ICTS

Cell division orchestrates the fundamental task of replication in living systems. The cleavage of a single mother cell into two daughter cells is driven by the active constriction of a cytokinetic ring containing actin filaments, crosslinkers, and myosin motors. The ingression kinetics of the cytokinetic ring can be either symmetric or asymmetric in the division plane. In other words, the cleavage of the mother cell can begin unilaterally from one end, with the other end ingressing in a delayed manner such that the cell is cleaved asymmetrically. Our aim is to develop an understanding of the physics that governs asymmetric ingression of the cytokinetic ring. On the timescales of cytokinesis, the actomyosin cortex can be considered as anisotropic viscous fluid with inherent active stresses. We will propose a simple and promising computational model of an isotropic active fluid that shows asymmetric ingression.

We examine the existence of Ricci solitons on (κ, µ)-almost cosymplectic (acs) manifolds. Our study shows that on (κ < 0, µ)-acs manifolds, Ricci solitons are actually expanding, and (κ = 0, µ)-acs manifolds admit arbitrary Ricci solitons. Additionally, we analyze the behavior of Ricci solitons on (κ, µ)-acs manifolds with the Jacobi-type potential vector field and with the potential vector field V aligned with the structure vector field ξ.

Magnetic fields in astrophysical objects can be explained by the con-version of turbulent kinetic energy into magnetic energy, a process known as dynamo. When the dynamo generates a magnetic field on a scalesmaller than the correlation scale of the flow, it is called fluctuation dy-namo. During its kinematic stage, when the Lorentz force is negligible, themagnetic field grows exponentially over time. Subsequently, the magnetic field reaches saturation in the non linear stage.Astrophysical plasmas undergo collapse in various contexts, including galaxy formation and star formation. How does this collapse influence the growth of magnetic field in the kinematic stage of the fluctuation dy-namo? Analytically, we show a super-exponential growth of the magnetic field during the kinematic stage of Fluctuation dynamo in a homologous collapse of plasma, assuming delta correlated velocity field in time. This growth in magnetic field is in addition to the magnetic field growth due to flux freezing. These calculations are carried out by solving the induction equation in comoving variables, which are introduced to account for the overall collapse. We have performed simulations of MHD equations in comoving vari-ables within a periodic box to study the fluctuation dynamo in collapsing plasma. These simulations confirmed the presence of super-exponential growth of the magnetic field, which can have influence on the structure formation in our universe.

The past decade has seen rapid developments in the study of Cosmological correlators. These correlators are mostly calculated on the Ground state wavefunctional of the Universe (Hartle-Hawking state). But on solving the Wheeler DeWitt equation in the large volume limit on an asymptotically de-Sitter spacetime we find states other than the Hartle-Hawking state. I will be discussing the general idea of cosmological correlators and computing them in Non-Hartle Hawking states.

In this talk, I will describe the promising prospects of probing, and investigating the strong field effects of black holes using Dynamical Horizons, Numerical Relativity, and their associated gravitational radiation.

We study the properties of massive fields extrapolated to the blowup of spatial infinity (\hat{i}^0), extending the program initiated in arXiv:2207.06406. In the free theory, we find an explicit representation for boundary two-point functions and boundary to bulk two point functions and also present an explicit smearing function that can be used to represent local bulk operators in terms of smeared boundary operators. We study interactions and find that, generically, interacting massive fields decay slower than free-fields as one approaches \hat{i}^0. We propose that meaningful correlators at \hat{i}^0 can be obtained through a LSZ-like prescription that isolates the on-shell part of bulk Wightman functions before extrapolating them to \hat{i}^0. We show that operators at \hat{i}^0, defined via this prescription, equal the average of “in” and “out” operators defined at i^+ and i^− respectively. We present several sample calculations.

