AstroPhysics Seminar

שלחו לחבר
Usual Time
Thursday 17:00
Place
Physics (Building 202), Room 301
More Details

The seminars have been adapted to Zoom

All recordings appear online, here: https://www.youtube.com/playlist?list=PLCq9Go28tBlCFfRXw5WAmFYc7ORznISXj

If you'd like to give a talk, email ofek.birnholtz@biu.ac.il and/or asaf.peer@biu.ac.il

Upcoming Lectures
- Forecasting the Solar Wind with Sequential Monte Carlo Assimilation of Satellite Data Grant Meadors, Los Alamos National Lab (LANL)
Grant Meadors, Los Alamos National Lab (LANL)
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Space weather affects life on Earth and in outer space. Human technologies are affected by coronal mass ejections and similar outbursts of solar activities. Accurate prediction of the solar wind and its polarity can help understand the Sun and its dynamic environment. The Wang-Sheeley-Arge (WSA) phenomenological model of the coronal magnetic field can estimate solar-wind speed and interplanetary magnetic field polarity in the inner heliosphere by using photospheric magnetic field maps. WSA has historically used two parameters, the source surface and interface radii, to tune its predictions. In this talk, I describe how our team used sequential Monte Carlo, also called particle filtering, in the assimilation of satellite data to adjust the values of these radii. Adaptive optimization, applied to week-long timescales across several months of historical data, yielded approximately double predictive performance. In addition to improved forecasts, this statistical study highlights challenges in parameter estimation for the nearest and most-observed solar-mass object: the Sun.

*Approved for Los Alamos Unlimited Release: LA-UR-21-21918.

- TBA Uri Kol, New York University (NYU)
Uri Kol, New York University (NYU)
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TBD

- TBA Vassilios Mewes, Oak Ridge National Lab
Vassilios Mewes, Oak Ridge National Lab
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Vassilios Mewes

National Center for Computational Sciences and Physics Division, Oak Ridge National Laboratory

- Enhancing gravitational waveform models through dynamic calibration and regression Yoshinta Setyawati, Max-Planck-Institut für Gravitationsphysik (Utrecht University & Albert-Einstein-Institut)
Yoshinta Setyawati, Max-Planck-Institut für Gravitationsphysik (Utrecht University & Albert-Einstein-Institut)
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Fast and accurate binary-black-hole (BBH) merger waveform models that span wide parameter ranges are crucial for future searches and parameter estimation of gravitational-wave data. To date, analytical waveforms that incorporate numerical-relativity information, such as effective-one-body and phenomenological models, play an important role in analysing LIGO and Virgo data. However, these models are not automatically updated every time new numerical waveforms become available. Here we present a new perspective on dynamically tuning waveform models by incorporating sparse information from a more accurate model. We also show the first attempts to use our method to include additional physical effects that were not present in the original model and investigate various techniques that include interpolation and regression implemented in the development of waveform modeling.
 
The talk will be given over Zoom, at https://zoom.us/j/9290951953

 

- Flux-based statistical prediction of three-body outcomes Barak Kol, Hebrew University of Jerusalem, Israel (HUJI)
Barak Kol, Hebrew University of Jerusalem, Israel (HUJI)
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The three-body problem in Newtonian gravity is one of the oldest and richest problems in physics. Giants have worked on it and it has been the source of numerous fields in theoretical physics and mathematics including perturbation theory, topology and chaos. Yet, it remains unsolved, and the associated statistical theory remains incomplete and flawed even after almost fifty years of work. Inspired by recent beautiful work of Nick Stone and his collaborator Leigh, I have developed a reduction of the outcome probability distribution. In this sense, I believe the problem has been cracked, as will described in the talk.

