The gravitational-wave spectrum

How we listen Searching — upper limits so far

Cosmic microwave background polarisation

10⁻¹⁸ Hz – 10⁻¹⁶ Hzone cycle every 320 million years to 32 billion years

Gravitational waves from the first instant after the Big Bang would have left a faint, swirling pattern — “B-modes” — in the polarisation of the cosmic microwave background. Telescopes at the South Pole and in Chile's Atacama Desert are hunting for it.

How we listen Evidence for a background, 2023

Pulsar timing arrays

1 nHz – 100 nHzone cycle every 120 days to 32 years

Millisecond pulsars are nature's most precise clocks. A passing gravitational wave nudges the arrival times of their radio pulses by tens to hundreds of nanoseconds, in a pattern that's correlated across the sky.

In 2023, pulsar timing arrays in North America, Europe and India, Australia and China reported evidence for a nanohertz gravitational-wave background.

How we listen Planned — launch around 2035

LISA

100 μHz – 100 mHzone cycle every 10 s to 2.8 hours

Three spacecraft, 2.5 million kilometres apart, trailing the Earth around the Sun. Laser links between them will listen in the millihertz band, out of reach of any detector on the ground.

How we listen Planned — 2030s

Cosmic Explorer and the Einstein Telescope

3 Hz – 4 kHzone cycle every 250 μs to 330 ms

The next generation of ground-based observatories: Cosmic Explorer, with 40-kilometre arms in the USA, and the Einstein Telescope, an underground observatory in Europe. They will be around ten times more sensitive than today's detectors.

How we listen Observing since 2015

LIGO, Virgo and KAGRA

10 Hz – 3 kHzone cycle every 330 μs to 100 ms

Laser interferometers with arms three to four kilometres long, measuring stretches and squeezes of spacetime a ten-thousandth the width of a proton.

First detection: GW150914, two black holes merging 1.3 billion light-years away. Hundreds of signals have been observed since.

How we listen Proposed (Australia)

NEMO — Neutron Star Extreme Matter Observatory

1 kHz – 4 kHzone cycle every 250 μs to 1 ms

A proposed Australian detector tuned to kilohertz frequencies, where the remnants of colliding neutron stars ring after they merge.

Source Not yet detected

Primordial background from inflation

10⁻¹⁸ Hz – 10 kHzone cycle every 100 μs to 32 billion years

Inflation — a burst of exponential expansion in the first instant after the Big Bang — should have filled the Universe with gravitational waves at every frequency. The signal is extraordinarily faint.

Source The likely source of the PTA background

Supermassive black-hole binaries

1 nHz – 100 nHzone cycle every 120 days to 32 years

When galaxies merge, their central black holes — millions to billions of times the mass of the Sun — sink together and orbit each other, emitting waves with periods of months to years.

Source A target for LISA

Massive black-hole mergers

100 μHz – 100 mHzone cycle every 10 s to 2.8 hours

Black holes of thousands to millions of solar masses, merging as their host galaxies collide. They will be loud enough for LISA to hear across the observable Universe.

Source Known from telescopes; a target for LISA

White-dwarf binaries in the Milky Way

100 μHz – 10 mHzone cycle every 1.7 min to 2.8 hours

Pairs of white dwarfs in our own Galaxy, orbiting each other every few minutes to hours. There are so many that together they form a background hum that LISA will have to see through.

Source A target for LISA

Extreme mass-ratio inspirals

1 mHz – 100 mHzone cycle every 10 s to 17 min

A star-sized black hole spiralling into a supermassive one, completing tens of thousands of orbits on the way in. The signal maps the warped spacetime around the big black hole in exquisite detail.

Source Detected — hundreds of mergers

Stellar-mass black-hole binaries

10 mHz – 1 kHzone cycle every 1 ms to 1.7 min

Black holes tens of times the mass of the Sun. They spiral together for years in the LISA band, then sweep up in frequency and merge in a fraction of a second in the LIGO band — a “chirp”.

Source Detected — GW170817 (2017)

Neutron-star mergers

10 Hz – 2 kHzone cycle every 500 μs to 100 ms

Colliding neutron stars: city-sized remnants of dead stars, denser than an atomic nucleus.

GW170817 was seen in gravitational waves and right across the electromagnetic spectrum, from a short gamma-ray burst to a kilonova.

Source Not yet detected

Continuous waves from spinning neutron stars

10 Hz – 2 kHzone cycle every 500 μs to 100 ms

A spinning neutron star with a tiny “mountain” — perhaps only millimetres high — would hum continuously at twice its spin frequency.

Source Not yet detected

Core-collapse supernovae

100 Hz – 2 kHzone cycle every 500 μs to 10 ms

When a massive star's core collapses, the bouncing, turbulent core should emit a brief burst of gravitational waves — detectable from a supernova in or near our Galaxy.

Source Not yet detected

Neutron-star post-merger remnants

1 kHz – 4 kHzone cycle every 250 μs to 1 ms

If a neutron-star collision leaves behind a massive, rapidly spinning neutron star, it rings at kilohertz frequencies, revealing how matter behaves at extreme densities and temperatures.