Tag Archives: #blackholes

Hierarchical Mergers Of Black Holes (Cosmology)

While most of the fusion events between extreme objects detected by Ligo and Virgo are produced by so-called “first generation” black holes formed by the collapse of stars, others could instead be second (or third) generation, in which one of the that melts already comes from one (or more) previous fusion of black holes. An article published today in Nature Astronomy reviews all the theoretical results, models and events of gravitational waves detected and coming from hierarchical mergers of stellar-mass black holes

Generations in comparison. Generations of black holes, which merge into a single entity producing space-time “vibrations”, detected on Earth thanks to the Ligo and Virgo interferometers . And if the first generation comes from two black holes formed by the collapse of massive stars at the end of their lives, it is possible that history will repeat itself, and the resulting black hole fuses again with a similar object, giving rise to a black hole of second generation. To recognize the exact position of these extreme stellar objects in the family tree of black holes, it is necessary to observe the imprint that mergers leave on gravitational waves. And that’s not all, because in reality such black holes may have already been observed. This is what we read in an articlereview published today in Nature Astronomy by David Gerosa , young astrophysicist at the University of Birmingham close to return to Italy with a one-way ticket signed ERC , which will see him in the role of associate professor at the University Milano-Bicocca. Gerosa deals with the astronomy of gravitational waves, both studying the dynamics of the sources (the binary pairs of black holes, in fact) from a theoretical point of view, statistically analyzing the data.

Your review talks about second or third generation black holes. Are there statistical estimates on how many such objects may exist, on what are the probabilities of forming them and, among these, how many are actually observable?

«To date, Ligo and Virgo have observed about 50 events. One of these ( Gw 190521 ) has characteristics (in particular the mass) typical of a second generation event. Based on the observed sample alone, a rough estimate of the incidence rate is therefore 1/50. However, we must take into account the observational bias – events involving high masses are easier to observe – and how different astrophysical environments can assemble successive generations of mergers . It is a very open problem, and I hope this transpires from our review ».

Are these newly formed objects, to be looked for in the nearby universe?

“Not necessarily, but with current equipment we are sensitive only to events that occur at redshifts less than 1″

What differentiates them from black holes that come from a first generation merger?

«The mass involved, first of all, which is higher for second or third generation events. This is particularly interesting because stellar evolution models predict an upper limit on the mass of black holes that form from the collapse of massive stars, which is about 50 times the mass of the Sun. If Ligo or Virgo measure a higher mass event, a different training scenario must be involved. We talked about this idea in a 2017 article  (and also presented in another independent article released the same day) and the 2019 event seems to have confirmed it. The mass, alone, however, can also have other origins ».

What else, then?

“The production of black holes of a generation other than the first also has a very characteristic effect on spin. It is a relativistic phenomenon called orbital hang up which regulates the number of orbits performed by the binary as the spin varies: hierarchical black holes typically have spin around the characteristic value 0.7 (in dimensionless units in which the spin varies between 0 and 1) ” .

Being a signal generated by massive objects, is it easier to detect with current detectors?

“And how. In fact, we tend to have already seen one, Gw 190521. The signal is easier to identify for more massive objects. Care must be taken that they are not too massive, otherwise the signal goes out of the band sampled by the detectors, even if these objects are intrinsically few ».

Are there any observed cases that have dubious characteristics and could belong to this category?

«In the case of Gw 190521, as we have said, the main interpretation is that it is a second generation merger . But it is not the only one . Another second generation event, albeit a little more uncertain, could be Gw 190412 ».

How come?

“Because the mass involved is lower, and in this case the ratio between the masses and the spin suggest that it is a mixed merger in which one black hole is second generation and the other is the first. These two data – relationship between masses and spin – are however “weaker” indicators: forming such an event using “normal” theoretical models is rare but not impossible ».

How to definitively confirm that the event derives from a second or third generation merging ?

“Certainty in science builds up slowly. With more events (we expect thousands within a few years) it will become clear whether a subpopulation of next-generation black holes is needed to explain the data. ‘

Featured image: Davide Gerosa, astrophysicist at the University of Birmingham and first author of the review article on generations of black holes © INAF


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Provided by INAF

How To Detect Direct Collapse Black Holes? (Cosmology)

B. Yue & A. Ferrara explored the possibility of detection of the continuum radio signal from direct collapse black holes (DCBHs) by upcoming radio telescopes such as the SKA and ngVLA. They showed that, if the jet power 𝑃jet ≳ 1042¯43  erg s¯1, it can be detectable by SKA/ngVLA, depending on the jet inclination angle. Their study recently appeared in the Journal Monthly Notices of the Royal Astronomical Society.

Direct collapse black holes (DCBHs) are high-mass black hole seeds, putatively formed within the redshift range (15 < z < 30), when the Universe was about 100-250 million years old. Unlike seeds formed from the first population of stars (also known as Population III stars), direct collapse black hole seeds are formed by a direct, general relativistic instability. They are very massive, with a typical mass at formation of ~ 105 M. DCBHs have not been detected until now, either because they are too faint and/or too rare. There are many promising detections techniques have been proposed for their detection such as the cosmic infrared background, multi-color sampling of the Spectral Energy Distribution (SED) combined with X-ray surveys, a specific Ly𝛼 signature, and the neutral hydrogen 𝜆 = 3 cm maser line.

