Category Archives: Cosmology

Have We Detected Dark Energy? Cambridge Scientists Say it’s A Possibility (Cosmology)

Dark energy, the mysterious force that causes the universe to accelerate, may have been responsible for unexpected results from the XENON1T experiment, deep below Italy’s Apennine Mountains.

It was surprising that this excess could in principle have been caused by dark energy rather than dark matter. When things click together like that, it’s really special.

Sunny Vagnozzi

A new study, led by researchers at the University of Cambridge and reported in the journal Physical Review D, suggests that some unexplained results from the XENON1T experiment in Italy may have been caused by dark energy, and not the dark matter the experiment was designed to detect.

They constructed a physical model to help explain the results, which may have originated from dark energy particles produced in a region of the Sun with strong magnetic fields, although future experiments will be required to confirm this explanation. The researchers say their study could be an important step toward the direct detection of dark energy.

Everything our eyes can see in the skies and in our everyday world – from tiny moons to massive galaxies, from ants to blue whales – makes up less than five percent of the universe. The rest is dark. About 27% is dark matter – the invisible force holding galaxies and the cosmic web together – while 68% is dark energy, which causes the universe to expand at an accelerated rate.

“Despite both components being invisible, we know a lot more about dark matter, since its existence was suggested as early as the 1920s, while dark energy wasn’t discovered until 1998,” said Dr Sunny Vagnozzi from Cambridge’s Kavli Institute for Cosmology, the paper’s first author. “Large-scale experiments like XENON1T have been designed to directly detect dark matter, by searching for signs of dark matter ‘hitting’ ordinary matter, but dark energy is even more elusive.”

To detect dark energy, scientists generally look for gravitational interactions: the way gravity pulls objects around. And on the largest scales, the gravitational effect of dark energy is repulsive, pulling things away from each other and making the universe’s expansion accelerate.

About a year ago, the XENON1T experiment reported an unexpected signal, or excess, over the expected background. “These sorts of excesses are often flukes, but once in a while they can also lead to fundamental discoveries,” said co-author Dr Luca Visinelli, from Frascati National Laboratories in Italy. “We explored a model in which this signal could be attributable to dark energy, rather than the dark matter the experiment was originally devised to detect.”

At the time, the most popular explanation for the excess were axions – hypothetical, extremely light particles – produced in the Sun. However, this explanation does not stand up to observations, since the amount of axions that would be required to explain the XENON1T signal would drastically alter the evolution of stars much heavier than the Sun, in conflict with what we observe.

We are far from fully understanding what dark energy is, but most physical models for dark energy would lead to the existence of a so-called fifth force. There are four fundamental forces in the universe, and anything that can’t be explained by one of these forces is sometimes referred to as the result of an unknown fifth force.

However, we know that Einstein’s theory of gravity works extremely well in the local universe. Therefore, any fifth force associated to dark energy is unwanted and must be hidden, or screened, when it comes to small scales, and can only operate on the largest scales where Einstein’s theory of gravity fails to explain the acceleration of the Universe. To hide the fifth force, many models for dark energy are equipped with so-called screening mechanisms, which dynamically hide the fifth force.

Vagnozzi and his co-authors constructed a physical model, which used a type of screening mechanism known as chameleon screening, to show that dark energy particles produced in the Sun’s strong magnetic fields could explain the XENON1T excess.

“Our chameleon screening shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions,” said Vagnozzi. “It also allows us to decouple what happens in the local very dense Universe from what happens on the largest scales, where the density is extremely low.”

The researchers used their model to show what would happen in the detector if the dark energy was produced in a region of the Sun called the tachocline, where the magnetic fields are particularly strong.

“It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter,” said Vagnozzi. “When things click together like that, it’s really special.”

Their calculations suggest that experiments like XENON1T, which are designed to detect dark matter, could also be used to detect dark energy. However, the original excess still needs to be convincingly confirmed. “We first need to know that this wasn’t simply a fluke,” said Visinelli. “If XENON1T actually saw something, you’d expect to see a similar excess again in future experiments, but this time with a much stronger signal.”

If the excess was the result of dark energy, upcoming upgrades to the XENON1T experiment, as well as experiments pursuing similar goals such as LUX-Zeplin and PandaX-xT, mean that it could be possible to directly detect dark energy within the next decade.


