23 Apr 2012

Credit:  NASA/STScl/Ann Feild
Credit: NASA/STScl/Ann Feild

Scientists differ on how the universe will end, some aren't even certain it will ever end.  What is generally accepted is that the universe is expanding and that the likely scenario is that of the Big Freeze.  While the Big Freeze may be the leading theory supported by many, other more radical theories take a different approach and show a different outcome.  Although for many scientists its comes down to either Dark Flow or Dark Energy.   

The Standard View and Dark Energy

So the question is what is driving the universe's expansion?  The answer to most is dark energy.  Dark energy is like anti-gravity working over large distances that is ultimately creating this expansion in a tug of war with gravity.  Observations since the 1990s show this expansion.  Dark energy currently accounts for 74% of the total mass-energy of the universe.  Two forms of dark energy are that of the cosmological constant (a constant energy density filling space evenly) and scalar fields (dynamic quantities with energy density that can vary in time and space). 

A 2011 survey of 200,000 galaxies seems to confirm the existence of dark energy, but the physics behind it are unknown.  Dark energy was originally discovered when scientists tried to find out how much the expansion was slowing down due to gravity pulling it back inward.  Instead they found that the expansion was accelerating.  This earned three astrophysicists the 2011 Nobel Prize.

The Big Freeze

Another theory on the fate of the universe is that continued expansion continues to move forward resulting in a universe that approaches absolute zero in temperature in the end.  Observations of the young, distant universe suggest that the strength of dark energy has remained steady throughout time, so scientists view this to be our most likely future for our universe.

The conventional thinking is that the acceleration will proceed at a constant rate, similar to a vehicle moving 20 mph faster with each mile traveled.  If there is nothing to stop the acceleration, all galaxies will eventually move away from each other at the speed of light, resulting in each galaxy being alone in a cold, dark universe within 100 billion years.  For us in the Milky Way galaxy, we would not be able to see any galaxies outside of our own, even with the most powerful telescopes.  Data from the Wilkinson Microwave Anisotropy Probe (WMAP) also help support the Big Freeze scenario by showing that the geometry of the universe is flat and will expand forever.

Inflation says the universe initially expanded faster than the speed of light and grew from sub-atomic size to golf-ball size instantaneously.  (Credit: NASA)
Inflation says the universe initially expanded faster than the speed of light and grew from sub-atomic size to golf-ball size instantaneously. (Credit: NASA)

The Big Crunch

Before scientists discovered that the universe was expanding and accelerating, many cosmologists believed the universe would actually reverse direction into a big crunch, where normal gravity wins and everything collapses back onto itself.  But more recent observational data has shown that this scenario of a crunch will not happen, instead there will be infinite expansion.

Credit:  HowStuffWorks
Credit: HowStuffWorks

Big Bounce

This theory revolves around the Big Bang but views the first Big Bang as the result of the collapse of a previous universe.  The Big Bang theory states that beginning of the universe had infinite density, contradicting everything else in physics.  If the universe were closed, then the Big Bounce predicts once this universe collapses it spawns another universe similar to the Big Bang after a universal singularity is reached or a repulsive quantum force causes another expansion.


In one multiverse hypothesis, our uni-"verse" is one Big Bang among infinite numbers of simultaneously expanding Big Bangs that are spread out over endless distances.

Each "verse" can be matter or antimatter.   The "verses" expand and then collide, matter and antimatter annihilate which releases energy.  The infinite size of the multiverse and infinite number of "verses" imply that new "verses" form as old ones are destroyed.

The Big Rip

One controversial theory that has been circulating since 2003 is that the universe will end in something called "The Big Rip".  This harrowing theory has the death of the universe where "phantom energy" rips apart stars, galaxies, and planets, and eventually everything that exists.  By contrast, the theory that states how the universe began, from nothing into something via a rapid inflation is known as the Big Bang postulated in 1927 by Georges Lemaitre.

The lead author of The Big Rip is Robert Caldwell of Dartmouth University.  Even the author of the theory describes this as a fantastic possibility.  The Big Rip is based on astronomical observations that were made in the late 1990s, that the universe is expanding at an increasing rate and there is something unknown moving everything outward.

What if this rate of expansion increased?  This is the question posed by Caldwell and his colleagues.

The answer to this is that the eventual and increasing pace would overtake the normal trusted effects of gravity on a localized level.  This means that even nuclear forces that bind things together would become ineffective in this situation.  All matter eventually rips apart.

To become a destructive phantom force, Caldwell and his colleagues had to toy with the cosmological constant, a mathematical fix that Einstein applied to general relativity, changing to it grow over time.

