It’s ironic that the mathematics for the very large grew out of quantum mechanics – the maths of the very small. Imagine the universe during the first tiny fraction of the first second. Random chance, arising from quantum uncertainties, allows for some areas of space to have slightly more matter and energy than other areas of space. This isn’t much different from the observation that the density of air has slight variations across a room. But then if space itself grows quickly, those slight density fluctuations will be turned into big density fluctuations which act as the seeds of large scale structure. This happens because the areas with greater density have stronger gravitational forces, and the particles in these regions attracted each other more rapidly than those in the areas of lower density. Over many billions of years, this led to the structure we see: galactic clusters formed from the high density regions, leaving behind the empty spaces.
The images are CMB satellite maps of the Universe, one from COBE – COsmic Background Explorer, the other from the Wilkinson Microwave Anisotropy probe, or WMAP, detecting temperature differences of 0.0002 degrees Celsius
If we look at the evidence from redshift, also the brightness of supernovae, we observe that for a particular redshift the supernova ought to be brighter than it actually is, suggesting that it’s not just moving away with constant speed, it’s also accelerating. Gravity wants to pull the pieces together, expansion (driven by who-knows-what – a fifth fundamental force, perhaps), wants to push it apart. if there’s enough matter, gravity will win and the Universe will collapse, if not, expansion will win and the Universe will continue to expand forever.
There’s more. Individual galaxies within clusters such as the Virgo Cluster move much faster than anticipated. Since the motion of the galaxies stems from gravitational forces, this observation indicates that there is more mass inside the cluster than can be seen. The gravitational forces of this greater amount of matter lead to higher velocities. This is the evidence for so-called ‘dark matter’.
Whatever the fifth force – if it exists – is, that drives the expansion of empty space, is invisible. And it has no gravitational effect; in fact it has an ANTI-gravitational effect, so it can’t be ordinary matter. For these reasons, and for a few more subtle reasons, cosmologists have given this mysterious stuff the name ‘dark energy’.
Recent results now indicate that 70% of the energy density of the universe is dark energy. The remaining 30% is matter, but only a tiny fraction of this, about 0.1%, is visible matter. The vast majority of the matter in the universe is dark matter, and the majority of the universe is dark energy.
What will happen? A blueshift and crunch or continued redshift? Until we’ve figured out more details about dark matter and dark energy, your guess is as good as mine…
Finally, guesswork over whether the Universe will collapse or continue expanding forever seems to hinge on knowing how dense it is. Back to Year 8 science – density is defined as mass divided by volume. One can measure the density of the universe by observing the local group of galaxies and assuming that the Universe is all the same. One can also calculate the density required such that the Universe will eventually stop expanding. This density is called the critical density, and the ratio of the observed density to the critical density is given by the Greek letter ω. If ω is less than one, the Universe will continue expanding until it is so large that the darkness and cold will come. If omega equals one the Universe will eventually stop expanding but will not collapse. In this case, the darkness and cold will still come. But, if omega is greater than one, then the Universe is doomed to collapse under it’s own gravitational mass, and will implode in a cosmic fireball of steadily decreasing size, disappearing down a singularity like a rabbit down a hole. But don’t worry, the ultimate fate of the Universe is a few billion years away.
|Omega (Density Ratio)||Fate of the Universe|
|ω<1||Open; Eternal Expansion, Cold Death|
|ω=1||Flat; Cold Static Death|
|ω>1||Closed; Big Crunch, Hot Death|
For theoretical reasons, at the moment cosmologists like to believe that ω = 1.
Unfortunately, attempts to measure it yield results ω=0.1, or thereabouts.