Comments on: Superhorizon Perturbations and the Cosmic Microwave Background

By: Mike

Mike — Wed, 20 Aug 2008 19:04:00 +0000

Does this mean the dipole that is measured is actually due to our peculiar velocity alone, and does not have a contribution from fluctuations in the inflaton? I think it’s pretty cool just to know our peculiar velocity!

By: Rich

Rich — Tue, 19 Aug 2008 16:14:45 +0000

I’m still amazed at how many cool new ideas CMB cosmology stimulates! Very much looking forward to the Planck data coming along (once ESA finally launch it; space missions seem to take for *ever*)

By: Lawrence B. Crowell

Lawrence B. Crowell — Mon, 18 Aug 2008 00:12:22 +0000

This is clearly a complex paper to read. I have only read the first two pages. The cancellations between dipole CMB temperature and amplitude in the adiabatic assumption means that any difference between them thermalized or went into equilibrium. So the Sach-Wolfe amplitude variations and any DT/T variation had sufficient time to cancel out. Since the two are determined by the same gravitational potential I presume the two are likely to deviate in the early universe by a small enough amount so as to be cancelled out if these deviations are adiabatic or sufficiently small.

I figure if I display my relative ignorance I will be corrected and learn something.

Lawrence B. Crowell

By: Eugene

Eugene — Sun, 17 Aug 2008 10:09:59 +0000

Sean,

You mean it only cancels when the supermode is adiabatic? I am not sure I understand a superhorizon adiabatic mode though. Are you saying that if the supermode is evolving outside the horizon (hence is effectively isocurvature when it reenters the horizon eventually), the dipole doesn’t cancel?

I should go read the paper…

By: Sean

Sean — Sat, 16 Aug 2008 19:39:35 +0000

wdjohns– there’s not much that we “know” about spacetime beyond the observable universe; we can only talk about what a certain model says, and that’s what I’m doing here. I know of models where there are disconnected bubbles of spacetime, but those bubbles would never collide, as they are not embedded in some bigger spacetime.

John– I’m not sure what you have in mind. Frequency is 1/time, so “Planckian” frequency would be 1/(Planck time), which is a maximum (if anything), not a minimum. I think what you have in mind is a photon whose wavelength is stretched beyond the Hubble radius. That’s no problem — it’s just an electromagnetic field, which dilutes away as the universe expands.

By: wdjohns

wdjohns — Sat, 16 Aug 2008 17:39:50 +0000

P.P.S.: Sorry for the multiverse multipost. My comment is specific to your post on Groeneboom, Eriksen, and the CMB anomaly.

My question then is could the observed polarity reflect mass and energy pulled in from the product of two previous big bangs?

And I wonder if you could look out through the blue spots (lower radiation areas) and see the product of other big bangs.

Thanks for the discussion space, and praise for your work!

By: wdjohns

wdjohns — Sat, 16 Aug 2008 17:03:01 +0000

I’m interested in the idea of other big bangs because I understand that there is not enough matter and energy in the product of our own big bang to cause it all to eventually collapse on itself. If it all goes outward forever, then “where do babies come from?” that is, what precipitated our big bang? and why is it the only one?

By: wdjohns

wdjohns — Sat, 16 Aug 2008 16:58:52 +0000

Thanks for the reply Sean. How do we know there is only one spacetime? Why could there not be multiple big bangs in a universe that is (much) bigger than the “universe” produced by our own big bang?

By: John R Ramsden

John R Ramsden — Sat, 16 Aug 2008 10:53:21 +0000

Congrats on the paper Sean, published in the nick of time by the sound of it!

Talking about ultra-long wavelengths, I was wondering the other day what would happen to a photon that appeared during inflation if space was expanding sufficiently fast to reduce its frequency to 1 Planck unit, which is obviously the minimum.

If that notion makes sense, might not “maximally stretched” photons like this oppose further increases in the cosmic expansion rate, and even slow it? Or would their existence in the first place be ruled out for some reason, such as the background temperature being so high at that stage, above the electroweak unification temperature for example?

By: Cormac O Raifeartaigh

Cormac O Raifeartaigh — Sat, 16 Aug 2008 10:48:02 +0000

Hi Sean, this is completely off-topic but I thought fellow bloggers and readers might be interested in a simple puzzle concerning Hubble’s law that I’ve posted on my blog at
http://coraifeartaigh.wordpress.com
The prize is a guest post for the correct answer in simple language!