Embedded Files
August - December, 2020
August - December, 2020
1. Rajeev Kumar Jain, Dept. of Physics, IISc, Bangalore
1. Rajeev Kumar Jain, Dept. of Physics, IISc, Bangalore
Nov. 18, 2020 @ 4 PM (Online, via Teams)
Nov. 18, 2020 @ 4 PM (Online, via Teams)
Title: Primordial black holes from single field inflation and their observational imprints
Title: Primordial black holes from single field inflation and their observational imprints
Abstract: Primordial black holes have recently gained a lot of attention as a novel dark matter candidate. I shall discuss their generation in single field inflationary models and their observational imprints such as an induced background of gravitational waves which can be detected with the future GW observatories such as LISA.
Abstract: Primordial black holes have recently gained a lot of attention as a novel dark matter candidate. I shall discuss their generation in single field inflationary models and their observational imprints such as an induced background of gravitational waves which can be detected with the future GW observatories such as LISA.
2. Chethan Krishnan, CHEP, IISc, Bangalore
2. Chethan Krishnan, CHEP, IISc, Bangalore
Nov. 25, 2020 @ 2:30 PM (Online, via Teams)
Nov. 25, 2020 @ 2:30 PM (Online, via Teams)
Title: H0 as a Universal Diagnostic
Title: H0 as a Universal Diagnostic
Abstract: Hubble Tension is usually presented as a mismatch between local H0 measurements and the value of H0 inferred via LCDM from observations at z~1100. We will reverse this logic to argue that the (non-)constancy of the inferred value of H0 with the redshift of observational data can be used as a diagnostic of the (in)validity of cosmological models within the FLRW paradigm. A corollary is that if the current H0 tension is cosmological in origin, it must necessarily be the case that there are other inferred values of H0 from intermediate redshifts: Hubble Tension must run.
Abstract: Hubble Tension is usually presented as a mismatch between local H0 measurements and the value of H0 inferred via LCDM from observations at z~1100. We will reverse this logic to argue that the (non-)constancy of the inferred value of H0 with the redshift of observational data can be used as a diagnostic of the (in)validity of cosmological models within the FLRW paradigm. A corollary is that if the current H0 tension is cosmological in origin, it must necessarily be the case that there are other inferred values of H0 from intermediate redshifts: Hubble Tension must run.
3. Ruta Kale, NCRA-TIFR, Pune CANCELLED
3. Ruta Kale, NCRA-TIFR, Pune CANCELLED
Dec. 2, 2020 @ 4 PM (Online)
Dec. 2, 2020 @ 4 PM (Online)
Title: Megaparsec-scale phenomena in galaxy clusters: A metrewavelength view
Title: Megaparsec-scale phenomena in galaxy clusters: A metrewavelength view
Abstract: Clusters of galaxies are the Universes' most massive gravitational potential wells that hold large reservoirs of baryons in the form of diffuse gas called the intra-cluster medium (ICM). The ICM is mainly thermal gas of temperature up to ten million Kelvin that is weakly magnetised with field strengths of 0.1 - a few micro Gauss. The cosmic rays and magnetic fields, referred to as the non-thermal components, elude detection in most spectral bands and thus have remained the least understood components of the ICM. The relativistic electrons in the ICM manifest the non-thermal components at low radio frequencies (<= GHz) via synchrotron radiation providing a direct probe of their life cycles. From the radio surveys with the GMRT in the past few years, it has emerged that cluster mass and re-acceleration of seed relativistic electrons by shocks and turbulence are the factors important in the generation of the megaparsec-scale diffuse radio sources termed as radio halos, mini-halos and relics. Cluster mergers are responsible for driving shocks and turbulence in the ICM and the hadronic collisions and radio galaxies are the likely sources of the seed relativistic electrons. Although cluster mergers and radio galaxies are fairly common in massive clusters the rarity of the diffuse radio sources indicates the gaps in our understanding. With the Upgraded Giant Metrewave Radio Telescope (uGMRT) it is now possible to make radio images that are a factor of a few better in sensitivity than the GMRT and to obtain a well sampled spectrum across the frequency range of 150 - 1400 MHz. I will describe our results towards characterising the cosmic rays and magnetic fields with the uGMRT to unravel the details on the connection between cluster mergers and the re-acceleration mechanisms.
