Urjit A. Yajnik
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Previous Organizations / Institutes
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Urjit A. Yajnik has a total of 150 co-authors. Here are the top co-authors by number of shared publications:
- Manu B. Paranjape
Groupe de Physique des Particules, Département de Physique, Université de Montréal, C.P. 6128, succ. centre-ville, Montréal, Québec, H3C 3J7, Canada
12 shared publications - Chayan Majumdar
Institute of Modern Physics
9 shared publications - Richard MacKenzie
Université de Montréal, Complexe des sciences
9 shared publications - Piyali Banerjee
School of Engineering and Technology Nagaland University 797112 Dimapur India
8 shared publications - Dong-Han Yeom
National Taiwan University
7 shared publications - Supriya Senapati
Department of Physics Indian Institute of Technology Bombay 400 076 Powai Mumbai Maharashtra India
7 shared publications
Latest Publications
- 8 August 2026
- Published by World Scientific Pub Co Pte Ltd in International Journal of Modern Physics D
Reflecting gravitons: The graviton laser and the Gertsenshtein effect
Graviton lasers have been considered in the past [A. Landry and M. B. Paranjape, Int. J. Mod. Phys. D 25 (2016) 1644016], but practical terrestrial implementations appear infeasible. The absence of any known mechanism to reflect gravitons means that it remains unclear how a graviton beam could be directed repeatedly through a putative lasing medium. Astrophysical graviton lasing is still a possibility as circular graviton orbits around black holes afford the possibility of an arbitrarily long path length through the lasing medium of ultra-light dark matter [E. Dupuis and M. B. Paranjape, Int. J. Mod. Phys. D 27 (2018) 1847009; B. Avila-Lopez, R. MacKenzie, F. Mendez and M. B. Paranjape, Int. J. Mod. Phys. D 34 (2025) 2544003]. In this essay, we consider the possibility of a graviton laser that could be constructed in a laboratory setting. The graviton lasing medium could be one of many possible gravitating systems, of which we give three possible examples. We calculate the possibility of reflecting the gravitons by using the conversion of gravitons into photons in an external magnetic field, the Gertsenshtein effect [M. E. Gertsenshtein, Sov. Phys. JETP 14 (1962) 84]. We may convert the gravitons to photons, then reflect the photons, then reconvert the photons into gravitons via the same effect, and then pass them through the graviton lasing medium. With an identical apparatus on the other side, we can essentially extend the path length of the gravitons through the lasing medium as arbitrarily long as desired.
- 8 August 2026
- Published by World Scientific Pub Co Pte Ltd in International Journal of Modern Physics D
Reflecting gravitons: The graviton laser and the Gertsenshtein effect
Graviton lasers have been considered in the past [A. Landry and M. B. Paranjape, Int. J. Mod. Phys. D 25 (2016) 1644016], but practical terrestrial implementations appear infeasible. The absence of any known mechanism to reflect gravitons means that it remains unclear how a graviton beam could be directed repeatedly through a putative lasing medium. Astrophysical graviton lasing is still a possibility as circular graviton orbits around black holes afford the possibility of an arbitrarily long path length through the lasing medium of ultra-light dark matter [E. Dupuis and M. B. Paranjape, Int. J. Mod. Phys. D 27 (2018) 1847009; B. Avila-Lopez, R. MacKenzie, F. Mendez and M. B. Paranjape, Int. J. Mod. Phys. D 34 (2025) 2544003]. In this essay, we consider the possibility of a graviton laser that could be constructed in a laboratory setting. The graviton lasing medium could be one of many possible gravitating systems, of which we give three possible examples. We calculate the possibility of reflecting the gravitons by using the conversion of gravitons into photons in an external magnetic field, the Gertsenshtein effect [M. E. Gertsenshtein, Sov. Phys. JETP 14 (1962) 84]. We may convert the gravitons to photons, then reflect the photons, then reconvert the photons into gravitons via the same effect, and then pass them through the graviton lasing medium. With an identical apparatus on the other side, we can essentially extend the path length of the gravitons through the lasing medium as arbitrarily long as desired.
