During the early years of the 20th century most astronomers believed that the universe was static. Even Albert Einstein incorporated the notion of the static and eternal universe in his equations. After Edwin Hubble conclusively proved that the Universe was in fact expanding, Einstein admitted that describing the universe as static was “the greatest blunder of my life”. Later observation revealed that the universe wasn’t just expanding – it was expanding at an accelerated pace. Astronomers were puzzled and had no idea how to explain the accelerated expansion of the universe because their calculations revealed that there should be enough matter in the universe to eventually slow the expansion. A hypothetical form of energy – dark energy – was used to explain the accelerated expansion. Although dark energy has never been directly observed it is thought to permeate the entire universe.
Recent observations have continued to make the picture more confusing. An international team of scientists revealed that there was a stage in the universe’s history when the expansion of the universe was decelerating. These observations were made using the Baryon Oscillation Spectroscopic Survey (BOSS), which is part of the Sloan Digital Sky Survey. The results indicate that when the universe was in between 2.5-7 billion years old, it was expanding at a decelerating pace. This is perhaps what would have been expected before the understanding that the universe is currently expanding at an accelerated pace. It now appears that for the first 2.5 billion years of the universe’s existence the force of the Big Bang caused rapid expansion, which was slowed in between 2.5-7 billion years due to the force of gravity. Astronomers call the first 7 billion years the “matter-dominated” universe.
However the divide between the transition from decelerated-expansion to the current state of accelerated-expansion was sharp. And if dark-energy is causing this expansion, how did it come to affect the universe when it did? As many astronomers are now realizing this is one of the biggest questions in all of cosmology and the answer to it could have deep implications for the fate of the entire universe. Most importantly, will dark energy continue to push the universe apart forever? Or will gravity eventually re-gain control over the expansion and start a phase of contraction?
The answers to these questions depend on three important variables:
- The overall shape of the universe
- How much dark energy it contains
- How dark energy density responds to continued expansion
It may surprise you to realize that scientists do not currently know for sure what the shape of the universe is. There are thought to be three possibilities: closed, open and flat. If the universe is closed (like a sphere) then it should eventually enter a phase of contraction, unless there are significantly more amounts of dark energy than presently hypothesized. If the universe is open (like a saddle) or flat, than the universe will continue to expand forever.
The results of continued expansion or eventual contraction have far reaching consequences for the ultimate fate of the universe. If the universe eventually enters a phase of contraction (i.e., gravity beats dark energy) than the universe will experience what is known as a Big Crunch. During the Big Crunch the universe would return to the state at which it began, as a dimensionless singularity. In this universe all of the galactic clusters and galaxies would start to get closer and closer together and the universe would start to get very warm, perhaps mirroring the state the universe was in during its infancy. However, if accelerated-expansion continues (i.e., dark energy beats gravity) than the universe will likely experience a Big Freeze or Big Rip. In a Big Freeze scenario everything would be so far apart that no more stars or galaxies would form and the universe would reach a temperature of absolute zero. Or it is even possible that the universe could experience a Big Rip. In this scenario all material objects in the universe would disintegrate because the universe would be accelerating so quickly that all objects would be ripped apart at a molecular level. Current expansion is only accelerating a pace that affects objects on a galactic cluster scale of reality.
Unfortunately, at this stage in our knowledge about the development of the universe it is still premature to conclude with any certainty that one scenario is more plausible than another. And the recent observations made by BOSS raise more questions and make our understanding of the universe more complicated than it was previously. Could dark energy’s effects on our universe decrease in the future? It appears as though dark energy’s effects have changed in the past (i.e. 6.5 billion years ago). Could we be in a dark energy phase? Could gravity start to re-assert control over accelerated-expansion? Or is the universe now too big for gravity to pull it back together again?
The only way to know for sure will be to continue making observations and developing new technologies that will be able to reveal aspects of the universe that are currently unknown to us. If the universe’s ‘adolescent’ pictures tell us anything, it is that we still have a lot to work out. Either way, whether the universe expands forever, or eventually starts to contract, it will be tens of billions, if not trillions of years, before complex life (like us!) should begin to worry.