Abstract: Clusters of galaxies are the Universes' most massive gravitational potential wells that hold large reservoirs of baryons in the form of diffuse gas called the intra-cluster medium (ICM). The ICM is mainly thermal gas of temperature up to ten million Kelvin that is weakly magnetised with field strengths of 0.1 - a few micro Gauss. The cosmic rays and magnetic fields, referred to as the non-thermal components, elude detection in most spectral bands and thus have remained the least understood components of the ICM. The relativistic electrons in the ICM manifest the non-thermal components at low radio frequencies (<= GHz) via synchrotron radiation providing a direct probe of their life cycles. From the radio surveys with the GMRT in the past few years, it has emerged that cluster mass and re-acceleration of seed relativistic electrons by shocks and turbulence are the factors important in the generation of the megaparsec-scale diffuse radio sources termed as radio halos, mini-halos and relics. Cluster mergers are responsible for driving shocks and turbulence in the ICM and the hadronic collisions and radio galaxies are the likely sources of the seed relativistic electrons. Although cluster mergers and radio galaxies are fairly common in massive clusters the rarity of the diffuse radio sources indicates the gaps in our understanding. With the Upgraded Giant Metrewave Radio Telescope (uGMRT) it is now possible to make radio images that are a factor of a few better in sensitivity than the GMRT and to obtain a well sampled spectrum across the frequency range of 150 - 1400 MHz. I will describe our results towards characterising the cosmic rays and magnetic fields with the uGMRT to unravel the details on the connection between cluster mergers and the re-acceleration mechanisms.
4. Indrani Banerjee (Department of Physics and Astronomy, National Institute of Technology, Rourkela)
4. Indrani Banerjee (Department of Physics and Astronomy, National Institute of Technology, Rourkela)
Dec. 3, 2020 @ 4 PM (Online)
Dec. 3, 2020 @ 4 PM (Online)
Title: Silhouette of M87*: A new window to peek into the world of hidden dimensions
Title: Silhouette of M87*: A new window to peek into the world of hidden dimensions
Abstract: The remarkable agreement of general relativity (GR) with a host of experimental tests only adds to its phenomenal success. However, its inability to satisfactorily explain observations like galactic rota- tion curves and the accelerated expansion of the universe makes the quest for a more complete theory increasingly compelling. This may either involve modifications in the gravity sector or additions to the matter sector or both. In this talk, I will consider modifications of the gravitational field equations by invoking extra dimensions and discuss its consequences and implications on the recent observation of the shadow of the supermassive black hole M87* by the Event Horizon Telescope collaboration.
Abstract: The remarkable agreement of general relativity (GR) with a host of experimental tests only adds to its phenomenal success. However, its inability to satisfactorily explain observations like galactic rota- tion curves and the accelerated expansion of the universe makes the quest for a more complete theory increasingly compelling. This may either involve modifications in the gravity sector or additions to the matter sector or both. In this talk, I will consider modifications of the gravitational field equations by invoking extra dimensions and discuss its consequences and implications on the recent observation of the shadow of the supermassive black hole M87* by the Event Horizon Telescope collaboration.
5. Sk Minhajur Rahaman, NCRA-TIFR, Pune
5. Sk Minhajur Rahaman, NCRA-TIFR, Pune
Dec. 9, 2020 @ 4 PM (Online)
Dec. 9, 2020 @ 4 PM (Online)
Title: On the origin of pulsar radio emission: Two-stream Langmuir instability in pulsar plasma
Title: On the origin of pulsar radio emission: Two-stream Langmuir instability in pulsar plasma
Abstract: Radio emission from pulsars has a very high brightness temperature (10^25 to 10^27 K). It is at least 12 orders of magnitude higher than the theoretical limit from an incoherent synchrotron radio source. The high brightness temperature requires in-phase emission from a collection of charged particles acting as a single entity viz., charge bunches. The physics of charge bunch formation has been an open-ended problem since the discovery of pulsars. However, theoretical and observational advances made in the last two decades have put a tight constraint on the plasma parameter space in the radio emission region. The first stage of charge bunch formation requires an electrostatic and longitudinal Langmuir two-stream instability in the pulsar beam-plasma system. Pulsar beam-plasma consists of a hot and dense electron-positron pair plasma tens of millions of kelvin hot; along with tenuous and charged beams of positron and ion. This composite of pair plasma and beams is restricted to a one-dimensional ultra-relativistic outflow strictly along the super-strong ambient magnetic field in the radio emission region. The physics of charge bunch formation requires a combination of the linear and non-linear stages of plasma instability. However, to invoke a non-linear regime, it is necessary to demonstrate the presence of high growth rates in the linear regime of instability. In this talk, I will explore two models of two-stream instability viz., longitudinal drift and cloud-cloud overlap. The former involves the relative separation of the bulk velocity of the electron-positron distribution functions of pair plasma in momentum space. The latter involves the spatial overlap of successive pair plasma clouds due to intermittent pair cascade discharges at the polar gap. For both models, we use hot plasma treatment and obtain very high growth rates within a spatial window of 100-1000 km from the neutron star surface. It provides the necessary justification for a higher-order plasma theory. I will briefly discuss the ingredients for such a non-linear approach.
