The Big Crunch, though it should be called the Great Crunch as per the fact that the event will not be large - but indeed quite great - but will appear to a disembodied observer to be the convergence of all matter and space-time fabric to a single -zero dimentional- point. There are a number of suggestions, speculations, and provable theories which explain this phenomenon, but I will only go into the three most popular - and the three that make the most sense.
  1. The Great Crunch will be caused by the universe -(Organ Universe)- ceasing to expand because the gravity generated by the mass of the universe would overpower the intertia of galaxies, clusters, etc... and the universe (Organ Universe) would begin to contract toward itself and end up turning around and crunching (Great Crunch) into itself at a single point. It is thought that the kinetic energy of this event will spawn another Big Bang and the cycle will continue. This concept of the Great Crunch relies on the critical mass of the universe being large enough to pull everything back unto itself. If one adds all observable matter, one finds that it is short of critical mass for a Great Crunch by a factor of almost 10. The answer to this problem is mass called dark matter, matter that is just like regular matter (if there is such a thing) exept that it cannot be seen. Its effects can be seen everywhere we see gravitational forces working but we cannot see the matter that would make such bends in space-time. Several explainations of dark matter include burnt out stars, neutrino clouds, and all sorts of things that make sense, solve the problem of critical mass, and have observable effects on the universe.
  2. Another view on how the Great Crunch might happen is if every galaxy, matter cloud, and speck of mass are moving away from eachother, then eventually - regardless of the critical mass and dark matter ideas - they will go full circle around the universe. Relativity explains the space-time fabric as unbounded but also as finite; this means that it is curved- somewhat like a sphere. So eventually all matter will travel a full course and end up in the same place it all started - thus, a Great Crunch, the same kinetic energy and the possibility for another Big Bang.
  3. Another -though unpopular but acceptable- idea is that matter (galaxies, or matter clouds) will start to move toward other pockets of mass which make larger bends in space-time. Eventually, it is thought, the little pockets of mass will collide with the larger pockets and so on and so forth until all mass collides and the resulting warp in the fabric of space time will tear a hole at a single point, and blah blah woof woof, etc... you get the idea

What exactly does this mean? "The result indicates that the universe if flat." I find myself acutely confused; wouldn't such a simple CMB probe disproving Relativistic concepts of space-time curvature make headlines and cause a massive disturbance within the scientific community? And as for the matter discrepancy, MACHOs, neutrino mass, and plain ol' dark matter make up for it more than adequately. Please ladies and gents, explain further what you mean, i'm curious? Did some hodge-podge NASA experiment negate relativistic concepts of spacetime curvature?

For now, at least, the Big Crunch hypothesis has been disproven by a group of scientists based in Europe and Australia. Using an infrared telescope, they measured the abundance of deuterium molecules in a certain part of the Orion Nebula. As a previous post suggests, they have determined that there is simply not enough matter in the universe to counteract the process of expansion that began with the Big Bang. What will happen instead? Most astronomers seem to think that the universe will end not in fire, but in ice. Billions, probably trillions, of years from now, all the stars will have burnt out, the nebulae will have exhausted their fuel, and the planets will be nothing more than cold rocks. Perhaps this is an anticlimactic ending, and it certainly lacks the attractive symmetry of the Big Crunch, but it seems to be the best anyone has to suggest.

More disproof of the Big Crunch came about two months ago with the first relaease of data from the Boomerang experiment. (i knew the answer since last summer hehe), The Boomerang experiment was a balloon based experiment that measured the cosmic microwave background(CMB). In finding the position of the first and second peaks in the power spectrum of the CMB the experiment measured the curvature of the universe. The result indicates that the universe if flat. That rules out the big crunch because you need a closed universe in order for it to recollapse. This is better than the Orion experiment which only probed baryonic matter. The CMB probes all contributions to stuff in the universe whatever it might be. The experiment was NASA funded so you can proabaly find info through the NASA page.

Scenarios for the end of the Universe have recently become much more varied, due to some old but neglected ideas being revived and some new ones cooked up. After going over the conventional wisdom on the Big Crunch, I will describe why it is still a possibility despite recent measurements indicating a "flat Universe". (To make one thing clear, these measurements do not disprove the existence of curved spacetime. What they mean is that the spatial curvature averages to zero over very large regions.)

The old story of the Crunch

It used to be assumed that only two types of stuff were relevant to the long-term evolution of the Universe, matter (including dark matter) and radiation. These are distinguished by the equation of state, in other words the pressure divided by the energy (or mass) density. For matter this is (to good approximation) zero, for radiation 1/3 (in units where c=1). By conservation of energy, this also tells us how the stuff behaves as the Universe expands: they both get diluted, but radiation dies off quicker (it's redshifted). Currently, the Universe is matter-dominated. Both matter and radiation tend to brake the expansion of the Universe.

