|This of course justifies uninhibited shopping in every brain downtime. For instance yesterday, I heard a talk by Sean Carroll, after which I was so confused about the direction of time that I went to get some chocolate as antidote. And that's how I found a model of the universe. It came in a bar of aero chocolate, one of the numerous children from the Nestlé family. It basically consists of:|
And then there are the imprints of the nature of CDM in the structure of our universe. Structure formation is a very involved topic, a big part of which is typically performed numerically, with just stunningly beautiful results.
If you have DivX, and a fast internet connection, look at these movies from the millenium simulation in low resolution (10.8 MB), med resolution(13.4 MB), high resolution (48.6 MB), -- more info on this website.
Wow, what a trip!
But here's the point: each point in this simulation corresponds to 106 solar masses. All smaller structures are not resolved. What Stefan Hofmann and collaborators showed in their work though was that CDM's smallest structures are 12 orders of magnitude smaller than that! This applies generally for those types of CDM that have been in thermal and chemical equilibrium with the radiation in the early universe. This is typically the case for neutralinos and binos, but not for axions, in which case the small scale structure would look differently (he says work is in progress).
Starting with a primordial initial power spectrum, they calculated the evolution of this spectrum, for the first time including collisional damping and free-streaming. As Stefan said in his talk, in principle there is a third contribution from heat conduction 'but heat is a pretty boring thing for cold stuff, so we drop this term'. In their work they showed that the spectrum has a sharp cut-off at about 10-6 solar masses, below which there are no smaller substructures. You find a very readable summary on the arxiv
- The first WIMPy halos
Anne M. Green, Stefan Hofmann amd Dominik J. Schwarz
Cosmologists measure time in redshift, commonly denoted with z. We are today at z=0. The larger z, the further in the past an event was. Hofmann's analytical calculations hold down to z approximately 60, where the linear perturbation theory can no longer be applied because the density contrast has become too large. These analytical results however, can then be used as input for numerical calculations. This has been reported in a Nature article
- Letters to Nature, Nature 433, 389-391 (27 January 2005)
Earth-mass dark-matter haloes as the first structures in the early Universe
J. Diemand, B. Moore and J. Stadel
where the numerical calculation goes down to approximately z=20. Results of this simulation are shown in the picture below, where the small structures are magnified
(If you have no access to Nature, the same article is also available at astro-ph/0501589)
These smallest CDM halos without further substructure are distributed over a size of roughly the solar system, which means they are extremely diluted. Their average velocity is approximately 1 meter per second. They are estimated to propagate through galaxies without being disrupted, which means that these CDM substructures could travel through our solar system and render the background we live in time-dependent!
To summarize: the microscopic nature of CDM has an imprint on the small scale structure of our universe. The examination of these small scale fluctuations therefore would allow us to distinguish between different candidates for CDM.
Update Nov. 14th: See also the PS on Dark Matter.
4. Further Reading
- More than you want to know about the WMAP measurements
- Audio, Slides and Video of Stefan Hofmann's talk: Go to Perimeter's Streaming Seminars, click on 'Seminar Series' on the left side, in the field 'Find presentations' type 'Missing Link' and click on the search button (they are working on an improvement..., no honestly, I have seen the upcoming new sites with my own eyes!)
- Summary article of the results by Hofmann et al
- Nature article: Earth-mass dark-matter haloes as the first structures in the early Universe
- Wikipedia entry on Dark Matter
- A Primer on Dark Matter
- WMAP: What is the universe made of?
- PhysicsWeb: The search for Dark Matter
- Scott I. Chase: What is Dark Matter?
- Dark Matter Dynamics and Indirect Detection
- Dark Matter: Introduction
In the October Issue of Physics Today, Burton Richter - an experimentalist and Nobel laureate - commented on 'Theory in particle physics'. About cosmology he wrote that it is in
"[...] a kind of intermediate state in which all that is missing to make it practical knowledge is a mathematically sound microscopic realization."
Well, yes, that is ' all ' that is missing ;-)
But as a theoretical physicist in the 21st century, I have to give credits to the experimental achievements. We have plenty of evidence for physics beyond the standard model. Astrophysics and cosmology provide us with numerous puzzles to keep our days busy. In case someone got the impression, we theoretical physicists are not sitting around being depressed about the trouble with physics. We just don't have the time! The universe is waiting to be explored. And if you aren't yet convinced of the beauty of it all, go get some chocolate.
Updated on Jan. 14th 2007.
TAGS: SCIENCE, PHYSICS, ASTROPHYSICS, COSMOLOGY, DARK MATTER