The talk is based on https://arxiv.org/abs/2002.11496

- TBA Daniel Wysocki, University of Wisconsin–Milwaukee / CGCA
Daniel Wysocki, University of Wisconsin–Milwaukee / CGCA
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TBD

- TBA Alex Nielsen, Stavanger University (Norway)
Alex Nielsen, Stavanger University (Norway)
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TBD

- CARMENES: a radial velocity search for planets around M dwarfs Lev Tal Or, Ariel
Lev Tal Or, Ariel
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CARMENES is a radial-velocity (RV) survey for exoplanets around nearby M dwarf stars.Using a high resolution dual-channel spectrograph, it provides M-star RV measurements with a precision down to ~1 m/s. In three years of surveying ~325 nearby M-dwarfs, we have detected about a dozen of new planets, including habitable-zone Earth-mass planets and planets that challenge planet-formation models. Some of the new planets are amenable for characterization by next-decade direct-imaging and astrometric instruments. CARMENES’ unique design allows addressing additional questions related to M dwarfs and close-in planets, such as the spectroscopic manifestation of photospheric activity and rotation, magnetic field strength of active stars, and the atmospheric composition of hot Jupiters. I will give a brief overview of the latest results from CARMENES. If time is left I will comment on a simple way of correcting for systematic errors in large RV surveys.

The talk is at 17:00 Jerusalem Time, on Zoom: https://zoom.us/j/9290951953  

Previous Lectures
- BH Spectroscopy and testing strong field gravity Swetha Bhagwat, La Sapienza University, Roma
Swetha Bhagwat, La Sapienza University, Roma
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In this talk I will talk about a recent work where we propose a new test of GR. The gravitational waves emitted during the coalescence of binary black holes offers an excellent probe to test the behaviour of strong gravity at different length scales. In this work, we propose a test called the merger-ringdown consistency test that focuses on probing horizon-scale dynamics of strong-gravity using the binary black hole ringdowns. This test is a modification of the more traditional inspiral-merger-ringdown consistency test. I will present a proof-of-concept study of this test using simulated binary black hole ringdowns embedded in the Einstein Telescope-like noise. Furthermore, we use a deep learning framework, setting a precedence for performing precision tests of gravity with neural networks.

The talk is at 17:00 Jerusalem Time (16:00 CET), on Zoom: https://zoom.us/j/9290951953  

- The deepening Mystery of the Vela Radio-Pulsar Glitch Greg Ashton, Monash University & Royal Holloway, University of London
Greg Ashton, Monash University & Royal Holloway, University of London
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The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

- Unification of Inflation and Dark Energy: Quintessential Inflation from Lorentzian Slow Roll David Benisty, Ben Gurion University & Frankfurt Institute for Advanced Studies
David Benisty, Ben Gurion University & Frankfurt Institute for Advanced Studies
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From the assumption that the slow roll parameter ϵ has a Lorentzian form as a function of the e-folds number N, a successful model of a quintessential inflation is obtained, as succinctly studied in \cite{Benisty:2020xqm}. The form corresponds to the vacuum energy both in the inflationary and in the dark energy epochs and satisfies the condition to climb from small values of ϵ to 1 at the end of the inflationary epoch. We find the corresponding scalar Quintessential Inflationary potential with two flat regions. Moreover, a reheating mechanism is suggested with numerical estimation for the homogeneous evolution of the universe. The suggested mechanism is consistent with the BBN bound.

Based on: https://arxiv.org/abs/2006.04129

The talk will be given on Zoom:

We will meet about 30 minutes before the official start time for virtual mingling and informal chat

Maayane Soumagnac
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We have entered a golden age for studying astrophysical “transients” (short and violent astrophysical events). Recent advances in the field of large-scale, high-cadence astronomical surveys have provided unprecedentedly rich and diverse data sets and transient astronomy is said to have entered a “big data era”. I will give a short overview of several on-going and future instruments which are real « game-changers » in the field and will show how combining their assets, as well as using tools from  "data science" - involving high-performance computing and data mining - can lead to promising new science. I will then share recent results from the first UV survey of the early evolution of interacting supernovae and from mining twenty years of X-ray archival data in search for the shortest transients, including the elusive signatures of the mysterious supernova "shock breakout".

 

Note the unusual date/time (Wednesday morning). 

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

- Strong Gravity Tests with Tidal Effects Yotam Sherf, Ben-Gurion University
Yotam Sherf, Ben-Gurion University
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In this talk, we discuss the exciting possibilities of exploring strong gravity with future GW observations. The properties of Black-Holes (BH) and exotic compact objects (ECOs) immersed in a tidal environment are discussed. In particular, we focus on the BH reaction and its induced quadrupole moment to an external tidal field. The no-Love number theorem for GR BHs is thoroughly reviewed and alternative explanations are suggested. Here, I'll present for the first time how simple quantum mechanical arguments support the existence of the Love number in quantum black-holes, moreover I'll show how the detection of these quantum induced effects is possible with future precision gravitational wave measurement.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The start time is 17:00 Jerusalem time