Abstract: Radio emission from pulsars has a very high brightness temperature (10^25 to 10^27 K). It is at least 12 orders of magnitude higher than the theoretical limit from an incoherent synchrotron radio source. The high brightness temperature requires in-phase emission from a collection of charged particles acting as a single entity viz., charge bunches. The physics of charge bunch formation has been an open-ended problem since the discovery of pulsars. However, theoretical and observational advances made in the last two decades have put a tight constraint on the plasma parameter space in the radio emission region. The first stage of charge bunch formation requires an electrostatic and longitudinal Langmuir two-stream instability in the pulsar beam-plasma system. Pulsar beam-plasma consists of a hot and dense electron-positron pair plasma tens of millions of kelvin hot; along with tenuous and charged beams of positron and ion. This composite of pair plasma and beams is restricted to a one-dimensional ultra-relativistic outflow strictly along the super-strong ambient magnetic field in the radio emission region. The physics of charge bunch formation requires a combination of the linear and non-linear stages of plasma instability. However, to invoke a non-linear regime, it is necessary to demonstrate the presence of high growth rates in the linear regime of instability. In this talk, I will explore two models of two-stream instability viz., longitudinal drift and cloud-cloud overlap. The former involves the relative separation of the bulk velocity of the electron-positron distribution functions of pair plasma in momentum space. The latter involves the spatial overlap of successive pair plasma clouds due to intermittent pair cascade discharges at the polar gap. For both models, we use hot plasma treatment and obtain very high growth rates within a spatial window of 100-1000 km from the neutron star surface. It provides the necessary justification for a higher-order plasma theory. I will briefly discuss the ingredients for such a non-linear approach.
6. Kartick C Sarkar, Racah Institute of Physics, Hebrew University of Jerusalem, Israel
6. Kartick C Sarkar, Racah Institute of Physics, Hebrew University of Jerusalem, Israel
Dec. 16, 2020 @ 4 PM (Online)
Dec. 16, 2020 @ 4 PM (Online)
Title: Non-equilibrium ionisation in a supernova remnant
Title: Non-equilibrium ionisation in a supernova remnant
Abstract: Supernovae remnants or shocks in general reveal a great deal about the interstellar medium (ISM), including hints to the galactic evolution. Dynamics and evolution of SN remnants is a classical problem and have been studied numerous times in the literature but never with a self-consistent atomic physics. In this talk, I will discuss our efforts to study the textbook problem but with a self-consistent ion network that is coupled to hydrodynamics and radiative transfer.
Abstract: Supernovae remnants or shocks in general reveal a great deal about the interstellar medium (ISM), including hints to the galactic evolution. Dynamics and evolution of SN remnants is a classical problem and have been studied numerous times in the literature but never with a self-consistent atomic physics. In this talk, I will discuss our efforts to study the textbook problem but with a self-consistent ion network that is coupled to hydrodynamics and radiative transfer.
7. Joydeep Bagchi, IUCAA Pune
7. Joydeep Bagchi, IUCAA Pune
Dec. 23, 2020 @ 4 PM (Online)
Dec. 23, 2020 @ 4 PM (Online)
Title: Unexplained Plasma Features in Galaxy Clusters
Title: Unexplained Plasma Features in Galaxy Clusters
Abstract: Ultra-sensitive next generation radio telescopes like the MeerKAT (a SKA precursor) and LOFAR have revealed new plasma features that have never been seen before in radio galaxies embedded in intra-cluster medium: such as long, collimated synchrotron threads of yet unknown origin, which link the extended and bent radio lobes of radio galaxies and plethora of other peculiar, faint features observed in galaxy clusters. I will describe such observations and discuss some possibilities, by way of sparking interest for understanding their origin.
Abstract: Ultra-sensitive next generation radio telescopes like the MeerKAT (a SKA precursor) and LOFAR have revealed new plasma features that have never been seen before in radio galaxies embedded in intra-cluster medium: such as long, collimated synchrotron threads of yet unknown origin, which link the extended and bent radio lobes of radio galaxies and plethora of other peculiar, faint features observed in galaxy clusters. I will describe such observations and discuss some possibilities, by way of sparking interest for understanding their origin.
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