Startling emptiness at heart of our galaxy

Scientists are puzzled by the vacancy they observe around the black hole at the center of our own galaxy. Though this is what I would have thought you'd expect, it turns out that physicists were expecting other stars to fill in the vacuum left by the stars sucked up by the black hole. This article, at New Scientist, briefly excerpted below, talks about the issues.

 ....

Let's not forget that, until now, there has only been indirect evidence for a black hole at the centre of our galaxy. We know that something massive lurks there because its gravity affects the motion of nearby stars, and the most likely culprit is a black hole. But we need direct evidence to be sure. Now the hope is that stars like S2 will not only provide that evidence but also allow us to test our most cherished ideas about black holes.

Among them is the idea, known as the no-hair theorem, that black holes are essentially so simple that they can be described adequately by their mass and how fast they spin. Theorist Clifford Will of Washington University in St Louis, Missouri, suggests that we could test the theorem, and therefore general relativity, by examining the orbits of stars close to the supermassive black hole. One way to do this would be to watch a star complete many orbits around the galactic centre. Einstein's theory predicts that the star's point of closest approach to the centre should progressively shift from one orbit to the next. If the no-hair theorem is correct, the rate of this "precession" depends on the mass and spin rate of the black hole, and nothing else. Even better, says Will, would be to track two stars (The Astrophysical Journal Letters, vol 674, p L25). That way, you can use the relationship between both stars' orbits to cancel out the mass of the black hole, so the precession depends only on its spin. If it turns out that the precession depends on something more complex, then the no-hair theorem is wrong. And if that is true, then general relativity is also wrong. ...

Another way to test relativity is to use pulsars. These super-dense remnants of supernova explosions spin very rapidly, sweeping a lighthouse beam of radio waves across the sky once every turn. This makes them fantastically precise timekeepers. If any exist in the centre of the galaxy, then we might be able to pick up another relativistic effect - gravitational time dilation, where the passage of time slows down in the warped space-time surrounding a massive object. Spot this and we would have evidence of a massive black hole.

Star-spotting

Unfortunately, pulsars are intrinsically faint, making them difficult to detect in the dusty galactic centre. But astronomers have just embarked on an attempt to detect all the pulsars in the Milky Way, and they are hopeful of observing pulsars in the centre of the galaxy ...

Marcus Chown, author of the above excerpted article,  is the author of We Need to Talk About Kelvin (Faber & Faber, 2010).