Scientific Series

I discuss how exact, non-perturbative results can be obtained for both optical transport and static susceptibilities in “Hertz-Millis” theories of Fermi surfaces coupled to critical bosons. Such models possess a large emergent symmetry and anomaly structure, which we leverage to fix these quantities. In particular, I will show that in the infrared limit, the boson self energy at zero wave vector is a constant independent of frequency, and the real part of the optical conductivity is purely a delta function Drude peak with no other corrections. I will also obtain exact relations between Fermi liquid parameters as the critical point is approached from the disordered phase. 

Zoom Link:  https://pitp.zoom.us/j/93340611986?pwd=cisrZmFxcEVWZVdrT2tMRVZiVTdRQT09

String theory suggests the existence of multiple axion species forming what is known as an “axiverse”. The axions in this model are thought to have logarithmically-distributed masses extending far below 10^{-21} eV, leading to the presence of ultralight axions. The latter have astrophysical de Broglie wavelength and affect the cosmic structures in which they cluster by erasing small-scale features. In contrast to other ultralight or fuzzy dark matter models, the string axiverse allows for a mixed dark sector with a subdominant ultralight component. Trace amounts of ultralight axions have been shown alleviate some observational discrepancies in cosmology such as the missing satellite problem and the Hubble-S8 tensions. Using an effective field theory approach and data from the Baryon Oscillation Spectroscopic Survey, we reach the strongest constraints on the axion relic density for axions with masses below 10^{-25} eV. To study heavier axions, we develop a new algorithm capable of simulating the formation of large-scale structure in the presence of more than one axion species. Making use of this code, we isolate new observational features in the cosmic web which may help us detect the presence of a plenitude of axions with weak lensing surveys.

Zoom Link: https://pitp.zoom.us/j/95089179013?pwd=NTBIeitScFRhYnBBSnlrcnoyVEhydz09

I will discuss the wide - range of new physics that can be probed with nuclear-spin comagnetometers, from EDM measurements to dark matter direct detection experiments.

I will outline the potential for improvement in these measurements, and how improved quantum control could have a significant impact in our sensitivity.  I will present some new dark matter detection results using comagnetometers.

In this talk I will discuss recent advances in Neural-Network parametrisations of pure and mixed quantum states that can be efficiently sampled.  In the first part I will generalise the Neural Density Operator ansatz originally proposed by Torlai and Melko by combining two general ingredients that can be used to construct deep, autoregressive ansatze that automatically enforce positive definitness.  In the second part, instead, I will show that any matrix product state can be exactly represented by a recurrent neural network with a linear memory update. I will then discuss how to generalise this linear RNN architecture to 2D lattices, comparing both approaches to standard DMRG calculations.

Zoom link:  https://pitp.zoom.us/j/98833163484?pwd=bEZTeTB0c1l1QmkrQXdEc3dBQ0dJZz09

The axion solution to the strong CP problem has many virtues but it suffers from the so-called quality problem. This problem is avoided in heavy axion realizations which, however, are harder to probe experimentally. I will show how generic heavy axion models can be tested in current and future gravitational wave (GW) observatories, due to the cosmological GW background produced by the axionic topological defects. Moreover, the resulting GW signal is correlated with a sizable strong CP phase, within reach of upcoming neutron electric dipole moment measurements.

Zoom link:  https://pitp.zoom.us/j/99150077172?pwd=UWhkZVdzV2lCQittWXVtMnludXBkUT09

In the standard cosmological paradigm, the initial condition follows Gaussian statistics. At later times, gravitational evolution induces nonlinearities in the large-scale structure, information that was fully captured by the two-point statistics at the early times gets spread into higher-order statistics. Whilst current standard cosmological analyses have focused on two-point statistics, higher-order statistics help further to tighten constraints by breaking parameter degeneracies as well as to probe the primordial Universe. In this talk, I will present our recent progress on the N-point Correlation Function (NPCF), including an analytical Gaussian covariance formalism, a first detection of the 4-point correlation function from nonlinear structure formation. Finally, I will focus on our recent analysis of parity-odd mode using the data from Baryon Oscillation Spectroscopic Survey (BOSS) and discuss the implication of parity-search at cosmological scales with large scale structure.

