Gravitational Waves spreading across the Universe

A team of Astronomers from across the globe, The NANOGrav collaboration, this week announced they'd observed a faint sign of a background signal or "hum" of Gravitational Waves that's spread through the Universe.

What's a Gravitational Wave?

Gravitational Waves are ripples in space and time caused by the acceleration of massive objects. These move away or "propagate" in all directions from their source at the speed of light. They contain information about the event that caused them and transport energy, in a similar way electromagnetic radiation or light does.

They'd been proposed by a number of people but were formally described by Albert Einstein as part of his Theory of General Relativity. Gravitational Waves shrink and then expand the very space they move through.

When did we first detect Gravitational Waves?

There'd been an indirect detection prior to the first direct detection in 2015 by the LIGO gravitational wave detector, for which the 2017 Nobel Prize in Physics was awarded. This event was caused by the collision of two Black Holes, some 1.3 billion light years from Earth.

If the very fabric of space around me is moving, why can't I feel it?

While events that cause Gravitational Waves are extremely energetic, this energy is propagated outwards and hard to detect after what could be millions of years of travel. The frequency of these waves is so low and the wavelength so incredibly long, they're beyond our ability to experience them directly.

Artist's impression of an array of pulsars being affected by gravitational waves produced by orbiting super massive black holes in a faraway galaxy (Image credit: Aurore Simonnet for the NANOGrav Collaboration)

How was the recent discovery made and how long did it take to gather all the data?

Rather than looking for individual Gravitational Wave events, the NANOGrav team collected data from a number of "cosmic clocks" to detect the ripples of space on vast scale.

Spread throughout galaxies are quickly spinning neutron stars called Pulsars. These are in the final stages of stellar evolution, leftovers after massive stars explode in Supernova events. Their rotating cores compress and shrink under their gravity and spin faster and faster. Think of an ice skater bringing their arms and legs in towards their body to twirl faster.

In the case of Pulsars, they spin with extremely regular times of milliseconds to seconds. Their rate of spin will change, but it will be over enormous spans of time. If their magnetic "beam" points towards us, we see Pulsars as some of the most accurate timekeeping devices in the Universe.

Astronomers have been studying and cataloging Pulsars for decades, with Murriyang - The CSIRO Radio Telescope outside of Parkes NSW - being a key facility in providing these Pulsar timing observations.

Image via Alex Cherne

Using a network of radio telescopes around the world, 67 different pulsars were observed for 15 years.

NANOGrav found small, but measurable changes to the distances from Earth to these Pulsars caused by Gravitational Waves.

The Universe is a noisy place at all frequencies of light, so even with the accuracy of Pulsar timing the NANOGrav was unable to extract individual events, but instead found the long-theorised "hum" of Gravitational Waves that are spread throughout the Universe - and this is not something "out there" either. Gravitational Waves are passing through everything around us and even through our bodies!

What causes Gravitational Waves?

It's likely the Gravitational Waves that makeup the background hum detected by the NANOGrav team was caused by the merger of Black Holes. A number of these mergers since the beginning of the Universe- possibly in the billions - send Gravitational Waves out into space, they combine and effect space around us.

“It’s like a choir, with all these supermassive black hole pairs chiming in at different frequencies,” Chiara Mingarelli, a NANOGrav scientist who worked on the new findings, said. “This is the first-ever evidence for the gravitational wave background. We’ve opened a new window of observation on the universe.”

Can astronomers observe using Gravitational Waves?

Very likely!  As Gravitational Wave are changes in the fabric or space and time itself, they're not effected by the presence of matter. This means the inner workings of Super Massive Black Holes, or certain eras of the early history of the Universe which is opaque to the wavelengths of lights astronomers use are "visible" to Gravitational Wave.

Every time Astronomers have worked out ways to observe the Universe - whether it's been in radio wave, infrared or ultraviolet etc - amazing new discoveries follow. Gravitational Waves will no doubt provide another way to learn about the space we live in.

Cheers,

Earl White

BINTEL

29th June 2023