Wherever they move through the universe, they compress and stretch space and alter the flow of time: gravitational waves. Predicted by Einstein as early as 1915, researchers have now succeeded in proving the existence of very long gravitational waves.
The existence of gravitational waves was already predicted by Albert Einstein in 1915. A century later, in 2015, they were measured for the first time. Now an international team has another success: For the first time, they’ve detected very long gravitational waves. They are now hoping to gain new insights into the world of black holes.
Waves 2015 im LIGO experience It was discovered from a massive cosmic event – the merger of two black holes. Before the merger, the two black holes had been orbiting each other closely for several seconds. The forces involved in this dance, and the subsequent merging of warped space and altered time, spread as waves across the universe for billions of years – until they accidentally hit Earth and were measured there. In 2017, the Nobel Prize in Physics was awarded for this achievement.
Over 25 years of observation
Researchers at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn report a similar success. Together with astronomers from other European observatories and with additional data from India, they have succeeded in detecting gravitational waves of very long wavelengths for the first time.
While an event recorded in LIGO lasted only a few seconds, a long gravitational wave would take several years to travel through a measuring instrument. The Bonn researchers had to be patient enough to gather the data to be able to prove the existence of long gravitational waves in space. Since 1995, and thus for 28 years, they have been using a very large instrument to track long gravitational waves. he is called European pulsar timing set (EPTA) is the size of our entire galaxy.
Stars send radio waves out into space
EPTA works like this: There are a number of pulsars in the universe. These are stars that regularly emit pulses of radiation into space like beacons. Not just pulses of light, but radio waves. These radio waves can be picked up by radio telescopes on the ground. If a very long gravitational wave runs between a radio telescope on Earth and a pulsar several light years away, the wave extends and compresses the space between Earth and the pulsar. With this, the pulsar is also moving closer to Earth and then a little further away from it.
In numbers: A star four light-years away is displaced by only about 100 metres. This relatively small change in the distance between the Earth and the star also results in a slight change in the propagation times of the radio pulses emitted by the star – and this change has now been detected in data from ground-based radio telescopes.
“Pulsars are excellent natural clocks,” explains Dr. David Champion, chief scientist at the Max Planck Institute for Radio Astronomy (MPIfR), in a press release from the institute. “We use the amazing regularity of their signals to search for subtle changes in their beats, allowing us to detect tiny stretches and pressure in space-time caused by gravitational waves from the distant universe.”
Important measurements in Germany
The most important telescope in the European EPTA was the Effelsberg radio telescope in the Eifel. The instrument, which is more than 50 years old, remains state of the art thanks to constant modifications and provided the longest series of data available anywhere in the world, explains Professor Michael Kramer, director of the Max Planck Institute for Radio Astronomy in Bonn. “It forms the backbone of this experiment,” says the astronomer.
The Effelsberg data were combined with measurements from radio telescopes in France, Italy, the Netherlands and Great Britain and with data from the Large Indian Telescope. The first indications that the waves we were looking for actually exist were discovered in 2016 during evaluations. After many years of high-precision measurements, researchers are now pretty sure that long gravitational waves do indeed move through the universe.
Research groups in North America, Australia and China can also show similar results. That’s why groups have now announced their discoveries simultaneously on four continents.
New insights into the world of black holes?
Long-wave gravitational waves are created when two supermassive black holes orbit each other. It is billions of times heavier than our sun. In contrast, the black holes whose mergers produce the short-wavelength gravitational waves recorded in LIGO are extremely small. Its mass is usually about 30 times greater than that of our Sun. Their cousins, who are billions of times heavier, sit in the centers of galaxies.
When galaxies collide, a pair dance of black holes at their centers can occur. The reciprocal orbit of the two most massive black holes does not take seconds, but several years. The gravitational wave emitted in this process slowly evolves and appears extended. The dance of supermassive black holes can span millions of years, and during all that time gravitational waves have been radiating out into space. Astronomers around the world now hope to use the newfound longwaves to study the behavior of supermassive black holes.
However, it is not yet possible to assign the measured wave trains to individual black holes. This step should work in the coming years, and then gravitational wave astronomy will have a powerful new tool at its disposal.
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