- The study of white dwarfs: a glimpse into our future Uri Malamud, Technion
Uri Malamud, Technion
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Several billion years into the future, our Sun will break away from the main sequence. It would expand, turning into a giant star, eventually shedding its outer layers to become a white dwarf -- a perpetually fading remnant of its former glory. We will not be around to see this. Planet Earth will be engulfed in the process. In the outer Solar system, planets and minor planets will be baked by the intense radiation from the giant Star, becoming active as comets do. Their orbits will expand, and this would give rise to rich dynamical interactions. In the aftermath of this calamity, many surviving objects would be injected into tidal crossing orbits of our Sun's ultra-dense successor, the white dwarf. As they do, they will be violently and repeatedly ripped apart, breaking into their smallest constituent building blocks. While we cannot hope to glimpse our own future, nature has given us a unique
opportunity to triumphantly jubilate as we watch the demise of other, less fortunate exo-planetary systems. In my talk I would briefly discuss various topics related to white dwarf atmospheric pollution by exo-planetary remnants: focusing on the properties and chemistry of the polluters ; the formation of debris discs and compact accretion discs; and the growing number of recent discoveries of individual (disintegrating or intact) minor and major exo-planets observed in orbit of white dwarfs.

The talk will be give over Zoom, at https://zoom.us/j/9290951953

 

 

- Dynamical Horizons in Binary Black Hole Mergers Daniel Pook-Kolb, Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut)
Daniel Pook-Kolb, Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut)
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Marginally outer trapped surfaces (MOTSs) are the main tool in numerical relativity to infer properties of black holes in simulations of dynamical systems. In this talk, I will present results that show how we can understand a merger of two black holes in terms of the evolution of these MOTSs. This closes a gap in our understanding of binary-black-hole mergers and provides the quasilocal analog of the famous "pair-of-pants" picture of the event horizon of two merging black holes. In particular, we will encounter three new phenomena: (i) the merger of MOTSs, (ii) the formation of self-intersecting MOTSs immediately after this merger, and (iii) a non-monotonicity result for the area of certain smoothly evolving MOTSs. Finally, I will show a remarkable correspondence between the evolution of the horizon geometry at late times and the quasi-normal modes which describe the ringdown signal measurable by far away observers.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

- Photon Ring Autocorrelations in Event Horizon Telescope Shahar Hadar, Harvard
Shahar Hadar, Harvard
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Abstract: In the presence of a black hole, light sources connect to observers along multiple paths. As a result, observed brightness fluctuations must be correlated across different times and positions in black hole images. Photons that execute multiple orbits around the black hole appear near a critical curve in the observer sky, giving rise to the photon ring. In the talk I will describe the structure of a Kerr black hole's photon ring. I will then discuss a novel observable we have recently proposed: the two-point correlation function of intensity fluctuations on the ring. This two-point function exhibits a universal, self-similar pattern consisting of multiple peaks of identical shape: while the profile of each peak encodes statistical properties of fluctuations in the source, the locations and heights of the peaks are determined purely by the black hole parameters. Measuring these peaks would demonstrate the existence of the photon ring without resolving its thickness, and would provide estimates of black hole mass and spin. With regular monitoring over sufficiently long timescales, this measurement could be possible via interferometric imaging with modest improvements to the Event Horizon Telescope.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

- Gravitational Wave detection challenges: past, present and future Marco Drago, Gran Sasso Science Institute, L'Aquila, Italy
Marco Drago, Gran Sasso Science Institute, L'Aquila, Italy
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A new astronomy started in 2015 with the first gravitational wave detection. Since then, LIGO-Virgo collaborations have confirmed several
events originating from the coalescence of binary systems composed by compact objects. While for these type of events the wave signature is
clear and well characterized, for other possible sources in the universe we are lacking of a complete modelization, so that it is necessary to
adopt alternative approaches to discover. We overview one of these approaches extensively used in the search for gravitational wave in the
LIGO-Virgo scientific runs, exploting its performances and possible applications. We also introduces the future challenges, when the next generation of detectors will become operative.