Reference:
Sunny Vagnozzi et al. ‘Direct detection of dark energy: the XENON1T excess and future prospects.’ Physical Review D (2021). DOI: 10.1103/PhysRevD.104.063023


Provided by University of Cambridge

How Would Be Rotating and Non Rotating Dark Energy Stars? (Cosmology)

By adopting the extended Chaplygin equation-of-state, Ilidio Lopes and colleagues carried out study on the isotropic and slowly-rotating dark energy stars. They found that the moment of inertia increases with the mass of the star and in the case of non-rotating objects the moment of inertia grows faster. Their study recently appeared in Arxiv.

A dark-energy star is a hypothetical compact astrophysical object, which a minority of physicists think might constitute an alternative explanation for observations of astronomical black hole candidates. The concept was proposed by physicist George Chapline. The theory states that infalling matter is converted into vacuum energy or dark energy, as the matter falls through the event horizon. The space within the event horizon would end up with a large value for the cosmological constant and have negative pressure to exert against gravity.

Now, Ilidio Lopes and colleagues carried out study on the isotropic and slowly-rotating dark energy stars. They computed the moment of inertia as a function of the mass of the stars, both for rotating and non-rotating objects. They have also shown a solution for the non-diagonal metric component as a function of the radial coordinate for three different star masses: i) a light star (M ∼ 1.4 M), ii) a heavy star (M ∼ 2 M) and iii) an average star (M ∼ 1.75 M).

© I. Lopes et al.

They found that, the moments of inertia increase with the mass of the star.

While, in the case of non-rotating objects the moment of inertia grows faster.

Finally, they found that the curve corresponding to rotation lies below the one corresponding to non-rotating stars. Therefore the deviation is smaller for light stars and larger for heavy stars.

Moreover, it has been found that, for a given mass, a rotating star has a lower moment of inertia than its non-rotating counterpart.

“The increase of observational data expected in the coming years will allow us to study the effect of rotation on the moments of inertia to validate or exclude this type of EoS models.”

— they concluded.

Reference: Grigoris Panotopoulos, Angel Rincon, Ilıdio Lopes, “Slowly rotating dark energy stars”, Arxiv, 2021.
arXiv:2109.05619


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Whats The Importance Of Black Hole Repositioning? (Cosmology)

Based on a suite of smoothed particle hydrodynamics simulations with the swift code and a Bondi-Hoyle-Lyttleton subgrid gas accretion model, Yannick Bahe and colleagues investigated the impact of repositioning on SMBH growth and on other baryonic components through AGN feedback. They found that repositioning has direct physical consequences such as it promotes SMBH mergers and thus accelerates their initial growth. In addition, it raises the peak density of the ambient gas and reduces the SMBH velocity relative to it, giving a combined boost to the accretion rate that can reach many orders of magnitude. Their study recently appeared in Arxiv.

Energy feedback from active galactic nuclei (AGN) that are powered by supermassive black holes (SMBHs) at the centres of massive galaxies is an essential ingredient of galaxy formation simulations. The orbital evolution of SMBHs is affected by dynamical friction that cannot be predicted self-consistently by contemporary simulations of galaxy formation in representative volumes. Instead, such simulations typically use a simple “repositioning” of SMBHs, but the effects of this approach on SMBH and galaxy properties have not yet been investigated systematically.

Now, Yannick Bahe and colleagues investigated the impact of repositioning on SMBH growth by using hydrodynamical simulations and a Bondi-Hoyle-Lyttleton subgrid gas accretion model.

They showed that, across at least a factor ∼1000 in mass resolution, SMBH repositioning (or an equivalent approach) is a necessary prerequisite for AGN feedback; without it, black hole growth is negligible. They also showed that, limiting the effective repositioning speed to ≲ 10 km s¯1 delays the onset of AGN feedback and severely limits its impact on stellar mass growth in the centre of massive galaxies.

Projected SMBH positions (circles, colour-coded by 𝑚BH) are plotted over the corresponding gas density-temperature map (brightness representing surface density and hue temperature, increasing from pink to yellow) of the most massive 𝑧 = 0 halo (𝑀200c = 1.7 × 10¹³ M) in the 25 Mpc eagle-resolution simulations with the Default (left) and NoRepositioning (centre) models, respectively. © Yannick Bahe et al.

