Dark matter, which can be detected only thanks to its gravitational influence, remains a mystery after decades of research. Because it helps explain most observations quite well, a majority of astronomers however supports the theory. Why then haven’t we been able to identify its nature, or just prove its existence? Two recent studies may offer an answer: they now suggest that dark matter might just not exist, at all.

The existence of dark matter was suggested to help explain why galaxies rotate so quickly, including our own. Completely transparent, astronomers can detect it only through its gravitational pull, and they assume that about 80% of the Universe’s mass is made of dark matter.

Astronomers from Chile measured precisely the movement of  400 stars, especially stars far away from the galactic plan (up to 13,000 light-years away), to determine the amount of matter around the Sun. The volume covered by this new survey was four times bigger than previously.

According to galaxy formation theory, the Milky Way should be surrounded by a halo of dark matter. Although its shape is hard to determine, astronomers expect dark matter to be present in important amounts around the Sun. However, according to this new study, there doesn’t seem to be any dark matter at all in the region around the Sun! The team’s calculations are in excellent agreement with observations, but there is no place left for dark matter in what’s making up the mass of the region around the Sun.

© ESO/L. Calçada

Some exotic configurations such as an egg-shaped halo could explain these observations, but they are unlikely. While this doesn’t rule out completely the existence of dark matter, it may at least explain why researchers have so far been unable to detect dark matter in laboratories…

Another team of astronomers, from the University of Bonn, in Germany, studied the neighborhood of our galaxy in order to better understand what surrounds it. Thanks to a lot of data from various origins (ranging from photographic plates to recent images from the Sloan Deep Sky Survey), they obtained a detailed map of the various satellite galaxies and globular clusters (spherical clusters of stars) orbiting the Milky Way.

Looking at the results, all of the objects appear to be located in a plane perpendicular to the galactic disk: the structure they form spreads from 33,000 up to a million light-years away. When moving around the galaxy, the objects are creating trailss of material along their path: these trails are in the same perpendicular plane, showing that these objects have been there for a very long time.

This is where a problem appears with dark matter theory. According to the model, all of the Milky Way’s satellites have formed independently and were later captured by our galaxy. Thus, they should have come from various directions, which is almost impossible according to the new observations. Some members of the team suggest that this could be explained by an important collision between our galaxy and another some 11 billion years ago: all the objects would have then formed simultaneously.

Finally, a member of the team explains that their results seem to rule out the existence of dark matter in the Universe, seeing here a paradigm shift that could lead to a new understanding of our Universe.

Of course, astronomers are still far from ruling out the existence of dark matter, and it may well exist anyway. However, the hunt for the mysterious substance hasn’t given any answer yet, and more and more questions are raised; several alternatives also try to account for observations without the need for dark matter. What if, in the end, dark matter was just an illusion?

 

References

Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood, Moni-Bidin et al. arXiv:1204.3924v1

The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way, M. S. Pawlowski, et al. http://arxiv.org/abs/1204.5176

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