- Open Access
Relativistic quantum mechanics and quantum field theory
Relativistic quantum mechanics can be considered to have begun with a search for wave equations corresponding to each intrinsic spin. However, relativistic quantum physics differs fundamentally from the non-relativistic wave mechanics. It requires a formalism allowing \ creation and destruction of particles. This gets proper treatment only in a framework called quantum field theory. This article is a semi-historic account of the intriguing new features which emerge as a part of quantum field theory. Such a discussion is impossible without a basic presentation of the formalism itself. Hence some mathematics is included in finer print. The article is directed mostly to those familiar with essential classical mechanics and basic quantum mechanics, though I strive to provide a flavour of the subject to the keenly interested non-physics reader.
Open AccessRelativistic quantum mechanics and quantum field theory
Relativistic quantum mechanics can be considered to have begun with a search for wave equations corresponding to each intrinsic spin. However, relativistic quantum physics differs fundamentally from the non-relativistic wave mechanics. It requires a formalism allowing \ creation and destruction of particles. This gets proper treatment only in a framework called quantum field theory. This article is a semi-historic account of the intriguing new features which emerge as a part of quantum field theory. Such a discussion is impossible without a basic presentation of the formalism itself. Hence some mathematics is included in finer print. The article is directed mostly to those familiar with essential classical mechanics and basic quantum mechanics, though I strive to provide a flavour of the subject to the keenly interested non-physics reader.
- 1 January 2025
- Published by Springer Nature in Journal of the Indian Institute of Science
- Vol. 105 (1) , 43-62
Relativistic Quantum Mechanics and Quantum Field Theory
- 1 January 2025
- Published by Springer Nature in Journal of the Indian Institute of Science
- Vol. 105 (1) , 43-62
Relativistic Quantum Mechanics and Quantum Field Theory
- 8 August 2024
- Published by American Physical Society (APS) in Physical Review D
- Vol. 110 (4) , 043016
Left-right symmetry breaking and gravitational waves: A tale of two phase transitions
We study possible ways gravitational waves (GW) are sourced in a theory with minimal left-right symmetry breaking. Generically first order phase transitions (FOPT) lead to gravitational waves sourced by bubble dynamics, while second order phase transitions (SOPT) do not. However, due the presence of two degenerate fields, we obtain domain walls in the putative SOPT case, giving rise to GW via disintegrating domain walls, testable at experiments such as IPTA, DECIGO, and LISA. On the other hand, for the case of FOPT, we get the usual signal from spontaneously created bubbles, but there also arises a late forming domain wall structure separating the two types of vacua. The disintegration of these walls provides an additional source of GW. Thus the parameter range signaling FOPT case gives rise to two distinct peaks in the spectrum of GW. This is verifiable for the low symmetry breaking scales , but a high scale such as remains beyond the reach of currently planned experiments. Finally, we point out that a version of the left-right symmetric model which separates the scale of parity breaking from that of gauge symmetry breaking is also subject to domain wall formation and amenable to GW observations.- 8 August 2024
- Published by American Physical Society (APS) in Physical Review D
- Vol. 110 (4) , 043016
Left-right symmetry breaking and gravitational waves: A tale of two phase transitions
We study possible ways gravitational waves (GW) are sourced in a theory with minimal left-right symmetry breaking. Generically first order phase transitions (FOPT) lead to gravitational waves sourced by bubble dynamics, while second order phase transitions (SOPT) do not. However, due the presence of two degenerate fields, we obtain domain walls in the putative SOPT case, giving rise to GW via disintegrating domain walls, testable at experiments such as IPTA, DECIGO, and LISA. On the other hand, for the case of FOPT, we get the usual signal from spontaneously created bubbles, but there also arises a late forming domain wall structure separating the two types of vacua. The disintegration of these walls provides an additional source of GW. Thus the parameter range signaling FOPT case gives rise to two distinct peaks in the spectrum of GW. This is verifiable for the low symmetry breaking scales , but a high scale such as remains beyond the reach of currently planned experiments. Finally, we point out that a version of the left-right symmetric model which separates the scale of parity breaking from that of gauge symmetry breaking is also subject to domain wall formation and amenable to GW observations.- Open Access
Gravitational production of massive scalars in the context of inflation
