Solar and Stellar Physics

The group is interested in unravelling the physics of the magnetic coupling and dynamics of the solar atmosphere including its role in shaping up the space weather and climate using imaging and spectroscopic observations aided with hydrodynamic, magneto-hydrodynamic simulations, and machine learning. Prof Durgesh Tripathi is the Principal Investigator of the the brand new Solar Ultraviolet Imaging Telescope (SUIT) onboard Aditya-L1 mission and providing, for the first time, observations of the lower and middle layers of the solar atmosphere in near and mid ultraviolet (200-400 nm) radiation using 11 different filters. These observations are providing wealth of information and constraints on various activities occurring in the magnetised solar atmosphere. Moreover, these are also providing information in the wavelength range central to the Sun climate relationship.

The IUCAA'STARS group is interested in stellar evolution and pulsation across the Hertzsprung-Russell diagram, Galactic archaeology, resolved stellar populations in star clusters and nearby galaxies, and near-field cosmology. The ongoing research is focussed on astronomical distances using stellar standard candles for improving the precision of the cosmic distance ladder and determining the present expansion rate of the Universe. We are involved in several ongoing ground and space-based time-domain surveys and the upcoming Vera C. Rubin Observatory's LSST survey. We are also interested in stellar evolution and pulsation modeling using open-source 1D code in Modules for Experiments in Stellar Astrophysics (MESA) software, and astrostatistics and data science applications in time-domain astronomy.

Quantum Metrology and Sensing

A. Precision and Quantum Measurement lab (PQM-lab) (https://pqmlab.iucaa.in) is expeimenting on

1. quantum metrology
2. precision measurements
3. developing quantum-phenomena based sensors/ technologies

The PQM-lab is dedicated to supporting India’s quantum mission, supporting mega-science projects (e.g., LIGO-India, TMT) and synergizing between precision atomic spectroscopy and astronomical studies to explore unanswered questions in fundamental science.

Indigenization of the quantum clock with unprecedented accuracy has national importance and also will be used for testing fundamental science and geodesy. In particular, our long-term interest is measuring the temporal constancy of the dimensionless fundamental constants and possible violation of fundamental symmetries. For that PQM-lab is working on three major inter-connecting areas,

1. establishing a lab-based reference optical clock
2. optical fiber-based quantum channel and
3. developing a chip ion-trap.

The heart of our experiment is a ytterbium-ion-based quantum clock. For this, a single ytterbium-ion will be trapped in an indigenously developed precision ion trap. The ion will be cooled to sub-mK temperature using the laser cooling technique, which will then be used to probe its highly forbidden electric octupole (E3) clock transition at the 467 nm wavelength. An ultra-stable sub-Hz linewidth clock laser will be produced by referencing a commercial laser to an indigenously developed ultra-stable Fabry-Pérot cavity. Further, the optical clock referenced phase & frequency preserved photons need to be disseminated to geographically distributed locations for establishing a network among the quantum clocks and intercomparison. To meet the aforementioned requirement, active phase noise cancellation of the optical fiber by stabilizing its length will be implemented, a critical technology under development at the PQM-lab.

B. Quantum-enhanced techniques in Precision Interferometer (QuPI): 

Precision interferometry is the basis for some of the most sensitive scientific instruments ever built, such as gravitational-wave detectors, atomic sensors, and optical clocks. At their core, gravitational-wave detectors measure extremely small changes in distance between test masses by exploiting interference of coherent laser light. However, the ultimate sensitivity of such devices is limited by fundamental sources of noise, especially quantum noise—comprising shot noise (photon counting uncertainty) and radiation-pressure noise (back-action from fluctuating photon momentum).

Quantum-enhanced techniques refer to the deliberate use of non-classical states of light and matter, as well as quantum measurement strategies, to overcome these limits. Key approaches include:

Squeezed Light Sources: By preparing light in a squeezed state, one can reduce fluctuations in one quadrature (phase or amplitude) at the expense of the other. This directly lowers shot noise or radiation-pressure noise, improving interferometer sensitivity beyond the standard quantum limit.

Quantum Non-Demolition (QND) Measurements: Design of measurement schemes that extract information without disturbing the observable of interest. Examples include variational readout or speed-meter interferometers.

