Dark Energy
I am currently taking a break from philosophy and my thesis-related work to write up an astronomy essay which is due at the end of the month. In the process, I went back to the one I wrote last year, to check out some stuff I wrote there.
It seemed a waste to leave it rotting in my hard drive (especially since science-related stuff tend to age very quickly), so I reformatted it, and uploaded on Scribd. It is an introduction to the history of, and current research around, dark energy: pretty basic, but I think it is still more detailed than the average New Scientist popular article on the topic. So well, if anyone is interested, I am posting it here.
EDIT – Thanks to the comment of Grad Student below I identified a couple of mistakes in the paper. Unfortunately it turns out that the original .doc format file is now corrupted so I can’t create a new corrected .pdf to upload on Scribd. Therefore, note that the equation of state on page 14 should have w and not omega on the left side and that all the cosmological constraints presented by Vikhlinin et al., which I discuss on page 16, were determined by observations of galaxy clusters and not superclusters.
Fabio Cunctator
Very interesting! Thanks for uploading that, Fabio.
Cheers,
Adrian
Thanks Adrian, I’m glad someone finds it somehow useful. One thing I meant to write is that most of the references are probably not as up-to-date as they were when I wrote it — the Plank satellite for example is now already up and running, even though it is still collecting data (there is no ‘Plank sky’ yet as there is the COBE and the WMAP ones).
An excellent essay, Fabio. The data that has most convinced me that acceleration is really happening is the cluster data that Vikhlinin and others have worked on. That gives us 3 roughly independent measurements of DE: CMB + matter, Type Ia supernovae, and clusters. Still, who knows if cosmic acceleration implies our theory of gravity is wrong, or if GR + cosmological constant is correct.
I hope you don’t mind if I mention a few details that may need correcting. On page 13 you seem to imply that the discovery of an accelerating universe means that the Hubble constant can no longer be considered to be a constant (and is now referred to as the Hubble parameter). However, even in cosmological models without dark energy/acceleration, the Hubble “constant” changes in time. The only universe I know of where Hubble constant is truly a constant is a flat dark energy dominated universe (Omega_Lambda = 1). Such dark energy dominated universes would be infinitely old.
When cosmologists have referred to the Hubble constant, they’ve always known that it changes in time, so they have defined the Hubble constant to be the value of the Hubble parameter at the present age of the universe.
Minor details:
On page 14, I assume you meant to write w = p_Lambda/rho_Lambda = -1, using Omega as the equation of state parameter is confusing since it’s used elsewhere as the density parameter.
On page 16, Vikhlinin and others study clusters of galaxies, not superclusters. Unlike clusters, superclusters haven’t really gravitationally collapsed and relaxed to an equilibrium state. I’m not sure, but I think elsewhere in your paper when you refer to superclusters, it should be replaced with clusters.
That’s great feedback, thanks a lot! I’ll correct it ASAP and upload the revised version. Thanks especially for the parameter/constant issue, I always had that a bit unclear.
As for the Omega instead of w, yeah you are right: the document went through various file formats (doc, docx, odt, pdf… because I’ve always been wary of larning to use LateX) and this screwed quite a lot of the equations formatting, so when i rewrote parts of them I messed up the w/omega thing
[...] and all) I can now post the second and final dissertation I wrote for the course (I posted the first one some months ago). Given the theme, this time it was much harder for me to ‘keep the [...]
Extraterrestrial Life « Hyper tiling said this on July 9, 2010 at 1:05 pm |