Now, by assuming that the DCBHs can launch and sustain powerful jets at the accretion stage after formation, B. Yue & A. Ferrara explored the possibility of detection of the continuum radio signal from direct collapse black holes (DCBHs) by upcoming radio telescopes such as the SKA and ngVLA.

“DCBHs may also produce radio signal during the collapse stage. However this stage is shorter therefore the detection probability is smaller. Our model supposes that DCBHs can launch powerful jets similar to those observed in the radio-loud AGNs.”

— they said.

They applied the jet properties of the observed blazars to the jetted high-𝑧 DCBHs, and used the publicly available GT09 jet model to predict the radio signal from the jet.

They showed that, if the jet power 𝑃jet ≳ 1042¯43  erg s¯1, it can be detectable by SKA/ngVLA with 100 integration hours, depending on the jet inclination angle. However, as the jet power depends on both black hole mass and spin, the DCBHs with mass ≳ 105 𝑀 can be detected. But, less than this value (i.e. 105 𝑀) it can be hard to detect DCBHs with SKA.

The probability to observe a DCBH with flux > 𝑆 at 17.09 GHz. In each row, for left to right the panels correspond to Γ = 5.0, 10.0 and 20.0 respectively. For each panel, the 𝐵 and 𝑟blob of each curve are given in parentheses in the right panel. The dashed and dashed-dotted lines are probabilities after marginalizing the 𝐵, , and 𝑟blob for d1 distribution model and d2 distribution model respectively. In each panel vertical lines refer to sensitivities of SKA2-mid (left), ngVLA (middle) and SKA1-mid (right) respectively. © Yue and Ferrara

Additionally, by considering the spin distribution they showed that, about 10¯3 of DCBHs with MBH = 106 M, if they are all jetted, are detectable. Moreover, if all DCBHs are jetted, for the most optimal case ~ 100 deg¯2𝑧¯1 at z = 10 would be detected by SKA1-mid with 100 hours integration time.

Finally, it has been suggested that, if the jet “blob” emitting most of the radio signal is dense (𝑟blob ∼ 50 𝑟s) and highly relativistic (i.e. has large bulk motion velocity (ν ∼ 10)), then the DCBH would only feebly emit in the SKA-low band, because of self-synchrotron absorption (SSA) and blueshift. Moreover, the free-free absorption in the DCBH envelope may further reduce the signal in the SKA-low band. Thus, combining SKA-low and SKA-mid observations might provide a potential tool to distinguish a DCBH from a normal star-forming galaxy.

However, their study contains some uncertainties such as it is not clear how many of DCBHs can launch jets, number density of the DCBHs and their mass function etc.

“Future theoretical investigations, for example the numerical simulations, would help to improve our predictions. However, the uncertainties could only be reduced by observations themselves. For example, if the radio signal from DCBHs is much lower than our predictions, then it is likely that most DCBH cannot launch strong jet. This will force us to investigate the difference between DCBHs and low-𝑧 black holes, for example on the magnetic field, the density of the envelope, or the spin.”

— they concluded.

Featured image: Observed radio flux from a jetted DCBH, for different jet powers, Lorentz factors and inclination angles, as indicated in the label. Sensitivities of different telescopes are given. For all sensitivities the integration time is 100 hours, and bandwidth is given in the text © B. Yue & A. Ferrara


Reference: B Yue, A Ferrara, Radio signals from early direct collapse black holes, Monthly Notices of the Royal Astronomical Society, 2021;, stab2121, https://doi.org/10.1093/mnras/stab2121


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Supermassive Black Holes Put A Brake on Stellar Births (Cosmology)

Black holes with masses equivalent to millions of suns do put a brake on the birth of new stars, say astronomers. Using machine learning and three state of the art simulations to back up results from a large sky survey, the researchers resolve a 20-year long debate on the formation of stars. Joanna Piotrowska, a PhD student at the University of Cambridge, will present the new work today (Tuesday 20 July) at the virtual National Astronomy Meeting (NAM 2021).

Star formation in galaxies has long been a focal point of astronomy research. Decades of successful observations and theoretical modelling resulted in our good understanding of how gas collapses to form new stars both in and beyond our own Milky Way. However, thanks to all-sky observing programmes like the Sloan Digital Sky Survey (SDSS), astronomers realised that not all galaxies in the local Universe are actively star-forming – there exists an abundant population of “quiescent” objects which form stars at significantly lower rates.

The question of what stops star formation in galaxies remains the biggest unknown in our understanding of galaxy evolution, debated over the past 20 years. Piotrowska and her team set up an experiment to find out what might be responsible.

Diagram showing the relative importance of supermassive black holes, supernova explosions, and dark matter haloes, in shutting down star formation in galaxies. © Joanna Piotrowska

Using three state-of-the-art cosmological simulations – EAGLE, Illustris and IllustrisTNG – the astronomers investigated what we would expect to see in the real Universe as observed by the SDSS, when different physical processes were halting star formation in massive galaxies.

The astronomers applied a machine learning algorithm to classify galaxies into star-forming and quiescent, asking which of three parameters: the mass of the supermassive black holes found at the centre of galaxies (these monster objects have typically millions or even billions of times the mass of our Sun), the total mass of stars in the galaxy, or the mass of the dark matter halo around galaxies, best predicts how galaxies turn out.

These parameters then enabled the team to work out which physical process: energy injection by supermassive black holes, supernova explosions or shock heating of gas in massive halos is responsible for forcing galaxies into semi-retirement.

The new simulations predict the supermassive black hole mass as the most important factor in putting the brakes on star formation. Crucially, the simulation results match observations of the local Universe, adding weight to the researchers’ findings.