Large cells often rely on cytoplasmic flows for intracellular transport, maintaining homeostasis, and positioning cellular components. Understanding the mechanisms of these flows is essential for gaining insights into cell function, developmental processes, and evolutionary adaptability. Here, we focus on a class of self-organized cytoplasmic stirring mechanisms that result from fluid-structure interactions between cytoskeletal elements at the cell cortex. Drawing inspiration from streaming flows in late-stage fruit fly oocytes, we propose an analytically tractable active carpet theory. This model deciphers the origins and three-dimensional spatio-temporal organization of such flows. Through a combination of simulations and weakly nonlinear theory, we establish the pathway of the streaming flow to its global attractor: a cell-spanning vortical twister. Our study reveals the inherent symmetries of this emergent flow, its low-dimensional structure, and illustrates how complex fluid-structure interaction aligns with classical solutions in Stokes flow. This framework can be easily adapted to elucidate a broad spectrum of self-organized, cortex-driven intracellular flows.

I will discuss the Hertz-Debye formalism, adapted to arbitrary dimensional de Sitter space, to solve electromagnetic field equations in an expanding spacetime.

Many-body chaos is an important characterization of dynamical systems. Recently, there has been a tremendous surge in the interest of studying chaos in many-body quantum or classical systems. In this talk, I shall discuss how chaos sets within the intermediate time in a symmetry-breaking phase of a classical interacting spin model. Starting with the recently developed formalism of the decorrelation function, we shall discuss how a localized quenched defect in the ordered phase can give rise to secondary lightcones. These are particularly interesting in the sense that in the long time limit, the scattering from the secondary lightcones gives rise to the actual propagation of a primary chaotic front. We shall also discuss the dynamical effects on the defect and link the emergence of chaos with low-energy spin-wave excitations.

In situ observations and satellite altimetry data have confirmed the coexistence of energetic internal waves and eddies in different parts of the ocean. Even though several aspects of eddy induced tracer transport in the ocean are reasonably explored, internal wave effects on tracer transport are yet to be understood in detail. This work investigates wave induced tracer transport by advecting passive tracers in three different flow regimes with varying wave-eddy energy ratio.We observe an order of magnitude increase in the downscale transfer rate of tracer variance in wave dominated flows which suggests an enhanced dispersion of tracers. We quantify this enhancement using an efficiency parameter and see that flows with very low wave-eddy energy ratio have an efficiency which is about 20 times less than that of wave dominated flows.Our results suggest that internal waves play a pivotal role in enhancing the dispersion of tracers.

We present a scaling theory of the many-body localisation transition in terms of characteristic energyscales which emerge from probing the multifractality (or lack thereof) of the spectral decomposition of eigenstates at different energyscales. These scales correspond to the ones, above which the spectral decompositions exhibit their global behaviour, namely full ergodicity in the ergodic phase and multifractality in the many-body localised phase. On the other hand, at scales below the characteristic ones, the decomposition in the ergodic phase shows finer (multi)fractal structures whereas in the localised phase, the decomposition picks out well-separated, localised resonant peaks. The scaling of these characteristic energyscales across the many-body localisation transition bears striking resemblances to that of inverse participation ratios of eigenstates and admits a scaling theory consistent with a Kosterlitz-Thouless type scenario at the transition.

We consider a harmonically confined 1d short-range Riesz gas consisting of $N$ particles in equilibrium at finite temperature. The particles interact with each other through a repulsive power-law interaction with an exponent $k>1$. We study the probability distribution of the number of particles in the $(-\infty, W]$ called index distribution, denoted by $\mathcal{I}(W, N)$. We analyze the probability distribution of $\mathcal{I}(W, N)$ and show that it exhibits a large deviation form for large $N$ characterised by a speed $N^{\frac{3k+2}{k+2}}$ and by a large deviation function of the fraction $c$ of the particles inside the domain and $W$. The density profiles that create the large deviations, display interesting shape transitions which gets manifested as a third-order phase tranistion. Our Monte-Carlo (MC) simulations show good agreement with the expression for density profiles. We find that the typical fluctuations of $\mathcal{I}(W, N)$, obtained from our field theoretic calculations are Gaussian distributed with a variance that scales as $N^{\nu_k}$, with $\nu_k = (2-k)/(2+k)$. We also present some numerical findings on the mean and the variance. Furthermore, we find the thermodynamic pressure and the bulk modulus.