*Incidentally, Marco Drago was the first person to see a gravitational wave - and he'll be available to chat before and after the talk

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

- A new Conservation law for Surface Explosions Almog Yalinewich, Canadian Institute for Theoretical Astrophysics, Toronto
Almog Yalinewich, Canadian Institute for Theoretical Astrophysics, Toronto
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Many astrophysical phenomena involve an abrupt release of a large amount of energy close to the surface of a large body. Examples include impacts on a terrestrial planet and outbursts from a neutron star while inside the common envelope of a giant star. In this talk I will present a new universal analytic solution that can describe the shock wave in all these scenarios. I will show that this shock wave satisfies a new kind of conservation law that lies somewhere in between energy and momentum conservation. This conservation law opens the door to a myriad of insights about a wide range of physical problems: the size and shapes of craters, atmospheric mass loss from giant impacts and oblique shock breakout from supernovae.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for virtual mingling and informal chat

- Generating initial data for Binary Neutron Star simulations using LORENE Tanmayee Gupte, Rochester Institue of Technology
Tanmayee Gupte, Rochester Institue of Technology
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The recent detection of gravitational waves (GW) from a system of binary neutron stars (BNS) in coincidence with electromagnetic observations has launched a new era of multimessenger astrophysics. As a result, BNS mergers are one of the main targets for GW interferometer detectors on earth. A particularly interesting challenge is to constraint the equation of state (EOS) of the nuclear matter inside the neutron star core, which is still theoretically unknown. In order to do parameter estimation and detect additional GW signals, we need to compare the observed signals to theoretical GW templates, which depend on different characteristics like total mass, EOS, mass ratio, etc. Limited work has been previously done with simulating unequal-mass BNS because of numerical difficulties. We have modified the LORENE code to advance our ability to construct unequal-mass BNS initial data, and used them to initiate dynamical evolutions of BNS mergers performed using the Einstein Toolkit. Here we discuss the importance of Initial Data and the modifications done to the LORENE code. 

 

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

- The Secret Life of Black Widow Companions Sivan Ginzburg, Berkeley
Sivan Ginzburg, Berkeley
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Black Widows are rapidly spinning magnetized neutron stars with companions that are only a few percent the mass of the Sun. I will present numerical stellar evolution tracks showing how main sequence stars are reduced to such low masses by magnetic braking and Roche-lobe overflow. The numerical results are explained by an analytical model, similar to the Hayashi track, but accounting for the pulsar’s gamma-ray irradiation. I will compare the theory to radio and gamma-ray observations of the pulsars, as well as to novel optical images of the companions themselves. I will demonstrate that the mass at which a Black Widow companion becomes fully convective is a simple function of its orbital period, allowing us to study stellar structure and magnetism away from the main sequence in a controlled manner. 

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

- Multi-messenger Constraints on the Equation-of-State of Dense Nuclear Matter Ben Margalit, UC Berkeley
Ben Margalit, UC Berkeley
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The equation-of-state of cold asymmetric (neutron-rich) matter at supra-nuclear densities is a long-standing open question. It cannot currently be calculated from first principles theory (QCD), nor can this regime be directly probed through terrestrial experiments. Neutron stars (NSs) therefore provide a unique laboratory for studying cold dense matter. In this talk I will describe progress in this field obtained by utilizing both gravitational-wave and electromagnetic observations of NS mergers. An overview will be given of several different methods of constraining the equation-of-state using NS mergers, the strengths and caveats associated with each will briefly be discussed, as will the future outlook for this newly-emerging field.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

- Constraining binary evolution via Interpolating detailed model likelihoods: GWTC-1 and StarTrack Vera Delfavero, Rochester Institute of Technology
Vera Delfavero, Rochester Institute of Technology
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Gravitational wave observations can now strongly differentiate between assumptions for how binary compact objects form. Different models for compact binary formation can be ranked by their similarity to GW observations, as a marginal likelihood. In this work, we show how to carefully interpolate this marginal likelihood between model parameters, enabling posterior distributions for these model parameters. Using the StarTrack binary evolution code, we compare one- and three-dimensional models to the compact binary mergers reported in GWTC-1. Consistent with prior work, with our one- dimensional models we infer that modest natal kicks are more consistent with the observed merger rates and mass distributions.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Tel-Aviv time, but we'll start gathering 30 minutes earlier for informal chat

Note: Israel switches to Winter Time on October 25th, and the US on November 1st, so 17:00 in Tel-Aviv means 11:00 in Rochester

- Star Formation in the Multiphase-Turbulent Interstellar Medium - from Early Cosmic Times to the Milky Way Shmuel Bialy, Harvard Center for Astrophysics, Boston
Shmuel Bialy, Harvard Center for Astrophysics, Boston
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*** Recording available here: https://www.youtube.com/watch?v=qwhaQQBQY4c&feature=youtu.be ***

 

I will start with a broad review of the field of star formation and galaxy evolution, and some pressing open questions. I will then dive into the star-forming interstellar medium (ISM), asking the question, what regulates the star formation process?

