States and Germany.

For the first time, the researchers have cataloged astronomical objects -- mapping over 51,863 Lyman-alpha-emitting galaxies at high redshift; 123,891 star forming galaxies at lower redshift; 5,274 non-emission line galaxies at low redshift; and 4,976 active galactic nuclei (AGN) -- bright spots that signal the presence of black holes.

The paper describing the catalog is published February 2023 in The Astrophysical Journal.

"We've just exploded in terms of the number of redshifts cataloged for the first time," said study co-author Erin Mentuch Cooper, a research scientist at The University of Texas at Austin (UT Austin). Cooper is the data manager for the HETDEX project.

"There is a gold mine of astronomy exploration in the HETDEX catalog. That's what I love about it," said study co-author Karl Gebhardt, the Herman and Joan Suit Professor in Astronomy, College of Natural Sciences, UT Austin. Gebhardt is project scientist and principal investigator of HETDEX.

A star's redshift tells astronomers how fast a star is moving away from the Earth because its frequency, akin to its color, gets lower as it moves away, much like the horn of a train as it passes by.

The faster a star moves away, the farther away it is. That relationship between speed and distance, called Hubble's Law, can pin down a galaxy's location and allows astronomers to create a 3D map of over 200,000 stars and galaxies with HETDEX.

"This is only a small percentage of what we will find, but it's a good start. Ultimately, HETDEX aims to map one million red-shifted galaxies," Cooper said.

HETDEX is unique from previous large sky surveys because it's a non-targeted survey, blanketing the sky and collecting spectra from the 35,000 fiber optic cables of the Visible Integral Field Replicable Unit Spectrograph (VIRUS).

VIRUS takes starlight from distant galaxies and splits the light into its component colors like a prism does. HETDEX tiles the sky, collecting 35,000 spectra in a moon-sized swath of sky and moving from spot to spot. It collects about 500-600 hours of observations each year for its survey data.

"We have the largest spectroscopic instrument on the planet, and we're doing one of the longest surveys in terms of time," Gebhardt said. "To analyze this data, we need the fastest computer we can get our hands on, and that's where TACC comes in. TACC does all the data storage and all the data analysis for this giant survey."

The data from the telescope goes straight to the TACC Corral data storage system via high speed lines at 100 Gigabits/second.

"TACC has worked hard with us to streamline our system, and it's just working fantastically. We can process years of data in a couple of days, maybe a week of time on TACC systems. And we do it multiple times because we keep adjusting and improving our methods," Gebhardt added.

HETDEX used the Maverick and Stampede2 supercomputers of the Texas Advanced Computing Center, a leading academic supercomputing center at UT Austin. Stampede2 is funded by the National Science Foundation as a shared resource for thousands of scientists across the US. They helped process and analyze about 60 terabytes of image data on TACC's Corral system.

What's more, Cooper and colleagues have worked with TACC to create JupyterHub public access to the data.

"Anyone with any academic credentials can get a TACC account and go on through a web browser to access our data. We're going to let them access all of our data. This is just the catalog right now. But, the future is going to offer a legacy potential of the science from HETDEX. TACC is helping setting that up," Cooper said.

One interesting highlight from the catalog is the identification of an active galactic nuclei (AGN) with strong Lyman-alpha light emission. Gebhadt co-authored a studyled by UT Austin astronomy post-doctoral researcher Chenxu Liu, published November 2022 in The Astrophysical Journal. It presents intriguing evidence of a black hole without a surrounding host galaxy.

"This is what I call 'naked black holes,'" Gebhardt said. "Nothing confirmed yet, but we suspect these could be out there. Only a survey like HETDEX will be able to find these."

The science generated from HETDEX adds to the bigger picture of understanding the expansion of the entire universe, unexpectedly growing much faster than expected based on precise observations from the Hubble Space Telescope in 2019 of supernovae that act like a cosmic yardstick.

The Holy Grail for HETDEX is an accurate measure of the Universe expansion rate 10 billion years ago that will reveal the physical model for dark energy.

Astronomers are at odds over how to explain the measure of the current expansion rate. Understanding it could require a modification in the theory of gravity, or a change in the fundamental Big Bang theory. It might be the handiwork of an undiscovered particle.

A precise value of the expansion rate early in the Universe can be compared to the expansion rate today. This comparison can determine if the Universe will continue to expand forever, or will someday collapse on itself many billions of years from now.

"The whole point of the HETDEX project is to measure the expansion of the universe," Gebhardt said.

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Ground-breaking number of brown dwarfs discovered

Brown dwarfs are mysterious astronomical objects that fill the gap between the heaviest planets and the lightest stars, with a mix of stellar and planetary characteristics. Due to this hybrid nature, these puzzling objects are crucial to improve our understanding of both stars and giant planets. Brown dwarfs orbiting a parent star from sufficiently far away are particularly valuable as they can be directly photographed -- unlike those that are too close to their star and are thus hidden by its brightness. This provides scientists with a unique opportunity to study the details of the cold, planet-like atmospheres of brown dwarf companions.

However, despite remarkable efforts in the development of new observing technologies and image processing techniques, direct detections of brown dwarf companions to stars have remained rather sparse, with only around 40 systems imaged in almost three decades of searches. Researchers led by Mariangela Bonavita from the Open University and Clémence Fontanive from the Center for Space and Habitability (CSH) and the NCCR PlanetS at the University of Bern directly imaged four new brown dwarfs as they report in a study that has just been published in the journal Monthly Notices of the Royal Astronomical Society MNRAS. This is the first time that multiple new systems with brown dwarf companions on wide orbital separations have been announced at the same time.

Innovative search method

"Wide-orbit brown dwarf companions are rare to start with, and detecting them directly poses huge technical challenges since the host stars completely blind our telescopes," says Mariangela Bonavita. Most surveys conducted so far have been blindly targeting random stars from young clusters. "An alternative approach to increase the number of detections is to only observe stars that show indications of an additional object in their system," explains Clémence Fontanive. For example, the way a star moves under the gravitational tug of a companion can be an indicator of the existence of that companion, whether it is a star, a planet or something in between.

"We developed the COPAINS tool which predicts the types of companions that could be responsible for observed anomalies in stellar motions," Clémence Fontanive continues. Applying the COPAINS tool the research team carefully selected 25 nearby stars that seemed promising for the direct detection of hidden, low-mass companions based on data from the Gaia spacecraft of the European Space Agency (ESA). Using then the SPHERE planet-finder at the Very Large Telescope in Chile to observe these stars, they successfully detected ten new companions with orbits ranging from that of Jupiter to beyond that of Pluto, including five low-mass stars, a white dwarf (a dense stellar remnant), and a remarkable four new brown dwarfs.

Major boost in detection rate

"These findings significantly advance the number of known brown dwarfs orbiting stars from large distances, with a major boost in detection rate compared to any previous imaging survey," as Mariangela Bonavita explains. While for now this approach is mostly limited to signatures from brown dwarf and stellar companions, future phases of the Gaia mission will push these methods to lower masses and allow for the discovery of new giant exoplanets. Clémence Fontanive adds: "On top of having so many new discoveries in one go, our program also demonstrates the power of these search strategies."

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