The enigmatic universe has long captivated the imagination of scientists and stargazers alike. Among its many profound phenomena, wormholes and black holes stand as cosmic mysteries that continue to beckon exploration. Recent research conducted by Ke Gao and Lei-Hua Liu delves deep into the intricate world of these celestial wonders, focusing on the rotational Simpson-Visser metric (RSV) as the key to unraveling their magnification effects.
The allure of wormholes and black holes lies not only in their perplexing existence but also in their ability to magnify the cosmos. Understanding the finite distance analysis of this magnification phenomenon has been the pursuit of many astronomers and physicists, and Gao and Liu’s work takes a significant step towards clarity.
By meticulously calculating the deflection of light within the RSV metric, the researchers were able to unveil the mesmerizing magnification effect. This groundbreaking approach enabled them to apply the RSV metric to specific examples, including the Ellis-Bronnikov wormhole, Schwarzschild black hole, and Kerr black hole (or wormhole), shedding light on their unique magnification characteristics.
The results of their study are as intriguing as the objects of their investigation. Notably, the Ellis-Bronnikov wormhole exhibited singular magnification peaks, a distinctive trait that sets it apart from its black hole counterparts. In contrast, Schwarzschild’s black hole, as the ADM mass increases, unfolds the astonishing spectacle of up to three peaks of magnification.
The story doesn’t end there; black holes with negative spin, known as Kerr Black holes, introduce a fascinating twist. As spin increases, these enigmatic entities transition from three magnification peaks to a solitary peak, a phenomenon that is mirrored in the case of positive spin. These findings open new vistas in our comprehension of black hole behavior.
Perhaps the most tantalizing revelation is the application of this research to the Central Black Hole of the Milky Way Galaxy. Here, the lensing effect showcases multiple peaks of magnification, offering a tantalizing glimpse into the cosmic wonders that lie at the heart of our galaxy. Regrettably, these captivating effects remain beyond the purview of observation from Earth, a testament to the vastness of the cosmos.
In essence, Gao and Liu’s research provides not only a discernible phenomenological difference in magnification between black holes and wormholes but also lays down a firm theoretical foundation for future explorations into the intricacies of these celestial enigmas. As humanity continues to gaze towards the heavens, such revelations bring us ever closer to unlocking the profound secrets of the universe, one cosmic puzzle at a time.
Reference: Ke Gao, Lei-Hua Liu, “Can wormholes and black holes be distinguished by magnification?”, Arxiv, 2023. https://arxiv.org/abs/2307.16627
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In the realm of theoretical physics, understanding the concept of a multiverse has been a subject of great interest and speculation. Gopal Yadav, a prominent physicist, has proposed an intriguing model based on wedge holography that offers insights into the nature of the multiverse. By considering gravitating baths with varying degrees of gravity, Yadav’s model delves into the composition, lifespan, and communication between universes within the multiverse. Additionally, the model addresses the possibility of obtaining the Page curve of black holes and even offers a resolution to the famous “grandfather paradox.” In this article, we will delve into the key findings and implications of Yadav’s model, shedding light on the exciting implications for our understanding of the multiverse.
Wedge Holography and Multiverse Construction:
Yadav’s model of wedge holography revolves around the consideration of two gravitating baths, one with strong gravity and the other with weak gravity. By embedding 2n Karch-Randall branes in the bulk, which may or may not contain black holes, the model describes the multiverse in various scenarios.
In the first case, the multiverse is constructed using d-dimensional Karch-Randall branes embedded in anti-de Sitter branes. Notably, Yadav found that once created, the multiverse consisting of AdS branes persists indefinitely. In contrast, the second case involves d-dimensional Karch-Randall branes embedded in de Sitter branes, resulting in a multiverse composed of de Sitter branes with a short lifespan. These branes are created and annihilated simultaneously. Furthermore, Yadav highlights the incompatibility of describing a multiverse as a mixture of d-dimensional de Sitter and anti-de Sitter spacetimes within the same bulk due to the different intersecting characteristics of these branes.
Exploring Black Holes and Radiation:
Within Yadav’s model, the possibility of obtaining the Page curve of black holes is also investigated. By employing two Karch-Randall branes, one serving as a black hole and the other as a bath, the model encounters challenges in defining the island surface and identifying the nature of the radiation emitted. For instance, when a Karch-Randall brane consists of a black hole and cosmological event horizons, such as a Schwarzschild de-Sitter black hole on the brane, the observer collecting radiation faces difficulty in distinguishing between Hawking radiation and Gibbons-Hawking radiation.
Perhaps one of the most intriguing implications of Yadav’s model is its potential to resolve the well-known “grandfather paradox.” In this setup, where universes communicate through transparent boundary conditions at the interface point, the paradox can be circumvented. Suppose an individual, Bob, resides on Q1 while his grandfather lives on Q-1. To avoid the paradox, Bob cannot travel to Q-1, where he could potentially prevent his own existence. However, he can travel to Q-2, Q-3, and so forth, where he can meet other individuals like Robert and Alice. Thus, within this model, the “grandfather paradox” finds resolution.
Neutron stars, the dense remnants of core-collapse supernovae, are known for their extreme physical properties and play a vital role in understanding the dynamics of stellar evolution. These fascinating celestial objects are born with a unique characteristic – natal kicks. These kicks are a consequence of asymmetric ejection of matter and possibly neutrinos during the supernova explosion, imparting both linear and rotational motion to the neutron star. Recent research by Loeb and colleagues delves into the intriguing possibility that these natal kicks may occur off-center, leading to a natal rotation. The paper explores the correlation between observed pulsar spin and transverse velocity in our galaxy and develops a model to constrain the natal rotation imparted to neutron stars.