Our planet is a self-sustaining ecosystem powered by light energy from the sun, but roughly closed to matter. Many ecosystems on Earth are also approximately closed to matter and recycle nutrients by self-organizing stable nutrient cycles, e.g., microbial mats, lakes, open ocean gyres. However, existing ecological models do not exhibit the self-organization and dynamical stability widely observed in such planetary-scale ecosystems. Here, we advance a conceptual model that explains the self-organization, stability, and emergent features of closed microbial ecosystems. Our model incorporates the bioenergetics of metabolism into an ecological framework. By studying this model, we uncover a crucial thermodynamic feedback loop that enables metabolically diverse communities to almost always stabilize nutrient cycles. Surprisingly, highly diverse communities self-organize to extract 10 of the maximum extractable energy, or 100 fold more than randomized communities. Further, with increasing diversity, distinct ecosystems show strongly correlated fluxes through nutrient cycles. However, as the driving force from light increases, the fluxes of nutrient cycles become more variable and species-dependent. Our results highlight that self-organization promotes the efficiency and stability of complex ecosystems at extracting energy from the environment, even in the absence of any centralized coordination.

Fluid inertia imparts rich dynamics to leaves falling from trees. We show that bodies with two planes of symmetry can display a range of behaviors even without inertia. Any such body supports a conserved quantity in its dynamics, and is either a settler, a drifter or a flutterer, depending only on its shape. At large time, settlers and drifters respectively fall vertically and obliquely, while flutterers rotate forever while executing intricate patterns. The dynamics of flutterers decouples into a periodic and a Floquet part with different time scales, giving periodicity or quasiperiodicity. We design a set of bodies and use the boundary integral method to show that settlers, drifters and flutterers, all lie in this set.

TBA

We analyze Yukawa interactions in bulk using real-time formalism (grSK geometry), focusing on Yukawa scattering against a black hole with Hawking radiation. Our approach involves deriving an EFT in the exterior of the black hole and developing a diagrammatic understanding of scattering processes. We study these scattering processes, using boundary correlators obtained from these diagrams. Here, we have explicitly evaluated these correlators at the tree level in the bulk.

We use data driven methodologies to parametrise a tracer field in a reduced flow model in rotating frame with forcing. A neural network is trained to parametrise the tracer field evolution as a function of the flow vorticity field. We then also explore symbolic expression in terms of functional operators to get another Parameterising function. This overcomes the non interpretability disadvantage of a neural network parameterisation.

We find that, in the presence of weak incoherent effects from surrounding environments, the zero temperature conductance of nearest neighbour tight-binding chains exhibits a counter-intuitive power-law growth with system length at band-edges, indicating superballistic scaling. This fascinating environment assisted superballistic scaling of conductance occurs over a finite but extended regime of system lengths. This scaling regime can be systematically expanded by decreasing the coupling to the surrounding environments. There is no corresponding analog of this behavior for isolated systems. This superballistic scaling stems from an intricate interplay of incoherent effects from surrounding environments and exceptional points of the system's transfer matrix that occur at every band-edge.

Classification problems are ubiquitous in mathematics. In algebraic geometry these are familiar as moduli problems. A moduli space may be understood as the parameter space of a class of objects: associative/commutative algebras, vector bundles, complex manifolds, topological branes, SUSY vacua are some examples. Deformation theory is an area of algebraic geometry which deals with the local study of moduli spaces. In this talk we see a way to associate to any deformation of an object an action of ‘symmetries of a point’ on that object. If time permits, we relate deformations to symmetries in the context of topological field theories.

I will discuss how the balanced flow in the ocean is affected by waves generated by the tides at different length scales.

Can a butterfly flapping its wings in Bangalore cause a hurricane in Texas? I will argue that in the macroscopic world small changes necessarily lead to small responses. Microscopic chaos does not lead to loss of predictability in the macroscopic world.