Piotrowska says: “It’s really exciting to see how the simulations predict exactly what we see in the real Universe. Supermassive black holes – objects with masses equivalent to millions or even billions of Suns – really do have a big effect on their surroundings. These monster objects force their host galaxies into a kind of semi-retirement from star formation.”

Video: University of Cambridge PhD student Joanna Piotrowska explains how supermassive black holes shut down star formation and put galaxies into semi-retirement. Credit: Joanna Piotrowska, with music from Transition by StreamBeats.

Featured image: A Hubble Space Telescope image of the quiescent elliptical galaxy NGC 4150. In this star formation has essentially shut down. © NASA, ESA, R.M. Crockett (University of Oxford, U.K.), S. Kaviraj (Imperial College London and University of Oxford, U.K.), J. Silk (University of Oxford), M. Mutchler (Space Telescope Science Institute, Baltimore), R. O’Connell (University of Virginia, Charlottesville), and the WFC3 Scientific Oversight Committee


Provided by Royal Astronomical Society

How Primordial Black Holes Production Take Place In Alpha-attractor Galileon Inflationary Scenario? (Cosmology)

As we have already seen before, there are several inflation models which discuss the possibility of the production of primordial black holes in inflation process of the early universe. But, the production of PBHs requires the violation of slow-roll condition. For this reason, slow-roll violating models become an interesting alternative, including ultra-slow-roll (USR) inflation, inflation with inflection points or bumps and others. In this scenario, the inflaton field experiences a very flat potential in the inflection point region of the potential. Such a flat region results in a so-called ultra slow-roll phase in which the inflaton velocity decreases with a faster rate than the slow-roll phase and inflaton has a friction dominated phase. Consequently, the curvature perturbation grows rapidly due to the great decrease of the Hubble slow-roll parameter.

Now, Zeinab Teimoori and colleagues proposed a novel mechanism yo achieve the ultra-roll inflation. In particular, they carried out study on the process of the Primordial Black Holes (PBHs) production in the novel framework, namely α-attractor Galileon inflation (G-inflation) model.

“The most important motivation for choosing the Galileon scalar field theory is that, the field equations driven from this theory include derivatives only up to second order.”

— they said.

In their framework, they take the Galileon function as G(ϕ)=GI(ϕ)(1+GII(ϕ)), where the part GI(ϕ) is motivated from the α-attractor inflationary scenario in its original non-canonical frame, and it ensures for the model to be consistent with the Planck 2018 observations at the CMB scales. The part GII(ϕ) is invoked to enhance the curvature perturbations at some smaller scales which in turn gives rise to PBHs formation.

Figure 1: The curvature power spectra calculated by solving the Mukhanov-Sasaki equation numerically as a function of the comoving wavenumber k for Case 1 (solid), Case 2 (dashed) and Case 3 (dotted). The light green shaded region shows the area excluded by the CMB observations. The orange, blue, and cyan shaded regions represent the excluded regions for the power spectrum by the µ distortion of CMB, the effect on the ratio between neutron and proton during the big bang nucleosynthesis (BBN), and the current PTA observations, respectively © Zeinab Teimoori et al.

By fine tuning of the model parameters φc, ω, and σ, they found 3 parameters sets which successfully produced a sufficiently large peak in the curvature power spectrum. They showed that, these parameter sets produce PBHs with masses 12.99 M (for set 3), 1.76 × 10¯5 M (for set 2), and 8.6 × 10¯12 M (for set 1) which can explain the LIGO events, the ultrashort-timescale microlensing events in OGLE data, and around 0.98% of the current Dark Matter (DM) content of the universe, respectively.

Figure 2: The present fractional energy density of the secondary GWs versus frequency. The solid, dashed, and dotted plots are corresponding to Case 1, Case 2, and Case 3, respectively © Zeinab Teimoori et al.

Additionally, they studied the induction of the secondary gravitational waves (GWs) accompanied by the PBHs formation in their α-attractor G-inflation setup, and in particular they computed the present fractional energy density (ΩGW0) for the three parameter sets of their model. The spectrum of ΩGW0 exhibits a peak in its shape, and the peaks height for all the three cases is of order 10¯8, but their frequencies are different. The frequencies of the peaks for Cases 1, 2, and 3 are 2.953 × 10¯3 Hz, 8.017 × 10¯7 Hz, and 5.848 × 10¯10 Hz, respectively. The spectrum of ΩGW0 for Case 1 can be placed within the sensitivity region of detectors like LISA, TaiJi, and TianQin, and for Case 3 within the sensitivity regions of EPTA and SKA, while for Case 2, the spectrum is located completely outside of the sensitivity curves.

“Since the predictions of our α-attractor G-inflation model can lie inside the sensitivity regions of some GWs detectors, therefore the viability of our model can be tested in light of the forthcoming observational data.”

— they added

Finally, they estimated the tilt of the spectrum of secondary GWs in their setting for different regions of the frequency band. Their findings confirmed that the power spectrum of ΩGW0 can be parameterized in terms of frequency as the power-law function Ω_GW0 ∼ f^n. They calculated the values of the constant ‘n’ for different frequency bands for each case of their model, and showed that the results in the infrared regime f ≪ fc satisfy properly the analytical expression ΩGW0 ∼ f^[3–2/ln(fc/f).