I will discuss the multiphase structure of the ISM, key heating-cooling processes and chemical processes in the ISM, and interstellar turbulence, all of which may play an important role in regulating star formation. I will focus on a particularly appealing theory for star formation where gas heating by far-UV radiation from young stars (and by cosmic-rays in some galaxies), may provide a natural feedback loop, and thus organically self-regulate star-formation in galactic disks, and present recent results (Bialy 2020, ApJ accepted) for the link between star-formation rate and far-UV radiation intensity.

I will conclude with future prospects: Charting new ways for constraining poorly known interstellar properties: turbulence, 3D ISM structure, low energy cosmic-ray spectra, and our plan to construct an improved star-formation model for next-generation large scale cosmological simulations (i.e., IllustrisTNG successors).

 

 

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for virtual mingling and informal chat

- Asymptotic Approximants for Closed-Form Models of Epidemics Nate Barlow, Rochester Institute of Technology (RIT), NY, USA
Nate Barlow, Rochester Institute of Technology (RIT), NY, USA
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An accurate closed-form solution is obtained to the SIR Epidemic Model and to the SEIR Epidemic Model through the use of Asymptotic Approximants (Barlow et al., 2017). An analytic solution is obtained to the SEIR Epidemic Model. The solution is created by constructing a single second-order nonlinear differential equation in ln(S) and analytically continuing its divergent power series solution such that it matches the correct long-time exponential damping of the epidemic model. The utility of the analytical form is demonstrated through its application to the COVID-19 pandemic.
Similar methods have been used to model geodesics around Kerr black holes, and are under development for modeling gravitational waveforms from binary black  hole mergers, from the inspiral to the ringdown stages.
 
The talk and discussion will be hosted on Zoom: https://zoom.us/j/9290951953
The start time is 8:30 EDT / 15:30 Jerusalem time.

 

- Gravitational waves from Inspiraling Eccentric Binaries: Modeling & Data analysis Srishti Tiwari, Tata Institute of Fundamental Research, Mumbai, India
Srishti Tiwari, Tata Institute of Fundamental Research, Mumbai, India
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PhD Thesis Synopsis for Srishti Tiwari, our incoming postdoc

Zoom link: https://zoom.us/j/9290951953 

Black Widow Evolution Sivan Ginzburg, Berkeley
Sivan Ginzburg, Berkeley

Black widows are millisecond pulsars with low-mass companions (~2% the mass of the sun) on short orbits of several hours. When the first black widow was discovered in 1988, it was proposed that its companion is the remnant of a main sequence star that had been evaporated by the pulsar’s high energy radiation. I will present new observations from the last decade that challenge this picture, and discuss how the growing population of black widows can be explained consistently.

The talk will be given over Zoom, at: https://zoom.us/j/9290951953

The official talk starts at 16:30 Jerusalem Time (06:30 PDT), but we'll start preparing and mingling on Zoom from 16:00 and onwards

Detection of gravitational-wave signals from binary neutron star mergers using machine learning Marlin B. Schäfer, Albert Einstein Institute (AEI) Hannover, Germany
Marlin B. Schäfer, Albert Einstein Institute (AEI) Hannover, Germany

As two neutron stars merge, they emit gravitational waves that can potentially be detected by earth bound detectors. Matched-filtering based algorithms have traditionally been used to extract quiet signals embedded in noise. We introduce a novel neural-network based machine learning algorithm that uses time series strain data from gravitational-wave detectors to detect signals from non-spinning binary neutron star mergers. For the Advanced LIGO design sensitivity, our network has an average sensitive distance of 130 Mpc at a false-alarm rate of 10 per month. Compared to other state-of-the-art machine learning algorithms, we find an improvement by a factor of 6 in sensitivity to signals with signal-to-noise ratio below 25. However, this approach is not yet competitive with traditional matched-filtering based methods. A conservative estimate indicates that our algorithm introduces on average 10.2 s of latency between signal arrival and generating an alert. We give an exact description of our testing procedure, which can not only be applied to machine learning based algorithms but all other search algorithms as well. We thereby improve the ability to compare machine learning and classical searches.