Orbital eccentricity of coalescing compact binaries produces a strong imprint in their gravitational waves (GWs). Its presence indicates at dynamically assembled binaries in dense stellar environments like globular clusters. Hence, detecting an eccentric merger will significantly enhance our knowledge about the formation channels of these binaries. However, GW detectors are yet to unambiguously detect eccentric binaries. But the prospects will improve in the near future as current detectors reach their design sensitivities and the proposed highly-sensitive 3rd-generation detectors join the global GW detector network. Their broader sensitive frequency band will help in not only detecting more GW events but also in observing much longer inspirals, thereby substantially improving the eccentricity measurements.
Theoretical GW waveform models in the near future will therefore require improvements not only in their physics content but also in their evaluation speeds so that accurate analyses of GW events can be arrived at in pace with the increasing amounts of incoming data. Surrogate models are fast and accurate approximations of waveform models created using data-driven methods like reduced bases and empirical interpolation. However, eccentric binary black hole waveforms have much more features than their quasi-circular counterparts and hence are trickier to model efficiently. In this talk, I will discuss some strategies for building their efficient and accurate surrogate models.

Gravitational lensing of gravitational waves (GWs) offers a compelling opportunity to investigate the spacetime geometry in the vicinity of the lens. In this talk, we look into the effects of lensing-induced diffraction modulations in the GW signal and the prospects of constraining the lens parameters. Parameter inference of the lensed waveforms requires the template generation for these modulated signals to be computationally expeditious. We introduce a method founded on a 'greedy algorithm' for rapidly generating microlensed GW signals, tailored to astrophysically relevant lens models.

We examine the dynamics of sub-Kolmogorov anisotropic particles sedimenting through turbulence. Such systems are ubiquitous in nature, for example, the ice-crystals in Cirrus and mixed-phase clouds. The orientation dynamics of these ice crystals, modelled as spheroids, plays a crucial role in the planetary greenhouse effect. We perform direct numerical simulations(DNS) of such spheroids settling through a homogeneous isotropic turbulent flow field, including the effects of gravity on both the particle translational and rotational degrees of freedom. Particle orientation distributions are obtained over a wide range of spheroid aspect ratios, Stokes numbers and particle settling velocities. For all cases examined, distributions peak at the broadside-on (to gravity) orientation, and depart significantly from Gaussianity. These DNS results have been compared against theoretical predictions in the inertialess rapid-settling limit, when a particle sediments through a Kolmogorov eddy much faster than the latter decorrelates. The DNS results deviate from theory for Stokes numbers of order unity due to a spatially inhomogeneous particle concentration field resulting from a preferential sweeping effect. The spatial inhomogeneity of the particle distribution is characterized via a correlation dimension D2, allowing an estimate of clustering effects down to the Kolmogorov scale. D2 is found to be shape sensitive, with the degree of clustering being more for extreme shapes. We also show that due to the effects of gravity, more extreme shaped spheroids approach each other faster for St<=0.4 as compared to the spherical particles. The aforementioned results on particle clustering and relative velocity of the particles can be used to model a collision kernel for anisotropic particles.

We provide an algorithm to get part of the spectrum of a Clock Model that describes a system of lattice parafermions. The novelty in this algorithm is that it reduces the complexity of the problem, albeit it only approximates some of the eigenvalues of the Hamiltonian.

In recent years, there has been a renewed interest in exploring deformations of the Veneziano amplitude within the framework of the modern S-matrix bootstrap program. In this talk, we will provide a brief overview of some of the recent progress in the study of the deformations of the Veneziano amplitude. We will briefly discuss the implications of the unitarity condition on these amplitudes, focussing particularly on the q-deformed Veneziano amplitude.

