Comments on: The View of the Universe from the South Side of Chicago http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/ Random samplings from a universe of ideas. Sun, 05 Jul 2009 21:25:31 +0000 http://wordpress.org/?v=2.3.1 By: Count Iblis http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17486 Count Iblis Thu, 22 Jun 2006 01:25:36 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17486 Shane, what Michael Duff means is that you can always eliminate the dimensional constants and formulate your results entirely in terms of dimensionless constants. Newton's constant enters the equations via dimensionless combinations of the Planck mass, Fermi's constant and masses of the proton, neutron etc. The ratio of the abundances of the light elements are dimensionless numbers, so these are functions of these dimensionless combinations. Shane, what Michael Duff means is that you can always eliminate the dimensional constants and formulate your results entirely in terms of dimensionless constants.

Newton’s constant enters the equations via dimensionless combinations of the Planck mass, Fermi’s constant and masses of the proton, neutron etc. The ratio of the abundances of the light elements are dimensionless numbers, so these are functions of these dimensionless combinations.

]]> By: Shane Caldwell http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17485 Shane Caldwell Tue, 20 Jun 2006 22:27:06 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17485 Science, I don't think gravitational compression has anything to do with primordial nucleosynthesis. It matters to nucleosynthesis in stars and I don't know where else. Primordial nucleosynthesis is supposed to depend on G in two ways: first, via the expansion rate H which determines the temperature at which the weak interactions, that interconvert protons and neutrons, freeze out and the relative neutron abundance is determined; second, by determining how long the neutron population spends decaying into protons while it waits for the universe to get cold enough that the formation of deuterium outpaces its photodissociation by photons in the high-energy tail of the Planck spectrum. Now those arguments involve dimensional quantities. If their measurement is "operationally meaningless" then it is news to me. I doubt if I quite understand the paper that Count links to, but I am not convinced by a quick persual of it. Shane Science,

I don’t think gravitational compression has anything to do with primordial nucleosynthesis. It matters to nucleosynthesis in stars and I don’t know where else.

Primordial nucleosynthesis is supposed to depend on G in two ways: first, via the expansion rate H which determines the temperature at which the weak interactions, that interconvert protons and neutrons, freeze out and the relative neutron abundance is determined; second, by determining how long the neutron population spends decaying into protons while it waits for the universe to get cold enough that the formation of deuterium outpaces its photodissociation by photons in the high-energy tail of the Planck spectrum.

Now those arguments involve dimensional quantities. If their measurement is “operationally meaningless” then it is news to me. I doubt if I quite understand the paper that Count links to, but I am not convinced by a quick persual of it.

Shane

]]> By: Yidun http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17500 Yidun Thu, 15 Jun 2006 21:40:55 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17500 Prof. Carroll: Thanks! I think your TASI lecture notes is very good for an intro course on cosmology, although students still need to read many reference papers. But they need to read references anyway. I strongly agree that cosmology is very hard to teach, unlike those regular grad courses such as QM, GR, and so on. However, I think a student should have better learned advanced GR and QFT before he sit in a real cosmology class. Otherwise, it would be hard for both of the student himself and the teacher. Prof. Carroll: Thanks! I think your TASI lecture notes is very good for an intro course on cosmology, although students still need to read many reference papers. But they need to read references anyway. I strongly agree that cosmology is very hard to teach, unlike those regular grad courses such as QM, GR, and so on. However, I think a student should have better learned advanced GR and QFT before he sit in a real cosmology class. Otherwise, it would be hard for both of the student himself and the teacher.

]]> By: Sean http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17499 Sean Thu, 15 Jun 2006 16:51:34 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17499 Thanks, Yidun. I haven't seen Mukhanov's book yet, I'm looking forward to it. Dodelson's book is fantastic, although it focuses on the microwave background -- it would be perfect for a course that took the opposite tack from mine, neglecting the early universe for the later universe. There is a relatively new book by Lars Bergstrom and Ariel Goobar that is quite good, although at crucial places it doesn't go into as much detail as you would like from a text. Thanks, Yidun. I haven’t seen Mukhanov’s book yet, I’m looking forward to it. Dodelson’s book is fantastic, although it focuses on the microwave background — it would be perfect for a course that took the opposite tack from mine, neglecting the early universe for the later universe. There is a relatively new book by Lars Bergstrom and Ariel Goobar that is quite good, although at crucial places it doesn’t go into as much detail as you would like from a text.