We set up a formalism for calculating the energy density generated in a quantized massive scalar field in the course of the drastic change in spacetime geometry at the end of the inflationary era. The calculation relies on the notion of adiabatic vacuum. The Bogolubov coefficients are computed by employing the sudden approximation. After obtaining a general formula, we calculate explicitly the energy density generated in a particle species with $m/H \ll1$, where $m$ is the particle mass and $H$ is the Hubble constant during the inflationary epoch. We find the contribution of the long-wavelength modes to be $\propto H^5/m$. If such particles are very weakly interacting, they can come to dominate the total energy density in the Universe. Other cosmological implications are also discussed.Open AccessGravitational production of massive scalars in the context of inflation
We set up a formalism for calculating the energy density generated in a quantized massive scalar field in the course of the drastic change in spacetime geometry at the end of the inflationary era. The calculation relies on the notion of adiabatic vacuum. The Bogolubov coefficients are computed by employing the sudden approximation. After obtaining a general formula, we calculate explicitly the energy density generated in a particle species with $m/H \ll1$, where $m$ is the particle mass and $H$ is the Hubble constant during the inflationary epoch. We find the contribution of the long-wavelength modes to be $\propto H^5/m$. If such particles are very weakly interacting, they can come to dominate the total energy density in the Universe. Other cosmological implications are also discussed. - Open Access
Left-Right Symmetry Breaking and Gravitational Waves : A Tale of Two Phase Transitions
We study possible ways gravitational waves (GW) are sourced in a theory with minimal left-right symmetry breaking. Generically first order phase transitions (FOPT) lead to gravitational waves sourced by bubble collisions, while second order phase transitions (SOPT) do not. However, due the presence of two degenerate fields, we obtain domain walls in the putative SOPT case, giving rise to GW via decaying domain walls, testable at experiments such as IPTA, DECIGO, and LISA. On the other hand, for the case of FOPT, we get the usual signal from spontaneously created bubbles, but also a late forming domain wall structure separating the two types of vacua. Thus the traditional FOPT case gives rise to two distinct peaks in the spectrum of GW, verifiable for the low symmetry breaking scales $10^4 - 10^6$ GeV, but beyond the reach of currently planned experiments for a high scale $\sim10^{10}$ GeV. Finally, we point out that a version of the left-right symmetric model which separates parity breaking from gauge symmetry breaking is also subject to domain wall formation and amenable to GW observations.Open AccessLeft-Right Symmetry Breaking and Gravitational Waves : A Tale of Two Phase Transitions
We study possible ways gravitational waves (GW) are sourced in a theory with minimal left-right symmetry breaking. Generically first order phase transitions (FOPT) lead to gravitational waves sourced by bubble collisions, while second order phase transitions (SOPT) do not. However, due the presence of two degenerate fields, we obtain domain walls in the putative SOPT case, giving rise to GW via decaying domain walls, testable at experiments such as IPTA, DECIGO, and LISA. On the other hand, for the case of FOPT, we get the usual signal from spontaneously created bubbles, but also a late forming domain wall structure separating the two types of vacua. Thus the traditional FOPT case gives rise to two distinct peaks in the spectrum of GW, verifiable for the low symmetry breaking scales $10^4 - 10^6$ GeV, but beyond the reach of currently planned experiments for a high scale $\sim10^{10}$ GeV. Finally, we point out that a version of the left-right symmetric model which separates parity breaking from gauge symmetry breaking is also subject to domain wall formation and amenable to GW observations. - Open Access
Gravitational wave signature of generic disappearance of $Z_2$-symmetry breaking domain walls
Breaking of discrete parity at high scale gives rise to $Z_2$-domain walls (DW). The metastability of such walls can make them relatively long lived and contradict standard cosmology. We consider two classes of theories with similar underlying feature, the left right symmetric theories and two Higgs doublet models. Both of them possess some breaking of $Z_2$ discrete symmetries. As a first step, domains form at a high energy scale during parity breaking. In the second step, these domains further decompose into subdomains due to $Z_2$ symmetry breaking in two Higgs doublet models closer to the electroweak scale. We show that after this two step formation of domains and subdomains, a QCD instanton induced energy difference can remove the domain walls as well as the subdomain walls at around the same time successfully. The removal occurs purely as the result of a chance event taking place with probability very close to 0.25, and does not require one to