Skip to comments.Spin rate of black holes pinned down
Posted on 08/08/2013 1:35:04 AM PDT by LibWhacker
Black holes can be described by just two fundamental characteristics: mass and spin. Astronomers have been able to measure the objects mass for decades, by looking for gravitational effects on the orbits of nearby stars. But measuring spin, which records the angular momentum of the matter that falls into the holes, has proved troublesome, particularly for the supermassive black holes that lie at the centres of galaxies. No light emanates from the black holes spinning event horizons, so astronomers instead look for proxies that emit X-rays, such as the swirling disks of matter that feed into some holes.
Such indirect spin measurements have now been made for 19 supermassive black holes for which the mass is also well known (see Spin off). On 29 July, astronomers reported that they had calculated the spin of another supermassive black hole, using a new technique that, although unproven, provides an alternative way to target the elusive quantity. Theres a significant number of us who think we are getting a coherent picture of black hole spin, says Andrew Fabian, an astronomer at the University of Cambridge, UK.
(Excerpt) Read more at nature.com ...
Oh, an astronomy article. I thought it would be about Obama.
i have a question on this — perhaps someone here can answer it. if the centre of each galaxy has a super black hole and ‘we’ are all slowly circling it — is it inevitable, over a huge span of time that every galaxy will disappear into the centre. i realise the distances are vast and the timeline is huge, but does this mean every spiral galaxy (isnt that them all?) are slowly under the gravitational influence of the centre..and therefore (for want of a better word) falling into the black hole at the centre? does this mean all planetary matter in the universe will eventually disappear leading to a cold dark (absolute zero i would guess) universe...never been able to get anyone to give an answer on that...so any help appreciated
Yes, and nebulae and dwarf stars will be hit hardest!
(Actually I don’t know. I’m curious too.)
I’m not entirely sure but I don’t see any reason that stars can’t orbit the center like planets around the sun, meaning they won’t necessarily get sucked in.
The reason I’m not sure is because, in spite of the high gravity, the distance from us makes the force on us very low, so I don’t know if we do actually orbit it.
On the other hand, you have dinky objects at the extemes of the solar system that are orbiting the sun so it seems at least possible that we do actually orbit the center of the galaxy.
Good question. Keplerian orbits (like the earth around the sun) are stable, according to Newtonian mechanics, gravity will not cause the earth to spiral into the sun. In general relativity an orbiting pair radiate gravity waves, which remove orbital energy and cause them to spiral inward, eventually. The fate of universe appears to be a long slow thermodynamic cool down, with every particle approaching absolute zero, gathered up in gravitationally-bound galaxy-cluster-sized lumps. I’ll miss the old universe.
But there’s a lot we don’t know, so no one really knows for certain.
Actually, for the immediate term (probably billions of years at least) there is no reason that the stars in a galaxy cannot orbit a central black hole. Gravitational radiation would be minimal because of weakness of the gravitational field (the stars are orbiting at large distance). Gravitational radiation would only really be significant for systems with high density objects orbiting each other a close distance (such as two neutron stars orbiting each other, for instance).
Contrary to popular perception, a black hole does not inexorably suck in everything in its vicinity. In fact, for most practical purposes a black hole behaves the same way as any ordinary massive object having the same mass as the black hole. For instance, if the sun were to suddenly turn into a black hole, we of course would die, but not because of being sucked in to the new black hole. We’d die because of the massive ice age caused by the lack of solar radiation, but the earth would continue to orbit the new black hole in exactly the same way as it currently orbits the sun.
The real difference between a black hole and an ordinary object is what happens when you get close to it. The gravitational field of the black hole warps spacetime to such a degree that really wierd-looking things happen to anything that starts to venture too close. For a certain distance, called the event horizon, the spacetime cooridinates change their character, ie time and space interchange, meaning that it is not possible for an object to move away from the black hole’s center. Practical upshot is that an object (or even light) cannot escape the black hole once it gets to the event horizon. Interestingly, an outside observer can never see this occurring. As I stated above, weird things happen near a black hole. To an outside observer, it would take an infinite amount of time for an object to reach the event horizon. (The outside observer would also see the object “smeared out” over the entire surface of the event horizon — told you things are weird near the black hole!)
Its the “Black Hole Era” which precedes the Dark Era. The only structured matter left in the universe are black holes and the time span we are talking about here is trillions of years. The universe is not going to fall back into itself in a big crunch, its just going to get very, very, very cold and fade away.
Eventually black holes dissipate due to Hawkings Radiation.
Actually the earth feels no effect of the central black holes gravity of the milky way. The spiral arms of the milky way are the product of gravitational waves created at the early creation of the galaxy but now are maintained by the combined gravitational fields of all the stars in the galaxy. Our star is not falling towards the gravitational center of the galaxy, but is being carried along on the gravity waves of the galaxy itself.
I despise the notion of an open universe. The very thought of the slow soul-sucking heat death of everything is just absolutely esthetically wrong. I may well, be wrong, but I just absolutely refuse to accept the notion of an open universe. Of course, I doubt I'll get to hang around to see it, but the eventual collapse into a new black hole, that will rebound in a new big bang is a more satisfying prospect IMO.