The seminar will be given online via Zoom: https://zoom.us/j/9290951953

The official talk time is 16:30 Jerusalem Time (15:30 CEST), but we'll be around from 16:00 (15:00) for connection tests and chatter

 

Time Travel and Its Paradoxes Barak Shoshany, Perimeter Institute of Theoretical Physics
Barak Shoshany, Perimeter Institute of Theoretical Physics

I will discuss the possibility and feasibility of time travel within the context of general relativity and quantum field theory, the paradoxes resulting from it, and possible ways to resolve these paradoxes. The talk will be based on arXiv:1907.04178 and arXiv:1911.11590.

 

The talk will be given over Zoom, at: https://zoom.us/j/9290951953

The official talk starts at 16:30 Jerusalem Time (09:30 EDT), but we'll hstart mingling on Zoom from 16:00 and onwards

Searching for unexpected signals in the LIGO gravitational wave events Paolo Marcoccia, University of Stavanger, Norway
Paolo Marcoccia, University of Stavanger, Norway

The first Gravitational Wave detection of GW150914 led to a revolution in the world of modern physics and astronomy,

beyond being an additional confirmation for the predictions of Einstein's General Relativity,

it opened the gates of Experimental Physics to data that couldn't have been observed in any other way before that,

and capable of testing theories that before the GW era had no other way of being proved.

In my talk, i'll analyze some of the LIGO results from a different point of view respect to the one commonly adopted by LIGO,

by looking for statistically significant correlations in the data between the LIGO gravitational wave detectors.

Said method, even though wouldn't be the best choice for a blind search of new gravitational wave events, may be used to infer  properties of already known detections,

as well as testing deviation of the data from the prediction of standard theories and test out new ones.

The seminar will be given over Zoom, at: https://zoom.us/j/9290951953

The time (16:30) is In Jerusalem Time (15:30 CEST)

- Companions and debris around white dwarfs Na'ama Hallakoun, Weizmann Institute of Science
Na'ama Hallakoun, Weizmann Institute of Science
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The immediate surroundings of white dwarfs (WDs) are key to our understanding of a number of puzzles. Observations of WDs can reveal the presence of stellar, substellar, and stellar-remnant companions, planets, dust, atmospheric heavy elements, and planetary debris, each of relevance to several important questions. The remains of the pre-WD-phase solar systems are revealed in the form of heavy element 'pollution' in WD atmospheres, excess emission from dust discs, and–only recently–in transits of planetary debris. In principle, WDs can host not only debris, but also whole planetary systems. Binary systems consisting of two WDs are important in a broad range of astrophysical contexts, from stellar evolution, through Type-Ia supernova (SN Ia) progenitors, to sources of gravitational waves.

SNe Ia–supernova explosions of WDs–are a major source of heavy elements, and, as 'standard candles', they have provided one of the fundamental methods for estimating distances in the Universe. However, the nature of the progenitor systems of SNe Ia is still unclear. A progenitor scenario that has been long considered is the double-degenerate scenario, in which a double WD binary loses energy and angular momentum to gravitational waves, until merger and possible explosion as a SN Ia. If most SN Ia explosions are the result of double WD mergers, then the observed double WD merger rate should be high enough to account for the observed SN Ia rate.

In my talk I will present some of the clues we have found for these questions.

The talk will be given over Zoom, meeting link is  https://zoom.us/j/9290951953 

Quantum Black Hole Seismology Niayesh Afshordi
Niayesh Afshordi

I will start by motivating why some observational probes of astrophysical black holes in a quantum theory might be radically different from their classical ones. I will then show that these signatures can be best probed by searching for low frequency harmonics in the gravitational wave spectrum of perturbed black holes, what we call "quantum black hole seismology". Finally, I will end by summarizing the (controversial) observational status of these searches and their future outlook. 