introduce any non-renormalisable $Z_2$-symmetry breaking term to the Lagrangian. We then investigate the gravitational waves arising from the collapse of such domain walls and show that the peak frequency of these waves lies in the $10^{-7}$--$10^{-6}~\mbox{Hz}$ band, corresponding to annihilation temperatures of $1$--$10$ GeV. This frequency band is sensitive to pulsar timing array based experiments such as SKA and NANOGrav. The recent NANOGrav results rule out our DW collapse model for higher values of parity breaking scale above $10^7$ GeV. Our DW collapse model with parity breaking scales below $10^7$ GeV remains consistent with the current NANOGrav results and has a good chance of being seriously tested in future pulsar timing based experiments.Open AccessGravitational wave signature of generic disappearance of $Z_2$-symmetry breaking domain walls
Breaking of discrete parity at high scale gives rise to $Z_2$-domain walls (DW). The metastability of such walls can make them relatively long lived and contradict standard cosmology. We consider two classes of theories with similar underlying feature, the left right symmetric theories and two Higgs doublet models. Both of them possess some breaking of $Z_2$ discrete symmetries. As a first step, domains form at a high energy scale during parity breaking. In the second step, these domains further decompose into subdomains due to $Z_2$ symmetry breaking in two Higgs doublet models closer to the electroweak scale. We show that after this two step formation of domains and subdomains, a QCD instanton induced energy difference can remove the domain walls as well as the subdomain walls at around the same time successfully. The removal occurs purely as the result of a chance event taking place with probability very close to 0.25, and does not require one to introduce any non-renormalisable $Z_2$-symmetry breaking term to the Lagrangian. We then investigate the gravitational waves arising from the collapse of such domain walls and show that the peak frequency of these waves lies in the $10^{-7}$--$10^{-6}~\mbox{Hz}$ band, corresponding to annihilation temperatures of $1$--$10$ GeV. This frequency band is sensitive to pulsar timing array based experiments such as SKA and NANOGrav. The recent NANOGrav results rule out our DW collapse model for higher values of parity breaking scale above $10^7$ GeV. Our DW collapse model with parity breaking scales below $10^7$ GeV remains consistent with the current NANOGrav results and has a good chance of being seriously tested in future pulsar timing based experiments. - Open Access
- 9 January 2024
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2024 (1) , 1-29
Multi-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals
We study multi-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals. The Rényi entropy evaluation involves computing the partition function of the theory on Riemann surfaces with genus g > 1. This makes Rényi entropy sensitive to the local conformal algebra of the theory. The free compact boson possesses a global U(1) symmetry with respect to which we resolve Rényi entropy. The multi-charged moments are obtained by studying the correlation function of flux-generating vertex operators on the associated Riemann surface. Symmetry-resolved Rényi entropy is then obtained from the Fourier transforms of the charged moments. Rényi entropy is shown to have the familiar equipartition into local charge sectors upto the leading order. The multi-charged moments are also essential in studying the symmetry resolution of mutual information. The multi-charged moments of the self-dual compact boson and massless Dirac fermion are also shown to match for the cases when the associated reduced density moments are known to be the same. Finally, we numerically check our results against the tight-binding model.
- 9 January 2024
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2024 (1) , 1-29
Open AccessMulti-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals
We study multi-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals. The Rényi entropy evaluation involves computing the partition function of the theory on Riemann surfaces with genus g > 1. This makes Rényi entropy sensitive to the local conformal algebra of the theory. The free compact boson possesses a global U(1) symmetry with respect to which we resolve Rényi entropy. The multi-charged moments are obtained by studying the correlation function of flux-generating vertex operators on the associated Riemann surface. Symmetry-resolved Rényi entropy is then obtained from the Fourier transforms of the charged moments. Rényi entropy is shown to have the familiar equipartition into local charge sectors upto the leading order. The multi-charged moments are also essential in studying the symmetry resolution of mutual information. The multi-charged moments of the self-dual compact boson and massless Dirac fermion are also shown to match for the cases when the associated reduced density moments are known to be the same. Finally, we numerically check our results against the tight-binding model.
