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Mysterious Gravitational Pull Puzzles AstronomersThe local group |
By Gabriel Gache, Science News Editor
30th of November 2007, 14:15 GMT
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By tracking our galaxy's movement through the vast immensity of space, astronomers found that the Milky Way is actually traveling with an excess speed of 120 kilometers per second, due to a mysterious gravitational pull exerted by an unseen body, which might be hiding away from our sight behind the galaxy's central core.
The measurements were made on the remnant background microwave radiation that fills all the space, by applying the Doppler effect on radiation coming from different directions. It seems that the wavelength of the received microwave radiation is slightly shorter in the direction in which the galaxy in moving. As the calculations accounting for all the gravitational fields exerted on the galaxy show that the speed should be smaller than that calculated with the Doppler effect, astronomers take in consideration the fact that a cluster of galaxies or a large galaxy in the close proximity could account for the offset of the Milky Way's motion.
The calculations made at the Harvard-Smithsonian Center for Astrophysics in Cambridge, based on the 2 Micros Redshift Survey, reveal that the galaxy is actually moving in the direction of the galactic nuclei, as it is seen from the surface of the Earth. Due to the fact that the Milky Way's core is filled with high amounts of matter, observation through it is close to impossible, so astronomers estimated that a galaxy slightly more massive than our own situated at about 3 million light years could be held responsible for the extra pull. If so, it would be the most massive in the close vicinity, aside from the M31 galaxy, that has a similar mass and is situated at about 2.5 million light years from Earth.
The alternative theory proposes a large cluster formed of more that a thousand galaxies, situated at about 70 million light year in the same general direction. The region of space in which the galaxies might be in would become visible from Earth in about 100 million years, when the spin of the galaxy will put the solar system is a position from which we would be able to see what is on the other side. However, 100 million year represents a long period of time and Avi Loeb does not want to wait until then. So he started a radio survey that just might be able to penetrate the galactic core, to see what's on the other side.
In 2006 Pirin Erdogdu form the University of Nottingham led a similar study in the X-ray spectrum that has revealed a cluster of galaxies similar to those described by Loeb, but ruled out the theory that they might be responsible for the observed effect. However the fact that we can only see within 10 degrees of the galactic core still leaves room for the theory to remain untested.
Better still, it is relatively difficult to evaluate the conclusion drawn by Loeb, since any structure packing large mass concentrations, situated at distances up to 400 million light years have low or no gravitational influence on the motion of our galaxy.
The third possibility, proposed by Brent Tully from the University of Hawaii, involves a supercluster of galaxies with a mass more than 10,000 times that of the Milky Way, which lies about 700 million light years away, in a visible part of the universe about 40 degrees over the central core.
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