Comments on: Cosmology FAQ Open Thread

By: Lab Lemming

Lab Lemming — Fri, 16 Mar 2007 08:30:44 +0000

Is this thing still on?

If so, I have a question.
Why can’t physicists use the standard model to calculate exact decay constants for radionuclides, thus alleviating us geologists from having to measure them?

By: DennyUA

DennyUA — Tue, 13 Mar 2007 06:45:16 +0000

The good forum, has found answers to many questions.
Thanks:-))

By: Catholic Priest Proposes New Model for Creation | Cosmic Variance

Catholic Priest Proposes New Model for Creation | Cosmic Variance — Mon, 12 Mar 2007 07:04:36 +0000

[...] All of which springs to mind because the Modern Mechanix blog has unearthed a Popular Science article from 1932 by Donald Menzel, an astronomer at Harvard, that explains LemaÃ®tre’s ideas. (The time between Hubble and Humason’s discovery and Menzel’s article is somewhat less than the time between the 1998 discovery of dark energy and yesterday’s New York Times article.) Menzel’s piece does a great job of explaining the basics of the Big Bang model, long before it was given that name by Fred Hoyle. Indeed, he touches on many of the questions that still arise in a good Cosmology FAQ! For example, he emphasizes that the redshift is due to the expansion of space, not to the Doppler effect. The case of the universe is analogous, except that the expansion, being of a three-dimensional volume, cannot be visualized. The phenomena are, however, comparable. The nebulae are not running away from us. Their recession is due to expansion of space. This may, perhaps, seem to be quibbling over terms, since it amounts to the same thing in the end. Nevertheless, the distinction is worth keeping. According to the relativity theory, there is a difference between the running away of the nebulae and expansion of the medium in which they are imbedded. [...]

By: spaceman

spaceman — Sun, 04 Mar 2007 23:40:47 +0000

Sean,

I am not sure if the study of the CMBR is your area of cosmological expertise, but I was wondering, what is your opinion of the conclusions reached in a recent paper titled “On the apparent lack of power in the CMB anisotropy at large angular scales” by Amir Hajian of Princeton University? (astro-ph/0702723) When the COBE data was released one of the puzzling unexpected aspects of the full-sky CMB maps was the absence of long wavelength fluctuations; this odd feature, odd in the sense that it is at odds with inflationary theory, also exists in the WMAP data (or does it?). In other words, if the Universe is infinite and homogeneous, as inflation predicts, it is awfully improbable that we should not be seeing long wavelengths in our tiny portion of the cosmos.

But now, as reported in the above mentioned paper, it appears that there is no deficit in large scale power in the full sky maps, and the explanation for the apparent lack of large scale power is that masking the sky near the galactic plane creates this appearance.

By: Sean

Sean — Sun, 04 Mar 2007 19:19:16 +0000

zeenia– There are different possible sources for a gravity-wave background, and searching for it is an ongoing project. Any major ruckus in the early universe is a candidate to make gravitational waves; for example, a strong phase transition (analogous to bubbles in boiling water) could stir things up and leave gravitational waves behind. Even better, gravitational waves could be created during inflation, just as density perturbations are. People are trying to detect the influence of such waves in the cosmic microwave background, and perhaps someday via a dedicated satellite.

Shantanu– Measuring the “speed of gravity” is like measuring the “speed of blue”; the concept just doesn’t apply. You can talk about the speed of propagation of gravitational radiation — in that case, the prediction of GR is unambiguous that gravitational waves propagate at the speed of light. Arguing about whether some particular measurement is really probing the “speed of gravity” is a waste of time; the important thing is that no observation yet performed is inconsistent with the predictions of GR.

By: Shantanu

Shantanu — Sun, 04 Mar 2007 16:07:00 +0000

Sean, I have brought this up before . maybe you and other readers of CV
can chime in again.
My queston is what exactly did
Kopeikin-Fomalant experiment measure?
Sergei insists that his measurement is a model-independent measurement of
speed of gravity and has provided (IMO) reasonable counterarguments
to his critics.
However even the papers which critiqued his measurement take completely
diffrenet point of view. See Samuel’s paper which says that this does not
even measure speed of gravity in GR.
What do readers of CV think?

By: zeenia

zeenia — Fri, 02 Mar 2007 17:29:10 +0000

hello guys, no I am disapppointed as no one has answered my ques. Come on, I want to know why should there should be a GWBR and how is that connected to inflation. Sean please! I will be looking forward to ur answer!

By: nick

nick — Fri, 02 Mar 2007 04:56:43 +0000

Dark Matter and Dark Energy seem to be “in these days”. If there is interest in another possible candidate for Dark Matter, read comment 30, in Sean’s “find of Dark Matter and Sterile neutrinos” February 10, 2007 blog. If not turned-off by what is shown and interested in “the rest of the story” (even for Dark Energy) more can be said!

By: zeenia

zeenia — Thu, 01 Mar 2007 20:11:52 +0000

hi,
can we discuss about gravity wave background, why should there be one and how is it related to inflation ?
and can it be seen ?

By: George

George — Wed, 28 Feb 2007 01:18:56 +0000

There was an article in physorg today, with the lead… “A hidden twist in the black hole information paradox”.

It ends with, “Quantum information cannot be completely hidden in correlations: Implications for the black-hole information paradox” appears in the latest Physical Review Letters.

Sorry for being so dumb on this, but just what does ‘information’ mean in this context?