The Big Rip theory has dark energy increasing with time, making it an out of control phantom energy.  Think of the accelerating vehicle adding an extra 20 mph every half mile, then every hundred yards, then every foot.  Before long the vehicle starts to come apart and eventually the engine too.

Caldwell's team provided a precise countdown to the demise of our universe.  The end is projected to be 20 billion years from now.  We probably don't have to worry about this too much.  20 billion years is a long time for a species to exist, let alone our own solar system.  Our Sun's lifetime is expected to end within 5 billion years or so given the average life cycle of about 10 billion years for this type of star.  Scientists have, however, given us some things to look for, if the Big Rip is to occur.

One billion years from the end:  all galaxies will have receded so far from our own, essentially erased from the sky and not visible.

60 million years before the end:  The Milky Way begins to fly apart.

3 months before the end:  Planets in our solar system will start to fly away from the Sun.

Minutes before the end:  The Earth explodes.

30 minutes left:  A brief interval before atoms and their nuclei break apart.

After this?  Caldwell is uncertain of what happens after this.  It's not known if the universe recycles itself or another one is created.  It would probably look like time has ended.

Recent studies with the Chandra X-ray observatory have tracked down how dark energy has stifled the growth of galaxy clusters.  Scientists have combined this data with previous studies and concluded that the universe won't rip apart.  The study shows that whatever is forcing the expansion of the universe to speed up, it is also forcing its development to slow down.

Dark Flow-The Illusion of the Accelerating Expansion

There is another theory that suggests that the accelerating expansion of the universe is an illusion called "Dark Flow".  We perceive it as accelerating because of the way our region in the cosmos drifts through the rest of space.  This according to Christos Tsagas, a cosmologist at Aristotle University of Thessaloniki in Greece.  According to his theory, our relative motion makes it seem as if the universe as a whole is expanding faster and faster, when in reality its slowing down, which is what we would expect of gravity. 

This theory would get rid of the issue of dark energy and avoid the other theory of the Big Rip.  Instead of the universe being ripped apart, this theory would have it come to a stop and then begin shrinking.  According to Tsagas, the acceleration of the universe in our area is caused by its motion alone.  The universe beyond our domain isn't accelerating outward, but rolling to a stop.

Tsagas' explanation of things builds on a recent discovery by Alexander Kashlinsky, a cosmologist at NASA.  Over several years, Kashlinsky and his colleagues have shown that our region of space-time, an area of around 2.5 billion light years across, is moving relative to the rest of the universe, fast.

Tsagas's theory is also partly supported by recent observations that have confused cosmologists.  Data collected from space, including the cosmic background radiation and light from supernovae seem to show the universe has a preferred axis, or that it stretches more in one direction than the other as it expands.

Light from stellar explosions have aided scientists to identify this preferred axis.  By measuring how much the light from the explosion is red-shifted (stretched), they can detect the rate of expansion in different regions of space.  The findings show the universe stretches more toward the constellation Vulpecula in the Northern sky than any other direction.

The CMB radiation also showed a preferred axis and pointed in this same direction.  Tsagas argues that this is no coincidence that the axis is another illusion of the "dark flow" of our region.

This can be demonstrated by imagining you are a swimmer in a river.  If you are with the current you move faster than trying to swim upstream.  Likewise, our galactic bubble is also "swimming."  Tsagas says this is why we perceive the expansion of space-time as faster in one direction, that of our motion, than any other.

Skeptics will argue that dark flow doesn't account for the spatial distribution of the CMB radiation or the observed pattern of galaxy clustering.  Tsagas suggests "There should be no extra effects on the CMB, since the very large-scale kinematics [motions] remain essentially unaffected [by my theory].  There might be some small effects on galaxy clustering, but one needs to look into it to make sure."

The Tech-Stew Take Home

While there are many unknowns in trying to predict the future of our universe, we do know several things for certain.  The universe is expanding, its about 13.7 billion years old and we can quantify 22% as dark matter and 74% as dark energy.  We also can see the Cosmic Background Radiation (CMB) from about 380,000 years after the Big Bang. 

At least this is what we seem to perceive.  While we have theories like the Big Freeze, other theories like the Big Crunch and Big Rip offer an intriguing look at how things may be unfolding.  If concepts like Tsagas' "dark flow" are true, it may force us to change how we perceive our place in the universe and ultimately lead to new theories.  We can't have both dark energy and dark flow, its either one or the other and each one paints a different picture of things and leads to different theories.  Hopefully new data from instruments can help us get a better grasp on things leading us to a grand picture on the fate of the universe.

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