We Are On the Verge of Hearing the Background Hum of the Universe. Here’s Why We Listen : ScienceAlert

We Are On the Verge of Hearing the Background Hum of the Universe.  Here's Why We Listen : ScienceAlert
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The universe must be humming.

Every supernova, every merger between neutron stars or black holesEven rapidly spinning solitary neutron stars can or should be a source. gravity waves.

The phenomenon of rapid inflation of space below Big Bang 13.8 billion years ago it should have produced its own gravitational waves.

Like a rock thrown into a pond, these massive events should send out ripples that echo through the fabric of space-time – weak expansions and contractions of space can be perceived by us as inconsistencies in signals that must have been precisely timed.

Collectively, this mix of signals combine to form a random or “stochastic” buzz. gravitational wave It is probably one of the most sought after determinations in background and gravitational wave astronomy.

New frontier in space exploration

Considered – just like his discovery cosmic microwave background He has done (and continues to do) that finding the gravitational wave background will completely clarify our understanding of the Universe and its evolution.

“Detecting a stochastic background of gravitational radiation can provide a wealth of information about astrophysical source populations and processes in the very early Universe, inaccessible by any other means,” explains theoretical physicist Susan Scott of the Australian National University and ARC Center. Excellence for Gravitational Wave Discovery.

“For example, electromagnetic radiation does not provide a picture of the Universe before its last scattering time (about 400,000 years after the Big Bang).-32 Seconds after the Big Bang.”

Diagram of the expanding universe after the big bang
Primordial gravitational waves may have originated from the expansion after the Big Bang. (NAOJ)

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

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

Any radiation that came along with it dissipated, preventing it from taking it too far.. Light was able to move freely in the Universe until these subatomic particles recombined into atoms, an era known as the Age of Recombination. and down through the ages.

The first flash of light exploded in space about 380,000 years after the Big Bang, and over the next billions of years, as the Universe got bigger and bigger, that light drifted 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.

The irregularities in this light, called anisotropies, were due to the small temperature fluctuations represented by that first light. It’s hard to overstate how extraordinary its discovery was: the CMB is one of the only probes we have of the state of the early Universe.

The discovery of the gravitational wave background would be a magnificent replica of this achievement.

“We expect the detection and analysis of the gravitational wave background to revolutionize our understanding of the Universe,” says Scott, “as pioneered by the observation of the cosmic microwave background and its anisotropies.”

The buzz beyond the boom-crash

This first detection gravitational waves were made a short time ago, in 2015.

Two black holes colliding about 1.4 billion years ago sent ripples propagating at the speed of light; On Earth, these expansions and contractions of space-time are very weakly associated. instrument designed and refined for decades, waiting to detect just such an event.

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

It was a monumental find for several reasons. It directly confirmed to us the existence of black holes for the first time.

confirmed a prediction made by General Theory of Relativity 100 years ago gravitational waves were real.

And that meant that this tool, the gravitational wave interferometer, which scientists have been working on for years, would revolutionize our understanding of black holes.

And he has. LIGO and Virgo interferometers, almost 100 Gravity wave events to date: those strong enough to produce a distinct signal in the data.

These interferometers use lasers that glow in special tunnels several kilometers long. These lasers are affected by the stretching and compression of space-time produced by gravitational waves, creating an interference pattern with which scientists can understand the properties of the compact objects that generate the signals.

But the gravitational wave background is a different beast.

“An astrophysical background is produced 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 stellar-mass binary has already been detected. black holes mergers are expected to be the main source of GWB for these current generation detectors. We know there are many of these mergers that cannot be resolved individually, and together they produce a random hum of noise in the detectors.”

The collision rate of binary black holes in the universe is unknown, but the speed with which we detect them gives us a basis on which to make an estimate.

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

Scientists believe it ranges from about one merge per minute to several merges per hour, with each detectable signal lasting only a fraction of a second. These individual, random signals will likely be too weak to be detected, but they will combine to form a static background noise; astrophysicists compare it popcorn popping sound.

This would be the source of a stochastic gravitational wave signal that we can expect to find with instruments like the LIGO and Virgo interferometers. These instruments are currently undergoing maintenance and preparation and will be joined by a third observatory, KAGRA in JapanIn a new observation study in March 2023. Detection of popcorn GWB with this collaboration out of question.

These aren’t the only tools in the gravitational wave kit, though. And other tools will be able to detect other sources of gravitational wave background. Such a vehicle that is still 15 years away, Laser Interferometer Space Antenna (LISA)It will be released in 2037.

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

schematic diagram of ligo's intersecting arms
LIGO consists of a beam splitter that splits a laser in vertical directions. Realigning the beams creates a pattern if any of them have changed the smallest amount. (Caltech/MIT/LIGO Lab)

“GWB isn’t always popcorn-like,” Scott tells ScienceAlert.

“It can also consist of individual deterministic signals that overlap over time and produce a clutter noise, similar to background conversations at a party. An example of clutter noise is gravitational radiation produced by a galactic population of compact white dwarf pairs. This is an important entanglement noise for LISA. source. In this case, the stochastic signal is so strong that it becomes a foreground, acting as an additional source of noise when trying to detect other weak gravitational wave signals in the same frequency band.”

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

Timing the pulse of the universe

There’s also a massive gravitational wave observatory of galactic scale that scientists are working on looking for clues to the gravitational wave background: pulsar timing sequences pulsars a kind of neutron starThe remnants of once massive stars that died in a spectacular supernova, leaving only a dense core behind.

Pulsars spin in such a way that radio rays emanating from their poles pass by the Earth like a cosmic lighthouse; some do this at incredibly precise intervals, which is useful for a number of applications such as navigation.

But the stretching and compression of spacetime should, in theory, produce minor perturbations in the timing of pulsar flashes.

A pulsar displaying slight timing discrepancies may not mean much, but if a group of pulsars show correlated timing discrepancies, it could be indicative of gravitational waves produced by the inspired supermassive black holes.

Scientists have found provocative tips This is the source of the gravitational wave background in pulsar timing sequences, but we don’t yet have enough data to determine if this is the case.

We are temptingly close to detecting the gravitational wave background: the astrophysical background that reveals the behavior of black holes in the universe; and the cosmological background – quantum fluctuations seen in the CMB, inflation, the Big Bang itself.

It’s the beluga whale, Scott says: the whale we’ll see only after the hard work of separating the background into discrete sources that make up the noisy whole.

“Observing gravitational waves from the Big Bang is indeed the ultimate goal of gravitational wave astronomy,” he says.

“By removing this binary black hole foreground, proposed third-generation ground-based detectors such as the Einstein Telescope and Cosmic Explorer can be sensitive to a cosmologically generated background by 5 years of observations, thereby entering the realm where important cosmological observations can be made.”

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