In this work we study the interaction between an eddy and wind generated near inertial waves. Using freely evolving numerical simulations, we investigate the transition in wave energy levels required to destroy an eddy completely, across different regimes ranging from small to O(1) Rossby numbers. By formulating a criterion to distinguish between partially and completely destroyed vortices, we found that anticyclones exhibit a lower threshold for destruction compared to cyclones. We also observe the differences in the mode of destruction between cyclones and anticyclones.

In this work, we investigate the transport properties of thin-film Weyl-semimetal using linear response theory. Specifically, we studied the temperature dependence of the Drude peak and the low-frequency characteristic of the conductivity with first-order Born approximation in the presence of weak disorder. Furthermore, we investigate the dimensional crossover of the conductivity in a different range of frequency in terms of the scaling rule with respect to the disorder strength and compared it with the one dimension, two dimensions, and three-dimensional cases separately.

Resetting is a renewal mechanism in which a process is intermittently repeated after a random or fixed time. This simple act of stop and repeat profoundly influences the behaviour of a system as exemplified by the emergence of non-equilibrium properties and expedition of search processes. Herein, we explore the ramifications of stochastic resetting in the context of a single-file system called random average process (RAP) in one dimension. In particular, we focus on the dynamics of tracer particles and analytically compute the variance, equal time correlation, autocorrelation and unequal time correlation between the positions of different tracer particles. Our study unveils that resetting gives rise to rather different behaviours depending on whether the particles move symmetrically or asymmetrically. For the asymmetric case, the system for instance exhibits a long-range correlation which is not seen in absence of the resetting. Similarly, in contrast to the reset-free RAP, the variance shows distinct scalings for symmetric and asymmetric cases. While for the symmetric case, it decays (towards its steady value) as $\sim e^{-r t} / \sqrt{t}$, we find $\sim t e^{-r t}$ decay for the asymmetric case ($r$ being the resetting rate). Finally, we examine the autocorrelation and unequal time correlation in the steady state and demonstrate that they obey interesting scaling forms at late times.

Our research investigates how magnetic fields impact the gravitational waves emitted by binary neutron stars during the late inspiral stage. Neutron star binaries formed through dynamical capture may possess strong magnetic fields and a substantial eccentricity during this stage, necessitating a thorough study of these systems. These magnetic fields can affect orbital dynamics by causing additional tidal deformation in the companion (corrections to tidal love numbers), radiating orbital energy (motion of magnetic dipoles), and inducing attractive or repulsive effects based on the orientation of the magnetic dipoles. To measure these effects, we analyze the changes in gravitational wave cycles, accumulated dephasing, and energy loss rates of gravitational and electromagnetic radiation.

We study 1D stochastic models with two conservation laws. One of the models is the coupled continuum stochastic Burgers equation, for which each current is a sum of quadratic non-linearities, linear diffusion, and spacetime white noise. The second model is a two-lane stochastic lattice gas. Here we tune the two conserved densities such that the flux Jacobian, a 2 × 2 matrix, has coinciding eigenvalues. In the steady state, we investigate spacetime correlations of the conserved fields and the time-integrated currents at the origin. For certain choice of couplings the dynamical exponent 3/2 is confirmed. Moreover, at these couplings, continuum stochastic Burgers equation and lattice gas are demonstrated to be in the same universality class. (This is based on arXiv:2401.06399.)

Third generation gravitational wave (GW) detectors are expected to detect millions of binary black hole (BBH) mergers during their operation period. A small fraction of them (∼ 1%) will be strongly lensed by intervening galaxies and clusters, producing multiple observable copies of the GW signals. The expected number of lensed events and the distribution of the time delay between lensed images strongly depend on the mass distribution of dark matter halos. Warm dark matter (WDM) or fuzzy dark matter (FDM) models predict lower abundances of small mass halos as compared to the standard cold dark matter. This will result in a reduction in the number of strongly lensed GW events, especially at small time delays. Using the total number of lensed events and the time delay distribution, we can put a lower bound on the mass of the WDM/FDM particle from a catalog of lensed GW events. The expected bounds from GW strong lensing from third-generation GW detectors are better than the current constraints.

