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 ENERGY
Now, cosmologists have another idea. If there was a curvature to space, it would bend light, like a lens. As a result, the images of very distant objects and the pattern of irregularities in the cosmic background radiation would be distorted in a particular way if the universe is closed, and in a different way if the universe is open.
The observations do not reveal any trace of such distortions, so cosmologists are sure that the universe is flat. This means that there must be a certain amount of mass in the universe, which translates as a certain density today. But the amount of matter in the universe, adding together baryonic (or ordinary) matter and dark matter, provides only about 27 per cent of this critical density.
So observations of the cosmic background radiation tell cosmologists that there must be another form of mass dominating the universe. This is called dark energy. Just as all mass has an energy equivalent, so all energy has a mass equivalent, and although dark energy is not matter it has mass and affects the curvature of space and the way the universe expands.
The fact that space is flat tells us how much mass-energy there must be altogether; if 27 per cent is matter, then 73 per cent must be dark energy.
Dark energy shows its influence on the universe directly by the way it affects the expansion. When the distances to remote galaxies are measured using observations of supernovae, it turns out that these galaxies are all a little bit farther away from us than they should be according to the simplest interpretation of their redshifts.
Everything falls into place, however, if the expansion of the universe is speeding up, so that distant galaxies are a bit farther away than the simple Hubble's Law implies. The effect is too small to be measured for nearby galaxies, which is why it was not noticed until the end of the twentieth century.
It is thought that dark energy acts as a kind of antigravity, stretching space, and that this effect will get bigger as the universe ages – as galaxies move farther apart, their gravitational bonds weaken, but the dark energy keeps on pushing.
If these new ideas are correct, then space will always be flat, but the expansion of the universe will get faster and faster until, in about 100 billion years from now, galaxies are so far apart that it will be impossible to see anything beyond the Milky Way and its companion galaxies in the Local Group. What happens then is a matter of guesswork.
How the universe actually began is also a matter of guesswork; but it is astonishing how far back in time we can go before cosmologists have to resort to educated guesswork.
Because we know the temperature of the background radiation today, and the overall density of the universe, and how fast the universe is expanding, we can calculate backwards in time to work out what the temperature and density were at any time in the past.
We know that the universe as we know it began just under 14 billion years ago in the Big Bang, and one of the objectives of cosmology is to work out what conditions were like as close as possible to that beginning, which is sometimes referred to as 'time zero'.
The most extreme conditions of density that exist on Earth today are in the nuclei at the hearts of atoms, and thanks to experiments involving giant particle accelerator machines ('atom smashers'), physicists are confident that they understand the physics of nuclear densities thoroughly – and, of course, they understand what goes on at lower densities at least as well.
If we wound back the expansion of the universe to a time when the density everywhere was the same as the density of the nucleus of an atom today, we would go all the way back to 0.0001 of a second (one ten-thousandth of a second) after time zero. Cosmologists are confident that they understand, in general terms, everything that has happened to the universe since then.
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.