Steinhardt Turok imitate Smolin (to get small Lambda)

[marcus]

http://arxiv.org/abs/astro-ph/0605173

Steinhardt Turok, inventors of colliding brane cyclic ("ekpyrotic") universe, have followed Smolin's lead in proposing an EVOLUTIONARY MECHANISM that relaxes the cosmological constant down to near zero.

It is not a "natural selection" mechanism with differential reproduction rates but it is a mechanism by which Lamda repeatedly evolves towards lower values in later editions of the universe.

Abstract: "... In this paper, we show that a cyclic model of the universe can naturally incorporate a dynamical mechanism that automatically relaxes the value of the cosmological constant,...

..., nearly every volume of space spends an overwhelming majority of the time at the stage when the cosmological constant is small and positive, as observed today."

Page 12, conclusions: "... our result is a universe in which the cosmological constant Lambda(t) is an ultra-slowly varying function of time t and in which virtually every patch of space proceeds through stages of evolution that include ones in which Lambda(t) is small enough to be habitable for life. It is interesting to contrast this situation with the anthropic picture, especially versions based on inflationary cosmology, for which the fraction of habitable space is infinitesimally small. All other things being equal, a theory that predicts that life can exist almost everywhere is overwhelmingly preferred by Bayesian analysis (or common sense) over a theory that predicts it can exists almost nowhere...."

This article of Steinhardt Turok was published in journal Science 5 May 2006.
Some buzz was made of it on space.com here:
http://www.space.com/scienceastronomy/060508_mm_cyclic_universe.html

 

[marcus]

It's great to see Steinhardt and Turok invoking "common sense".

 

[marcus]

In the same issue of Science magazine with the Steinhardt Turok paper (which one could say was "anti-anthropic")
was this opinion piece of Villenkin (which one could say also in a rough sense is "pro-anthropic"---maybe the editors wanted balance)

http://arxiv.org/abs/astro-ph/0605242
The vacuum energy crisis
Alexander Vilenkin
4 pages; invited "perspective" article in "Science" (4 May 2006)

"The smallness of the vacuum energy density and its near coincidence with the average matter density of the universe are naturally explained by anthropic selection. An alternative explanation, based on the cyclic model of Steinhardt and Turok, does not address the coincidence problem and is therefore less convincing. This article appeared in Science (4 May 2006) as a 'perspective' for Steinhardt and Turok's paper in the same issue (astro-ph/0605173)."

the two pieces in Science got this notice by New Scientist
http://www.newscientistspace.com/ar...iverse-can-explain-cosmological-constant.html

 

[Dcase]

The Steinhardt Turok paper [colliding cyclic branes of an ekpyrotic universe] would make more sense to me if the branes were able to tell time by oscillating as helicoids.

Helicoids would appear to have some relationship to the helicity of Freedman [1994 Dirac Lecture], Penrose [twistor theory] and Witten and to ‘The Kinematic Origin of Complex Wave Functions” by David Hestenes. Helical trajectories appear to be prominent in both QM and GR. Such helicoids could either collide due to different helical angles or perhaps have an accordion motion.

‘Plane’ branes [helical angle zero] would seem to require some type of pendulum mechanics.

 

[hellfire]

I was reading Vilenkin's great book https://www.amazon.com/dp/0809095238/?tag=pfamazon01-20.

In the paper he has formulated a theorem that basically shows that every model that in average expands is geodesically incomplete in past direction, regardless of any energy conditions. The paper focuses in inflationary models, arguing for necessary initial conditions, but, also, in the conclusions, the proven theorem that in average expanding models cannot be eternal is applied to Steinhardts cyclic model.

The paper is:

http://arxiv.org/gr-qc/0110012
Arvind Borde, Alan H. Guth, Alexander Vilenkin

Abstract:

Many inflating spacetimes are likely to violate the weak energy condition, a key assumption of singularity theorems. Here we offer a simple kinematical argument, requiring no energy condition, that a cosmological model which is inflating -- or just expanding sufficiently fast -- must be incomplete in null and timelike past directions. Specifically, we obtain a bound on the integral of the Hubble parameter over a past-directed timelike or null geodesic. Thus inflationary models require physics other than inflation to describe the past boundary of the inflating region of spacetime.

At the end of the paper, they claim:

In some versions of the cyclic model the brane space-times are everywhere expanding, so our theorem immediately implies the existence of a past boundary at which boundary conditions must be imposed. In other versions, there are brief periods of contraction, but the net result of each cycle is an expansion. For null geodesics each cycle is identical to the others, except for the overall normalization of the affine parameter. Thus, as long as Hav > 0 for a null geodesic when averaged over one cycle, then Hav > 0 for any number of cycles, and our theorem would imply that the geodesic is incomplete.

Does anyone know about any response of Steinhard to this?