]]> By: Yidun http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17498 Yidun Thu, 15 Jun 2006 16:47:05 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17498 Dear Prof. Carroll, I think a combination of the two books: Mukhanov's new cosmology book and Dodelson's "Modern cosmology" is good for students. However, they won't fit in a quater of UC. :-) Best, Y Dear Prof. Carroll,

I think a combination of the two books: Mukhanov’s new cosmology book and Dodelson’s “Modern cosmology” is good for students. However, they won’t fit in a quater of UC. :-)
Best, Y

]]> By: Science http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17497 Science Thu, 15 Jun 2006 16:28:22 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17497 Count Iblis, Thanks for that reference, as it confirms what I just said: "The paper is very useful when you take it as showing that the ratio of gravity to electromagnetism was constant. (This is quite a different claim than saying gravity is constant.)" A ratio of force strengths is dimensionless, and this ratio isn't changing. The absolute strengths of the forces can vary. Count Iblis,

Thanks for that reference, as it confirms what I just said:

“The paper is very useful when you take it as showing that the ratio of gravity to electromagnetism was constant. (This is quite a different claim than saying gravity is constant.)”

A ratio of force strengths is dimensionless, and this ratio isn’t changing. The absolute strengths of the forces can vary.

]]> By: Jacques Distler http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17496 Jacques Distler Thu, 15 Jun 2006 14:04:02 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17496 Wikipedia does <em>not</em> currently output MathML. There is a <a href="http://blahtex.org/" rel="nofollow">project</a> to add MathML support to MediaWiki (the wiki software that powers Wikipedia). Currently, MediaWiki just outputs PNGs. BlahTeX will add the option of outputting MathML instead. Wikipedia does not currently output MathML. There is a project to add MathML support to MediaWiki (the wiki software that powers Wikipedia). Currently, MediaWiki just outputs PNGs. BlahTeX will add the option of outputting MathML instead.

]]> By: Count Iblis http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17495 Count Iblis Thu, 15 Jun 2006 12:30:10 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17495 It's actually not a change in G but a change in a dimensionless coupling that is constrained, <a href="http://arxiv.org/abs/hep-th/0208093" rel="nofollow">see here.</a> It’s actually not a change in G but a change in a dimensionless coupling that is constrained, see here.

]]> By: Science http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17494 Science Thu, 15 Jun 2006 09:44:35 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17494 The most interesting paper is http://pancake.uchicago.edu/~carroll/371/papers06/Caldwell-BBN.pdf Gravitational compression and electrostatic repulsion of charges control the fusion rate. So the analysis is showing that the ratio of gravity to electromagnetic force was similar in the first seconds of the BB. So the evidence that gravity strength constant G was the similar to the current value in the big bang is not there. Example: suppose gravity was x times weaker at 1 second after BB. Nucleosynthesis would be the same, because electromagnetism and gravity strength is a constant. Changing G doesn't vary the fusion rate of charges, because the effect of extra gravitational compression is simply offset by the extra Coulomb repulsion. Fusion can only occur where charges approach closely enough for the strong attractive nuclear force to cause they to fuse together. This means attractive gravity must overcome the repulsive Coulomb barrier. The paper is very useful when you take it as showing that the ratio of gravity to electromagnetism was constant. (This is quite a different claim than saying gravity is constant.) The most interesting paper is http://pancake.uchicago.edu/~carroll/371/papers06/Caldwell-BBN.pdf

Gravitational compression and electrostatic repulsion of charges control the fusion rate. So the analysis is showing that the ratio of gravity to electromagnetic force was similar in the first seconds of the BB.

So the evidence that gravity strength constant G was the similar to the current value in the big bang is not there.

Example: suppose gravity was x times weaker at 1 second after BB. Nucleosynthesis would be the same, because electromagnetism and gravity strength is a constant.

Changing G doesn’t vary the fusion rate of charges, because the effect of extra gravitational compression is simply offset by the extra Coulomb repulsion.

Fusion can only occur where charges approach closely enough for the strong attractive nuclear force to cause they to fuse together. This means attractive gravity must overcome the repulsive Coulomb barrier.

The paper is very useful when you take it as showing that the ratio of gravity to electromagnetism was constant. (This is quite a different claim than saying gravity is constant.)

]]> By: Amara http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17493 Amara Thu, 15 Jun 2006 07:55:49 +0000 http://blogs.discovermagazine.com/cosmicvariance/2006/06/13/the-view-of-the-universe-from-the-south-side-of-chicago/#comment-17493 PK: Thanks for the MathML info regarding Wikipedia. I knew that there was a reason that I should upgrade my MathType. Sean: Your students' papers (all of the ones I looked at so far) are really great. As a person who works in a science field 10^{34] scales away from cosmology, the papers give me something with meat to chew on to learn some of the key methods/questions/answers. PK: Thanks for the MathML info regarding Wikipedia. I knew that there was a reason that I should upgrade my MathType. Sean: Your students’ papers (all of the ones I looked at so far) are really great. As a person who works in a science field 10^{34] scales away from cosmology, the papers give me something with meat to chew on to learn some of the key methods/questions/answers.

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