The Nature of Natal Kicks and Rotation:
During a core-collapse supernova, the asymmetric expulsion of matter and neutrinos generates a recoil force, propelling the newly formed neutron star away from its birthplace. If this force is exerted even slightly off-center, it imparts a rotational motion to the neutron star. As a result, neutron stars born with off-center kicks may possess a certain degree of rotation.
Exploring the Pulsar Population:
Loeb and colleagues’ study aims to investigate the implications of off-center natal kicks on the population of pulsars in our galaxy. Pulsars, rapidly spinning neutron stars emitting beams of radiation, provide valuable insights into the physics of these stellar remnants. By analyzing the spin properties and transverse velocities of pulsars, the researchers seek to establish a correlation that can shed light on the presence and characteristics of off-center natal kicks.
Modeling Natal Rotation:
To examine the effects of natal rotation, Loeb and colleagues developed a comprehensive model that incorporates the observed population of pulsars in the galaxy. By comparing the model’s predictions with the available data on pulsar spin periods, transverse velocities, and ages, the researchers were able to constrain the location of the off-center kick.
Key Findings:
After thorough analysis and modeling, the study by Loeb and colleagues presents a significant finding. The researchers determine, with a confidence level of 90%, that the location of the off-center kick, referred to as Rkick, is approximately 1.12 kilometers. This result suggests that the off-center kicks responsible for imparting natal rotation to neutron stars tend to occur at a consistent distance from the center of the star.
Implications and Future Directions: The findings of this study hold considerable implications for our understanding of core-collapse supernovae and the resulting neutron stars. By constraining the location of off-center natal kicks, this research provides valuable guidance for future simulations of massive star core-collapse, aiding in the refinement of models that replicate the complex dynamics of these cataclysmic events.
Moreover, the observed correlation between pulsar spin and transverse velocity offers insights into the mechanisms governing the formation and evolution of neutron stars. This knowledge contributes to our understanding of stellar astrophysics and the intricate processes that shape the behavior and properties of these enigmatic cosmic objects.
Neutrinos, elusive particles that interact weakly with matter, have long captured the imagination of scientists. These subatomic particles, produced in various astrophysical processes, can provide valuable insights into the mysteries of the universe.
Recent research done by Gaspert and colleagues has revealed an intriguing possibility: the Moon could serve as an ideal platform for detecting dark matter through neutrino flux. While this opens up exciting avenues for scientific discovery, practical challenges must be overcome to realize this ambitious endeavor.
Many neutrino fluxes on the Moon are nearly the same as on the Earth, but a few are radically different. For direct detection of electroweak scale dark matter on the Earth, the most important flux sources are solar neutrinos, diffuse supernova background neutrinos (DSNB) & atmospheric neutrinos.
Surprisingly, the solar and DSNB neutrino fluxes on the Moon closely resemble those on Earth. However, it is the flux of atmospheric neutrinos that exhibits a stark contrast due to the Moon’s unique characteristics. Unlike the Earth, the Moon lacks a substantial atmosphere, and cosmic rays bombarding its surface undergo direct collisions, generating a distinct neutrino spectrum.
This key difference has prompted Gaspert and colleagues to estimate the total flux and spectrum of neutrinos near the lunar surface. Their findings suggest that a large-scale liquid xenon or argon detector stationed on the Moon could offer significantly enhanced sensitivity to dark matter with masses above and approximate to 50 GeV, surpassing the capabilities of a comparable detector on Earth. This heightened sensitivity could even extend to detecting the elusive mχ = 1.1 TeV thermal Higgsino dark matter.
While these results fuel the motivation for lunar-based dark matter direct detection experiments, it is important to acknowledge the practical challenges associated with realizing such an apparatus. Several factors need to be considered, including transportation logistics, cosmic activation concerns, local background interference, infrastructure requirements, and the allocation of human resources. These obstacles must be addressed and overcome for the vision of Moon-based dark matter detection to become a reality.
Transporting a large-scale detector to the Moon is a formidable task that requires careful planning and execution. Launching and safely landing such a sensitive instrument without compromising its functionality poses significant engineering challenges. Additionally, the issue of cosmic activation arises, as prolonged exposure to cosmic rays during transit could lead to unwanted activation of detector components. Addressing this concern necessitates shielding measures and thorough safety protocols.
Local backgrounds, originating from lunar materials and natural radioactivity, could introduce unwanted noise into the dark matter signal. Rigorous studies and mitigation strategies are required to ensure that these local sources do not overshadow the sought-after signatures. Infrastructure development on the Moon, such as establishing power supply systems and maintaining a stable environment for the detector, presents further logistical hurdles that must be overcome.
Lastly, human resources play a vital role in the success of lunar-based experiments. Skilled personnel will be needed to operate and maintain the detector, troubleshoot issues, and analyze the vast amount of data collected. Collaborative efforts among scientists, engineers, and astronauts will be necessary to tackle the intricacies of such an ambitious scientific endeavor.
Despite the practical challenges, the current momentum towards lunar exploration offers an opportune moment to consider the potential applications of Moon-based experiments for scientific discovery. The quest to unravel the mysteries of dark matter, one of the most enigmatic aspects of our universe, is a formidable task that requires innovative approaches. The unique neutrino flux on the Moon, with its reduced backgrounds, provides an enticing opportunity for advancing our understanding of this elusive cosmic entity.