- Open Access
- 3 January 2024
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2024 (1) , 1-24
Gravitational wave signature of generic disappearance of Z2-symmetry breaking domain walls
Breaking of discrete parity at high scale gives rise to Z 2-domain walls (DW). The metastability of such walls can make them relatively long lived and contradict standard cosmology. We consider two classes of theories with similar underlying feature, the left right symmetric theories and two Higgs doublet models. Both of them possess some breaking of Z 2 discrete symmetries. As a first step, domains form at a high energy scale during parity breaking. In the second step, these domains further decompose into subdomains due to Z 2 symmetry breaking in two Higgs doublet models closer to the electroweak scale. We show that after this two step formation of domains and subdomains, a QCD instanton induced energy difference can remove the domain walls as well as the subdomain walls at around the same time successfully. The removal occurs purely as the result of a chance event taking place with probability very close to 0.25, and does not require one to introduce any non-renormalisable Z 2-symmetry breaking term to the Lagrangian. We then investigate the gravitational waves arising from the collapse of such domain walls and show that the peak frequency of these waves lies in the 10 −7–10 −6 Hz band, corresponding to annihilation temperatures of 1–10 GeV. This frequency band is sensitive to pulsar timing array based experiments such as SKA and NANOGrav. The recent NANOGrav results rule out our DW collapse model for higher values of parity breaking scale above 107 GeV. Our DW collapse model with parity breaking scales below 107 GeV remains consistent with the current NANOGrav results and has a good chance of being seriously tested in future pulsar timing based experiments.
- 3 January 2024
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2024 (1) , 1-24
Open AccessGravitational wave signature of generic disappearance of Z2-symmetry breaking domain walls
Breaking of discrete parity at high scale gives rise to Z 2-domain walls (DW). The metastability of such walls can make them relatively long lived and contradict standard cosmology. We consider two classes of theories with similar underlying feature, the left right symmetric theories and two Higgs doublet models. Both of them possess some breaking of Z 2 discrete symmetries. As a first step, domains form at a high energy scale during parity breaking. In the second step, these domains further decompose into subdomains due to Z 2 symmetry breaking in two Higgs doublet models closer to the electroweak scale. We show that after this two step formation of domains and subdomains, a QCD instanton induced energy difference can remove the domain walls as well as the subdomain walls at around the same time successfully. The removal occurs purely as the result of a chance event taking place with probability very close to 0.25, and does not require one to introduce any non-renormalisable Z 2-symmetry breaking term to the Lagrangian. We then investigate the gravitational waves arising from the collapse of such domain walls and show that the peak frequency of these waves lies in the 10 −7–10 −6 Hz band, corresponding to annihilation temperatures of 1–10 GeV. This frequency band is sensitive to pulsar timing array based experiments such as SKA and NANOGrav. The recent NANOGrav results rule out our DW collapse model for higher values of parity breaking scale above 107 GeV. Our DW collapse model with parity breaking scales below 107 GeV remains consistent with the current NANOGrav results and has a good chance of being seriously tested in future pulsar timing based experiments.
- Open Access
Multi-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals
We study multi-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals. The Rényi entropy evaluation involves computing the partition function of the theory on Riemann surfaces with genus g>1. This makes Rényi entropy sensitive to the local conformal algebra of the theory. The free compact boson possesses a global U(1) symmetry with respect to which we resolve Rényi entropy. The multi-charged moments are obtained by studying the correlation function of flux-generating vertex operators on the associated Riemann surface. Symmetry-resolved Rényi entropy is then obtained from the Fourier transforms of the charged moments. Rényi entropy is shown to have the familiar equipartition into local charge sectors up to the leading order. The multi-charged moments are also essential in studying the symmetry resolution of mutual information. The multi-charged moments of the self-dual compact boson and massless Dirac fermion are also shown to match for the cases when the associated reduced density matrix moments are known to be the same. Finally, we numerically check our results against the tight-binding model.
Open AccessMulti-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals
We study multi-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals. The Rényi entropy evaluation involves computing the partition function of the theory on Riemann surfaces with genus g>1. This makes Rényi entropy sensitive to the local conformal algebra of the theory. The free compact boson possesses a global U(1) symmetry with respect to which we resolve Rényi entropy. The multi-charged moments are obtained by studying the correlation function of flux-generating vertex operators on the associated Riemann surface. Symmetry-resolved Rényi entropy is then obtained from the Fourier transforms of the charged moments. Rényi entropy is shown to have the familiar equipartition into local charge sectors up to the leading order. The multi-charged moments are also essential in studying the symmetry resolution of mutual information. The multi-charged moments of the self-dual compact boson and massless Dirac fermion are also shown to match for the cases when the associated reduced density matrix moments are known to be the same. Finally, we numerically check our results against the tight-binding model.