A Universe from Nothing

Study matter and its motion through spacetime...

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Re:

#21  Postby twistor59 » Mar 03, 2010 7:56 am

Geraint wrote:Well, ns is almost always used for the scalar spectral index of density perturbations, so it would be wilfully confusing to use the same notation for the spectral index of gravity waves. I think in the original snippets they must be talking about curvature perturbations (well, I'm pretty sure, since their mention of the prediction for ns from simple models of inflation seems to tally with what I remember from elsewhere).



Thanks, yes I had a quick Google around - as you say, ns seems to be used, as far as I can tell, in connection with perturbations of the scalar curvature about the uniform curvature that you'd have in the standard FRW models. I've seen nT (presumably for "tensor") used for the gravitational wave equivalent.
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Re: A Universe from Nothing

#22  Postby crank » Mar 03, 2010 8:34 am

Calilasseia wrote: possessing the local geometry of (2d compactified Milne)/Z2 × R3, times a finite-volume Calabi-Yau manifold in the vicinity of the collision.


There are many here, like especially myself, that such a statement is more convoluted than any Calabi-Yau illustration,
Image
please don't make my head hurt so much. I am reading Turok and Steinhardt book for dummies, but just started, maybe that will help.


Calilasseia wrote:
fostering an appropriate braneworld collision.[/i]

Do their papers suppose braneworlds to have always existed, or where do they come from to begin with before the instantiation of a universe? Maybe I am looking at this incorrectly?
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Re: A Universe from Nothing

#23  Postby tnjrp » Mar 03, 2010 8:44 am

I've understood Steinhardt & Turok (and Baum & Frampton) specifically posit, as the title of their popular science book implies, that the universe, or perhaps more specifically the underlying topology, is eternal.
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Re: A Universe from Nothing

#24  Postby crank » Mar 03, 2010 8:54 am

tnjrp wrote:I've understood Steinhardt & Turok (and Baum & Frampton) specifically posit, as the title of their popular science book implies, that the universe, or perhaps more specifically the underlying topology, is eternal.

"Endless"--I think if I thought about it I would have assumed they meant no end in the future, but strictly, it should mean either end. Titles are often not of the authors choosing and/or are picked more for cleverness or sales, not always of the greatest import. I would be a lot further into it if I didn't spend so much time here goddamn it :crazy:
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Re: A Universe from Nothing

#25  Postby tnjrp » Mar 03, 2010 9:01 am

I haven't read the book at all but rather the arXiv papers and articles on the subject so I can't really comment on how well the book presents the idea of "no begining and no end".
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Re: A Universe from Nothing

#26  Postby newolder » Mar 03, 2010 11:43 am

twistor59 wrote:... I've seen nT (presumably for "tensor") used for the gravitational wave equivalent.

The relevant number quoted at the WMAP parameter summary page and in the arxiv work is called r, the ratio of tensor(gravitational) to scalar perturbations:
http://arxiv.org/PS_cache/astro-ph/pdf/0507/0507455v3.pdf wrote:Inflation predicts nearly scale-invariant spectra of primordial scalar (energy density) and tensor (gravitational wave) perturbations. What has been less clear is the precise prediction for the scalar spectral index ns and the tensor/scalar ratio r. In particular, is ns likely to be distinguishable from pure Harrison-Zel’dovich (ns = 1)? And is r likely to be large enough for the tensor perturbations to be detected (r >~ 10−2)?

crank wrote:Do their papers suppose braneworlds to have always existed, or where do they come from to begin with before the instantiation of a universe? Maybe I am looking at this incorrectly?

If a brane collision should ever result in no separation thereafter (the branes collide inelastically and remain stuck together) then this would be the end of the Universe (an 'end of the world' scenario), otherwise the cycles of the theory's postulated branes are eternal. It remains possible, in this theory, that the cosmological 'constant' was different (higher) in the remote past than it is now but the dynamics of the cycle forces the cc to small values at late times - it's a 'stable attractor' solution, without which the model would seem fine-tuned and unstable.
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Re: A Universe from Nothing

#27  Postby Mr Ashbo » Mar 15, 2010 12:52 pm

I thought that a Big Bounce rather than a Big Bang based on the Loop Quantum Gravity (LQG) theory prosecuted by Martin Bojowald (amongst others) was an alternative to creating anything from nothing - but I guess it doesn't really since whether we are in the first or billionth universe in a series, the first one still had to come to exist somehow - wouldn't it?

From Wikipedia - http://en.wikipedia.org/wiki/Big_Bounce

"Peter Lynds has recently put forward a new cosmology model in which time is cyclic. In his theory our Universe will eventually stop expanding and then contract. Before becoming a singularity, as one would expect from Hawking's black hole theory, the Universe would bounce just before it became a singularity. Lynds feels that a singularity would violate the second law of thermodynamics and this stops the Universe from being bounded by singularities. The Big Crunch would be avoided with a new Big Bang. Lynds feels the exact history of the Universe would be repeated in each cycle. Some critics feel that while the Universe may be cyclic, the histories would all be variants"

On a purely personal, non-scientific basis, I would prefer this to be true - somehow the avoidance of a singularity and the presence of bounce once the universe contracts to a certain size seems more acceptable /plausible to me.
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