Reference: Reference: Andrea Gaspert, Pietro Giampa, Navin McGinnis, David E. Morrissey, “Dark Matter Direct Detection on the Moon”, Arxiv, 2023.
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New distance measurements of the diffuse spheroid galaxy Ngc 1052-Df2 place this galaxy at a distance of 72 million light years and confirm that the galaxy is practically devoid of dark matter, a very rare case in the galaxy landscape. This absence of dark matter compared to other galaxies suggests that dark matter exists as a real physical entity and not as a result of a different law of gravitation on a galactic scale.
According to the current paradigm, dark matter makes up about 86% of all matter in our Universe. Its peculiarity is that it does not interact electromagnetically like ordinary matter, but only by gravitational way . For this reason it is difficult to study it, in fact it can only be detected on a large scale by observing the gravitational effects it causes on ordinary matter: unfortunately there is no experimental detection.of dark matter particles. The presence of this matter has been deduced thanks to studies on the velocity curves of spiral galaxies: as we move away from the nucleus of a galaxy, the stars do not decrease their speed as one might expect, but continue to move. quickly. If Newton’s law of gravity holds, this excess of speed indicates that most of the mass of galaxies is made up of invisible matter capable of holding the stars of which they are composed bound together with its own force of gravity: unlike galaxies. they would fall apart. Dark matter in the evolution of the Universe is very important because it is thanks to its intense gravitational effects that, within immense haloes of dark matter, galaxies were formed . Otherwise, after the Big Bang, ordinary matter would never have undergone any process of gravitational collapse and galaxies would not have formed. From this theoretical framework it is expected that each galaxy contains a consistent amount of dark matter: for example the value of the average ratio between dark matter and ordinary , measured for galaxies such as our Milky Way, is of the order of 30 times and increases both for more massive galaxies, and for less massive galaxies.
However, things seem more complex than that, at least as far as the galaxy Ngc 1052-Df2 is concerned . It is an ultra-diffuse galaxy with low surface brightness that is prospectively located in the constellation of the Whale, identified thanks to a large-field survey of the group of galaxies of Ngc 1052. The galaxy contains so little ordinary matter that it is practically transparent, so much is it It is true that in the images that portray it you can see the background galaxies much further away. Morphologically, this galaxy has a spheroidal appearance and does not appear to have a core, spiral arms or a disk of stars. The geometric dimensions are similar to those of the Milky Way.
In a March 2018 article published in Nature, the results of the radial velocity measurements of 10 luminous globular clusters belonging to this evanescent galaxy were published for the estimation of the total mass of the system. The result was that the ratio of dark to bright matter in Ngc 1052-Df2 was about 1, a value about 400 times lower than expected and in stark contrast to what is observed in other galaxies. Put simply, the case of NGC1052-DF2 showed that dark matter is not always coupled with baryon matter , at least on a galactic scale. To confirm this incredible result, the discovery team, led by Pieter van Dokkum of Yale University, focused on precise distance measurementby Ngc 1052-Df2, publishing a new paper in The Astrophysical Journal Letters . In the work of 2018, the distance of the galaxy was assumed to be similar to that of the group of galaxies to which it seemed to belong, namely that of Ngc 1052 at about 65 million light years from us. How does distance fit into estimating the relationship between dark and ordinary matter? To understand this, just think of the fact that the estimation of the mass of a star can be done by measuring its intrinsic brightness and this is obtained by measuring both the apparent brightness and the distance at which the star is located. By scaling this reasoning on a galactic scale we understand that if Df2 were closer to Earth than the 65 million light years adopted, thenits stars would be intrinsically weaker and less massive , so the luminous matter would make a minor contribution to the total mass (which is measured with the radial velocity of globular clusters) and the ratio between dark and luminous matter would increase accordingly. Distance measurement thus becomes a crucial parameter for determining the amount of luminous matter in the galaxy.
To measure the distance of a galaxy you need ” standard candles “, ie stars whose intrinsic brightness is known a priori . The team of astronomers, using the “Hubble” space telescope, focused on measuring the apparent brightness of the red giants located on the periphery of Ngc 1052-Df2 and which, during their evolution, all reach the same brightness peak. In this way, the difference between intrinsic and apparent brightness can be used to measure large intergalactic distances. The new distance estimate tells us that Df2 is 72 million light years awaythat is, the galaxy is further away than the original estimate of 65 million light years. From here it follows that Df2 is really devoid of dark matter, it is not an observational bias .
Moreover, Df2 is not the only galaxy without dark matter, another galaxy, Ngc 1052-Df4 , is also devoid of dark matter. In this case, however, some scientists suggest that dark matter may have been removed from the galaxy due to tidal forces exerted by another passing galaxy.
The discovery of these galaxies devoid of dark matter, paradoxically, confirms that dark matter really exists. In fact, if dark matter were only an effect of a gravitational law different from the Newtonian one, all galaxies should show its presence. The fact that there are galaxies without dark matter means that something is really missing in their structure. Understanding why Df2 is devoid of dark matter will require further observation, the mystery continues.
Featured image: The galaxy poor in dark matter Ngc 1052-Df2 taken with the Hubble Advanced Camera for Surveys between December 2020 and March 2021. The galaxy is so poor in matter that, through it, you can see the background galaxies (Credits: Nasa , Esa, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (Ias), Alyssa Pagan (STScI))
Leveraging the Australian forerunner of the Ska project, the first map of a section of the Milky Way’s galactic plane (about 40 square degrees wide) was created with an angle of sensitivity and resolution never before achieved by an infrastructure in the southern hemisphere. The results were published in Mnras by a group of radio astronomers led by INAF and the University of Macquarie in Sydney.