Reference: Zeinab Teimoori, Kazem Rezazadeh, Mariwan Ahmed Rasheed, Kayoomars Karami, “Mechanism of primordial black holes production and secondary gravitational waves in α-attractor Galileon inflationary scenario”, Arxiv, pp. 1-31, 2021. https://arxiv.org/abs/2107.07620


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Jets Remain A Mystery (Cosmology)

FAU involved in DFG research group focusing on plasma rays from black holes.

A new DFG research group led by Julius-Maximilians-Universität Würzburg (JMU) is investigating ultra-high energy jets. Two astrophysicists from FAU are also involved. The research group will be funded with a total of 3.6 million euros over the next four years.

Black holes can be found in the centre of nearly all galaxies. They have an unimaginably large mass and attract material, gas and even light as a result. Just recently, astronomical images showing the accumulation of material on a super-massive black hole caused a stir among the general public.

Black holes like this can release immense energy out into their surroundings, energy which was originally stored in their rotation or the potential energy of collected material. Energy is released in jets. Jets are bundles of plasma rays which accelerate particles to tremendous energies before propelling them from the centre of the galaxy at speeds nearing the speed of light. Such jets can reach several hundred thousand light years into space and radiate bright radio, gamma and X-rays.

A number of mysteries remain

However, these jets still baffle researchers. What are they made of? How are they launched from the direct vicinity of supermassive black holes? Which processes are responsible for their high-energy radiation and how do they interact with the mother galaxy? These are the questions the new DFG research group ‘Relativistic Jets in Active Galaxies’ hopes to answer.

The German Research Foundation (DFG) is funding the project with 3.6 million euros over the next four years, with the option to continue into a second phase of funding lasting another four years. The speaker of the group is the astrophysicist Professor Dr. Matthias Kadler from JMU. Dr. Thomas Dauser and Prof. Dr. Jörn Wilms from Dr. Karl Remeis observatory, the Astronomical Institute at FAU, are also involved. Further projects are located at the universities of Hamburg and Heidelberg, the Leibnitz Institute for Astrophysics Potsdam and the Max Planck Institutes for Astronomy and Radio Astronomy in Heidelberg and Bonn.

Improving the combination of observations and modelling

The researchers have set themselves the ambitious goal of designing a model of the jets that can explain the physics behind the phenomenon and that fits in with all observations. In order to do so, they plan to overcome the traditional split between various scientific approaches to the problem, for example by coordinating observations and theoretical modelling to a much greater extent than is currently the case.

‘Impressive breakthroughs in observational astronomy and astroparticle physics in recent years have shifted the focus of modern research onto jets even more than before,’ explains Prof. Dr. Matthias Kadler. ‘At the same time, theoretical and numerical modelling have made enormous advances.’ Our research group is the first to combine these approaches in this way and to such an extent.’

Dr. Thomas Dauser explains, ‘In the FAU sub-project we will combine measurements in the X-ray range with satellites and measurements in the radio range in order to study the structure of the region directly around the black hole, in which jets are created.’

Prof. Wilms emphasises: ‘In this project, the research group effectively combines the existing expertise of the universities in Erlangen-Nuremberg and Würzburg when it comes to investigating black holes and their jets in the entire electromagnetic spectrum.’

Funding focused on supporting young researchers

DFG research groups are intended to enable researchers to focus on current and current questions in their field of research and to establish innovative areas of research. The funds provided by DFG ought to be largely used to create project positions for young researchers.

Featured image: Visualisation of the holistic approach taken by the research group: Observations (right) and theoretical modelling (left) of jets are combined at the smallest and largest scales. (Source: Matthias Kadler (JMU); based on individual images by C. Fromm (JMU), A. Baczko (MPIfR), R. Perley and W. Cotton (NRAO/AUI/NSF))


Provided by FAU

Danish Student Solves How The Universe is Reflected Near Black Holes (Cosmology)

In the vicinity of black holes, space is so warped that even light rays may curve around them several times. This phenomenon may enable us to see multiple versions of the same thing. While this has been known for decades, only now do we have an exact, mathematical expression, thanks to Albert Sneppen, student at the Niels Bohr Institute. The result, which even is more useful in realistic black holes, has just been published in the journal Scientific Reports.

You have probably heard of black holes — the marvelous lumps of gravity from which not even light can escape. You may also have heard that space itself and even time behave oddly near black holes; space is warped.

In the vicinity of a black hole, space curves so much that light rays are deflected, and very nearby light can be deflected so much that it travels several times around the black hole. Hence, when we observe a distant background galaxy (or some other celestial body), we may be lucky to see the same image of the galaxy multiple times, albeit more and more distorted.

Galaxies in multiple versions

The mechanism is shown on the figure below: A distant galaxy shines in all directions — some of its light comes close to the black hole and is lightly deflected; some light comes even closer and circumvolves the hole a single time before escaping down to us, and so on. Looking near the black hole, we see more and more versions of the same galaxy, the closer to the edge of the hole we are looking.

Light from the background galaxy circles a black hole an increasing number of times, the closer it passes the hole, and we therefore see the same galaxy in several directions (credit: Peter Laursen).
Light from the background galaxy circles a black hole an increasing number of times, the closer it passes the hole, and we therefore see the same galaxy in several directions (credit: Peter Laursen).

How much closer to the black hole do you have to look from one image to see the next image? The result has been known for over 40 years, and is some 500 times (for the math aficionados, it is more accurately the “exponential function of two pi“, written e2π).