Finally, they shed light on three mechanisms through which repositioning affects SMBH growth, and hence AGN feedback. Firstly, it enables SMBH mergers – which lead to higher SMBH masses and hence increase the Bondi-Hoyle-Lyttleton accretion rate. Secondly, it moves SMBHs to regions of higher gas density, by up to several orders of magnitude. Thirdly, it (indirectly) slows SMBHs down by an order of magnitude with respect to their ambient gas.

“Our results suggest that a more sophisticated and/or better calibrated treatment of SMBH repositioning is a critical step towards more predictive galaxy formation simulations.”, they conclude.


Reference: Yannick M. Bahé, Joop Schaye, Matthieu Schaller, Richard G. Bower, Josh Borrow, Evgenii Chaikin, Folkert Nobels, Sylvia Ploeckinger, “The importance of black hole repositioning for galaxy formation simulations”, Arxiv, 2021. https://arxiv.org/abs/2109.01489


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What Is The Structure & Properties Of Accretion Disk Around A Rotating Black Hole Without Reflection Symmetry? (Cosmology)

By adopting a theory-agnostic approach and considering a recently proposed Kerr-like black hole model, Chen and Yang investigated the structure and properties of accretion disk around a rotating black hole without reflection symmetry. They showed that, in the absence of reflection symmetry, the accretion disk is curved surface in shape. Furthermore, they found that the parameter ϵ would shrink the size of the innermost stable circular orbits (ISCO), and enhance the efficiency of the black hole in converting rest-mass energy to radiation during accretion. Their study recently appeared in Arxiv.

Rotating black holes without equatorial reflection symmetry i.e. Z2 symmetry, can naturally arise in effective low-energy theories of fundamental quantum gravity, in particular, when parity-violating interactions are introduced. Due to the complexity of the theories, the rotating black hole solutions do not have analytic expressions and they can only be studied using numerical or perturbative approaches.

Now, Chen and Yang adopted a theory-agnostic approach and considered a relatively simple Kerr-like black hole model to investigate the astrophysical implications of Z2 asymmetry.

“The Kerr-like metric we considered is relatively simple in that its geodesic equations are designed to be completely separable. Therefore, the metric can be a good approximation of those complicated solutions in effective theories and can be very useful in studying the astrophysical implications of reflection asymmetry in a phenomenological manner.”

At first, they investigated the properties of accretion disk around the Kerr-like black hole and found that, in the absence of reflection symmetry, the accretion disk is curved surface in shape, rather than a flat disk lying on the equatorial plane.

They also explored the astrophysical implications of Z2 asymmetry on the accretion disk properties around a Kerr-like black hole. In particular, they found that, in a toy model with a specific choice of the deviation function, the parameter ϵ would shrink the size of the innermost stable circular orbits (ISCO), and enhance the efficiency of the black hole in converting rest-mass energy to radiation during accretion.

Finally, they investigated the gravitational redshift effect and computed the g-factor associated with the emission coming from the ISCO in the Kerr-like spacetime. They suggested that the spin measurements based on the redshift g-factor observations should be analyzed with great care and assuming the Kerr hypothesis, such measurements could overestimate the true spin value of the black hole if the black hole is actually the Kerr-like one with a large deviation parameter |ϵ| (at a level of ∼ 16% if |ϵ| ∼ 17).

“There are other important observables of the accretion disk that we have not explored in this study, such as the disk temperature and luminosity. In the cases in which the spacetime possesses equatorial reflection symmetry, one can adopt the thin-disk model and the calculations of these observables can be quite straightforward. However, in the absence of reflection symmetry, the disk is a curved surface and one has to rebuild the corresponding curved thin-disk model. This is beyond the scope of the present paper. In addition, it will be interesting to investigate the detailed motions of particles after they cross the ISCO and enter the plunging phase. All the above explorations would give further insights into the fundamental differences between Kerr and Kerr-like black holes, and possibly their observable signatures. We will leave these interesting topics to future works.”, they conclude.


Reference: Che-Yu Chen, Hsiang-Yi Karen Yang, “Curved accretion disks around rotating black holes without reflection symmetry”, Arxiv, pp. 1-20, 2021. https://arxiv.org/abs/2109.00564


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Could 3-Form Wormholes Be Black Hole Mimickers? (Cosmology)

Mariam Bouhmadi-Lopez and colleagues have constructed a symmetric wormhole solution in General Relativity (GR), which is supported by a 3-form field with a potential that contains a quartic self-interaction term. They hint towards the possibility that, the 3-form wormholes could be potential black hole mimickers, as long as the coefficient of the quartic self-interaction term (Λ) is sufficiently large, precisely when NEC is weakly violated. Their study recently appeared in Arxiv.