A group of radio astronomers, led by the National Institute of Astrophysics (INAF) and the University of Macquarie in Sydney, made the first radio observations of a large section of the galactic plane of the Milky Way with the Australian Ska Pathfinder (Askap) , developed and managed by the Commonwealth Scientific and Industrial Research Organization (Csiro). Specifically, the region mapped by the researchers includes the entire area of the Scorpio survey (Stellar Continuum Originating from Radio Physics In Ourgalaxy), one of the numerous exploration projects of the broader Evolutionary Map of the Universe (Emu) program, which consists of observation of the whole southern hemisphere with Askap, one of theprecursors of the Ska project . The observations, reported in two articles published in the Monthly Notices of the Royal Astronomical Society , were made in 2018 with the interferometer not yet fully deployed (15 of the 36 antennas were operational at the time), covering a total area of about 40 degrees squares.
As part of the preparatory activities for the EMU survey , Askap’s antennas were pointed towards the tail of the constellation of Scorpio. The so-called Scorpio field was included among the first targetsscientific studies of Askap, thanks to the preliminary work carried out by the Italian team of INAF using the Australia Telescope Compact Array (Atca), which allowed to achieve a series of important scientific results and to develop skills in the reduction and analysis of radio data of the plan galactic. More than 3,600 compact radio sources have been extracted from the Scorpio field, many of which are unclassified. All the sources previously classified as HII regions, areas rich in ionized hydrogen associated with star formation sites, or as planetary nebulae, the last evolutionary phases of stars of intermediate mass, have been revealed and new ones have been discovered, significantly increasing the number of objects belonging to these different galactic populations.radio quiet and to discover numerous extended sources, not classified and belonging to the class of so-called “galactic bubbles”, which constitute a new sample within which to identify new supernova remains.
Composite image of a portion of the Scorpio field. In green the infrared data collected by Spitzer / Glimpse, in red those collected by Herschel / Hi-Gal and in blue the radio data collected by Askap. By superimposing infrared data (which track dust) on radio maps (which track either ionized gas or synchrotron), radio and infrared coincide in star-forming regions, while in the case of supernova remnants (Snr) only radium is visible . Credits: G. Umana / Inaf
“Scorpio is the only galactic field observed so far with Askap and is therefore particularly important for the characterization of some galactic populations”, explains Grazia Umana , principal investigator of the survey and first author of one of the two articles, as well as a researcher at INAF of Catania, «because it provides a solid base level from which to start to better design some aspects of the EMU survey . In addition to the discovery of numerous galactic radio sources, these observations have highlighted Askap’s unique feature of mapping complex objects at various angular scales, an extremely useful feature especially in the case of studies of the galactic plane. This is the result of a skilful design byarray that is sensitive to both compact objects and extended and diffuse emission. On the basis of these first results of observations of the galactic plane with Askap we can have only a small taste of the potential of the Ska project in the field of galactic radio astronomy ».
The galactic plane is the place in the Milky Way where the solar system resides: it contains countless stars, dust and gas clouds, as well as a significant amount of dark matter. Studying the plan of the Milky Way has always been one of the most important objectives of radio astronomers, but the presence of diffuse emission in the galaxy makes it difficult to obtain artifact-free images: this effectively reduces the quality of the final images making data analysis a task. particularly challenging. Many of these problems have been mitigated using different approaches and increasingly complex algorithms, but due to the large amount of data provided by tools such as Askap, human intervention at each stage of data reduction is not possible and this requires a different approach. .
” Numerous difficulties have arisen in the data reduction phase”, underlines Simone Riggi , researcher at INAF in Catania and first author of the survey catalog article , “because the standard techniques are currently optimized for extragalactic fields in which the emission diffuse that permeates the galactic plane is not present. It was therefore necessary to develop new procedures for data calibration and define new strategies in the data acquisition phase. What we have learned will allow us to contribute to the design of the EMU survey , optimizing its scientific return also for galactic science.An important part of the work done with the Ska precursors is to gain experience in managing the representative data of the Ska project. A significant challenge with these huge datasets, beyond data reduction itself, is to automatically find and classify radio sources. ‘
The Milky Way extending over the Askap radio telescope operated by Csiro (the Australian Scientific Agency) at the Murchison Radio-astronomy Observatory in Western Australia. Askap is one of the forerunners of the Ska project. Credits: Csiro / A. Cherney
The Scorpio field was used as a test bench to test the Caesar source extraction tool developed by the Italian team on real data and in the presence of diffuse emission . A first catalog of compact radio sources and their components was produced, which will be subsequently updated when the new Askap observations of the Scorpio field with the complete array are available, scheduled for the end of 2021. “Multi-frequency data and the expected increase in sensitivity and spatial resolution will allow us to measure the spectral index for all sources, enabling further progress in our classification studies, ”adds Riggi.
“The data collected at that early stage of Askap also demonstrates its excellent sensitivity to extended radio emissions,” says Andrew Hopkins , head of the EMU project for Macquarie University. «A fundamental result to allow us to detect these important structures in the Milky Way, allowing us to deepen our knowledge on the formation and evolution of stars in the galaxy».
