We are on the brink of Hearing the Background Tyranny of the Universe. Here’s why we’re listening: ScienceAlert

We are on the brink of Hearing the Background Tyranny of the Universe.  Here's why we're listening: ScienceAlert
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The universe must be humming.

Every supernova, every merger between neutron stars or black holesit may or should even be the source of rapidly rotating single neutron stars gravitational waves.

It follows the rapid inflation of space big bang It should have created its own cascade of gravitational waves 13.8 billion years ago.

Like a rock dropped into a pond, these massive events should send ripples reverberating through the fabric of spacetime—faint expansions and contractions of space that can be detected to us as inconsistencies in precisely timed signals.

Collectively, this mixture of signals combine to form random or “stochastic” noise. gravitational wave background and perhaps one of the most sought after detections in gravitational wave astronomy.

A new frontier in space exploration

There is this idea – just like discovery cosmic microwave background it did (and continues to do) before it – the discovery of the gravitational wave background would completely shake up our understanding of the Universe and its evolution.

“Detecting the stochastic background of gravitational radiation can provide a wealth of information about astrophysical source populations and processes in the very early Universe,” explains theoretical physicist Susan Scott from the Australian National University and ARC Centre.

“For example, electromagnetic radiation does not provide a picture of the Universe earlier than the time of the last scattering (about 400,000 years after the Big Bang). Gravitational waves can tell us all the way back to the beginning of inflation, just ~10-32 Seconds after the Big Bang.”

Diagram of the expanding universe after the big bang
The initial gravitational waves could have been caused by the expansion after the Big Bang. (NAOJ)

To understand the importance of the gravitational wave background, we need to talk a little about another remnant of the Big Bang: the cosmic microwave background, or CMB.

Moments after our universe began to tick and space cooled, the bubbling foam that was everything turned into an opaque soup of subatomic particles in the form of ionized plasma.

Any radiation that came up with it was dissipated, preventing it from traveling a great distance. Light was able to travel freely through the Universe until these subatomic particles recombined into atoms, known as the Recombination Period. and for centuries.

About 380,000 years after the Big Bang, the first flash of light exploded into space, and over the next billion years, as the Universe grew and grew, that light was swept into every corner. It is still around us today. This radiation is extremely weak, but can be detected especially at microwave wavelengths. This CMB is the first light in the Universe.

Irregularities in this light, called anisotropies, were caused by the small temperature fluctuations represented by the first light. It’s hard to overstate how phenomenal his discovery is: the CMB is one of the only tests we have of the state of the early Universe.

The discovery of the gravitational wave background would be a spectacular repeat of this achievement.

“We expect that the detection and analysis of the gravitational wave background will revolutionize our understanding of the Universe, just as it was pioneered by the observation of the cosmic microwave background and its anisotropies.”

Noise beyond the boom

The first detection Gravitational waves were created a short time ago, in 2015.

Two black holes that collided about 1.4 billion years ago sent waves propagating at the speed of light; On Earth, these expansions and contractions of space-time are very weakly triggered instrument developed and refined over decades, waiting to reveal just such an event.

Two side-by-side black holes surrounded by a glow of orange radiation
Artist’s rendering of two colliding black holes. (Caltech/R. Damage / IPAC)

This was a monumental reveal for several reasons. This gave us direct confirmation of the existence of black holes for the first time.

confirmed the prediction made by General Theory of Relativity 100 years ago gravitational waves are real.

And that meant that the gravitational wave interferometer, an instrument scientists had been working on for years, would revolutionize our understanding of black holes.

And there is. Discovered LIGO and Virgo interferometers About 100 gravitational wave events to date: those strong enough to produce the signal recorded in the data.

These interferometers use lasers that illuminate special tunnels several kilometers long. These lasers are affected by the stretching and compression of spacetime caused by gravitational waves, creating an interference pattern from which scientists can infer the properties of the compact objects that generate the signals.

But the gravitational wave background is a different animal.

“The astrophysical background is created by the mixed noise of many weak, independent and unresolved astrophysical sources,” says Scott.