General Relativity (GR) is a well-tested theory, so it would be interesting to find traversable wormhole solutions in GR with “physically reasonable” matter field to support the throat. Such a matter field should preferably possess a correct sign for its kinetic energy though necessarily still violate some energy conditions. It would be even better if such a matter field is in some sense natural (e.g., it can also be applied to explain cosmological puzzles such as the accelerated expansion of the Universe). One natural candidate is the 3-form field which is ubiquitous to string theory within a cosmological framework and beyond.

FIG. 1: (a) The embedding diagram of the wormhole spacetime supported by the 3-form field. The red curve indicates the wormhole throat. (b) This schematic plot shows the evolution of the 3-form field on the potential in the wormhole spacetime. The shaded regions represent the regions where the NEC is violated. © Mariam Bouhmadi-Lopez et al.

Now, Mariam Bouhmadi-Lopez and colleagues numerically constructed a symmetric wormhole solution in pure Einstein gravity supported by a massive 3-form field with a potential that contains a quartic self-interaction term.

They found that, the wormhole spacetimes have only a single throat and they are everywhere regular and asymptotically flat. Furthermore, their mass and throat circumference increase almost linearly as the coefficient of the quartic self-interaction term Λ increases.

The amount of violation of the null energy condition (NEC) is proportional to the magnitude of 3-form, thus the NEC is less violated as Λ increases, since the magnitude of 3-form decreases with Λ.

In addition, they have investigated the geodesic equations for null particles and timelike particles moving around the wormhole. It is found that the unstable photon sphere/orbit, on which photons can undergo circular motions around the wormhole, is exactly at the wormhole throat.

In addition, they have investigated the geodesics of particles moving around the wormhole and found that the unstable photon orbit is located at the throat. They also found that the wormhole can cast a shadow whose apparent size is smaller than that cast by the Schwarzschild black hole, but reduces to it when Λ acquires a large value.

Moreover, they also discussed the behavior of the innermost stable circular orbit (ISCO) around this wormhole and found that the radius of ISCO deviates from the Schwarzschild counterpart when Λ is small, but reduces to it for a larger Λ. Thus, their wormholes can be a black hole mimicker when Λ is large, precisely when NEC is less violated.

“Of course, most astrophysical black holes rotate, so it remains to be seen if this mimicry still holds when rotation is considered.”

— authors of the study.

“Future investigation will look into the radial perturbation on the background metric and the form field to check stability, among other considerations. Indeed, the wormhole solutions supported by the complex phantom scalar field are found to be unstable against linear perturbations. It would be interesting to check whether our wormholes suffer from the same instability, and if so, for which range of Λ.”, they concluded.


Reference: Mariam Bouhmadi-López, Che-Yu Chen, Xiao Yan Chew, Yen Chin Ong, Dong-han Yeom, “Traversable Wormhole in Einstein 3-Form Theory With Self-Interacting Potential”, Arxiv, pp. 1-12, 2021.
arXiv:2108.07302


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Found A “Splinter” in the Milky Way (Cosmology)

Astronomers have identified a strange feature never before seen in our galaxy: a cluster of young stars and gas clouds protruding from Sagittarius’ arm like a splinter sticking out of a wooden plank. About three thousand light years long, it is the first large structure identified with an orientation so drastically different from that of the arm. All the details on A&A

You will know the saying Devil is in the details . Its origin is very ancient and is thought to derive from the form le bon Dieu est dans le détail , generally attributed to Gustave Flaubert. The sentence reminds us that the more you examine something, the more you look at the details, the more you appreciate its complexity and get closer to the truth. It also applies to its most negative meaning, the one that recalls the devil: something may seem simple at first or on the surface, but when you look at it in detail, a complexity can emerge that requires much more time and effort to be understood.

It seems that this is the case of the Milky Way , where scientists – studying its details with NASA’s Spitzer Space Telescope and ESA’s Gaia mission – have identified for the first time a strange feature: a “contingent” of young stars and clouds of gas protruding from one of its spiral arms, like a splinter sticking out of a wooden plank. About 3000 light years long , it is the first large structure identified with an orientation so drastically different from that of the arm.