The EMU project will also extend to part of the northern hemisphere, covering 75 percent of the sky observed at the 1.4 GHz frequency, with better angular resolution and sensitivity than achieved so far. The researchers will observe a large fraction of the galactic plane and will be able to produce a wide-field atlas of the Milky Way’s continuous radio emission, with unprecedented results in terms of sensitivity and angular resolution, which will have a major impact in star formation studies. of the galactic structure and stellar evolution.
«New Askap observations of the galactic plane as part of the EMU survey and subsequently with the Ska project will allow us to explore a whole series of observational parameters with a very high probability of discovering new classes of objects. Our final goal is to acquire and consolidate the skills and competences in view of the development of the entire antenna array of the Ska project, in order to be ready and competitive to lead and participate in the Ska Key Science Projects (Ksp) and to full exploitation of data », Umana concludes.
Featured image: Askap image of the Scorpio field at 912 MHz. The mosaic covers a region of approximately 40 square degrees. The shape of the galactic equator is defined by a series of compact sources and regions of ionized hydrogen (H II regions), associated with star formation sites. Several supernova remnants (Snr) are also visible. Outside the galactic plane some large and bright structures are evident. Among these, the region in the upper center of the field includes the regions H II G345.45 + 1.50 and IC 4628. The white frames are zoom of some representative objects, clockwise, a Snr, a star-forming region with a Massive Young stellar Object, another star-forming region and a pair of Snr. Credits: G. Umana / Inaf
To know more:
Read on Monthly Notices of the Royal Astronomical Society the article ” A first glimpse at the Galactic Plane with the ASKAP: the SCORPIO field “, by G. Umana, C. Trigilio, A. Ingallinera, S. Riggi, F. Cavallaro, J. Marvil, RP Norris, AM Hopkins, CS Buemi, F. Bufano, P. Leto, S. Loru, C. Bordiu, JD Bunton, JD Collier, M. Filipovic, TMO Franzen, MA Thompson, H. Andernach, E . Carretti, S. Dai, A. Kapinska, BS Koribalski, R. Kothes, D. Leahy, D. Mcconnell, N. Tothill and MJ Michałowski
Read on Monthly Notices of the Royal Astronomical Society the article ” Evolutionary map of the Universe (EMU): Compact radio sources in the SCORPIO field towards the galactic plane “, by S Riggi, G Umana, C Trigilio, F Cavallaro, A Ingallinera , P Leto, F Bufano, RP Norris, AM Hopkins, MD Filipović, H Andernach, J Th van Loon, MJ Michałowski, C Bordiu, T An, C Buemi, E Carretti, JD Collier, T Joseph, BS Koribalski, R Kothes , S Loru, D McConnell, M Pommier, E Sciacca, F Schillirò, F Vitello, K Warhurst and M Whiting,
Askap is a network of radio telescopes located at the Murchison Radio Astronomy Observatory in the desert region of Western Australia, where the Aboriginal Wajarri Yamatji ethnic group has resided for millennia. Managed and operated by the Australian scientific agency Csiro, Askap has 36 parabolic antennas of 12 meters in diameter with a collection area of 4000 square meters: each of the antennas required 13 to 18 hours of assembly. Thirty antennas are arranged in a circle of 2 kilometers in diameter, while the remaining 6 antennas are arranged to form a Reuleaux triangle with a maximum distance from the center of 6 kilometers. Askap is one of the forerunners of the SKA project and has been operational since 2012, but official scientific observations began only from 2020.
In a new article published in the Monthly Notices of the Royal Astronomical Society, an ICRA-ICRANet research team (some of them INAF associates) sheds light on the mass and spin of stellar-mass BHs from an extensive analysis of long-duration GRBs
What is the fate of very massive binary stars, which kind of signatures/observables are associated with their stepwise evolution, which kind of new physical laws are revealed, represent the most relevant questions at the heart of relativistic astrophysics. The answer to these questions is intimately related to the explanation of the most powerful transients in the Universe, supernovae (SNe) and gamma-ray bursts (GRBs), and with the formation of neutron star-black hole (NS-BH), of neutron star-neutron star (NS-NS), and possibly BH-BH binaries. A crucial question then arises: how large are the mass and how fast are the rotational spin of those astrophysical BHs and NSs?
A clue to this answer comes out from decades of electromagnetic observations of X-ray binaries in which a BH accretes mass from a stellar companion. From their continuous monitoring, it has turned out that these BH have masses ranging ∼ 5–20 solar masses, where the upper edge is given by the very recently updated mass of the BH harbored by the X-ray binary Cygnus X-1 [1]. While the origin of X-ray binaries is well established, focus is needed to identify the evolutionary channels leading to the onset of GRBs, to their time evolution, as well as to the new physical laws and astrophysical regimes envisaged for their description.
In a new article published in the Monthly Notices of the Royal Astronomical Society [2], an ICRA-ICRANet research team (some of them INAF associates) sheds light on the mass and spin of stellar-mass BHs from an extensive analysis of long-duration GRBs. This has been allowed by fifty years of exponential growth of multiwavelength observations of GRBs and theoretical progress, from which it has been possible to identify the “inner engine” of the GRB, and verify the validity of the BH mass-energy formula established fifty years ago. The subject of study are 380 energetic long GRBs with energy release above 1052 erg in gamma-rays, all with a measured cosmological redshift, and an X-ray afterglow. These systems are accompanied by an SN of type Ic, namely an SN produced by a star which has lost its hydrogen and helium layers. The binary-driven hypernova (BdHN) scenario of long GRBs bridges what we know from binary evolution, with high-energy relativistic astrophysics to explain these extreme systems.