Extragalactic Astronomy

1. IGM: Most of the bayrons created in the Big bang reside in regions between galaxies. These regions are called Intergalactic medium (IGM).  Absorption lines imprinted in the spectra of distant bright objects such as quasars or Gamma ray bursts allow us to probe the physics of the IGM and its redshift evolution. In particular the astronomers at IUCAA have made important contributions to this field such as: (1) Measuring the HI photoionization rate as at low-z, (2) Computing consistent UV background as a function of redshift, (3) Constraining the UV escape fraction from star forming galaxies, (4) Accurately measuring the thermal evolution of the IGM that allow one to probe the physical condition during He II reionization, (5) First measurement of redshift space three-point correlation function at low-z, (6) Accurate measurement of redshift evolution of two- and three-point correlation function  (7) large survey of OVI at high-z (8) accurate modeling of metal line absorbers and (9) heating of the IGM using cosmic rays. At present the team is involved in the hydrodynamical modeling of the IGM using Gadget 3.
   
   
2. Quasar absorption lines - Galaxy evolution and Cosmology: Strong absorption lines seen in spectra of high-z quasars allow us to probe cosmology, fundamental physics and galaxy evolution. IUCAA astronomers measure the temperature of the cosmic microwave background at different redshifts to confirm the big-bang model. The team also provided one of the most stringent measurements on the time variation of electromagnetic coupling constant and electron-to-proton mass ratio using quasar absorption lines.
   
   Detection of HI 21-cm absorption and or molecular species at high-z allow one to probe the physical state of the interstellar medium in protogalaxies. IUCAA astronomers (and their collaborators) have carried out different surveys using GMRT and VLT to build the largest sample of such absorbers. Some IUCAA faculty members are now carrying out one of the largest surveys (MeerKAT Absorption Line Survey, MALS) to detect HI 21cm and OH absorption over the redshift range 0<z<2.
   
   Establishing the connection between the absorption lines seen in the spectrum of quasars and their host galaxies are very important to understand the galaxy evolution in a luminosity unbiased way. Several students in IUCAA are involved in such an activity using large telescopes and most advanced instruments such as MUSE.
   
   
3. Galaxy evolution: Galaxies, primarily made up of stars, gas and invisible dark matter, are the fundamental building blocks of our universe. When and how these galaxies have formed remain as one of the outstanding challenges in modern astronomy. The challenges arise because galaxies are not isolated systems like “Island universes”; they are highly interacting, they merge with another, accrete gas/satellites from surrounding or the cosmic web, they also pollute the intergalactic medium by ejecting materials outwards. But when one inspects a galaxy at a given redshift slice or say in the local universe (already about 13.7 billion years old), several of these factors may get erased due to the dynamics of stars or hard to disentangle. 
   
   At IUCAA, galaxy research ranges from high-redshift, young, star-forming galaxies to the local, matured ones like our Milky Way. Researchers use state-of-the-art N-body simulation carried out at IUCAA HPC to understand their dynamical evolution. They also use multi-wavelength data from various large ground-based surveys like SDSS; deep surveys from space-based telescopes like HST, Spitzer and now AstroSat’s UVIT for far and near-UV observations as well as Integral Field Spectroscopic data from surveys like MANGA to address the above mentioned challenges.
   
   
4. Circumgalactic Medium (CGM): It is now well-established that all galaxies are embedded in diffuse gaseous haloes known as the Circumgalactic Medium (CGM). Lying at the interface between a galaxy and its wider environment, the CGM modulates not only the accretion and ejection of material in the galaxy, but also the interaction of the galaxy with the larger-scale environment. Moreover, the CGM is a major reservoir of baryons in the Universe, and plays a key role in the star formation and evolution of galaxies. Therefore, in order to fully understand the physical processes at work within galaxies, it is crucial to have a robust understanding of the gaseous haloes surrounding them. At IUCAA, using multi-wavelength observations from state-of-the-art observing facilities such as the Multi-Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) and the Hubble Space Telescope (HST), research is ongoing to address some of the key open questions: (i) How is the multi-phase CGM gas distributed around galaxies? (ii) How are the properties of the CGM connected with that of the galaxies and the environment? (iii) How does the connection between the CGM gas and galaxies evolve with redshift?