NANOGrav announced that it has discovered a signal feature that may be derived from low-frequency gravitational waves. If confirmed, this will be another milestone in gravitational wave astronomy. .
The signals discovered by NANOGrav came from distant pulsars. These pulsars are fast-rotating dense celestial bodies, and their poles emit light beams that pass over the earth like lighthouses in the universe. Researchers used radio telescopes to collect signal data that may be generated by gravitational waves.
"This news is very exciting, but the research results need to be further verified." said Zhang Chengmin, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences.
Finding the pulsar signal time difference
"The signals that appear in the data are incredible but exciting." said Joseph Simon, the lead researcher of the above-mentioned study.
Since 2015, scientists have used the Laser Interferometer Gravitational Wave Observatory (LIGO) and the European Virgo Gravitational Wave Observatory (Virgo) to detect gravitational wave signals many times. Why is the appearance of the suspected low-frequency gravitational wave signal still so exciting?
"The gravitational waves detected by LIGO have frequencies ranging from tens to hundreds of hertz, which belong to high-frequency gravitational waves. The gravitational wave signal that NANOGrav looks for has a frequency of nanohertz and a wavelength spanning several light-years. It belongs to low-frequency gravitational waves. "Zhang Chengmin said that in terms of frequency, there is a difference of more than ten orders of magnitude between the two.
According to Zhang Chengmin, the detection of NHz gravitational wave signals is of great significance for studying the history of the early universe, verifying the Big Bang theory, obtaining information on supermassive black hole collisions and mergers, studying galaxy mergers, and further studying the properties of various types of gravitational waves in the universe.
Gravitational waves are called ripples in time and space. Astronomers cannot "see" it directly through a telescope, but they can look for it by measuring the effects of passing through time and space—small changes in the precise position of objects.
NANOGrav's research object is pulsar signals. A pulsar is a reliable cosmic timer that we can detect. These small, dense, compact celestial bodies rotate rapidly, sending out radio wave pulses at precise time intervals.
Gravitational waves will interfere with this regularity, because the space-time ripples produced by gravitational waves will cause space-time to produce tiny stretches and shrinks. These ripples in time and space can cause very small deviations between the actual time when the pulsar signal arrives on Earth and the expected time.
NANOGrav investigates the time characteristics of the regular signals emitted by many millisecond pulsars scattered in the Milky Way, the so-called pulsar timing array, to detect minute changes in time caused by gravitational waves stretching and contracting time and space, and then obtain the existence of gravitational waves. Clues.
The selected satellite can be described as cream of the crop
According to reports, NANOGrav created the pulsar timing arrays by studying the 47 most stable rotating millisecond pulsars.
Why are these 47 pulsars? Because not all pulsars can be used to detect such low-frequency gravitational wave signals. Only pulsars with the most stable rotation and a long time to be studied can meet the detection requirements. These pulsars rotate hundreds of times per second and are incredibly stable to ensure the accuracy required to detect gravitational waves.
"At present, astronomers have discovered more than 3,000 radio pulsars. The stability of ordinary pulsars is not enough, but the stability of millisecond pulsars is very high, as high as hundreds of millions of years or even billions of years. Use millisecond pulsars for high-precision measurement." Zhang Chengmin explained that the number of millisecond pulsars discovered so far is about 400, and astronomers will further select very stable pulsars as observation objects. These 47 millisecond pulsars are the source Here.
NANOGrav said that of the 47 pulsars it studied, 45 have at least 3 years of data sets for analysis. In this study, the researchers found a low-frequency noise characteristic in these data sets, which is the same on multiple pulsars. The time variation they found is so small that when studying any single pulsar, the evidence is not obvious. But when taken as a whole, these unobvious evidence means an important signal feature.
"The motions of pulsars have no correlation with each other, but gravitational waves passing through the Milky Way will cause their motions to have a correlation or regularity. This study hopes to eliminate all kinds of noise and eliminate this regular motion. Find out the information to find out the low-frequency gravitational wave signal." Zhang Chengmin said.
It will take a few years to
reach an exact conclusion. NANOGrav claims that the newly discovered signal features have excluded some sources other than gravitational waves, such as interference from solar system matter or some errors in data collection.
In order to verify this suspected low-frequency gravitational wave signal, researchers must find a unique correlation between different pulsar data-the degree of correlation between the two pulsar data is related to their sky position relative to the earth. However, because the signal is too weak, no significant evidence of this correlation has been found so far. To enhance the signal, NANOGrav needs to expand its data set to include a larger number of pulsars with longer research time, which requires an increase in the number of telescope arrays. Sensitivity. In addition, by combining NANOGrav's data with data from other pulsar timing array experiments, the launch of the International Pulsar Timing Array (IPTA) cooperation program may help reveal this particular correlation.
Currently, NANOGrav is developing new technologies to ensure that the detected signals do not come from other sources. They are building a computer model to help detect whether the signal is caused by effects other than gravitational waves to avoid misjudgments.
"Detecting gravitational waves with a pulsar timing array requires patience. We are analyzing data for more than a decade, but it may take several years to get a definitive conclusion," said Scott Lanson, current chairman of NANOGrav.
We need more large telescopes to join
whether the signal comes from the low-frequency gravitational waves really? Zhang Chengmin believes that: "The current research results are still in the preliminary stage and cannot be concluded for the time being."
"This research actually analyzes the observation data of more than 40 millisecond pulsars, which is the result of data processing. Timing data processing methods, The choice of millisecond pulsars will have a huge impact on the research results. To confirm that it is indeed a low-frequency gravitational wave signal, further verification is needed." Zhang Chengmin said.
He told reporters that the detection of low-frequency gravitational waves requires high-precision measurement, as well as long-term data accumulation and data analysis. The larger the number of large telescopes added to the observation, the higher the sensitivity of the telescope, the higher the quality of the data, the more reliable data is accumulated, and the smaller the error, the higher the reliability of the data.
Zhang Chengmin introduced that in addition to North America, Australia and Europe currently have telescopes to monitor millisecond pulsars. Through the addition of other radio telescopes around the world, to provide the research team with more high-quality data, it is possible to further verify this discovery in the future.
But there is also bad news. In the entire research process, NANOGrav used the observation data of the US Green Shore Radio Telescope and Arecibo Telescope. The 305-meter Arecibo telescope recently collapsed.
NANOGrav said that the research team will seek other data sources and strengthen cooperation with international counterparts. However, the loss of the Arecibo telescope will still affect NANOGrav's description of this background noise and detection of gravitational wave signals in the future.
"The accuracy of the Arecibo telescope is relatively high, and its collapse has affected the accumulation of new data and increased the difficulty of verification. In the future, the research team will need to cooperate with other telescopes to conduct observations to further test this discovery." Zhang Chengmin Say.
