Wednesday, April 25, 2012

Dark Matter Theories Come Up Short

By studying the motion of the Milky Way's stars, researchers have learned there is not a lot of evidence for dark matter around the Sun. Current hypothesis have the solar neighborhood teeming with dark matter - an invisible enigma which is detected through its gravitational force. However, new research done by a team of astronomers in Chile have pointed out that the theories just don't match the facts. Chances are good that we'll never be able to directly detect dark matter particles with terrestrially based equipment.

By employing several telescopes, including the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory, astronomers have mapped the proper motion of more than 400 stars up to 13 000 light-years from the Sun. From these figures they were able to calculate the mass of material near our Sun... a volume four hundred percent larger than previously studied.

“The amount of mass that we derive matches very well with what we see - stars, dust and gas - in the region around the Sun,” says team leader Christian Moni Bidin (Departamento de Astronom√≠a, Universidad de Concepci√≥n, Chile). “But this leaves no room for the extra material - dark matter - that we were expecting. Our calculations show that it should have shown up very clearly in our measurements. But it was just not there!”

To understand dark matter, we widely accept the theory that it cannot be seen, yet can be detected from the gravitational effects which it exerts on material near it. This elusive substance was originally suggested as a means to explain why the perimeter of galaxies, including our own, have a speedy rotation. With time, dark matter and dark energy was also suggested to be a integral part of how galaxies form and evolve. The concept is so popular that it is generally accepted that dark matter makes up about 80% of universal mass. All this from a particle that cannot be detected in a laboratory and eludes clarification!

Through the use of the FEROS spectrograph on the MPG/ESO 2.2-meter telescope, the Coralie instrument on the Swiss 1.2-meter Leonhard Euler Telescope, the MIKE instrument on the Magellan II Telescope and the Echelle Spectrograph on the Irene du Pont Telescope, astronomers took exacting measurements of stellar motion. Their targets specialized in stars located away from the Milky Way's plane - allowing the team to work backwards to surmise the amount of dark matter present. These motions are commonly believed to be the mutual gravitational attraction of all materials - be it stellar in nature, or dark matter.

By adding computer modeling of how galaxies form and rotate, the researchers were able to hypothesize our Milky Way is encircled by dark matter... a type of halo. Although they are unable to predict what shape this halo might take, they fully expected to find a concentration in the area around the Sun. Imagine their surprise when hard data pointed to the absence or what they were looking for! They were expecting a range 0.4-1.0 kilograms of dark matter in a volume the size of the Earth, but the new measurements find 0.00±0.07 kilograms of dark matter in a volume the size of the Earth. These types of results could mean that detecting sparse interactions of dark matter particles and "normal" matter cannot be achieved on Earth.

“Despite the new results, the Milky Way certainly rotates much faster than the visible matter alone can account for. So, if dark matter is not present where we expected it, a new solution for the missing mass problem must be found. Our results contradict the currently accepted models. The mystery of dark matter has just become even more mysterious. Future surveys, such as the ESA Gaia mission, will be crucial to move beyond this point.” concludes Christian Moni Bidin.

Written by: Tammy Plotner. Image: Artist’s impression of the expected dark matter distribution around the Milky Way. Credit: ESO. Original Story Source: ESO Press Release.

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