Astronomers have a rough idea of ​​the size and shape of the Milky Way’s arms, but much remains unknown as it is not possible to see the entire structure as the Earth is inside it. It’s like trying to draw a map of Rome from inside the Colosseum. It is not possible to measure distances accurately enough to know if two buildings were on the same block or a few streets away… or to hope to see as far as the ring road, with so many buildings along the line of sight.

To learn more, the authors of the new study focused on a nearby portion of one of the galaxy’s arms, the Sagittarius Arm . Using the Spitzer Space Telescope – prior to its retirement in January 2020 – they searched for newborn stars, nestled in the gas and dust nebulae where they form. Spitzer detected infrared light that can penetrate those clouds, while visible light is blocked.

Young stars and nebulae are thought to be closely aligned with the shape of the arms in which they reside. To get a 3D view of the arm segment, the scientists used the latest data released by the Gaia mission to accurately measure the distances to the stars. The combined data revealed that the long, thin structure associated with Sagittarius’ arm is made up of young stars moving at almost the same speed and in the same direction through space.

“A key property of spiral arms is how tightly they wrap around the galaxy,” says Michael Kuhn , Caltech astrophysicist and first author of the study. This characteristic is measured by the pitch angle of the arm, the angle between the tangent to the spiral arm at a point and the tangent to the circle passing through the same point. A circle has a pitch angle of 0 degrees and as the spiral becomes more open, the pitch angle increases. “Most of the models of the Milky Way suggest that the Sagittarius arm forms a spiral at an angle of pitchof about 12 degrees, but the structure we have examined stands out at an angle of almost 60 degrees ».

Similar structures – sometimes called spurs or feathers – have been found in the arms of other spiral galaxies. For decades, scientists have wondered if the spiral arms of our Milky Way also had them, and now one has been found.

The newly discovered feature contains four nebulae known for their breathtaking beauty: the Eagle Nebula (inside which are the famous Pillars of Creation ), the Omega Nebula , the Trifid Nebula and the Lagoon Nebula . In the 1950s, a team of astronomers made rough measurements of the distance of some of the stars in these nebulae and were able to deduce the existence of the Sagittarius arm. Their work provided some of the earliest evidence of our galaxy’s spiral structure.

The Aquila, Omega, Trifida and Laguna nebulae are shown from the left, taken by NASA’s Spitzer Space Telescope. These nebulae are part of a structure within the Sagittarius arm of the Milky Way that protrudes from the arm at a very high angle. Credits: Nasa / Jpl-Caltech

“Distances are among the hardest things to measure in astronomy,” said co  author Alberto Krone-Martins , an astrophysicist and professor of computer science at the University of California, Irvine and a member of the Gaia Data Processing and Analysis Consortium (Dpac). “It’s only Gaia’s recent direct distance measurements that make the geometry of this new structure so obvious.”

In the new study, the researchers also drew on a catalog of more than 100,000 newborn stars discovered by Spitzer in an investigation of the galaxy called the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (Glimpse). “When we put together the data from Gaia and Spitzer and finally see this detailed three-dimensional map, we can see that this region is more complex than previously,” reports Kuhn.

Astronomers have not yet fully understood what causes the spiral arms to form in galaxies like ours. While we can’t see the complete structure of the Milky Way, the ability to measure the motion of individual stars is useful for understanding this phenomenon: the stars in the newly discovered structure probably formed around the same time, in the same area, and were affected. by the forces acting within the galaxy, including gravity and the shear due to the rotation of the galaxy.

“Ultimately, this reminds us that there are many uncertainties about the large-scale structure of the Milky Way, and we need to look at the details if we are to understand that bigger picture,” concludes one of the study’s co-authors, Robert Benjamin , an astrophysicist at the University of Wisconsin-Whitewater and principal investigator of the Glimpse survey. “This structure is a small piece of the Milky Way, but it could tell us something significant about the Galaxy as a whole.”

