The GMRT Near Pune: What 30 Giant Dishes Are Quietly Listening For

Thirty parabolic dishes, each 45 metres across, sit in the dry scrub of Khodad village in Pune district. From a passing highway, they look like something agricultural, enormous white bowls tilted at the sky, unhurried. There is no dome, no eyepiece, no visible beam of light. The Giant Metrewave Radio Telescope, operated by the National Centre for Radio Astrophysics under TIFR, does not collect light at all. It collects radio waves, the longest and laziest end of the electromagnetic spectrum, at wavelengths between 0.21 metres and 6 metres. At those wavelengths, the universe sounds nothing like silence.

The dishes are arranged in a Y-shape, fourteen along each arm, two at the centre, spread over a baseline of roughly 25 kilometres. That spread is the point. A single dish would give a blurry signal. Spread across 25 kilometres and combined through a technique called aperture synthesis, the array resolves detail as if it were a single telescope 25 kilometres wide. The signals from every dish travel by fibre-optic cable to a central correlator, which combines them in real time. What arrives at the correlator is not an image but a river of numbers that astronomers convert, over hours of computation, into maps of the radio sky.

The GMRT's most important frequency is 1420 megahertz, the emission line of neutral hydrogen, the most abundant atom in the universe. When a hydrogen atom's electron flips its spin, it releases a photon at exactly that frequency. Every cloud of hydrogen in every galaxy broadcasts this tone. Because the universe is expanding, hydrogen in a distant galaxy broadcasts the same tone shifted to a lower frequency, redshifted, by an amount that tells you exactly how far away the galaxy is and how long ago the light, or in this case the radio wave, left it.

The GMRT has used this to map hydrogen in galaxies up to 9 billion light-years away, observing the universe as it was when it was roughly a third of its current age. One landmark survey, the GMRT Cold HI survey, tracked how the total amount of neutral hydrogen in the universe has changed across cosmic time. The answer matters because hydrogen is the raw fuel of star formation. Understanding when galaxies had it, used it, and ran out of it is understanding the biography of stars across the entire history of the universe.

A pulsar is a neutron star, the collapsed core of a dead massive star, spinning and emitting a beam of radio waves from its magnetic poles. As it spins, the beam sweeps past Earth like a lighthouse, arriving with a regularity that rivals atomic clocks. The GMRT monitors dozens of pulsars. Some spin hundreds of times per second; these are millisecond pulsars, spun up by stealing mass from a companion star over millions of years.

Millisecond pulsars are useful far beyond their own strangeness. Arrays of them across the sky form a galactic-scale detector for gravitational waves, ripples in spacetime at nanohertz frequencies, far below what LIGO can hear. The Indian Pulsar Timing Array, which uses GMRT data, is part of a global collaboration that in 2023 reported strong evidence for a gravitational wave background: a low hum of spacetime distortions likely produced by pairs of supermassive black holes spiralling toward each other across the universe. The GMRT's sensitivity at low frequencies made it a contributing instrument to that finding.

Between 2011 and 2017, NCRA carried out a major upgrade, the upgraded GMRT, or uGMRT, replacing the original analog receivers with wideband digital systems. The original GMRT could observe in five narrow frequency bands. The uGMRT observes continuously from 120 megahertz to 1460 megahertz, covering the full range in four broad bands with no gaps. Sensitivity improved by a factor of roughly three to ten depending on the frequency. A source that previously required eight hours of observation to detect could now be found in under two.

The upgrade was designed and built almost entirely in India, at NCRA's Pune campus. The digital backend, the system that samples, filters, and correlates the signals, was developed in-house. This matters beyond institutional pride: it means NCRA can modify the instrument for new science without waiting for external vendors. Since the upgrade, the uGMRT has been used to detect fast radio bursts, to study the magnetic fields threading the interstellar medium, and to follow up gravitational wave events detected by LIGO, searching the radio sky for any accompanying signal from the collision that caused the wave.

Radio telescopes need radio quiet, freedom from the interference that mobile networks, aircraft, and industrial equipment produce. Khodad sits in a shallow valley that provides natural shielding on several sides. The site was chosen in the 1980s after a survey of candidate locations across Maharashtra and Karnataka. The land is dry enough that the dishes do not sink, flat enough that the arms of the Y can be laid out without major earthworks, and remote enough that the radio frequency environment is still, by the standards of the twenty-first century, relatively clean.

Metre-wave radio astronomy is a niche that most of the world's large telescopes avoid. The Very Large Array in New Mexico and the Australian Square Kilometre Array Pathfinder work primarily at centimetre wavelengths. The GMRT was designed specifically for the metre-wave window, which gives it access to phenomena those instruments cannot study as well: low-frequency emission from galaxy clusters, the extended radio lobes of active galactic nuclei, and the redshifted hydrogen line from the most distant galaxies. When the Square Kilometre Array, the next generation of radio telescope, being built in South Africa and Australia, comes online, the GMRT's role will shift toward the transient sky and the deep hydrogen surveys that require its particular combination of sensitivity and frequency coverage. Until then, the dishes at Khodad remain among the most productive square metres of scientific real estate in the country, listening to frequencies the rest of the world mostly ignores.