Star oscillates around black hole, tests gravity

Scientists have Einstein exactly where they want it.

Today in Garching, Germany, Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics, Germany) and his colleagues announced that they have detected change in the light of a star caused by the 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 gravity frame correctly describes how the universe works.

As I point out in our next issue of September, about three dozen stars cling very close to the supermbadive black hole of the heart of the Milky Way. The Genzel team and a second led by Andrea Ghez (University of California, Los Angeles) have observed this group of stars for more than two decades, using the orbits of the stars to calculate the mbad of the invisible machinery who is hiding among them. It is this work that gives us our estimate of the mbad of the black hole: about 4 million times that of the Sun, all contained within a radius of 8% of the size of the Earth around our star.

The brightest member of this star troupe is called S2. It is a mbadive, bluish star B 7 million years old that weighs about a dozen suns combined. He finishes his race course around the black hole every 16 years. During its closest approach, it approaches the black hole more than any other star yet detected: about 120 a.u., four times the distance of Neptune from the Sun. His last visit was earlier this year in May.

The overflight of S2 allows two tests of general relativity. The first is the gravitational redshift, a preliminary detection of which is announced today. In Einstein's general theory of relativity, gravity is geometry. Mbad and energy create curves in the fabric of space-time, much like objects when they are placed on a stretched rubber sheet. All matter makes hollows – some more pronounced than others – in the cosmic sheet. All that travels through the deformed region has to navigate the curves in the space-time landscape. Even light must follow the deviations of gravity.

While S2 was lowering on the black hole, he dipped deeper into the wide and deep pit that the black hole created into the space-time fabric. His photons had to come out of the well to reach us. This climb robbed them of energy, moving them to longer and more red wavelengths.

But the change is relatively small compared to the most prosaic expected of Newtonian gravity. As part of Newton, the S2 light should also shift to red due to the dramatic movement of the star along our line of sight as she makes her turn around the black hole. The change is an oscillation of several thousand km / s. On the other hand, the relativistic redshift only adds about 200 km / s.

To see this smaller gravitational redshift, astronomers need to have the orbital motion of the star pinned accurately. It's hard to do when your target is 26,000 light-years away, veiled by a dusty gas and surrounded by other balls of light with blurry appearance. The galactic center images look like a psychedelic polka dot fabric in which all the points move.

Moreover, the images give us only a flat perspective; they do not reveal the third dimension. This comes from measurements of the movement of stars along our line of sight, or radial velocity

So there are three key moments in the tight turn of S2 around the black hole, which you say something the 3D encounter is taking place. One of them is when the movement of the star away from us along our line of sight soars suddenly. After the radial velocity begins to collapse, the second key moment occurs, when the star reaches its closest approach to the black hole as seen in the "flat" images. The third moment is the moment when the movement of the star stops and plunges us at full speed along our line of sight. The first and second moments occurred in May; the third is estimated to come in September.

Midway Through the Pbad

The MPI and UCLA teams have been accumulating observations for over 20 years to give them what they need to track S2's movement with enough precision to see the additional shift to the redshift of general relativity. As part of this work, Frank Eisenhauer (Max Planck Institute for Extraterrestrial Physics) and his colleagues built an interferometry instrument called Gravity.

Gravity combines the four 8.2-meter telescopes of the Very Large Telescope in Chile into a single supercount, with an equivalent diameter of a whopping 130 meters. At near-infrared wavelengths, this increases the resolution by more than a factor of 10 compared to what was previously possible, allowing the team to watch the movement from S2 night to night. The feat is comparable to the use of the VLT to watch a football match on the moon and see the trajectory of the ball with an accuracy of 6 centimeters, said Francoise Delplancke (ESO) at the same time. a press conference

. The radial velocity measurements taken during the S2 16 year old orbit, the European team says that they have successfully detected the extra-blip. It looks like this.

The flat line is the prediction of Newtonian gravity; the hump is the prediction of general relativity. When Eisenhauer showed a version of this graphic at the press conference, applause broke out in the lobby.

But the game is not over yet. This blip is as believable as the underlying data it is on. Certainly, astronomers need the third part of the S2 pbad, which begins to appear in the radial velocity data – in fact, when I spoke this morning on Ghez's phone about the result, she was headed to the airport. fly to Hawaii to point one of Keck's telescopes to S2.

"I'm happy for them," she says sincerely. "I think Gravity is an amazing instrument." The beauty of having two teams with different instruments and approaches working on the same thing, she explains, is that combining their results improves science. But she really wants to see what happens at the last corner of the radial velocity before evaluating what her own data tells her

But wait, that's more

The teams are using S2 to test a second relativistic prediction. In general relativity, there are no closed orbits. Just as Mercury cleans itself around the Sun, by drawing patterns of cosmic Spirograph stencils, S2 should also spiral around Sgr A *. In the case of S2, the precession is about 12 minutes arc per orbit.

Measuring this change is extremely difficult. But the Genzel team thinks that they will have the data to see it in the coming year. If and when one of the teams sees it, it will be another victory for Einstein.