S. K. Gupta

In a courtroom, there isn't always a video recording of the crime. Sometimes the verdict is based on witness testimony: what happened before, what happened after. That's exactly how physicists treated the signal GW250114, detected on January 14, 2025.

Every second, invisible particles from space, cosmic rays, strike Earth's atmosphere. Colliding with air molecules, they produce showers of secondary particles, among which muons are especially abundant. A muon is a heavy cousin of the electron that lives only a couple of microseconds but still manages to reach the ground.

After a supernova explosion, a neutron star neutron star remains: a city-sized ball of unimaginably dense matter, spinning hundreds of times per second. Any tiny bump on its surface would make it wobble like an off-balance spinning top, humming faintly through the fabric of space.

During a thunderstorm, the sky acts like a giant battery: charge builds up in the cloud, and a lightning bolt discharges it. That charge is invisible, but cosmic particles (muons, which constantly rain down on Earth from space) can sense it. As a muon flies through a charged cloud, it speeds up or slows down slightly, and these changes reveal the voltage inside the cloud.

LIGO, Virgo, and KAGRA detectors act like giant microphones, listening in on gravitational waves — vibrations of space itself. The new GWTC-5.0 catalog recorded 161 black hole mergers, bringing the total count to 390 events.

Imagine a giant inflatable balloon with galaxies painted on it. As it inflates, the pattern stretches and neighboring dots move apart. The rate of that stretching is the Hubble constant. Astronomers usually catch light from distant stars and see how much it has reddened due to the stretching of space.

A huge bowl of water. Drop a stone into it — ripples spread across the surface. The fabric of spacetime works much the same way: when two massive stars merge, they create ripples — gravitational waves. These waves travel to Earth and are picked up by giant detectors like LIGO and Virgo.

Gravitational-wave detectors are like extremely sensitive ears: they pick up the trembling of space when black holes or neutron stars collide somewhere in the universe. But even the most sophisticated "ears" cannot hear everything. The new LIGO A# project is like replacing the eardrum with a thinner, more sensitive membrane.