Featured image: A cluster of stars and star-forming clouds have been found protruding from the Sagittarius arm of the Milky Way. The inset shows the size of the structure and the distance from the Sun. Each orange star indicates star-forming regions that can contain dozens to thousands of stars. Credits: Nasa / Jpl-Caltech


To know more:

  • Read on Astronomy & Astrophysics the article ” A high pitch angle structure in the Sagittarius Arm ” by A. Kuhn, RA Benjamin, C. Zucker, A. Krone-Martins, RS de Souza, A. Castro-Ginard, EEO Ishida, MS Povich and LA Hillenbrand for the COIN Collaboration

Provided by INAF

Black Hole Size Revealed by its Eating Pattern (Cosmology)

The feeding patterns of black holes offer insight into their size, researchers report. A new study revealed that the flickering in the brightness observed in actively feeding supermassive black holes is related to their mass.

Supermassive black holes are millions to billions of times more massive than the sun and usually reside at the center of massive galaxies. When dormant and not feeding on the gas and stars surrounding them, SMBHs emit very little light; the only way astronomers can detect them is through their gravitational influences on stars and gas in their vicinity. However, in the early universe, when SMBHs were rapidly growing, they were actively feeding – or accreting – materials at intensive rates and emitting an enormous amount of radiation – sometimes outshining the entire galaxy in which they reside, the researchers said.  

Portrait of astronomy professor Yue Shen, who led the study.
Illinois Astronomy professor Yue Shen. Photo by L. Brian Stauffer

The new study, led by the University of Illinois Urbana-Champaign astronomy graduate student Colin Burke and professor Yue Shen, uncovered a definitive relationship between the mass of actively feeding SMBHs and the characteristic timescale in the light-flickering pattern. The findings are published in the journal Science.

The observed light from an accreting SMBH is not constant. Due to physical processes that are not yet understood, it displays a ubiquitous flickering over timescales ranging from hours to decades. “There have been many studies that explored possible relations of the observed flickering and the mass of the SMBH, but the results have been inconclusive and sometimes controversial,” Burke said.

The team compiled a large data set of actively feeding SMBHs to study the variability pattern of flickering. They identified a characteristic timescale, over which the pattern changes, that tightly correlates with the mass of the SMBH. The researchers then compared the results with accreting white dwarfs, the remnants of stars like our sun, and found that the same timescale-mass relation holds, even though white dwarfs are millions to billions times less massive than SMBHs.

The light flickers are random fluctuations in a black hole’s feeding process, the researchers said. Astronomers can quantify this flickering pattern by measuring the power of the variability as a function of timescales. For accreting SMBHs, the variability pattern changes from short timescales to long timescales. This transition of variability pattern happens at a characteristic timescale that is longer for more massive black holes.

The team compared black hole feeding to our eating or drinking activity by equating this transition to a human belch. Babies frequently burp while drinking milk, while adults can hold in the burp for a more extended amount of time. Black holes kind of do the same thing while feeding, they said.

Illinois Astronomy professor Yue Shen.
Photo by L. Brian Stauffer

“These results suggest that the processes driving the flickering during accretion are universal, whether the central object is a supermassive black hole or a much more lightweight white dwarf,” Shen said.

“The firm establishment of a connection between the observed light flicker and fundamental properties of the accretor will certainly help us better understand accretion processes,” said Yan-Fei Jiang, a researcher at the Flatiron Institute and study co-author.

Astrophysical black holes come in a broad spectrum of mass and size. In between the population of stellar-mass black holes, which weigh less than several tens of times the mass of the sun, and SMBHs, there is a population of black holes called intermediate-mass black holes that weigh between about 100 and 100,000 times the mass of the sun.

IMBHs are expected to form in large numbers through the history of the universe, and they may provide the seeds necessary to grow into SMBHs later. However, observationally this population of IMBHs is surprisingly elusive. There is only one indisputably confirmed IMBH that weighs about 150 times the mass of the sun. But that IMBH was serendipitously discovered by the gravitational wave radiation from the coalescence of two less-massive black holes.

“Now that there is a correlation between the flickering pattern and the mass of the central accreting object, we can use it to predict what the flickering signal from an IMBH might look like,” Burke said.

Astronomers worldwide are waiting for the official kickoff of an era of massive surveys that monitor the dynamic and variable sky. The Vera C. Rubin Observatory in Chile’s Legacy Survey of Space and Time will survey the sky over a decade and collect light flickering data for billions of objects, starting in late 2023.

“Mining the LSST data set to search for flickering patterns that are consistent with accreting IMBHs has the potential to discover and fully understand this long-sought mysterious population of black holes,” said co-author Xin Liu, an astronomy professor at the U. of I.