Calculating this is so complicated that, until recently, we had not yet developed a mathematical and physical intuition as to why it happens to be this exact factor. But using some clever, mathematical tricks, master’s student Albert Sneppen from the Cosmic Dawn Center — a basic research center under both the Niels Bohr Institute and DTU Space — has now succeeded in proving why.

There is something fantastically beautiful in now understanding why the images repeat themselves in such an elegant way. On top of that, it provides new opportunities to test our understanding of gravity and black holes,” Albert Sneppen clarifies.

Proving something mathematically is not only satisfying in itself; indeed, it brings us closer to an understanding of this marvelous phenomenon. The factor “500” follows directly from how black holes and gravity work, so the repetitions of the images now become a way to examine and test gravity.

Spinning black holes

As a completely new feature, Sneppen’s method can also be generalized to apply not only to “trivial” black holes, but also to black holes that rotate. Which, in fact, they all do.

The situation seen "face-on", i.e. how we would actually observe it from Earth. The extra images of the galaxy become increasingly squeezed and distorted, the closer we look at the black hole (credit: Peter Laursen).
The situation seen “face-on”, i.e. how we would actually observe it from Earth. The extra images of the galaxy become increasingly squeezed and distorted, the closer we look at the black hole (credit: Peter Laursen).

“It turns out that when the it rotates really fast, you no longer have to get closer to the black hole by a factor 500, but significantly less. In fact, each image is now only 50, or 5, or  even down to just 2 times closer to the edge of the black hole“, explains Albert Sneppen.

Having to look 500 times closer to the black hole for each new image, means that the images are quickly “squeezed” into one annular image, as seen in the figure on the right. In practice, the many images will be difficult to observe. But when black holes rotate, there is more room for the “extra” images, so we can hope to confirm the theory observationally in a not-too-distant future. In this way, we can learn about not just black holes, but also the galaxies behind them:

The travel time of the light increases, the more times it has to go around the black hole, so the images become increasingly “delayed”. If, for example, a star explodes as a supernova in a background galaxy, one would be able to see this explosion again and again.

Albert Sneppen’s article has just been accepted for publication in the journal Scientific Reports, and can be read here: Divergent reflections around the photon sphere of a black hole.

Featured image: A disk of glowing gas swirls into the black hole “Gargantua” from the movie Interstellar. Because space curves around the black hole, it is possible to look round its far side and see the part of the gas disk that would otherwise be hidden by the hole. Our understanding of this mechanism has now been increased by Danish master’s student at NBI, Albert Sneppen (credit: interstellar.wiki/CC BY-NC License).


Provided by University of Copenhagen

Discovered A Population of Large Black Holes in the Star Cluster Palomar 5 (Cosmology)

Palomar 5 is a unique star cluster. This is due, in the first place, to the fact that it is one of the “spongiest” in the Milky Way halo, with an average distance of a few light-years between the stars, comparable to the distance from the Sun to the brightest star. next. Second, because it is associated with a specular stellar current that covers more than 20 degrees in the sky. In an article published in Nature Astronomy , an international team of astronomers and astrophysicists led by the University of Barcelona shows that the two distinctive features of Palomar 5 are probably the result of a population of more than one hundred black holes in the center of the cluster.

“The number of black holes is about three times greater than would be expected from the number of stars in the cluster, which means that more than 20% of the total mass of the cluster is made up of black holes,” explains Mark Gieles . professor at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and lead author of the work. “Each has a mass twenty times the mass of the Sun,” says the expert, “and they formed in supernova explosions at the end of the life of massive stars, when the cluster was still very young.”

Tidal currents are associations of stars that were ejected from star clusters or dwarf galaxies. In recent years, about thirty narrow stellar currents have been discovered in the halo of the Milky Way. “We don’t know how these currents form, but one idea is that they are star clusters that have suffered some disturbance,” explains Gieles. However, none of the recently discovered currents have an associated star cluster, so researchers cannot be sure of this theory. To understand how these currents formed, one must study one with an associated stellar system: “Palomar 5 is the only case, and this makes it a kind of Rosetta Stone, which will allow us to understand the formation of stellar currents. · Lars », points out Gieles. “That’s why we studied it in detail,” he explains.

CCUB researcher Mark Gieles. 
Image: ICCUB

In this study, the authors simulated the orbits and evolution of each star from cluster formation to final dissolution. Their initial properties varied until they found that the observations of the current and the cluster matched. The astronomer team believes that Palomar 5 was formed from a smaller black hole fraction, but the stars were able to escape more efficiently than the black holes, so the black hole fraction increased gradually.

The black holes dynamically inflated the cluster through gravitational assistance interactions with the stars, which caused more stars to escape and the current to form. Just before it dissolves completely — about a billion years from now — the cluster will be completely made up of black holes. “This work has helped us to understand that, although the Palomar 5 cluster has the brightest and longest tails than any other cluster in the Milky Way, it is not unique. Instead, we believe that many similarly dominated black hole clusters have already disintegrated in the tides of the Milky Way to form the first newly discovered stellar currents, “said Denis Erkal , co-author of the study. , researcher at the University of Surrey (UK).

Gieles notes that the study shows “that the presence of a large population of black holes may have been common in all groups that formed the currents.” This is important for understanding the formation of globular clusters, initial star masses, and the evolution of massive stars. This work also has important implications for gravitational waves. “A large portion of binary black hole fusions are believed to form in star clusters. A big unknown in this scenario is the number of black holes in the clusters, which is difficult to delimit observationally because we can not observe the black holes, “says Fabio Antonini, Professor at Cardiff University (Wales, UK), and also co-author of the paper. “Our method provides a way to know how many black holes there are in a star cluster by looking at the stars that are ejected from it,” he said.

