CosmoCoffee

A very interesting idea, to be sure. Does anyone have any comments on the paper by David Wiltshire, gr-qc/0503099 ? He seems to take a very different approach (and gets different answers).

Alessandro Melchiorri wrote:Hi Niayesh,

Good point! certainly curvature should be investigated, but how you can use the CMB constraint on curvature ? this has been obtained under
the assumption of L-CDM. Anyway I think it would be difficult to reproduce
the CMB peaks positions in the SHCDM model.

Ale

As long as [tex]r^2 \nabla^2\varphi [/tex] is small, which is the assumption in Kolb et al. papers as well (what you call SHCDM), the correction is exactly similar to a small curvature in an OCDM universe. Now, the CMB constraints (i.e. location of the Doppler peaks) on [tex]\Omega_{K} [/tex] are slightly different in OCDM and LCDM universes, but, I believe the difference is small, as it is often said that CMB constrains the geometry of the universe ([tex]\Omega_{tot}= 1- \Omega_K[/tex]), in a rather model-indepent way.

There is an interesting point in astro-ph/0503553 according to which second order PT on a FRW background tells you that the local curvature and deceleration parameter are positive and given by second order derivatives of the gravitational potential from density inhomogeneities
on the largest scales. Thus, assuming there is no dark-energy, the universe must be decelerating rather than accelerating.

Unless there is a loophole in this argument, this seem to indicate that the viability of the Kolb et al model for superhorizon modes driving the universe to an accelerated expansion (without invoking dark energy) is
seriously threatened.
So, I was wondering what other people think about this point.

You have probably seen it, but anyway: This paper [astro-ph/0503582] seems to rebuff it all.

Anze Slosar wrote:You have probably seen it, but anyway: This paper [astro-ph/0503582] seems to rebuff it all.

Seems like missing higher order terms were important. Thanks for the ref.

Hi,
here in Rome (BTW we are setting up a website freya.phys.uniroma1.it -
comments are very wellcome !) we will have a seminar by Toni Riotto in a couple of weeks and then by Uros Seljak in a month. I'll try to understand what is the situation of the debate and let you know...

A note by me on backreaction and spatial curvature, with regard to the papers by Kolb et al and Geshnizjani, Chung and Afshordi is out as astro-ph/0504005. I also put down some general observations about a successful backreaction explanation to the late-time acceleration.

Hi all,

Toni Riotto gave a very nice talk here in Rome yesterday. It seems that they are writing a paper which will reply to all the points raised against their model. He also showed a plot claiming that they can have a good fit with the CMB first peak.

cheers
Alessandro

This is a very interesting discussion. However, I just wanted to make one remark about this topic. We are all assuming here that the physical mass in the observable universe actually interacts with the membrane of the expanding universe.

Call me controversial, wrong or just crazy, but there is real doubt in my mind that the expansion of the universe is actually affected by the summed mass we can observe. A lot of what we see and measure that leads to speculation on dark energy and matter could be attributed to a decreasing rate of acceleration of expansion with time, without needing to rely on the influence of matter (normal, dark or otherwise) or a need to invent dark energy.

The paper Kolb et al have produced is a bit of 'cold towel around the head' in terms of why need dark energy, but I agree with a number of comments everyone has made here about the implications for curvature. Perhaps the most interesting question to ask of this paper is whether galaxies as we see them at large z would form if we believe in super-Hubble perturbations - to my way of thinking early galaxies would just fly apart, unless G was greater in the early universe.

Just a thought, anyway...