This study is a collaboration with astronomy and physics professor Charles Gammie and astronomy postdoctoral researcher Qian Yang, the Illinois Center for Advanced Study of the Universe, and researchers at the University of California, Santa Barbara; the University of St. Andrews, U.K.; the Flatiron Institute; the University of Southampton, U.K.; the United States Naval Academy; and Durham University, U.K.

Burke, Shen and Liu also are affiliated with the Center for Astrophysical Surveys at the National Center for Supercomputing Applications at Illinois.

The National Science Foundation, the Science and Technology Facilities Council and the Illinois Graduate Survey Science Fellowship supported this research.

Featured image: An artist’s impression of an accretion disk rotating around an unseen supermassive black hole. The accretion process produces random fluctuations in luminosity from the disk over time, a pattern found to be related to the mass of the black hole in a new study led by University of Illinois Urbana-Champaign researchers. Graphic courtesy Mark A. Garlick/Simons Foundation


Editor’s notes:

The paper “A characteristic optical variability timescale in astrophysical accretion disks” is available online and from the U. of I. News Bureau. DOI: 10.1126/science.abg9933.


Provided by University of Illinois

How Many Types Of Singularities Exist In The Gravitational Waves? (Cosmology)

Yu-Zhu Chen and colleagues discussed gravitational waves with the exact cylindrical gravitational wave solutions. They showed that, there are two kinds of singularities in gravitational waves: source singularity and resonance singularity. Their study recently appeared in the Journal Symmetry.

In the weak field or say, linear approximation, gravitational waves are regarded as linear waves, which ignores the spacetime singularities. Most results about gravitational waves are deduced in this approximation, such as the gravitational quadrupole radiation, the resonance between the gravitational wave and the detector, and the linear superposition of two gravitational waves. But, there’s one another interesting theory called nonlinear theory—exact wave solutions of the Einstein equation. When you consider this theory, some new properties of gravitational waves come into sight.

Now, Yu-Zhu Chen and colleagues discussed gravitational waves with the exact cylindrical gravitational wave solutions rather than gravitational wave solutions in the linear approximation.

“Our paper is motivated by problems such as, the behavior of singularities in gravitational wave solutions and the new physical effects of gravitational wave solutions in addition to, e.g., the reflexion and the transmission.”

— they wrote.

Based on the exact solution, they analyzed singularities in gravitational waves. They showed that there are two kinds of singularities in gravitational waves.

The first kind of singularities lies at a fixed spatial position which corresponds to a source. They called it the “source singularity”.

While, by considering a cylindrical gravitational wave as a complete solution, they showed that singularities in cylindrical gravitational waves carry the information about the source. The second kind of singularities arise as time proceeds to infinity. They recognized this singularity as a resonance and called it the “resonance singularity”.

Unlike other researchers, who considered a resonance between gravitational radiation and the matter (especially the gravitational radiation detectors), Yu-Zhu Chen and colleagues suggested that a gravitational wave resonates with other gravitational waves. They mentioned that the resonance singularity only emerges when a gravitational wave with a source singularity and a gravitational wave without a source singularity possess the same frequency. Two gravitational waves with source singularities or two gravitational waves without source singularities do not resonate. The resonance also indicates that the gravitational wave with sources and the gravitational wave without sources are two of different kinds.

“We suppose that the resonance between gravitational waves is irrelevant to the symmetry of the system. In recent years, gravitational wave detection has produced rapid progress. We expect that the resonance between gravitational waves will be found in the future.”

— they wrote.

Moreover, they investigated the interference of two gravitational waves. They showed, how the interference terms of two cylindrical gravitational waves behave. Interference appears in both the metric and the energy-momentum tensor. Specifically, they showed that the interference term in the source vanishes in the sense of time-averaging.

“A gravitational wave with a source should be regarded as a gravitational radiation. Gravitational radiations will result in the energy loss of the source. With the conservation law of the energy, we may define the energy of the cylindrical gravitational radiation in our framework. We can also consider the resonance between matter waves and gravitational waves based on our previous works on scattering.”

— they concluded.

