Special & General Relativity Questions and Answers
How is gravitational lensing used to search for dark matter?
Because all forms of matter and energy 'gravitate', the lensing phenomenon is sensitive to all of these hidden and luminous forms of gravitational energy. By modeling the lensing geometry carefully, it is possible to determine the total gravitating mass of the lensing galaxy. When compared to its luminous mass in ordinary stars and gas, you then get a handle on just how much unseen mass there is in the galaxy that is doing the lensing. It is a very powerful technique. Another technique is to determine the temperature and density of X-ray emitting gas in distant clusters of galaxies. From the temperature and density you then can determine how deep the gravity well of the cluster has to be to contain the hot gas, and this also weighs both the seen and unseen mass components of the galaxies in the cluster. Both methods show that in some instances 1/2 of the gravitating mass is 'invisible' or 'dark'.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.