Our own universe may be but one bubble of space-time in a much bigger multiverse.
Credit: Greg Smye-Rumsby
This article is an edited extract of the book From Here to Infinity: The Royal Observatories Greenwich Guide to Astronomy, published by the University of Western Australia Press.
Credit: University of Western Australia Press
DARK MATTER
This may seem almost insignificant but, in fact, the detailed studies of the CBR show exactly the right pattern of hotter and colder patches to match the pattern of density variations that would lead to the kind of structure we see in the universe around us.
But there is one more thing required to do the job of making galaxies. There has to be a lot of dark matter in the universe to explain the nature of galaxies like the Milky Way.
Even more dark matter is needed to explain why clusters of galaxies don't fly apart. Clusters of galaxies are like swarms of bees, with each individual moving around within the cluster under the influence of gravity, while the whole cluster moves as a unit, carried by the universal expansion.
By measuring the velocities of individual galaxies within clusters using the Doppler effect, it is straightforward to calculate how much mass there must be in the cluster to stop the galaxies escaping. This is always a lot more than the amount of matter we can actually see in bright galaxies in the cluster.
The background radiation independently tells us the same thing.
Computer simulations of how structures can develop in the universe as it expands show that the pattern of galaxies and clusters we see can only have grown from fluctuations the size of the ones revealed by the background radiation if there is about six times as much dark matter as there is everyday atomic matter. Then, everything fits together beautifully.
Even this, though, is not the end of the story of what we can learn from the CBR. In the 1930s, soon after the expansion of the universe was discovered, cosmologists began to puzzle over the question of whether the expansion will continue forever, or whether it will stop one day, and perhaps even go into reverse. The answer depends on the way space is curved, as defined by the general theory of relativity.
There are three possibilities. If there is more than a certain amount of matter in the universe, corresponding to a particular density at the present day, then three-dimensional space is curved in the same way that the two-dimensional surface of a sphere is curved, so that it folds back on itself and has no edge.
In such a situation, if you head off in one direction and keep going long enough, you will end up back where you started, just as if you keep going in the same direction on the surface of the Earth you will get back to where you started after travelling right round the globe.
This is called a closed universe, and if the real universe is like that, it will expand for a while, slow down as gravity works against the expansion, and then collapse back on itself. This is like the way a ball thrown straight up in the air slows down under the influence of gravity and falls back to Earth.
At the other extreme, if there is less than the critical amount of mass in the universe, it is said to be open, and will expand forever. This is like the way in which a spacecraft launched with sufficient speed will escape entirely from the Earth's gravity.
The geometry is harder to picture in this case but it is the three-dimensional equivalent of a saddle surface, or a mountain pass, extending out to infinity in all directions.
Just at the dividing line between these two possibilities is the third alternative, that space is flat in three dimensions in the same way that a piece of paper smoothed out on a table is flat in two dimensions.
If the real universe is like that, it will keep expanding but at a slower and slower rate until eventually it hovers on the brink of collapse, but never collapses. At least, it used to be thought that that was the fate of a flat universe.
The shape of the universe
Regarding the way we see the shape of the universe,"open","closed","flat".Maybe it's possible that there are other geometries,just not avilable to the common eye.Of course this point does spill over into 'religion',but maybe for to long now we have been seeing the universe through scientific blinkers. Maybe there is a need to take a more "holistic" view?
Excellent science writing
Thanks to John Gribbon for laying out all these ideas with exceptional clarity.