MadSci Network: Astronomy

Re: Why can Bell Labs rule out COld Dark Matter?

Date: Thu Sep 28 07:23:59 2000
Posted By: Meghan Gray, Grad student, Astronomy, Cambridge University
Area of science: Astronomy
ID: 969384673.As

Hi Tassy!
Thanks for your question. March 2000 was a very exciting month for astronomers studying "cosmic shear," as several groups released the first claims of a detection of this effect. The paper you mentioned was published in the journal Nature (you could look for it in the library, or find it under D. Wittman et al, "Detection of weak gravitational lensing distortions of distant galaxies by cosmic dark matter at large scales").

The authors imaged large patches of sky and carefully measured the shapes of hundreds of thousands of distant galaxies. If you're unfamiliar with gravitational lensing, the idea is that the light from these distant galaxies gets bent due to the gravitational effects of dark matter distributed throughout the universe. This causes the galaxies to appear slightly distorted or elliptical to us. As astronomers we are very curious to understand how much dark matter is around and how it is distributed, and gravitational lensing is one of the only ways we can directly determine this. The effects on this scale are very small, which is why they needed to measure so many galaxies to detect it. On average, the gravitational lensing will cause galaxies in the same patch of sky to have very slightly more similar ellipticities than you would expect to occur randomly.

Cold Dark Matter, or CDM, is a theory that allows astronomers to model how structures in the universe might have evolved due to gravitation causing small structures to collapse into progressively larger ones over time. "Standard" CDM is one of several flavours. Now if the dark matter in the universe were distributed as the CDM simulations predict, we can calculate how strong a gravitational lensing effect we would expect from those dark matter structures. The authors found that the amount of lensing (correlated ellipticities of the background galaxies) they detected was significantly smaller than we would expect if Standard CDM were correct, meaning that the universe isn't as "clumpy" as that model predicts. Interestingly enough, the other groups who released results around the same time came to very similar conclusions, independently!


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