Ripples in space-time – gravitational waves
In 1915, Einstein proposed general relativity, in which he argued that gravity is the result of a distortion of space-time. Over the past century, the seemingly whimsical prophecies of general relativity have been validated one by one. One of the most striking predictions is that the structure of space-time fluctuates when massive objects such as neutron stars and black holes collide with each other. The ripples caused by such events can penetrate into space-time and spread far away.
In fact, when a distant gravitational wave passes through the Earth, it stretches or squeezes the Earth. But the effect is so small that gravitational waves were thought to be nearly impossible to detect for a long time. It wasn't until the advent of large precision detectors like LIGO and Virgo that it was possible to detect these tiny effects. In 2015, LIGO successfully detected gravitational waves directly for the first time, one of the biggest highlights of the astronomical community in the past few decades.
However, the story of the search for gravitational waves is not over. So far, LIGO and Virgo have discovered not only gravitational waves produced by the merger of binary black holes, but also gravitational waves produced by binary neutron stars and black holes devouring neutron stars. In addition to these individual gravitational wave events, physicists speculate that there should be a background signal in the universe that persists and permeates the entire universe of space-time, also known as the gravitational wave background. The gravitational wave background is the superposition of ripples created by many pairs of supermassive black holes orbiting each other over the past billion years.
Not all gravitational waves are the same, just as light waves have different frequencies, gravitational waves from different sources also have different frequencies. For example, the gravitational waves produced by the merger of supermassive black holes at the center of distant galaxies are much lower than the frequency of the merger of binary black holes detected by LIGO. | Image source: ESA
Recently, an international team of scientists announced their latest detection results for the interference signal of millisecond pulsars. They think this could be a big step toward proving the existence of a background of low-frequency gravitational waves.
Galaxy Clock
When a massive star runs out of fuel, its core eventually collapses to form one of the densest objects in the universe, a neutron star. Neutron stars have a radius of only about 10 kilometers , but they have a mass comparable to that of the Sun. In the universe, they are the most densely populated objects outside of black holes, and a teaspoon of neutron star material can weigh about 1 billion tons.
Pulsars are special types of neutron stars. From Earth, pulsars appear to flicker on and off. But in fact, when the pulsar rotates at high speed, the radio signal from the poles will pass across the earth at certain intervals like the light emitted by the lighthouse. These signal frequencies are very regular and can even be used as a kind of "galactic clock". The most accurate pulsars, known as millisecond pulsars, spin hundreds of times per second.
If there are gravitational waves passing between the pulsar and the Earth, then a slight stretch and squeeze of space-time introduces a slight deviation in the normal timing of the pulsar.
Pulsar arrays around Earth. If the universe is filled with a gravitational wave background, the pulsar's pulsating signal will be disturbed by gravitational waves as it travels. | Image credit: Tonia Klein/NANOGrav
The new study comes from an international cooperative group called the International Pulsar Chronograph Array (IPTA). In their latest official dataset (DR2), they incorporate independent probe data from multiple arrays, including precise timing information from 65 millisecond pulsars.
Strange perturbations
In simple terms, they found some strange perturbation patterns in these pulsar signals. They believe that the characteristics of such common signals between pulsars may be caused by interference from the background of gravitational waves. A similar pattern has been observed in other single datasets, and this integrated dataset seems to reinforce this conclusion.
Some optimistic scientists believe that this is a very exciting signal, most likely indicating that we have begun to detect traces of gravitational waves in the background.
But the team is still relatively conservative about this finding. There is not yet enough definitive evidence, and they are still studying other possibilities for this signal. For example, perhaps it may be caused by noise present in individual pulsar data that may have been improperly simulated in the analysis.
To determine that the gravitational wave background is the source of this ultra-low frequency signal, the IPTA must also probe for spatial correlations between pulsars. This means that each pair of pulsars must respond to gravitational waves in a very specific way, which is related to how separate they are in the sky.
These characteristic correlations between pulsar pairs are conclusive evidence of gravitational wave background detection. Without them, it is difficult to prove that some other process is not the cause of the signal.
It also means that scientists must further collect data, expand the array of monitoring pulsars, and continue to search to find larger data sets. If the ensuing signal becomes more pronounced and exhibits spatial correlation, the presence of a gravitational wave background can only be confirmed.
Explore the golden age of the universe
The detection of gravitational waves from double supermassive black holes or other cosmic sources could give us unprecedented information, such as a better understanding of how galaxies form and grow, and an understanding of the cosmological processes that occur in the nascent universe.
Some scientists say that achieving this goal will also require large-scale international cooperation efforts. In the next few years, we may usher in a golden age of exploration of the universe.
#创作团队:
Written by: Takeko
Typography: Wenwen
#参考来源:
https://www.birmingham.ac.uk/news/latest/2022/01/international-collaboration-offers-new-evidence-of-a-gravitational-wave-background.aspx
https://www.space.com/millisecond-pulsars-gravitational-wave-backgroun
https://academic.oup.com/mnras/article-abstract/510/4/4873/6503453
#图片来源:
Cover/Header: Principle