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Detecting Darkness
06.09.2009 6:09 PM
Parker Fagrelius

By Parker Fagrelius
Astronomy and Physics Missions Concepts

Well, the title is a bit of a misnomer perhaps. We are actually trying to define dark matter and dark energy properties through detection of distant galaxies. Dark matter and dark energy are the creative terms used to describe ~95% of all the mass/energy in the universe (remember, E=mc2). They are called dark in part because we can’t see them - they don’t interact with electromagnetic forces and therefore we are unable to directly measure their properties. One method that has been identified to indirectly measure the large-scale properties of dark matter and dark energy is Weak Gravitational Lensing. Similar to a glass lens bending the light from some distant light bulb due to its refractive index, matter can bend light because of its gravitational force. If there is a clump of matter between a telescope and a distant light bulb (let’s say a galaxy), the clump of matter will bend the light rays in such a way that the image of the galaxy appears slightly skewed. If we can determine in which way the image was skewed we can understand some of the properties of this clump of matter (dark matter). When we understand the distribution of theses clumps of dark matter we can draw inferences to the nature of dark energy, but I’ll leave that for another day.

I am currently working on a mission concept that will enable Weak Lensing measurements of thousands of galaxies. The problem I am addressing right now is how to detect and capture the light from those distant galaxies. Even with a telescope out in space, only about 1 photon per second from a distant galaxy will land on our detectors. When a photon lands on a detector pixel it generates an electron that will be collected by the detector electronics, but due to the detector material, only ~85% of the incoming flux will be converted to an electric signal. In addition to the signal from the distant galaxy, there will also be zodiacal background noise from the sun reflecting off of space dust, electrons generated in the detector material, additional noise caused by reading the detector (i.e. counting up all of the electrons on each pixel), and thermal radiation from the spacecraft. We have to ensure that all of this extra noise isn’t brighter than the source signal so we can actually see the galaxy. The first step to detecting the dark out there is making sure we can see the *relatively* bright objects.

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