Skwinty
14th April 2009, 12:01 PM
This is an excerpt from the LCDM woo thread.
I would like to discuss these models outside of that thread as I don’t believe that either of the models qualify in any manner as woo.
Originally Posted by Sol Invictus
Their idea is to introduce a pre-big bang phase, i.e. a phase during which the universe is contracting. When it gets very dense it "bounces", meaning the contraction reverses and the universe begins to expand. It turns out that one can construct models in which, during the contracting phase, the conditions are such as to generate some scale-invariant perturbations. Then with certain assumptions about the bounce, those perturbations form appropriate initial conditions (similar to those produced by inflation) to feed into standard Lambda-CDM and match observations today.
There are a number of very serious problems with this idea. The worst is that not only do we not know of a force that would reverse the contraction, the required properties of such a force make it almost certainly impossible. It's a little hard to explain this, but essentially any force which grows strong enough to resist gravity when the universe is so dense must have extremely bizarre properties, and those properties appear to rule it out (i.e. it would have huge effects even today, which we do not observe). There is some controversy about that, but my impression is that most people in the field do not think such a bounce is possible (or more accurately it might be possible, but it will require some totally unknown and new physics that somehow avoids the problems, and since it's so uncertain how that would work one cannot trust any model that relies on it).
I'll also add that this is an old idea, one that's been proposed many times in many forms since 1915. It never succeeds, because it always runs into this same problem.
I have read “The Endless Universe” as well as the papers below.
http://arxiv.org/abs/astro-ph/0404480v1 (http://arxiv.org/abs/astro-ph/0404480v1)
http://www.physics.princeton.edu/~steinh/cyclicFAQS/index.html (http://www.physics.princeton.edu/~steinh/cyclicFAQS/index.html)
http://arxiv.org/PS_cache/hep-th/pdf/0408/0408083v1.pdf (http://arxiv.org/PS_cache/hep-th/pdf/0408/0408083v1.pdf)
http://arxiv.org/abs/astro-ph/0204479v1 (http://arxiv.org/abs/astro-ph/0204479v1)
The points I would like to raise are as follows.
1.I agree that earlier models of a cyclic universe had loads of problems as indicated by Tolman and others.
2.I agree that string and M-theory are still in their infancy and are not complete as yet, as far as a theory of everything.
3.However, the cyclic model as posited by Steinhardt and Turok seems to overcome or explain away the problems experienced by earlier models.
4.The cyclic model agrees with the predictions of the inflation model: these being near scale invariance of temperature (COBE), the flatness of space through numerous ground based experiments and WMAP, matter and dark matter distribution in CBMR (Adiabaticity) and the random noise characteristics of this distribution (Gaussianity). The jury is still out on the gravitational wave predictions, hopefully the Planck mission will produce the goods one way or the other.
5.The cyclic model does away with inflationary energy and replaces it with dark energy, it is parsimonious with no ad hoc additions and seems to be the only serious contender to inflation.
Any comments appreciated.
I would like to discuss these models outside of that thread as I don’t believe that either of the models qualify in any manner as woo.
Originally Posted by Sol Invictus
Their idea is to introduce a pre-big bang phase, i.e. a phase during which the universe is contracting. When it gets very dense it "bounces", meaning the contraction reverses and the universe begins to expand. It turns out that one can construct models in which, during the contracting phase, the conditions are such as to generate some scale-invariant perturbations. Then with certain assumptions about the bounce, those perturbations form appropriate initial conditions (similar to those produced by inflation) to feed into standard Lambda-CDM and match observations today.
There are a number of very serious problems with this idea. The worst is that not only do we not know of a force that would reverse the contraction, the required properties of such a force make it almost certainly impossible. It's a little hard to explain this, but essentially any force which grows strong enough to resist gravity when the universe is so dense must have extremely bizarre properties, and those properties appear to rule it out (i.e. it would have huge effects even today, which we do not observe). There is some controversy about that, but my impression is that most people in the field do not think such a bounce is possible (or more accurately it might be possible, but it will require some totally unknown and new physics that somehow avoids the problems, and since it's so uncertain how that would work one cannot trust any model that relies on it).
I'll also add that this is an old idea, one that's been proposed many times in many forms since 1915. It never succeeds, because it always runs into this same problem.
I have read “The Endless Universe” as well as the papers below.
http://arxiv.org/abs/astro-ph/0404480v1 (http://arxiv.org/abs/astro-ph/0404480v1)
http://www.physics.princeton.edu/~steinh/cyclicFAQS/index.html (http://www.physics.princeton.edu/~steinh/cyclicFAQS/index.html)
http://arxiv.org/PS_cache/hep-th/pdf/0408/0408083v1.pdf (http://arxiv.org/PS_cache/hep-th/pdf/0408/0408083v1.pdf)
http://arxiv.org/abs/astro-ph/0204479v1 (http://arxiv.org/abs/astro-ph/0204479v1)
The points I would like to raise are as follows.
1.I agree that earlier models of a cyclic universe had loads of problems as indicated by Tolman and others.
2.I agree that string and M-theory are still in their infancy and are not complete as yet, as far as a theory of everything.
3.However, the cyclic model as posited by Steinhardt and Turok seems to overcome or explain away the problems experienced by earlier models.
4.The cyclic model agrees with the predictions of the inflation model: these being near scale invariance of temperature (COBE), the flatness of space through numerous ground based experiments and WMAP, matter and dark matter distribution in CBMR (Adiabaticity) and the random noise characteristics of this distribution (Gaussianity). The jury is still out on the gravitational wave predictions, hopefully the Planck mission will produce the goods one way or the other.
5.The cyclic model does away with inflationary energy and replaces it with dark energy, it is parsimonious with no ad hoc additions and seems to be the only serious contender to inflation.
Any comments appreciated.