The seminar will be given over Zoom, at: https://zoom.us/j/9290951953

The time (16:30) is In Jerusalem Time (09:30 EDT)

Universal signatures of a black hole’s photon ring Shahar Hadar, Harvard
Shahar Hadar, Harvard

The Event Horizon Telescope image of the supermassive black hole in the galaxy M87 is dominated by a bright, unresolved ring. General relativity predicts that embedded within this image lies a thin “photon ring,” which is composed of an infinite sequence of self-similar subrings that are indexed by the number of photon orbits around the black hole. The subrings approach the edge of the black hole “shadow,” becoming exponentially narrower but weaker with increasing orbit number, with seemingly negligible contributions from high order subrings. In the talk, I will discuss the structure of the photon ring, starting with non-rotating black holes, and then proceed to the complex patterns that emerge when rotation is taken into account. Subsequently I will argue that the subrings produce strong and universal signatures on long interferometric baselines. These signatures offer the possibility of precise measurements of black hole mass and spin, as well as tests of general relativity, using only a sparse interferometric array.

 

The talk will be given over Zoom, meeting link is  https://zoom.us/j/9290951953 

- Emission channels from perturbed black-holes Yotam Sherf, Ben-Gurion University
Yotam Sherf, Ben-Gurion University
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We study the emission of gravitational waves, gravitons, photons and neutrinos from a perturbed Schwarzschild blackhole (BH).The perturbation can be due to either classical or quantum sources and therefore the injected energy can be either positive or negative.The emission can be classical in nature, as in the case of gravitational waves, or of quantum nature, for gravitons and the additional fields. We first setup the theoretical framework for calculating the emission by treating the case of a minimally coupled scalar field and then present the results for the other fields. We perform the calculations in the horizon-locking gauge in which the BH horizonis deformed, following similar calculations of tidal deformations of BH horizons.The classical emission can be interpreted as due to a partial exposure of a nonempty BH interior, while the quantum emission can be interpreted as an increased Hawking radiation flux due to the partial exposure of the BH interior. At the end we demonstrate that the quantum emission in BHs that are far away from equilibrium is comparable and even larger than the Hawking radiation

- How (and what) can we learn about exo-solar moons, dwarf and minor planets Uri Malamud
Uri Malamud
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Exomoons orbiting terrestrial or super-terrestrial exoplanets have not yet been discovered; their possible existence and properties are therefore still an unresolved question. I will present results from a recent study about the collision-formation of massive exomoons, and discuss the plausibility of detecting them currently or in the future. We are also able to infer the existence of exo-solar moons, dwarf and minor planets from the observation of polluted white dwarf atmospheres, which probe their bulk composition. Small planetary objects with tidal crossing orbits disrupt around the white dwarf and form a disk of debris, which later accrete onto the white dwarf and pollute its atmosphere. Progress in the last decade, has shown this material to be typically dry, i.e., with terrestrial-like chemical composition and lacking in water. l will discuss results from a series of studies which examine whether water-bearing small planetary objects even have the potential to retain their water, as they undergo thermal, physical, chemical and orbital evolution during the high luminosity stellar evolution phases of their host stars. If time permits I will talk briefly about new hybrid approaches for modeling the aforementioned tidal disruptions and generating debris disks.

- Virialization of Gas in Dark Matter Halos and its Implications for Galaxy Evolution Dr. Jonathan Stern
Dr. Jonathan Stern
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Gas in dark matter halos, known as the circumgalactic medium (CGM), is both the source of fuel for star formation in the galaxy and the destination of galaxy outflows. The CGM is thus expected to play a fundamental role in galaxy evolution. I will revisit the question of the virialization of the CGM, in which the volume-filling gas phase transitions from being predominantly cool and free-falling at low halo masses to predominantly hot and quasi-static at high halo masses. Using both an idealized model and the FIRE cosmological simulations, I will demonstrate that several aspects of this process have not been previously appreciated despite over four decades of research. I will then show that CGM virialization in FIRE is associated with an abrupt change in the properties of star formation and outflows in the central galaxy, and with the formation of a galaxy disc. 
David Eichler

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On the origin of the ultra-high energy cosmic-rays נעמי גלובוס
נעמי גלובוס

Cosmic-rays are one of the most fascinating phenomena in the universe. They consist of energetic particles with an out-of-equilibrium power-law spectrum extending over at least eleven orders of magnitude in energy, from ~1 GeV to 10^11 GeV. In the past decade, new measurements by experiments such as the Pierre Auger observatory and Telescope Array, have greatly improved our knowledge of the highest energy domain of the cosmic-ray spectrum, the "ultra-high energy cosmic-rays" (UHECR), with energies > 10^9 GeV. At these energies, cosmic-rays are thought to be of extragalactic origin and they are highly challenging by questions with respect to their origins and their acceleration processes. 

