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Message 4475 - Posted: 17 Jan 2008, 1:07:10 UTC

Does anyone know if the the \"Big Rip\" scenario is still a viable hypothesis or has it been ruled out?
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Profile Benjamin Wandelt
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Message 4478 - Posted: 17 Jan 2008, 6:18:18 UTC - in response to Message 4475.  
Last modified: 17 Jan 2008, 14:58:11 UTC

Does anyone know if the the \"Big Rip\" scenario is still a viable hypothesis or has it been ruled out?


Hi-

It\'s not been ruled out. In fact some probes of the acceleration of the Universe seem to prefer Big Rip cosmologies - but they absolutely do not exclude less extreme dark energy. In fact click on the plot on the PICO homepage. One of the parameters on there is w_DE (same as w, look on the diagonal) and you can see that models with w<-1 are definitely still allowed by the data we analyzed there.

For those of you who haven\'t heard, the Big Rip is a name for a catastrophic end for the Universe which occurs in models with a particular type of dark energy where the Universe expands so fast that it literally rips apart. It occurs for dark energy models with an \"equation of state parameter w\" less than -1. The \"cosmological constant scenario\" corresponds to a dark energy equation of state w=-1.

FIRST SOME BACKGROUND ON w:

What does this w mean? It basically describes how quickly a substance dilutes when you stretch it. Think about ordinary matter, say some gas. Ordinary matter, like gas has an equation of state parameter w of 0. If you pull gas apart it becomes less dense. In fact if you stretch it by some factor f, say, in all three directions then the gas becomes f^3 times less dense.

Any material with w larger than 0 becomes less dense faster than matter. For example, radiation has w=1/3. If you stretch radiation by a factor of f in all three directions, it will actually reduce in density by a factor of f^4.

Dark energy is special, because it reduces in density less than ordinary matter if you stretch it. In fact if w=-1, the material DOES NOT REDUCE IN DENSITY AT ALL AS YOU STRETCH IT!

So if w<-1, it turns out that the material becomes DENSER when you stretch it. So dark energy with w<-1 in an expanding Universe becomes denser the more the Universe expands and therefore takes over the Universe.

It causes not just accelerated expansion (any w<-1/3), or exponential expansion (w=-1), but superexponential expansion. So you pick any volume in the Universe - it will become infinitely large in finite time. This is what\'s called the Big Rip.

There are many potential problems with the Big Rip from a theoretical point of view, but some of the data seem to prefer a very negative equation of state compared to -1 or larger. One of the problems is that spacetime itself is unstable in some of the models that have w<-1 and lead to a Big Rip.

But do not despair: even if w is measured to be less than -1, it does not have to be because the dark energy is so nefarious as to cause the demise of the Universe as we know it. It could be an apparent effect. For example, in my paper with my postdoc Greg Huey, called \"Interacting quintessence, the coincidence problem, and cosmic acceleration, we showed that by making dark energy and dark matter interact with each other, one can create a model of the Universe which would make w from observations appear to be less than -1, while all the components of the Universe are perfectly well-behaved.

Stuff that has w<-1 and produces a Big Rip is called phantom dark energy. And yes, it\'s a Star Wars reference - my good friend Rob Caldwell who first had the idea somehow managed to get the title \"A Phantom Menace?\" past the referees and editors at Physics Letters B...

Since w is one of the parameters we are exploring at Cosmology@Home, some of your very own computers have simulated Universes with phantom dark energy and Big Rip cosmologies...

Ben


Edits: corrected the title of my paper with Greg, and fixed a typo.
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Message 4481 - Posted: 17 Jan 2008, 10:38:23 UTC

Interesting read.

Perhaps we should get a StarWars logo on the project to attract even more crunchers? :-p
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Message 4493 - Posted: 18 Jan 2008, 0:32:09 UTC - in response to Message 4478.  

Does anyone know if the the \"Big Rip\" scenario is still a viable hypothesis or has it been ruled out?


Hi-

It\'s not been ruled out. In fact some probes of the acceleration of the Universe seem to prefer Big Rip cosmologies - but they absolutely do not exclude less extreme dark energy. In fact click on the plot on the PICO homepage. One of the parameters on there is w_DE (same as w, look on the diagonal) and you can see that models with w<-1 are definitely still allowed by the data we analyzed there.

For those of you who haven\'t heard, the Big Rip is a name for a catastrophic end for the Universe which occurs in models with a particular type of dark energy where the Universe expands so fast that it literally rips apart. It occurs for dark energy models with an \"equation of state parameter w\" less than -1. The \"cosmological constant scenario\" corresponds to a dark energy equation of state w=-1.

FIRST SOME BACKGROUND ON w:

What does this w mean? It basically describes how quickly a substance dilutes when you stretch it. Think about ordinary matter, say some gas. Ordinary matter, like gas has an equation of state parameter w of 0. If you pull gas apart it becomes less dense. In fact if you stretch it by some factor f, say, in all three directions then the gas becomes f^3 times less dense.

Any material with w larger than 0 becomes less dense faster than matter. For example, radiation has w=1/3. If you stretch radiation by a factor of f in all three directions, it will actually reduce in density by a factor of f^4.

Dark energy is special, because it reduces in density less than ordinary matter if you stretch it. In fact if w=-1, the material DOES NOT REDUCE IN DENSITY AT ALL AS YOU STRETCH IT!

So if w<-1, it turns out that the material becomes DENSER when you stretch it. So dark energy with w<-1 in an expanding Universe becomes denser the more the Universe expands and therefore takes over the Universe.

It causes not just accelerated expansion (any w<-1/3), or exponential expansion (w=-1), but superexponential expansion. So you pick any volume in the Universe - it will become infinitely large in finite time. This is what\'s called the Big Rip.

There are many potential problems with the Big Rip from a theoretical point of view, but some of the data seem to prefer a very negative equation of state compared to -1 or larger. One of the problems is that spacetime itself is unstable in some of the models that have w<-1 and lead to a Big Rip.

But do not despair: even if w is measured to be less than -1, it does not have to be because the dark energy is so nefarious as to cause the demise of the Universe as we know it. It could be an apparent effect. For example, in my paper with my postdoc Greg Huey, called \"Interacting quintessence, the coincidence problem, and cosmic acceleration, we showed that by making dark energy and dark matter interact with each other, one can create a model of the Universe which would make w from observations appear to be less than -1, while all the components of the Universe are perfectly well-behaved.

Stuff that has w<-1 and produces a Big Rip is called phantom dark energy. And yes, it\'s a Star Wars reference - my good friend Rob Caldwell who first had the idea somehow managed to get the title \"A Phantom Menace?\" past the referees and editors at Physics Letters B...

Since w is one of the parameters we are exploring at Cosmology@Home, some of your very own computers have simulated Universes with phantom dark energy and Big Rip cosmologies...

Ben


Edits: corrected the title of my paper with Greg, and fixed a typo.


Thanks for update. Very interesting post. It seems that a Big Rip would eventuate absolute zero temperature....
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