Einstein is right (again)

Orbit precession

The orbits of star S2 around the central black hole of the Milky Way resemble a rosette. (Image: ESO / L. Calçada)

The general theory of relativity forms the basis of our physical worldview. Now it has passed another test of its validity. Because astronomers have observed that the gravity of the black hole in the Milky Way center shifts the orbit of a nearby star over time. He draws a kind of rosette in the sky. Einstein had already predicted this so-called Schwarzschild precession through the curvature of a black hole – and now, a good 100 years later, researchers have proven this. The degree of orbit shift they observed corresponds to what the general theory of relativity predicts for this case.

According to Johannes Kepler’s classic celestial mechanics, planets follow an ellipse that is always the same when orbiting stars or moons when orbiting planets. The periapsis – the point on the track closest to the heavier object – always stays in the same place. But even in Albert Einstein’s time, observations of the planet Mercury show that this cannot be true: its point closest to the sun and the entire ellipse of its orbit shift over time. In the long term, the orbits of Mercury thus resemble a rosette. While Kepler’s mechanics could not explain this behavior, it perfectly matched the predictions of Einstein’s general theory of relativity. According to this, this so-called perihelial rotation of Mercury is caused by the curvature of space-time caused by the sun. “This famous effect was the first proof of the validity of general relativity,” explains Reinhard Genzel from the Max Planck Institute for Extraterrestrial Physics in Garching.

View of the Milky Way Center

But if Einstein’s theory is correct, the same effect should also occur in the vicinity of a black hole. The curvature of its enormous gravity would also have to shift the orbits of nearby stars like a rosette. Astronomers in the GRAVITY collaboration have now been able to check for the first time whether this so-called Schwarzschild precession exists. This was made possible because in the center of the Milky Way a star, S2, circles relatively close to the supermassive black hole Sagittarius A *. During its 16-year orbit, it comes close to the black hole up to almost 20 billion kilometers – its distance during these periapses corresponds to only about 17 light hours. If Einstein’s theory is correct, the position of these periapses and the position of the orbits of S2 would have to shift slightly from orbit to orbit.

To check this, the astronomers of the GRAVITY collaboration have now analyzed the observation data of the Very Large Telescope (VLT) in Chile, which goes back almost 30 years. The four telescopes of the VLT can be interconnected using interferometry in such a way that their resolution corresponds to that of a 130-meter single telescope. Only this makes it possible to track the position of S2 and the position of its orbit with the required accuracy. Thanks to an additional instrument christened GRAVITY, which further increases the resolution of the coupled telescopes through adaptive optics, the star positions can be measured with an accuracy of up to 30 millionths of an arc second. This would allow the instrument to distinguish two fireflies from each other, which sit six centimeters apart on the moon.

Stern follows Einstein’s rosette

The evaluations of the measurement data showed that the orbit of the star S2 shifts between 0.196 and 0.272 degrees per orbit. Thus, as predicted by Einstein, this star shows a Schwarzschild precession: its orbit rotates over time and thus forms a rosette. As the researchers explain, the observed extent of this trajectory agrees well with the value of 0.202 determined by the theory of relativity. “This means that our results are in full agreement with the general theory of relativity,” say the astronomers. Her observations thus prove for the first time that the relativistic orbit effects predicted by Einstein are also valid in the vicinity of a supermassive black hole.

The orbit precession measured at S2 also provides information about what else is hiding near the central black hole. “Because the S2 measurements follow the general theory of relativity so well, we can set strict limits on how much invisible material, such as distributed dark matter or smaller black holes, is present around Sagittarius A *,” explain the astronomers of the GRAVITY collaboration . Because if there were large mass accumulations there, they would have to disrupt the precession. But that is not the case. The researchers conclude that another black hole is likely to have a maximum of 100 solar masses. This is significantly less than some researchers postulate for such a hidden companion. A diffuse cloud of gas or accumulation of dark matter should not exceed 8,000 solar masses.

Source: GRAVITY Collaboration, Astronomy & Astrophysics, doi: 10.1051 / 0004-6361 / 202037813

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