Understanding the intricate network of nonlinear interactions crucial for the development and sustenance of turbulence induced by magnetorotational instability (MRI) has proven challenging. A large-scale dynamo, generating dominant azimuthal magnetic fields, emerges as a pivotal component of this turbulence. Direct numerical simulations of MRI dynamo have revealed statistical self-organization into large-scale cyclic dynamics. However, comprehending the underlying physics of these statistical states and assessing their astrophysical significance present theoretical hurdles. Through our newly developed direct statistical simulations, we have successfully identified several new dynamo mechanisms responsible for generating different components of the large-scale magnetic fields. In this talk, I will delve into the fundamental physics associated with the dynamo cycle and elucidate the properties and implications of the resulting cyclic patterns within turbulent accretion disks.

Machine learning (ML) based parametrisation for submesoscale geophysical flows We construct a d-dimensional Eddy Damped Quasi-Normal Markovian (EDQNM) Closure Model to study properties of magnetohydrodynamic (MHD) turbulence in arbitrary dimensions for a wide range of Prandtl num- bers and compressibility. In particular, we use this model to find the lower dL and upper dU critical dimensions for sustained dynamo action in the incompressible problem. Through numerical simulations of this EDQNM model, we find that the lower critical dimension dL ≈ 2.03 is very close to 2 while dU ≈ 6.0.

In this talk we consider the two-type Moran model with $N$ individuals. Each individual is assigned a resampling rate, drawn independently from a probability distribution $\P$ on $\R_+$, and a type, either $\h$ or $\s$. Each individual resamples its type at its assigned rate, by adopting the type of an individual drawn uniformly at random. Let $Y^N(t)$ denote the empirical distribution of the resampling rates of the individuals with type $\h$ at time $Nt$. We show that if $\P$ has countable support and satisfies certain tail and moment conditions, then in the limit as $N\to\infty$ the process $(Y^N(t))_{t \geq 0}$ converges in law to the process $(S(t)\,\P)_{t \geq 0}$, in the so-called Meyer-Zheng topology, where $(S(t))_{t \geq 0}$ is the Fisher-Wright diffusion with diffusion constant $D$ given by $1/D = \int_{\R_+} (1/r)\,\P(\d r)$. This is ijoint work with Frank den Hollander and Adrian Roellin

One of the challenges in numerical relativity is to include future null infinity in the computational domain with a well-posed formulation. Success will not only enable us to evolve any system of astrophysical interest, e.g. binary black holes and extracting the gravitational wave signal at future null infinity, with any desired accuracy, but also help in studying various phenomena of fundamental interest. One proposal is to use hyperboloidal slices. In this talk, I shall give an alternative approach to numerical relativity that uses hyperboloidal slices, present our ongoing efforts for obtaining a well-posed formulation of the Einstein Field Equations on these slices, and finally propose numerical schemes that assure stability and convergence for linear hyperbolic systems on these slices for long times. A natural extension will be to generalize these numerical methods to full Einstein Field Equations with suitable initial data.

Gravitational waves (GWs) emitted from astrophysical sources can get lensed on their way to Earth, similar to electromagnetic waves. Claims have emerged suggesting that detections made by LIGO and Virgo in earlier observational runs exhibit signs of lensing phenomena. Lensing has been primarily invoked to explain the discovered high-mass events, the bimodal mass distribution of black holes, and objects in the mass-gap region. A merger rate model (BDS model) has been proposed to account for these claims. In this work, we critically examine these arguments and see if they are consistent with a variety of observational data. We performed astrophysical simulations assuming galactic black hole mass distribution (across the cosmological distances) and the proposed merger rate model, considering both weak and strong lensing scenarios. The inferred mass and redshift distributions of compact binaries are then compared with the GWTC-3 catalog. Furthermore, we analyze the stochastic GW background across the parameter space of the merger rate model to evaluate its validity with existing upper limits. Our preliminary findings suggest that the mass distributions inferred from LIGO-Virgo observations may be devoid of lensing effects.