Palomar 5 is a globular cluster discovered in 1950 by Walter Baade. It is located in the constellation of the Serpent, at a distance of about 65,000 light-years, and is one of approximately 150 globular clusters orbiting the Milky Way. It is over ten billion years old, like most other globular clusters, meaning it formed in the early stages of galaxy formation. It is approximately ten times less massive and five times more extensive than a typical globular cluster and in the final stages of dissolution.

Simulation showing the formation of tidal currents in the Palomar 5 star cluster and the distribution of black holes. In yellow you see the stars and in black the black holes. More than 20% of the mass of Palomar 5 is made up of black holes.

Featured image: Map of the Milky Way plan obtained from data in the Gaia catalog (eDR3). At the top is a region where the star cluster of Palomar 5 and its tidal tails are observed (data obtained thanks to the DESI Legacy Imaging Survey, DECaLS). Image: E. Balbinot, Gaia, DECaLS-DESI


Reference article :

M. Gieles et al. “A supra-massive population of Stella-massblack holes in the globular cluster Palomar 5” . Nature Astronomy , July 5, 2021. DOI: 10.1038 / s41550-021-01392-2


Provided by University of Barcelona

Physicists Observationally Confirm Hawking’s Black Hole Theorem For The First Time (Cosmology)

Study offers evidence, based on gravitational waves, to show that the total area of a black hole’s event horizon can never decrease.

There are certain rules that even the most extreme objects in the universe must obey. A central law for black holes predicts that the area of their event horizons — the boundary beyond which nothing can ever escape — should never shrink. This law is Hawking’s area theorem, named after physicist Stephen Hawking, who derived the theorem in 1971.

Fifty years later, physicists at MIT and elsewhere have now confirmed Hawking’s area theorem for the first time, using observations of gravitational waves. Their results appear today in Physical Review Letters.

In the study, the researchers take a closer look at GW150914, the first gravitational wave signal detected by the Laser Interferometer Gravitational-wave Observatory (LIGO), in 2015. The signal was a product of two inspiraling black holes that generated a new black hole, along with a huge amount of energy that rippled across space-time as gravitational waves.

If Hawking’s area theorem holds, then the horizon area of the new black hole should not be smaller than the total horizon area of its parent black holes. In the new study, the physicists reanalyzed the signal from GW150914 before and after the cosmic collision and found that indeed, the total event horizon area did not decrease after the merger — a result that they report with 95 percent confidence.

Their findings mark the first direct observational confirmation of Hawking’s area theorem, which has been proven mathematically but never observed in nature until now. The team plans to test future gravitational-wave signals to see if they might further confirm Hawking’s theorem or be a sign of new, law-bending physics.

“It is possible that there’s a zoo of different compact objects, and while some of them are the black holes that follow Einstein and Hawking’s laws, others may be slightly different beasts,” says lead author Maximiliano Isi, a NASA Einstein Postdoctoral Fellow in MIT’s Kavli Institute for Astrophysics and Space Research. “So, it’s not like you do this test once and it’s over. You do this once, and it’s the beginning.”

Isi’s co-authors on the paper are Will Farr of Stony Brook University and the Flatiron Institute’s Center for Computational Astrophysics, Matthew Giesler of Cornell University, Mark Scheel of Caltech, and Saul Teukolsky of Cornell University and Caltech.

An age of insights

In 1971, Stephen Hawking proposed the area theorem, which set off a series of fundamental insights about black hole mechanics. The theorem predicts that the total area of a black hole’s event horizon — and all black holes in the universe, for that matter — should never decrease. The statement was a curious parallel of the second law of thermodynamics, which states that the entropy, or degree of disorder within an object, should also never decrease.

The similarity between the two theories suggested that black holes could behave as thermal, heat-emitting objects — a confounding proposition, as black holes by their very nature were thought to never let energy escape, or radiate. Hawking eventually squared the two ideas in 1974, showing that black holes could have entropy and emit radiation over very long timescales if their quantum effects were taken into account. This phenomenon was dubbed “Hawking radiation” and remains one of the most fundamental revelations about black holes.

“It all started with Hawking’s realization that the total horizon area in black holes can never go down,” Isi says. “The area law encapsulates a golden age in the ’70s where all these insights were being produced.”

Hawking and others have since shown that the area theorem works out mathematically, but there had been no way to check it against nature until LIGO’s first detection of gravitational waves.

Hawking, on hearing of the result, quickly contacted LIGO co-founder Kip Thorne, the Feynman Professor of Theoretical Physics at Caltech. His question: Could the detection confirm the area theorem?

At the time, researchers did not have the ability to pick out the necessary information within the signal, before and after the merger, to determine whether the final horizon area did not decrease, as Hawking’s theorem would assume. It wasn’t until several years later, and the development of a technique by Isi and his colleagues, when testing the area law became feasible.

Before and after

In 2019, Isi and his colleagues developed a technique to extract the reverberations immediately following GW150914’s peak — the moment when the two parent black holes collided to form a new black hole. The team used the technique to pick out specific frequencies, or tones of the otherwise noisy aftermath, that they could use to calculate the final black hole’s mass and spin.

A black hole’s mass and spin are directly related to the area of its event horizon, and Thorne, recalling Hawking’s query, approached them with a follow-up: Could they use the same technique to compare the signal before and after the merger, and confirm the area theorem?