I will first review the observational data on the cosmic-ray spectrum, composition and arrival directions. I will show that the spectrum and composition can be explained by a generic model having one Galactic component and one extragalactic component. I will review the multi-messenger constrains brought by neutrino and gamma-ray experiments on UHECR origin. Finally, I will discuss the origin of the UHECR dipole anisotropy recently reported by the Pierre Auger Observatory.

אופק בירנהולץ
Introducing the physics of gravitational waves and compact binary coalescences, and their detection and analysis by LIGO. Focusing on the catalog from LIGO's first 2 Observation runs O1+O2, as well as engagement opportunities for new students and researchers towards O3, and the future of LIGO and next generation detectors.
Gravitational Lensing by Galaxy Clusters: Unveiling the Dark Universe אלינור מידז׳ינסקי
אלינור מידז׳ינסקי

The most fundamental question in observational cosmology today is what is the nature of dark energy and dark matter. As the most massive gravitationally bound bodies in the Universe, clusters of galaxies serve as beacons to the growth of structure over cosmic scales, making them a sensitive cosmological tool. However, accurately measuring their masses has been notoriously difficult. Weak lensing provides the best direct probe of the cluster mass, both the baryonic and dark components, but it requires high-quality wide-field imaging. With its unprecedentedly deep and exquisite seeing, the Subaru Hyper Suprime-Cam (HSC) survey is an ongoing campaign to observe 1,400 square degrees. In this talk, I will present our new field-leading results from the first HSC data release of ~150 square degrees that encompass thousands of clusters. Harnessing our new HSC survey, I measure benchmark weak lensing cluster masses, and reconcile previous tension on cosmological parameters between the SZ and CMB within the Planck survey. The next generation of wide-field surveys is almost upon us, with the Large Synoptic Survey Telescope (LSST), WFIRST and several more coming online. They will discover hundreds of thousands of galaxy clusters, peering deep to the epoch of formation. I will describe these exciting new surveys and the multifold breakthrough science we will achieve in the new era of astronomy.

Exoplanets: From Detection to Characterization אביב אופיר
אביב אופיר

Exoplanets are almost never visible and thus remained unknown over centuries of astronomical research.  In this talk, I will explain how exciting discoveries of new worlds are now made, and surprising aspects of their characteristics are determined. This is accomplished by creative methods and dedicated telescopes on Earth and in space.  I will review the observational techniques for studying exoplanets and focus on transits – the passage of an exoplanet in front of its host star.  This seemingly simple geometry allows a surprising array of insights: from detailed transit analysis, we constrain the most fundamental planetary properties relevant for the system architecture, theories of planet formation, evolution, composition, global weather patterns, and some day, even biomarkers.  The relentless pace of discovery during the past two decades is expected not only to continue but even intensify in the future.

יוסי שוורצולד

Over the last three decades, our knowledge about planetary systems has increased dramatically, from one example with eight planets (our own Solar system) to over 2800 planetary systems hosting more than 3700 planets. While occurrence rate studies show that exoplanets are the rule rather than an exception, our understanding of the physical processes forming these planets is still very limited. Fortunately, we are now on the verge of the next revolution in exoplanet science. TESS, PLATO, JWST, WFIRST, and LSST will complete the demographic census of planets across a wide range of environments, and will allow detailed characterization of their atmospheres and structure.

   In this talk I will discuss the important role of microlensing in the forefront of exoplanetary studies. Gravitational microlensing is unique in its ability to probe several important but relatively untapped reservoirs of exoplanet parameter space, including the abundance and mass-function of cold planets, planet-formation efficiency in different Galactic environments, and the population of free-floating planets. A wealth of new and upcoming microlensing campaigns, both from ground and space, will allow the full exploration of the exoplanet demographics unique to microlensing, potentially revolutionizing our understanding of planet formation. In addition to studying planets, these surveys allow to study important regimes of the stellar mass function (e.g., massive remnants, isolated brown-dwarfs) and to to study the Galactic structure and evolution.