ASTRONOMY

Oct. 1, 1996
Computer-generated simulated three-dimensional image of matter distribution in the nearby universe is based on observations of the velocities of more than 2000 galaxies from which the mean space density is derived. The image includes otherwise invisible matter, if there is any. The dark areas correspond to regions with little or no matter at all--empty voids--and the brighter areas correspond to regions of enhanced density. The Milky Way galaxy in which we live is at the center. A region of exce

ASTRONOMY

Voids in the universe are empty

Stephen G. Anderson and Roland Roux

Computer-generated simulated three-dimensional image of matter distribution in the nearby universe is based on observations of the velocities of more than 2000 galaxies from which the mean space density is derived. The image includes otherwise invisible matter, if there is any. The dark areas correspond to regions with little or no matter at all--empty voids--and the brighter areas correspond to regions of enhanced density. The Milky Way galaxy in which we live is at the center. A region of excess mass known from earlier studies-- called the Great Attractor--is clearly visible.

The apparent size of these empty voids suggests that current theories regarding formation of galaxies will need revision. Cosmic Background Radiation measurements from after the "Big Bang" indicate that no holes existed then. Holes, therefore, must have formed later, and astronomers would consequently expect to find regions of the universe with large numbers of galaxies that "compensate" for the lack of matter in the large holes. Although clusters of galaxies have been observed in which the mean mass density is higher than in the surroundings, there is not enough matter to explain the emptiness of the newly discovered holes.

Advanced optical telescopes such as those operated by the European Southern Observatory (ESO; La Salle, Chile) can easily detect normal galaxies at large distances. Voids were first noted in the 1980s as regions where few galaxies could be seen. Proving the absence of matter, however, has been difficult; matter may "hide" from the astronomers` view by not forming any stars, existing in a form not visible with a telescope.

The velocity measurements were made by a team consisting of astronomers from the ESO, Dartmouth College (Dartmouth, England), Cornell University (Ithaca, NY ), and Wesleyan University (Middletown, CT). A computer program then determined how the matter is distributed in the corresponding region of space by checking which distribution of matter will best reproduce the observed galaxy velocities.

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