Zoom Link: https://pitp.zoom.us/j/92321535422?pwd=VlF4cHpvUit4bmR0eXYyczI5Qmw4dz09

An exciting development in outer solar system studies is the discovery of a new class of Kuiper belt objects with orbits that lie outside that of Neptune and have semimajor axes in excess of 250 A.U. The alignment of the major axes of these objects and other orbital anomalies are the basis for the Planet Nine hypothesis that an undiscovered giant planet orbits the sun at a distance of around 500 A.U. We show that a modified gravity theory known as MOND (Modified Newtonian Dynamics) provides an alternative explanation for the observed alignment, owing to significant quadrupolar and octupolar terms in the MOND galactic field within the solar system that are absent in Newtonian gravity. Using the well-established secular approximation of solar system dynamics we predict a population of Kuiper belt objects whose major axes are aligned with the direction to the center of the galaxy and that have additional clustering in orbital parameters. These features are exhibited by known Kuiper belt objects of the newly discovered class in support of the MOND hypothesis. Thus MOND, originally developed to explain galaxy rotation without invoking dark matter, may also be observable in the outer solar system.

Understanding the reason behind the observed accelerating expansion of the Universe is one of the most notable puzzles in modern cosmology, and conceivably in fundamental physics. In the upcoming years, near future surveys will probe structure formation with unprecedented precision and will put firm constraints on the cosmological parameters, including those that describe properties of dark energy. In light of this, in the first part of my talk, I'm going to show a systematic extension of the Effective Field Theory of Dark Energy framework to non-linear clustering. As a first step, we have studied the k-essence model and have developed a relativistic N-body code, k-evolution.

I'm going to talk about the k-evolution results, including the effect of k-essence perturbations on the matter and gravitational potential power spectra and the k-essence structures formed around the dark matter halos. In the second part of my talk, I'm going to show that for some choice of parameters the k-essence non-linearities suffer from a new instability and blow up in finite time.

This talk is based on: arXiv:2204.13098, arXiv:2205.01055, arXiv:2107.14215, arXiv:2007.04968, arXiv:1910.01105, arXiv:1910.01104.

Zoom Link: https://pitp.zoom.us/j/99797451101?pwd=dituM2d2MDFCbDgyVXJ4c2s1NVoyUT09

Any consistent regularization scheme induces an apparent violation of gauge invariance in non-anomalous chiral gauge theories. This violation shows up in perturbative calculations, and can be removed by including appropriate finite counterterms. In this talk I will discuss the derivation of such counterterms for a renormalizable chiral gauge theory.  I will use the background field method, which ensures background gauge invariance in the quantized theory. As a concrete application, I will show the finite counterterm at one loop in the Standard Model, within dimensional regularization and the Breitenlohner-Maison-'t Hooft-Veltman prescription for gamma5. 

Zoom Link: https://pitp.zoom.us/j/95507223984?pwd=Y0FDTEJpNDlVaE9Ba1ZNMURXeS9rdz09

Inflation can be viewed as a natural "cosmological particle detector" which can probe energies as high as its Hubble scale. In this talk, I study the imprints of heavy relativistic particles during inflation on primordial correlators in situations where the scalar fluctuations have a reduced speed of sound. Breaking dS boosts allows new types of footprints of massive fields to emerge. In particular, I show that heavy particles that are lighter than Hubble divided by the speed of sound leave smoking gun imprints in the three-point function of curvature perturbations (due to the exchange of those fields) in the form of resonances in the squeezed limit which are vividly distinct from the previously explored signatures of heavy fields in de Sitter correlators. Throughout I use and extend the cosmological bootstrap techniques derived from locality, unitarity, and analyticity in order to find fully analytical formulae for the desired boost breaking correlators.

Quantum mechanics is a cornerstone of modern physics. Just as the 19th century was called the Machine Age and the 20th century the Information Age, the 21st century promises to go down in history as the Quantum Age. Quantum Computing promises unprecedented speed in solving certain classes of problems while Quantum Cryptography promises unconditional security in communications. In this talk, I will discuss the world of single and entangled photons and also discuss ongoing work towards quantum computing, quantum information and quantum cryptography in our Quantum Information and Computing lab at the Raman Research Institute, Bengaluru. I will end with our broad vision for the future, which includes establishment of long distance secure quantum communications in India and beyond involving satellite based, fibre based as well as integrated photonics based approaches towards the global quantum internet.

Zoom Link: https://pitp.zoom.us/j/94788493307?pwd=QTlUaTY4Nm1IS3hyeUFIbVFKV3RMQT09

A black hole quantum state (Hartle-Hawking or Boulware)  is usually defined in a fixed background of a classical black hole. In my talk I will discuss the corresponding space-time geometry when the back-reaction is taken into account. The important questions include: does the back-reacted geometry always contain a horizon?; how it depends on the choice of the quantum state?;  and what is the right choice for the quantum state for the non-physical fields such as ghosts? I will answer these and other questions in the context of a two-dimensional dilaton gravity. The talk is based on a joint recent work with D. Sarkar and Y. Potaux, arXiv:2112.03855.

Zoom Link: https://pitp.zoom.us/j/5053597405?pwd=YjU2VVF1L3A1dmR1VWpPMHZFQW9rdz09