Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard

Science - The End of the Universe

This theory suggests that the average density of the Universe is sufficient, ultimately, to stop the expansion of the Universe gravitationally, and cause it to start to collapse back on itself.  Just as the Big Bang started with a singularity, so the Big Crunch ends with one, although the exact nature of either singularity is not explained, and must certainly consider Quantum effects.  The fact that the Universe is expanding at an increasing rate does not bode well for this theory.  There are, however, ways around this, a good example being the Steinhardt-Turok cyclic universe discussed in Alternatives to the Big Bang. 

The Big Crunch

The idea of the Universe ending in a Big Rip was first postulated in 2003.  If the ratio of the dark energy pressure to its energy density, its equation of state, has a value less than -1, the Universe ends in a Big Rip.  This scenario results in the Universe expanding at an increasing rate, as has been observed.  As the Universe expands, the point at which the rate of the expansion exceeds the speed of light gets closer to us.  Thus, the size of the observable Universe shrinks.  Over time, this means that, for example, the other galaxies in the local group would be outside of the observable Universe as their recession speed exceeds the speed of light.  Extrapolating this, the solar system itself would become unobservable.  Eventually, individual atoms would be torn apart and even individual particles.  This is the Big Rip, and essentially results in an enormous Universe in which there is no matter an no energy.  If Dark Energy is actually a cosmological constant, rather than something like quintessence, then this is the most likely scenario based on current observations as it would not be possible for the rate of expansion to slow. 

The Big Rip

The ultimate fate of the Universe depends crucially on its density and shape. It could be either positively or negatively curved, or flat.  Positive curvature implies a density greater than the critical density, so the Universe would eventually collapse on itself in something like the big crunch.  Negative or flat curvature implies a density less than or equal to the critical density, so the Universe would continue expanding forever.  The WMAP project data implies that the Universe is flat to within about 1% to 2%, but that the rate of the expansion is accelerating, so points towards the existence of some form of energy that has negative pressure, and that this may override the affect of gravity, and that renders these musings moot! 

The theories discussed below have appeared at varying times through the 20th, and into the 21st, centuries.  Thus, some earlier theories, based on the information then available, are now seen to be unlikely.  I have included some of them for completeness.  Current evidence of the rate of expansion of the Universe, and calculations of the mass density, imply that the most likely scenario is a "Big Freeze", often called the "heat Death of the Universe".  However, new theories of cyclic or oscillating universes are avoiding many of the earlier problems, like entropy, and may well prevail.  The "how, or if, the Universe started" question is intimately linked to how, or if, it will end.  Do read the sections on the "early Universe" and alternative theories to the Big Bang before looking at this section. 

Introduction

The Big Freeze

This theory requires the Universe to be either flat or negatively curved.   It maintains that the Universe will expand forever, getting colder and colder.  The theory does, however, require that the rate of expansion slows down, contrary to current observations.  It is possible that, depending on the nature of Dark Energy, the Universe's expansion rate could slow in the distant future; for example, if the Dark Energy was Quintessence, it does vary in its affect over time.