| MadSci Network: Astronomy |
Charles
Whilst it is generally thought that space-time is expanding, the effect is only noticeable over large scales. In general the velocity due to expansion has to exceed the peculiar motion of the object being mentioned. Typically the peculiar motion is taken at 500Kms-1 and is due to the motion of galaxies in a group. I assume from your questionion that by local you mean within a few light years. Typically cosmologists talk about the Universe's expansion on scales of billions of light years. Essentially the effect is negligble on a local scale. I'll try to analogize. Our Milky May Galaxy is on the outskirts of the Virgo group of galaxies. This group contains a few million galaxies seperated by millions of light years. The gravitaional interactions of the group are so strong that some of these galaxies are seen to be moving towards us, not away. It is generally thought that most galaxies harbour Black Holes in the center, with a mass of millions solar masses. Even this effect does not drastically affect local space, except in the region of a few light years.
It is possible consider a scenario where expansion is halted locally, only providing we define local as a mere few Megaparsec. This local region of space would then not expand but the rest of the Universe would continue to do so. What happens then at the boundary? Eventually space time is so distorted at the boundary that something must `snap'. This scenario would therefore allow any major clustering of Galaxies to exist separately from the Universe. Essentially space would rip itself apart. Therefore, you can not stop expansion at any scale.
What is important to Cosmologists in stopping inflation is the value of the density of the Universe? One definition of mass is density times volume and as Gravity is caused by Mass, the overall density of the Universe defines whether expansion stops or not. Local high density spots, in Black Holes e.g., have no overall effect. To try and make the point stronger, a black hole is typically under 10 Km radius and has a mass of a few solar masses. Consider that a Galaxy has a few tens of billions solar masses and a radius of tens of thousands of light years. A large cluster will have a mass a few billion times more than this and a `radius' thousands of times greater, a Black Hole in comparison is negligible. Granted there are thought to be Holes a few million Solar Mass, but again, Galaxies are much bigger and more massive. In fact compared to the overall Universe, Cosmologists often consider Galaxies to be `point masses' which have negligible effect
Your next question on Hawking Radiation is far more tricky to answer. To prepare my answer let me first explain an effect of Quantum Mechanics, so that other readers are aware of what Virtual Particles are. Heisesnberg's Uncertainty Principle says that large amounts of energy can be created for small amounts of time. This is manifested as virtual particles. They exist just long enough to transmit information of events without affecting the outside Universe. Typically we are talking in terms of 10-33 s, that is a billionth of a billionth of a billionth of a millionth of a second. Now Hawking Radiation postulates that if such a pair of virtual particles where created at the event horizon of a Black hole, one of the pair may be absorbed, the other could escape. Though the escaping particle causes an overall energy flow from the hole it will also interact with the surrounding medium very quickly and as quickly be re-emitted and so on. The point is that virtual particles exist below the level of meaurement and as such can not contribute to the Microwave Background. If nothing else they are the wrong energy. As you say the Cosmic Microwave background is omnipresent and also, to a factor of one part in a million, the same strength. For Hawking Radiation to cause this you would need to have Black Holes spread evenly throughout space emitting exactly the same radiation. This is is not observed as Black Holes emit mostly X-Rays in two well defined jets.