The GRB progenitor system is a binary composed of a carbon-oxygen (CO) star and a companion NS. During their long lifetime, a very massive binary experiences several stages, each one characterized by specific physical phenomena and observables (see left side of Figure 1). The more massive of the two stellar components evolves faster through the nuclear burning phases, leading it to make a first SN explosion, with consequent formation of a NS. Mass-transfer from the ordinary stellar component to the NS leads to an X-ray binary stage. Further binary interactions lead to multiple common envelope phases in which mass loss is enhanced and the ordinary star gets rid of its outer low-density envelope, forming a CO star. The binary orbit shrinks while thermonuclear evolution of the CO star proceeds until its iron core becomes unstable against gravitational collapse, forming a new NS (νNS) at its center, and driving an SN explosion. At this point, a powerful transient starts and its ultimate fate depends crucially on the distance separating the exploding CO star and the NS companion. The SN ejected material triggers a massive accretion process onto the NS companion as well as onto the νNS by matter fallback (see Figure 2).
For compact binaries with orbital periods of the order of 5 minutes (see right side of Figure 1), the companion NS accretes sufficient matter to trigger its gravitational collapse, forming a BH which emanates a distinct, associated emission at high-energies (GeV) characterized by a luminosity as a function of time that follows a power-law. The fallback accretion onto the νNS and its pulsar emission power the GRB X-ray and optical afterglow, characterized by power-law luminosities, different from the one of the GeV emission. BdHNe forming a BH have been called of type I.
From the statistics of the GeV emission, it has been inferred the morphology of the GRBs emission process: it occurs within a conical region of 60◦ measured from the normal to the orbital plane. No GeV radiation is observable outside such a conical region. The X-ray afterglow is instead present in all the BdHN I, independently of the inclination angle of the GRB with respect to the orbital plane. This detailed understanding have allowed the team to infer, from the analysis of the X-ray afterglow, the spin and magnetic field of the νNS. The analysis of the GeV emission have led, for the first time in about fifty years of GRB observations, to directly evaluate the precise mass and spin of the BHs formed in these powerful transients. The specific mass and spin of 11 BHs have been obtained and they range 2.3–8.9 solar masses and 0.27–0.87 solar masses, respectively.
This treatment of long GRBs, originating from the very massive binary stars, makes ample use of a description based on the four fundamental interactions: relativistic gravity and electrodynamics describe the “inner engine”, weak interactions drive the neutrino emission in the accretion process, and the strong interactions shape the inner structure of the NSs responsible of the X-ray afterglow.
Since the pioneering observations of BATSE instrument on board the Compton satellite [3], we know that GRBs are isotropically distributed when mapped in galactic coordinates. Similarly, following the discovery of their cosmological redshift thanks to BeppoSAX [4], observations of BdHN I have occurred all the way to z = 8.2 (e.g. GRB 090423 [5, 6]). We can safely assert that GRBs, also thanks to their outstanding energetics, have a fundamental role in relativistic astrophysics processes in the 95.5% of our known Universe. Their prolonged emission of polarized synchrotron radiation in the X-rays and in the GeV regime may well have a fundamental role in the life in and of our Universe.
Having said all the above, it comes as a surprise the vision carried forward by the LIGO-Virgo observatories that very massive binary stars should rapidly gravitationally collapse, evolve in into two BHs, crossing the spacetime of our Universe, finally merging into a larger BH. Such a vision avoids the role of any fundamental interactions with the sole exception of gravity, which seems at odds with the field of relativistic astrophysics.
fig. 1: Taken from [7]. Schematic evolutionary path of a massive binary up to the emission of a BdHN. (a) Binary system composed of two main-sequence stars, say 15 and 12 solar masse, respectively. (b) At a given time, the more massive star undergoes the core-collapse SN and forms a NS (which might have a magnetic field B ∼ 10¹³ G). (c) The system enters the X-ray binary phase. (d) The core of the remaining evolved star, rich in carbon and oxygen, for short CO star, is left exposed since the hydrogen and helium envelope have been striped by binary interactions and possibly multiple common-envelope phases (not shown in this diagram). The system is, at this stage, a CO-NS binary, which is taken as the initial configuration of the BdHN model [8]. (e) The CO star explodes as SN when the binary period is of the order of few minutes, the SN ejecta of a few solar masses start to expand and a fast rotating, newborn NS, for short νNS, is left in the center. (f) The SN ejecta accrete onto the NS companion, forming a massive NS (BdHN II) or a BH (BdHN I; this example), depending on the initial NS mass and the binary separation. Conservation of magnetic flux and possibly additional MHD processes amplify the magnetic field from the NS value to B ∼ 10¹⁴ G around the newborn BH. At this stage the system is a νNS-BH binary surrounded by ionized matter of the expanding ejecta. (g) The accretion, the formation and the activities of the BH contribute to the GRB prompt gamma-ray emission and GeV emission. (h) X-ray afterglow powered by the fallback accretion and pulsar-like emission of the νNS. (i) Optical emission of the SN due to nickel decay in the ejecta.Fig. 2: A SPH simulation from Becerra et al. [8] of the exploding CO-star as the SN in the presence of a companion NS. The CO-star is obtained from the evolution of a 25 solar masses zero-age main-sequence (ZAMS) progenitor which leads to a pre-SN CO-star mass MCO = 6.85 solar masses. The initial mass of the νNS (formed at the center of the SN) is 1.85solar masses and the one of the NS companion is MNS = 2 solar masses. The initial orbital period is 4.8 min. The panels show the mass density on the binary equatorial plane at two selected times from the SN explosion (t = 0 of the simulation), 159 s and 259 s. The reference system is rotated and translated so that the x-axis is along the line that joins the νNS and the NS, and the axis origin (0, 0) is located at the NS position. In this simulation, the NS collapses when it reaches 2.26 solar masses and angular momentum 1.24GM²/c, while the νNS is stable with mass and angular momentum, respectively, 2.04 solar masses and 1.24GM²/c. Up to the final simulation time, the binary system kept bound although the binary orbit widens, reaching an orbital period of 16.5 min and an eccentricity of e = 0.6. The collapse of the NS to the newly-formed BH, characteristic of a BdHN I, occurs at t = 21.6 min.