The ultimate state of the Universe would be decided by balancing the matter against the expansion. If the matter density is less than a certain number called the critical density, the Universe expands for ever and is called an "open Universe". This also means that it has negative spatial curvature. If the density is more than the critical density, the Universe is "closed", has positive spatial curvature and at some point will stop expanding and begin to contract. Topologically, such a Universe is a four-sphere or hypersphere, with the crossover between expansion and contraction at the "equator".

The Big Crunch is then the hypothetical point (at the "South Pole") at which the scale factor by which all cosmic distances are multiplied goes to zero. As one approaches this point, density and temperature increase without limit. At some point the Universe will become radiation-dominated again, but this will not save it from its fate; in fact the extra pressure (counterintuitively) tends to accelerate the collapse. Soon, I am in trouble because the temperature exceeds the Planck energy, and even a theoretical physicist can't tell you what happens after that. "Maybe the Universe undergoes some sort of phase transition and comes out in a new Big Bang!" I hear you cry. Fine, if you want to speculate on such things, but you'll quickly pass beyond the pale of anything that is scientifically meaningful. On the way there you will be cruelly mugged by the Second Law of Thermodynamics, which points out that the entropy of the Big Crunch is enormously greater than that of the Big Bang, so apart from the brain-fryingly unlikely occurrence of a huge negative fluctuation in entropy there is no way you can connect up the one to the other. The entropy problem effectively ruled out the possibility of a cyclic Universe (until very recently, and it's still very dodgy).

The Big Bang was very smooth, but the Big Crunch would not be. The role of gravity on scales smaller than the size of the Universe is to amplify small variations in density and cause stuff to clump together, forming lumps and voids. If enough stuff clumps together at once, you get gravitational collapse into a black hole. In a contracting Universe, density increases and the intensity and frequency of sound waves (which are nothing but density fluctuations) and gravitational waves also increase: so we can expect more and bigger black holes, which may end up eating most of the matter. Anything that passes the event horizon of a black hole is en route to its own Big Crunch in a finite proper time. Black holes may also merge together to form bigger holes, all the while emitting radiation and increasing the oerall entropy. So, rather than being a nice smooth South Pole, the Crunch would be a violent, writhing mess of black holes and wildly fluctuating spacetime wibbles. The sound of the Big Crunch would indeed differ from that of the Big Bang.

New Crunches

Now on to the recent developments. (Bear in mind, though, that even in the old picture, there would be a Big Crunch even if the total density was even slightly over critical. This has not been ruled out.) With the measurement of the luminosity of distant supernovae, it has become clear that the expansion of the Universe has not been slowing down as expected. In fact, it appears to be accelerating! If this observation is correct, one needs a new form of stuff in the Universe with a negative pressure, which is actually dominating the energy density today. Negative pressure implies that the stuff doesn't get diluted when you increase the volume that it occupies! The simplest possibility is the cosmological constant, which is a possible extra term in the Einstein equation. This has pressure equal to minus the energy density and is physically equivalent to a constant vacuum energy (which may arise from quantum field theory). However this is theoretically unattractive because the value of the constant would have to be incredibly small compared to the energies commonly encountered in particle physics, and would have to be fine-tuned to come to dominate just at the present epoch. If there is a true cosmological constant, the Universe will continue to expand and ever faster and will end up big, empty and cold. No Crunch.

The most general possibility is a scalar field (like the Higgs boson) which evolves (changes its value) gradually over the course of cosmological time. If this happens slowly enough, it mimics the equation of state of vacuum energy, but with a slowly-varying value. One can imagine a ball rolling down a very gentle hill, the height above sea level being the effective value of the cosmological constant. This goes under the name quintessence and can also explain the observations. The idea was popularized by Paul Steinhardt, one of the pioneers of inflation - not surprisingly, since quintessence is essentially the same thing as inflation, except much, much slower (and we don't know how it ends). The advantage of quintessence is that it can explain why the accelerated expansion only started fairly recently. Now, the fate of the Universe depends on what happens to the quintessence field in the far future. If the potential energy is always positive then again the Universe accelerates away forever.

But, it is conceivable that the value of the potential will become negative at some point - analogous to the ball rolling down the beach into the sea. (This of course depends on the overall shape of the potential energy.) Then, the Universe is doomed to recollapse into a Big Crunch. Andrei Linde and collaborators have recently emphasized such a possibility and shown that it can come out of some simple (but somewhat unrealistic) supergravity models. See

. ---------- .0 <- Now : slowly rolling . . . . . .o <- Soon / \ / \ / \o <- Later w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^w^ \ \o <- Doomed to collapse!
Horizontal axis: value of Q (quintessence field). Vertical axis: potential energy. The "waves" mark zero.

Despite the apparent flatness of the Universe and the current accelerated expansion, it is not ruled out that some time in the future (it may be as soon as ten billion years from now!) there will be a Big Crunch. However I have to admit that the theories that allow for one look, at the moment, fairly contrived.