Featured image credit: Getty Images


Reference: Chen, Y.-Z.; Li, S.-L.; Chen, Y.-J.; Dai, W.-S. Cylindrical Gravitational Wave: Source and Resonance. Symmetry 2021, 13, 1425. https://doi.org/10.3390/sym13081425


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How Primordial Blackholes Forms From The Gauss-Bonnet-Corrected Single Field Inflation? (Cosmology)

Countless number of primordial blackholes formation scenarios have been already discussed by us on our website. Now, Shinsuke Kawai and Jinsu Kim proposed another interesting mechanism for the primordial blackhole formation. They considered a model in which a scalar field is coupled to the Gauss-Bonnet term, and showed that primordial blackholes may be seeded when a scalar potential term and the Gauss-Bonnet coupling term are nearly balanced. Large curvature perturbation in this model not only leads to the production of primordial blackholes but it also sources gravitational waves at the second order. Their study recently appeared in Arxiv.

Cosmic inflation provides a natural framework for the production of primordial blackholes. Single field inflation is capable of generating large primordial curvature perturbation in small scales compared to the scale of the cosmic microwave background. In the single field inflation models for which the primordial blackhole production and the secondary gravitational waves are studied, the gravity sector is usually assumed to be the Einstein gravity. The Einstein gravity however is by no means a complete theory. From the effective field theory viewpoint, for example, higher curvature terms are expected to arise. One such higher curvature term is the Gauss-Bonnet term,

which leads to a relatively well-behaved theory of higher curvature gravity.

Previously, Shinsuke Kawai and Jinsu Kim investigated a model in which a scalar field ϕ is coupled to the Gauss-Bonnet term and discussed the features of a de Sitter-like fixed point as an alternative to cosmic inflation; in the presence of the Gauss-Bonnet coupling term there may exist a nontrivial de Sitter-like fixed point where the scalar potential term is balanced with the higher curvature Gauss-Bonnet term. Near the nontrivial fixed point, the standard slow-roll approximation is invalid and the ultra-slow-roll regime of inflation naturally arises. Furthermore, they pointed out that the primordial curvature power spectrum may become enhanced near the nontrivial de Sitter-like fixed point, which potentially leads to production of primordial blackholes.

Now, they investigated the production of primordial blackholes and the scalar-induced second-order gravitational waves in such a setup.

FIG. 1. The curvature power spectrum is shown for their two benchmark parameter sets. The enhancement is observed as the inflaton enters the ultra-slow-roll regime near the non-trivial fixed point. Here k∗ = 0.05 Mpc¯1 © Kawai and Kim

By considering two benchmark parameter sets they showed that, a large enhancement occurs in the curvature power spectrum by numerically solving the equations of motion.

A mode with large enhancement of the curvature perturbation may experience gravitational collapse when reentering the horizon, thereby producing primordial blackholes. For their two benchmark sets, they computed the present abundance of primordial blackholes. One set accounts for the totality of the dark matter relic density today, while in the other case primordial blackholes constitute only a portion of the present dark matter relic abundance.

FIG. 2. The density parameter of the scalar-induced second-order gravitational waves is shown for their two benchmark sets. The gravitational wave signal of Set 1 is well within the reach of the sensitivity bound of future experiments such as LISA, DECIGO, and BBO. In the case of Set 2, the signal crosses the sensitivity bound of SKA as well. © Kawai and Kim

A large curvature perturbation that leads to the production of primordial blackholes inevitably source the scalar-induced second-order gravitational waves. They also obtained the present density parameter of the gravitational waves by utilizing the approximated analytical expression together with their numerical results of the curvature power spectrum. Both of their two benchmark sets are found to be within the sensitivity bounds of future gravitational wave experiments such as LISA, DECIGO, BBO, and SKA.

“While we focused on the scalar potential of the natural inflation model and assumed a smeared step function for the Gauss-Bonnet coupling function in this work, some of the features that we have found are generic. When there is a balance between a scalar potential term and a Gauss-Bonnet coupling term, a nontrivial fixed point may exist. Near the nontrivial fixed point the ultra-slow-roll inflation generically occurs, during which period a large enhancement of the curvature perturbation is guaranteed. We thus expect that the production of primordial blackholes and the secondary gravitational wave signals are natural in higher curvature gravity theories.”

— they concluded.

Reference: Shinsuke Kawai, Jinsu Kim, “Primordial blackholes from Gauss-Bonnet-corrected single field inflation”, Arxiv, pp. 1-9, 2021. https://arxiv.org/abs/2108.01340


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