References: [1] J. C. A. Miller-Jones, A. Bahramian, J. A. Orosz, I. Mandel, L. Gou, T. J. Maccarone, C. J. Neijssel, X. Zhao, J. Zi´o lkowski, M. J. Reid, et al., Science 371, 1046 (2021), 2102.09091. [2] R. Ruffini, R. Moradi, J. A. Rueda, L. Li, N. Sahakyan, Y. C. Chen, Y. Wang, Y. Aimuratov, L. Becerra, C. L. Bianco, et al., MNRAS (2021), 2103.09142. [3] W. S. Paciesas, C. A. Meegan, G. N. Pendleton, M. S. Briggs, C. Kouveliotou, T. M. Koshut, J. P. Lestrade, M. L. McCollough, J. J. Brainerd, J. Hakkila, et al., Astroph. J. Supp. 122, 465 (1999), astro-ph/9903205. [4] M. R. Metzger, S. G. Djorgovski, S. R. Kulkarni, C. C. Steidel, K. L. Adelberger, D. A. Frail, E. Costa, and F. Frontera, Nature (London) 387, 878 (1997). [5] R. Salvaterra, M. Della Valle, S. Campana, G. Chincarini, S. Covino, P. D’Avanzo, A. Fernandez-Soto, C. Guidorzi, F. Mannucci, R. Margutti, et al., Nature (London) 461, 1258 (2009), 0906.1578. [6] R. Ruffini, L. Izzo, M. Muccino, G. B. Pisani, J. A. Rueda, Y. Wang, C. Barbarino, C. L. Bianco, M. Enderli, and M. Kovacevic, Astron. Astroph. 569, A39 (2014), 1404.1840. [7] J. A. Rueda, R. Ruffini, M. Karlica, R. Moradi, and Y. Wang, Astroph. J. 893, 148 (2020), 1905.11339. [8] L. Becerra, C. L. Ellinger, C. L. Fryer, J. A. Rueda, and R. Ruffini, Astroph. J. 871, 14 (2019), 1803.04356.
Numerous compelling evidences from astroparticle physics and cosmology indicate that the major matter component in the Universe is dark matter, accounting for about 85% with the remaining 15% is the ordinary matter. Nevertheless, people still know little about the dark matter, including its mass and other properties. Many models predict dark matter particles could couple to ordinary particle at weak interaction level, so it is possible to capture the signal of dark matter particle in the direct detection experiment. The scientific goals of the China Dark matter Experiment (CDEX) are on direct detection of light dark matter and neutrino-less double beta decay with p-type point contact germanium (PPCGe) detectors at the China Jinping Underground Laboratory (CJPL). The measurable energy spectra induced by the elastic scattering between dark matter particles and target nucleons in CDEX detector system could give us the information of dark matter mass, spin and other properties.
The analysis of the current dark matter experiments is usually model dependent, and many models beyond the standard model have predicted the existence of dark matter, such as super-symmetry model and extra-dimension model. Due to the variety of physics models, the constraints obtained from same experimental data cannot be applied directly to other models, which brings complications to physical interpretations. Cosmology observations have verified that the major part of dark matter is the non-relativistic cold dark matter, and as a result, the momentum transfer in the scattering process between dark matter particles and nucleons is only about hundreds of MeV, much lower than the electroweak scale (~250 GeV). It is therefore suitable to use effective field theory to analyze the interaction between dark matter and ordinary matter. Two alternative schemes have been proposed in recent years to study different possible interactions, namely non-relativistic effective field theory (NREFT) and chiral effective field theory (ChEFT). An effective theory contains all possible interactions allowed by given symmetric principles, so it can model-independently reduce the complicacy of analysis.
In the dark matter direct detection experiments, what are mostly focused on are the spin-independent (SI) and spin-dependent (SD) scattering analysis, while EFT can give more momentum-dependent or velocity-dependent interaction which are not taken into consideration usually. Benefiting from the low electrical noise of PCCGe, the analysis threshold of CDEX-1B and CDEX-10 both reach 160 eV, which can largely improve the detection sensitivity for light dark matter.
Based on the data set of CDEX-1B and CDEX-10, CDEX collaboration presents new limits for the couplings of WIMP-nucleon arising from NREFT and ChEFT. In the nonrelativistic effective field theory approach, they improve over the current bounds in the low mχ region. In the chiral effective field theory approach, they for the first time extended the limit on WIMP-pion coupling to the mχ< 6 GeV/c2 region.
Related results have been published online entitled “First experimental constraints on WIMP couplings in the effective field theory framework from CDEX” on Science China-Physics, Mechanics & Astronomy (Sci. China-Phys. Mech. Astron. 64, 281011 (2021))[1]. Prof. Y. F. Zhou from the Institute of Theoretical Physics, Chinese Academy of Sciences wrote a review article for this publication[2].