The researchers took on the challenge, and again split the GW150914 signal at its peak. They developed a model to analyze the signal before the peak, corresponding to the two inspiraling black holes, and to identify the mass and spin of both black holes before they merged. From these estimates, they calculated their total horizon areas — an estimate roughly equal to about 235,000 square kilometers, or roughly nine times the area of Massachusetts.

They then used their previous technique to extract the “ringdown,” or reverberations of the newly formed black hole, from which they calculated its mass and spin, and ultimately its horizon area, which they found was equivalent to 367,000 square kilometers (approximately 13 times the Bay State’s area).

“The data show with overwhelming confidence that the horizon area increased after the merger, and that the area law is satisfied with very high probability,” Isi says. “It was a relief that our result does agree with the paradigm that we expect, and does confirm our understanding of these complicated black hole mergers.”

The team plans to further test Hawking’s area theorem, and other longstanding theories of black hole mechanics, using data from LIGO and Virgo, its counterpart in Italy.

“It’s encouraging that we can think in new, creative ways about gravitational-wave data, and reach questions we thought we couldn’t before,” Isi says. “We can keep teasing out pieces of information that speak directly to the pillars of what we think we understand. One day, this data may reveal something we didn’t expect.”

This research was supported, in part, by NASA, the Simons Foundation, and the National Science Foundation.

Featured image: Physicists at MIT and elsewhere have used gravitational waves to observationally confirm Hawking’s black hole area theorem for the first time. This computer simulation shows the collision of two black holes that produced the gravitational wave signal, GW150914. Credit: Simulating eXtreme Spacetimes (SXS) project. Courtesy of LIGO


Reference: Maximiliano Isi, Will M. Farr, Matthew Giesler, Mark A. Scheel, and Saul A. Teukolsky, “Testing the Black-Hole Area Law with GW150914”, Phys. Rev. Lett. 127, 011103 – Published 1 July 2021. DOI: https://doi.org/10.1103/PhysRevLett.127.011103


Provided by MIT

Which Energy Sources Type II Civilization Will Utilize If They Aim To Build A Dyson Sphere Around A Black Hole? (Astronomy)

You may have came across different concepts related to advanced civilizations such as Alderson disk, Matrioshka brain etc. But, one of the famous concepts is the Dyson sphere. Its concept relies on the fact that, if an extraterrestrial intelligence has reached a level of supercivilization, it might consume an energy of its own star. For this purpose, to have the maximum efficiency of energy transformation, it would be better to construct a thin shell completely surrounding the star. For instance, if a dyson sphere is built around the Sun, the total luminosity of the Sun (L ∼ 4 × 1026 W) can be utilised, which is approximately nine orders of magnitude larger than the power intercepted by the Earth (∼ 1.7 × 1017 W). After receiving the energy from the star, the civilisation would be able to convert the energy from low-entropy to high-entropy and emanate the waste heat (e.g., in mid-infrared wavelengths) into the background, suggesting this kind of energy waste is detectable.

However, it is almost impossible to build a rigid Dyson Sphere due to the gravity and the pressure from the central star. Hence, some sorts of the concept were proposed. Such as, a Dyson Swarm, a group of collectors that orbits the central energy source. This method allows the civilisation to grow incrementally. Nevertheless, due to the orbital mechanics, the arrangement of such collectors would be extraordinarily complex. Another variant is a Dyson Bubble which contains a host of collectors as well. However, the collectors are instead stationary in space assuming equilibrium between the outward radiative pressure and the inward gravity, which are usually called “solar sails” or “light sails”.

Based on energy consumption, Kardashev classified the hypothetical civilisations into three categories (Kardashev scale). A Type I civilisation uses infant technology and consumes 4 × 1016 W (or 4 × 1023 erg s¯1), the energy of its planet. A Type II civilisation harvests all the energy of its parent star, namely 4 × 1026 W (or 4×1033 erg s¯1). A Type III civilisation represents the highest technological level, which can engulf its entire galaxy as its energy source. A typical energy used by a Type III civilisation is about 4 × 1037 W (or 4×1044 erg s¯1). To represent the civilisations that have not been able to use all of their available energy sources, Sagan suggested a logarithmic interpolation in the form of 𝐾 = 0.1(logP − 6), where 𝐾 is the Kardashev index and 𝑃 (in the unit of Watts) indicates the energy consumption. Currently, our civilisation level is approximately 0.735 and we may take 5000-10000 years to become type II.

After a type II civilization absorbs all the energy from the parent star, they would seek another energy source to maintain itself. What if that energy sources are nothing but the black holes? But, the question is, if black holes can be regarded as proper energy sources?, or if they are inefficient to provide ample energy for civilizations to thrive. Hsiao and colleagues recently answered this question in their recent paper.

They considered and discussed six types of energy sources: the Cosmic Microwave Background (CMB), the Hawking radiation, an accretion disk, Bondi accretion, a corona, and the relativistic jets from two types of black holes: a non-rotating black hole (Schwarzschild black hole) and a rotating black hole (Kerr black hole), ranging from micro, stellar-mass, intermediate-mass to Supermassive Black Hole (SMBH).

© Hsiao et al.

They showed that the collectable energy from the CMB at present by the Inverse Dyson Sphere would be too low (∼ 1015 W). Next, the Hawking radiation as a source seems to be rather infeasible since the Hawking luminosity cannot provide adequate energy (e.g., for 5 M, 𝐿Hawking ∼ 10¯30 W << 1026 W (Type II)).