“Our ground-based gravitational wave detectors LIGO and Virgo have already detected gravitational waves from dozens of individual mergers of a pair of black holes, but the astrophysical background from the binary gravity of a stellar mass. black hole compounds are expected to be the main source of GWB for this current generation of detectors. We know that there are many of these couplings that cannot be resolved individually, and together they create random noise in the detectors.”

The rate at which binary black holes collide in the universe is unknown, but the rate at which we can detect them gives us a baseline from which to estimate.

rainbow colors of waves surrounding a dark cluster representing merging black holes
Numerical simulation of binary merger of black holes. (N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulation of eXtreme Spacetimes (SXS) Collaboration)

Scientists believe this occurs between one per minute and several per hour, with each detectable signal lasting only a fraction of a second. These individual, random signals will probably be too weak to detect, but will combine to form static background noise; astrophysicists compare it the sound of popcorn.

This would be the source of the stochastic gravitational wave signal we might expect to find with instruments like LIGO and the Virgo interferometers. These instruments are currently under repair and preparation and will be connected to the third observatory. KAGRA in JapanIn a new observation in March 2023. Detection of popcorn GWB with this collaboration cannot be said.

These aren’t the only tools in the gravitational wave kit, though. Other instruments will be able to detect other sources of the gravitational wave background. One such tool is still 15 years old Laser Interferometer Space Antenna (LISA)It will be launched in 2037.

It is based on the same technology as LIGO and Virgo, but with “arms” 2.5 million kilometers long. It will operate at a lower frequency than LIGO and Virgo and therefore detect different types of gravitational wave events.

schematic diagram of the intersecting arms of the ligo
LIGO consists of a beam splitter that splits the laser into perpendicular directions. Aligning the rays creates a pattern if any of them have changed by the smallest amount. (Caltech/MIT/LIGO Lab)

“GWB doesn’t always look like popcorn,” Scott told ScienceAlert.

“It can also consist of individual deterministic signals that overlap in time and create jumble noise, similar to the background chatter at a party. An example of jumble noise is the gravitational radiation produced by the galactic population of compact white dwarf binaries. The importance of jumble noise for LISA In this case, the stochastic signal is so strong that it comes to the fore, acting as an additional source of noise when trying to detect other weak gravitational wave signals in the same frequency range.”

LISA could also theoretically detect cosmological sources of the gravitational wave background, such as cosmic inflation right after the Big Bang or cosmic strings – Theoretical cracks in the universe could have formed at the end of inflation by losing energy through gravitational waves.

Timing the pulse of space

There’s also a large, galactic-scale gravitational-wave observatory that scientists study to look for hints of the gravitational-wave background: to pulse timing arrays. pulsar is a type neutron starthe remnants of once-massive stars that died in a spectacular supernova, leaving behind only a dense core.

Pulsars rotate in such a way that beams of radio emission from their poles sweep across the Earth like cosmic beacons; some of them do so at incredibly precise intervals, which is useful for a number of applications, such as navigation.

But the stretching and compression of spacetime should theoretically cause small perturbations in the timing of pulsar flashes.

A single pulsar showing slight timing discrepancies might not mean much, but if a bunch of pulsars showed correlated timing discrepancies, it could be indicative of gravitational waves inspired by supermassive black holes.

Scientists have found charming hints about this source of the gravitational wave background in pulsar timing arrays, but we do not yet know enough to determine whether this is the case.

We are tantalizingly close to discovering the gravitational wave background: the astrophysical background that reveals the behavior of black holes throughout the Universe; and the cosmological background – quantum fluctuations observed in the CMB, inflation, the Big Bang itself.

Scott says it’s a white whale: we’ll only see it after the hard work of separating the background into discrete sources in the noisy whole.

“While we look forward to the wealth of information to be gained from detecting the astrophysically generated background, observing gravitational waves from the Big Bang is really the ultimate goal of gravitational wave astronomy,” he says.

“By removing the foreground of this binary black hole, proposed third-generation ground-based detectors such as the Einstein Telescope and Space Probe could be sensitive to the cosmologically created background with 5 years of observations, thereby entering the field where important cosmological observations can be made. well.”

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