| From: Sue | 3/02/99
18:55:44 |
| Subject: Shape of the Universe | post id:
164 |
| Is it true that the universe has
a spherical shape? How do we know? Is it less dense at the centre? Is the
expansion of the universe presently slowing down? Will there be a point
where it will all collapse back together? If so, would it form a black
hole? And is it theoretically possible under special circumstances for
black holes to explode, casting all the matter they contain back out into
space? I'd be grateful for detailed explanations on this
topic. | |
| From: Ian Allen (the Lab) | 4/02/99
0:18:11 |
| Subject: re: Shape of the Universe | post id:
184 |
| Sue, There was recently a story about the shape of the Universe in the Lab's 'News in Science' section. Astronomers' colossal cartographic creation Tuesday, 2 February 1999 The biggest map ever drawn not only plots the location of 15,500 galaxies, it also says something important about space - it's lumpy.  A team of astronomers from several British universities has created the map using an Infra-Red Astronomical Satellite (IRAS) and observatories in Chile, the Canary Islands and Coonabarabran in New South Wales. The map is three dimensional with the Earth at the centre of a sphere 700 million light years in radius. "It's by far the biggest map of anything. There is nothing like it in its scale" claimed Professor Carlos Frenk from Durham University in Britain. The map took ten years to compile and shows clumps of galaxies in clusters and superclusters separated by vast areas of void. The ratio of the size of the superclusters to the size of the voids provides some clue to the eventual fate of the universe - and it looks like the universe will keep on expanding indefinitely. "It's an historic map," said Professor Frenk. "Some day it will hang from every kid's wall." http://www.abc.net.au/science/news/stories/s18604.htm | |
| From: Chris (Avatar) | 4/02/99
9:23:10 |
| Subject: re: Shape of the Universe | post id:
193 |
| Is it
true that the universe has a spherical shape? How do we know? Is it less
dense at the centre? No, the universe is not exactly spherical in shape, it actually has a topology akin to the surface of a sphere. What does this mean? Well, consider the following three spatial topologies: a flat piece of paper, a flat piece of paper rolled into a cylindrical shape, and the surface of a sphere. Now imagine that you are a two dimensional creature confined to each surface - how can you tell the global topology of your "universe" from a point on it? It would simple is we could step off the surface (into extra dimensions) and view the surface that way - but we need to be more inventive if we're stuck on the surface. The answer is by looking at the local topology carefully. A good test is to examine the natural worldlines of two "parallel" photons. In the first two cases the worldlines will remain parallel! The implication is that the cylindrically curved topology is indistinguishable from a flat topology to an observer constrained to the surface. However in a spherically curved topology the worldlines will eventually diverge or converge - they won't stay parallel. It turns out that this is the topology we inhabit. The thing to remember, of course, is that these are two dimensional examples, but the universe "curves" in three dimensions - the exterior topology would only be viewable from a fourth or higher dimension. As for density, on a large scale the universe is remarkably homogenous. As far as we can tell the distribution of the largest scale structures is practically uniform in every direction. The final problem is that our local topology is so finely curved - so close to flat - that we can't tell for sure if the universe is open curved or closed curved. The difference between the two is the difference between the final heat death or cold death of the universe. Is the expansion of the universe presently slowing down? Will there be a point where it will all collapse back together? If so, would it form a black hole? Yes. The universal expansion rate is decreasing. We can't tell exactly how quickly it is slowing, however, just as we can't agree on an exact value for the Hubble constant (co-efficient of expansion) yet. Also, we're not sure exactly how much mass there is in the universe (enter the dark matter debate). Put these together and the result is that we don't know who will win the tug of war between universal expansion and gravitation. If there is enough mass to halt the expansion, then the universe will begin to contract again. (This is represented by a closed curved topology). If there is not, the universe will expand until it cools off to a few fractions of a degree above absolute zero (this is an open curved topology scenario). In the former case, the so called Big Crunch, the universe won't form a Black Hole - just as it didn't start with a Black Hole. A Black Hole is the region of space inside the event horizon of a space-time singularity. The Big Bang may well have been a naked singularity, but it was not a Black Hole. And is it theoretically possible under special circumstances for black holes to explode, casting all the matter they contain back out into space? The special circumstance is that the BH is very small - perhaps even quantum scale. At this point the Hawking Radiation which leaches away its mass energy becomes an important influence. Eventually that hole might lose enough mass to explode. The average BH which might result from a stellar death process can't explode. In forming, the BH has overpowered the greatest explosive pressure in the universe - called neutron degeneracy pressure - there is no "force" which can make it explode. The neutron degenracy pressure is that pressure which results from relativistic neutrons - and stops neutron stars from collapsing into BHs. Hope this helps! Chris | |
| From: Sue | 4/02/99
15:37:52 |
| Subject: re: Shape of the Universe | post id:
355 |
| Re: ""the tug of war between
universal expansion and gravitation" - as I understand it, as objects move
further apart, the gravitational attraction between them becomes weaker.