The operation and analysis of CDEX-1B and CDEX-10 are coming to the end, and the next generation of experiments CDEX-100/CDEX-1T are under preparation now. The lower background level and improvement of PPCGe performance can raise the sensitivity of direct detection experiment. While the next generation experiment of CDEX can discover dark matter remains unknown, but the mystery of dark matter will encourage more and more researchers to pursue its studies until the day when this profound mystery of the Universe will be solved.
[1] Y. Wang et al., (CDEX Collaboration), First experimental constraints on WIMP couplings in the effective field theory framework from CDEX, Sci. China-Phys. Mech. Astron. 64, 281011 (2021), https://doi.org/10.1007/s11433-020-1666-8 [2] Y.-F. Zhou, Improved constraints on dark matter effective interactions from CDEX, Sci. China-Phys. Mech. Astron. 64, 2841031 (2021), https://doi.org/10.1007/s11433-021-1679-4
⦿ There are two types of inflation potential we generally prefer: Concave and convex. The Planck data on cosmic microwave background indicates that the Starobinsky-type model with concave inflation potential is favored over the convex-type chaotic inflation. But why? This reason is still unclear.
⦿ Now, Chen and Yeom investigated Euclidean wormholes in the context of the inflationary scenario in order to answer the question on the preference of a specific shape of the inflaton potential.
⦿ They argued that if our universe began with a Euclidean wormhole, then the Starobinsky-type inflation is probabilistically favored.
⦿ They showed that only one end of the wormhole can be classicalized for a convex potential, while both ends can be classicalized for a concave potential. The latter is therefore more probable.
⦿ Their study point towards the fact that its not the universe but the wormhole which is expanding
How did the universe begin? This has long been one of the most fundamental questions in physics. The Big Bang scenario, when tracing back to the Planck time, indicates that the universe should start from a regime of quantum gravity that is describable by a wave function of the universe governed by the Wheeler-DeWitt (WDW) equation. The WDW equation is a partial differential equation and hence it requires a boundary condition. This boundary condition allows one to assign the probability of the initial condition of our universe. As is well known, to overcome some drawbacks of the Big Bang scenario, an era of inflation has been introduced. Presumably, the boundary condition of the WDW equation would dictate the nature of the inflation.
The Planck data on cosmic microwave background (CMB) indicates that certain inflation models are more favored than some others. In particular, the Starobinsky-type model with concave inflation potential (V” < 0 when the inflation is dominant.) appears to be favored over the convex-type (V” > 0) chaotic inflation. Is there any reason for this? Now, Chen and Yeom argued that if our universe began with a Euclidean wormhole, then the Starobinsky-type inflation is probabilistically favored.
One reasonable assumption for the boundary condition of the WDW equation was suggested by Hartle and Hawking, where the ground state of the universe is represented by the Euclidean path integral between two hypersurfaces. The Euclidean propagator can be described as follows:
where gµν is the metric, φ is an inflaton field, SE is the Euclidean action, and h (Sys. (a,b), stat. (µν)) and χ^a,b are the boundary values of gµν and φ on the initial (say, a) and the final (say, b) hypersurfaces, respectively. Using the steepestdescent approximation, this path integral can be well approximated by a sum of instantons, where the probability of each instanton becomes P ∝ e^−S_E. This approach has been applied to different issues with success: (1) It is consistent with the WKB approximation, (2) It has good correspondences with perturbative quantum field theory in curved space, (3) It renders correct thermodynamic relations of black hole physics and cosmology. These provide Chen and Yeom the confidence that the eventual quantum theory of gravity should retain this notion as an effective description.
In their original proposal, Hartle and Hawking considered only compact instantons. In that case it is proper to assign the condition for only one boundary; this is the so-called no-boundary proposal. In general, however, the path integral should have two boundaries. If the arrow of time is symmetric between positive and negative time for classical histories, then one may interpret this situation as having two universes created from nothing, where the probability is determined by the instanton that connects the two classical universes. Such a process can be well described by the Euclidean wormholes.
Now, Chen and Yeom investigated Euclidean wormholes in the context of the inflationary scenario in order to answer the question on the preference of a specific shape of the inflaton potential. They showed that only one end of the wormhole can be classicalized for a convex potential, while both ends can be classicalized for a concave potential. The latter is therefore more probable.
“We investigated Euclidean wormholes with a non-trivial inflaton potential. We showed that in terms of probability, the Euclidean path-integral is dominated by Euclidean wormholes, and only the concave potential explains the classicality of Euclidean wormholes. This helps to explain, in our view, why our universe prefers the Starobinsky-like model rather than the convex-type chaotic inflation model.“
— told Chen, first author of the study
It should be mentioned that there exist other attempts to explain the origin of the concave inflation potential. For example, it was reported by Hertog in his paper that, the Starobinsky-like concave potential is preferred if a volume-weighted term is added to the measure. Chen and Yeom note that the same principle can be applied not only to compact instantons but also to Euclidean wormholes; hence, this proposal may support their result as well. They must caution, however, that the justification of such a volume-weighted term is theoretically subtle.
This is of course not the end of the story. One needs to further investigate whether this Euclidean wormhole methodology is compatible with other aspects of inflation. It will also be interesting to explore the relation between the probability distribution of wormholes and the detailed shapes of various inflaton potentials. Furthermore, if this Euclidean wormhole creates any bias from the Bunch-Davies state, then it may in principle be confirmed or falsified by future observations. They left these topics for future investigations.
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