On the other hand, it has been suggested that, an accretion disk, a corona, and relativistic jets could be potential power stations for a Type II civilisation.

“Our results suggest that for a stellar-mass black hole, even at a low Eddington ratio, the accretion disk could provide hundreds of times more luminosity than a main sequence star.”

If a Type II civilisation collects the energy from the accretion disk of a SMBH, the energy could boost the Kardashev index, 𝐾 ∼ 2.9. Moreover, the energy reserved in a corona and jets can provide additional energy (∼ 30%− ∼ 50% for the corona luminosity and ∼ 60%− ∼ 80% for the jets luminosity) aside from the accretion disk.

“Our results suggest that if a Type II civilisation collects the energy from jets and electromagnetic radiation simultaneously, for a SMBH with a mass similar to Sgr A*, the Kardashev index can reach ∼ 3.”

Overall, their results suggested that a black hole can be a promising source and is more efficient than harvesting from a main sequence star.

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Figure 1. Example spectra of Scenario A (𝑀 = 5 M and 𝐿disk = 1600 L) (upper panel:) A hot Dyson Sphere with covering fraction 𝑅c = 20%, transfer efficiency 𝜂 = 50% , and 𝑅DS = 5000𝑅Sch. (lower panel:) A solid Dyson Sphere with covering fraction 𝑅c = 2%, transfer efficiency 𝜂 = 80%, and 𝑅DS = 7.12×106 𝑅Sch. The yellow curve suggests the accretion disk flux with the absorption of the Dyson Sphere ((1–𝑅c) times the original accretion disk flux) while the purple dot curve is the original flux of accretion disk without surrounding a Dyson Sphere. The blue curve shows the flux of the waste heat from the Dyson Sphere while the red dot-line curve indicates the total flux. The black circles, green diamonds, burgundy asterisks, blue stars, and magenta hexagrams indicate the limiting magnitude of GALEX, PAN STARRS1, VHS, WISE, and SDSS survey, respectively. © Hsiao et al.

They also discussed a possible location of a Dyson Sphere around a black hole. To absorb the accretion disk luminosity, a Dyson ring or a Dyson Swarm could be a possible structure. A Dyson Sphere should be located outside of the accretion disk, ∼ 103𝑅sch − 105𝑅sch. However, in this region, the hot temperature would melt the solid structure. In order to avoid melting of the solid Dyson Sphere, the solid Dyson Sphere (𝑇 = 3000 K) should be located at 𝑅DS ≳ 107 𝑅Sch and 𝑅DS ≳ 103 𝑅Sch for a stellar mass black hole and a SMBH, respectively. (RDS – radius of dyson sphere & RSch- Radius of Schwarzschild black hole).

Figure 2: Possible wavelengths for a hot Dyson Sphere to be detected in different black hole mass. The purple, blue, green, orange, and red regions indicate X-ray(0.01–10 nm), UV(10–400 nm), optical(400 0760 nm), NIR(760 nm – 5 𝜇m), and MIR(5 – 40 𝜇m) wavelength, respectively. The shaded region is the peak wavelength from black body radiation of the waste heat, covering a wide range of parameters of a Dyson Sphere. The region enclosed by blue, magenta, and black lines indicate stellar-mass, intermediate-mass and SMBH. Three arrows show how the peak wavelengths change with increasing parameters. The arrow lengths are arbitrary. © Hsiao et al.

The size of an accretion disk for a stellar-mass black hole is smaller than the size of the Sun, which means that a Dyson Sphere around the stellar-mass black hole can be smaller than that around the Sun. In terms of relativistic jets, a possible form of a Dyson Sphere is a Dyson Bubble. Balancing the pressure and the gravity from the black hole, a light sail could be stationary in space and can continuously collect the energy from the jets. Light sails also absorb the luminosity from the accretion disk at the same time. Their results suggested that the best way to place a light sail is close to the origin of the jets, which could save the materials to build a light sail.

Moreover, a hot Dyson Sphere around a stellar-mass black hole in the Milky Way (10 kpc away from us) is detectable in the UV(10−400 nm), optical(400−760 nm), NIR(760 nm−5 𝜇m), and MIR(5 − 40 𝜇m), which can be detected by our current telescopes (e.g., WFC3/HST and GALEX survey). For a solid Dyson Sphere, the limiting magnitude of the sky surveys such as Pan STARRS1, VISTA Hemisphere Survey (VHS) Wide-Field Infrared Survey Explore (WISE) and Sloan Digital Sky Survey (SDSS) are smaller than the flux density from the solid Dyson Sphere, which indicates that the solid Dyson Sphere is bright enough to be detected.

“The presence of Dyson Spheres may be imprinted in spectra. Performing model fitting and measuring the radial velocity will help us to identify these possible artificial structures.”, concluded authors of the study.

Featured image: Advanced Civilization Using A Dyson Sphere © Getty images


Reference: Tiger Yu-Yang Hsiao, Tomotsugu Goto, Tetsuya Hashimoto, Daryl Joe D. Santos, Alvina Y. L. On, Ece Kilerci-Eser, Yi Hang Valerie Wong, Seong Jin Kim, Cossas K.-W. Wu, Simon C.-C. Ho, Ting-Yi Lu, “A Dyson Sphere around a black hole”, MNRAS, 2021. DOI:10.1093/mnras/stab1832 preprint: https://arxiv.org/abs/2106.15181


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