So why would gravity be able to suddenly collapse an expanding universe?
| |
| From: steve | 4/02/99
15:40:42 |
| Subject: re: Shape of the Universe | post id:
356 |
| The idea, as far as I understand
it, is that gravitational attraction is acting as a frictional force,
slowing down the expansion to the point where the force driving the
expansion (ie the big bang) and the force of gravity get cancelled out,
resulting in a contraction and eventual
collapse. | |
| From: Cass | 4/02/99
17:50:49 |
| Subject: re: Shape of the Universe | post id:
369 |
| What, specifically, is a "naked"
singularity? Chris, what do you mean by "world
lines"? | |
| From: Brendan | 4/02/99
19:46:47 |
| Subject: re: Shape of the Universe | post id:
390 |
| Sue, I'm no cosmologist, just an interested lay person who's muddled through most of the cosmic mysteries via analogy (I'm hopeless at the maths!!) But Chris' answer to the question is pretty much how I would have put it, ie: imagining you are a two-dimensional creature "stuck" to the surface of a flat surface and proceeding from there. The name Big bang is somewhat misleading, though...as I understand it, a better name would be the Big Inflation...that is, the cosmos did not so much "explode" from its parent singularity as "inflated" outward (albeit at a very rapid rate)much like a balloon - with the "universe" being the "skin" of that expanding balloon. If, like myself, you are a lay person with an interest, I can recommend you to an excellent essay by Isaac Asimov that explains the concept in clearly understandable analogous terms. It's entitled "Beyond the Universe" and appeared in a collection of his essays called "The Roving Mind", published in paperback by Oxford University Press. Best wishes, Brendan | |
| From: James Richmond | 4/02/99
23:26:52 |
| Subject: re: Shape of the Universe | post id:
405 |
| Cass, the world line of an object
is the line traced out by the object on a space-time graph. Since we live
in a four (or more) dimensional universe, world lines can be kind of hard
to visualise without dropping a dimension or two. To give you an idea,
consider a particle constrained to move along a straight line (one
dimension) - call this line the x axis. We graph time on the y axis. Then,
if the particle justs sits in the same place doing nothing, its world line
will be a straight vertical line, parallel to the y (time) axis. If it
moves with constant velocity, the world line will be a straight line of
constant slope. Physicists also like to talk about light cones. In our two dimensional (one space, one time) picture, sketch the world lines of two photons travelling from a particular point in opposite directions. Now, imagine adding an extra space dimension - the light moves along the boundaries of a cone. Any massive particle starting off from the same point as the light must move within the light cone, since it can't travel faster than light. To get to the real world, we need to add one more dimension to this picture, which is a problem since no-one can visualise a four dimensional graph with all four axes at 90 degrees to each other. Spacetime diagrams can be helpful anyway, since we can often safely ignore one or more dimensions. | |
| From: Chris (Avatar) | 9/02/99
14:32:46 |
| Subject: re: Shape of the Universe | post id:
643 |
Cass (sorry about the delay - I missed this one...) A naked singularity is one unmasked by an event horizon. In a sense the EH "clothes" the singularity, because it shields any information from escaping. In the past physicists considered it anethematic that a naked singularity might exist, simply because it was destructive to the uniformity of the world view of space time. Recently it has been shown that a singularity might exist without an event horizon. Hope this helps! Chris | |