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Star swings around Black Hole, testing gravity



Scientists have Einstein right where they want it.

Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics, Germany) and his colleagues today announced in Garching that they have discovered the shift of a star light through the sun's extreme gravity of the black hole in the center of our galaxy. This change, called gravitational redshift is one of the long sought confirmations that Einstein's Gravitation Master correctly describes how the universe works.

As I explain in our upcoming September issue, about three dozen bright stars are crowding close to the supermassive black hole in the heart of the Milky Way. Both Genzel's team and a second, led by Andrea Ghez (University of California, Los Angeles), have been observing this star cluster for more than two decades, using the orbits of the stars to calculate the mass of the invisible thing. It is this work that gives us our estimate of the mass of the black hole: about 4 million times the sun, all within a radius of 8% of the size of the Earth around our star.

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The artist's impression shows the path of the S2 star swung through the supermassive black hole in the center of the Milky Way. As the star approached the black hole, its light had to fight against the immense gravitational field of the black hole. The light lost energy, causing the color of the star to shift slightly towards red (exaggerated here for the effect). Object sizes are not scalable. ESO / M. Kornmesser

The brightest member of this star group is called S2. It's a 7 million-year-old massive, bluish B type that weighs about as much as a dozen Suns together. He finishes his racetrack every 16 years around the black hole. During its closest approach, it comes closer to the black hole than any other star yet to be discovered: about 120 a.u. or four times Neptune's distance from the sun. His last round was earlier this year in May.

S2 Flyby allows two tests of general relativity. The first is the gravitational redshift, whose preliminary detection is announced today. In Einstein's general theory of relativity, gravity is geometry. Mass and energy create curves in space-time, much like objects when placed on a stretched rubber band. All matter makes dips – some stronger than others – in the cosmic leaf. Anything moving through the warped region must navigate through the curves in the spacetime landscape. Even light has to follow the redirections of gravity.

When S2 hit the black hole, it plunged further into the wide, deep well that the black hole created in space-time tissue. Their photons had to rise from the well to reach us. This ascent deprived them of energy and shifted them to longer, redder wavelengths.

  close-approach-pic2

This diagram shows the movement of the star S2 as it passed near the black hole, based on data from the gravity interferometer. At that time, the star was moving at almost 3% of the speed of light. ESO / MPE / GRAVITY Collaboration

But the change is relatively small compared to the more prosaic one expected of Newtonian gravity. In Newton's frame, the light from S2 should also shift backward along our line of sight due to the dramatic movement of the star as it spins around the black hole. The change manifests itself as a momentum of several thousand km / s. In contrast, the relativistic redshift adds only about 200 km / s.

To see this smaller, gravitational redshift, astronomers must exquisitely capture the orbital motion of the star. This is hard to do when the target is about 26,000 light-years away, surrounded by dusty gas and surrounded by other blurred light globules. Images of the galactic center look like a psychedelic polka dot fabric in which all dots move.

Besides, pictures only give us a flat perspective; they do not reveal the third dimension. That comes from measurements of the motion of stars along our line of sight, or radial velocity .

So there are three key moments in S2's breakneck turn around the Black Hole, all of which tell you something about the 3D encounter unfolding. One is when the motion of the star of us suddenly shoots up along our line of sight. After the radial velocity begins to decrease, the second key moment occurs as the star approaches its closest approach to the black hole, as seen in "flat" images. The third moment is when the star's movement ends its steep fall right here and slows down along our line of sight. The first and the second moment took place in May; the third is expected to arrive in September.

In the Center of the Passport

Both the MPI and UCLA teams have been gathering observations for more than 20 years to give them enough information to track the movement of S2's additional redshift blip from general relativity , As part of this work, Frank Eisenhauer (Max Planck Institute for Extraterrestrial Physics) and his colleagues built an interferometric instrument called Gravity.

Gravity combines the four 8.2-meter telescopes of the Very Large Telescope in Chile into a single subscope equivalent diameter of a whopping 130 meters. At near-infrared wavelengths, this increases resolution by more than a factor of 10 over what has been possible so far, allowing the team to observe the movement of S2 from night to night. The performance is akin to using the VLT to watch a football game on the moon and see the trajectory of the ball with an accuracy of 6 centimeters, said Françoise Delplancke (ESO) during a press conference

Combining gravitational data with Images and radial velocity measurements taken over S2's 16-year orbit have found that Europeans have successfully detected the extra blip. It looks like that.

  redshift-graph-pic3

GRAVITY Collaboration / Astronomy & Astrophysics 2018

The flat line is the prediction from Newtonian gravity; The bulge is the prediction from the general theory of relativity. When Eisenhauer showed a version of this diagram at the press conference, applause broke out in the hall.

The game is not over yet. This blip is only as credible as the underlying data it is on. To be sure, the astronomers need the third part of the S2 passport, which now appears in the radial velocity data – in fact, when I talked to Ghez over the phone on the phone this morning, she was on her way to the airport for Hawai'i i fly to point with one of the Keck telescopes on S2.

"I'm happy for her," she says sincerely. "I think Gravity is an amazing instrument." The beauty of having two teams working on the same thing with different tools and approaches explains that the combination of their results makes science better. But she really wants to see what happens at the last revolution of the radial speed before she rates what her own data tells her.

But wait, there is more

The teams use S2 to test a second relativistic prediction. In the general theory of relativity there are no closed paths. Just as Mercury moves around the Sun and extracts cosmic spirograph template patterns, S2 should also wind around Sgr A *. In the case of S2, the precession is about 12 arc minutes per revolution.

The measurement of this shift is immensely demanding. But Genzel's team thinks they'll have the data to see it next year. If and when one of the teams sees it, it is another victory for Einstein.


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