“Everything we know and love about the universe and all the laws of physics as they apply, apply to four percent of the universe. That’s stunning,” says astronomer and “Cosmos” host, Neil deGrasse Tyson. Most of the mass in the universe is missing, hidden in some exotic, as yet undetectable form from the visible matter that makes up galaxies, stars, planets, and Homo sapiens.
The problem of the missing mass, observed NASA, “has gotten to the point where it is more than just a problem. It is an embarrassment, an obstacle to understanding such things as the structure of galaxies, the evolution of clusters of galaxies, and the ultimate fate of the universe.”
Much to the surprise and consternation of astronomers, reports NASA, “as radio and optical observations have extended the velocity measurements for the stars and gas to the outer regions of spiral galaxies, they have found that the stars and gas clouds are moving at the same speed as the ones closer in, with a substantial part of the mass of the galaxy not concentrated toward the center of the galaxy but must distributed in a dark, unseen halo surrounding the visible galaxy.”
These outer regions of galaxies, faintly seen in a photograph, may actually contain most of the matter. In the words of astronomers Margaret and Geoffrey Burbidge —former director of Kitt Peak National Observatory in Arizona and one of the last giants of the postwar era of astronomy who discovered that life leads to stardust– it appears that “the tail wags the dog.” The Burbidges, reported Dennis Overbye for the New York Times, unveiled a universe “more diverse and violent than anybody had dreamed: radio galaxies and quasars erupting with gargantuan amounts of energy, pulsars and black holes pinpricking the cosmos, and lacy chains of galaxies rushing endlessly away into eternity.”
While the biggest dark matter haloes contain huge galaxy clusters, which weigh over a quadrillion times as much as our Sun, are well documented, the masses of the smallest dark matter haloes –hypothesized to be about the mass of the Earth–are unknown reports an international research team led by Wang Jie from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC).
Deepest Zoom–500 Times Size of Our Solar System
The dark matter density map shown below was created using a simulation measuring 2.4 billion light years on each side. The intermediate square (top right) is just under a million light years across. The smallest square (bottom left) is the deepest zoom: it is only 783 light years across, equivalent to 500 times the size of the solar system. In the intermediate square(top right) the largest dark matter haloes have a mass similar to that of a rich galaxy cluster (a million trillion times the mass of the Sun). In the smallest square (bottom right) the smallest clearly visible haloes have a mass comparable to that of the Earth (0.000003 the mass of the Sun). (Dr Sownak Bose, Center for Astrophysics, Harvard University).
The research team, based at the National Observatory of the Chinese Academy of Sciences in China, Durham University, the Max Planck Institute for Astrophysics in Germany, and the Center for Astrophysics at Harvard University, took five years to develop, test and “zoom in on a typical region of a virtual universe as if zooming in on an image of the Moon to see a flea on its surface.”
Such small haloes would be extremely numerous, according to the study, containing a substantial fraction of all the dark matter in the universe. However, they would remain dark throughout cosmic history because stars and galaxies grow only in haloes more than a million times as massive as the Sun. “These small haloes can only be studied by simulating the evolution of the Universe in a large supercomputer,” said Wang.
Haloes Too Small to Contain Stars
“By zooming in on these relatively tiny dark matter haloes we can calculate the amount of radiation expected to come from different sized haloes,” said co-author Carlos Frenk, Ogden Professor of Fundamental Physics at the Institute for Computational Cosmology, at Durham University. “Most of this radiation would be emitted by dark matter haloes too small to contain stars and future gamma-ray observatories might be able to detect these emissions, making these small objects individually or collectively ‘visible’. This would confirm the hypothesized nature of the dark matter, which may not be entirely dark after all.”
It enabled them to study the structure of dark matter haloes of all masses between that of the Earth and that of a big galaxy cluster. In number, the zoom covers a mass range of 10 to the power 30 (that is a one followed by 30 zeroes), which is equivalent to the number of kilograms in the Sun.
A Virtual Universe in Microscopic Detail
By zooming-in on the virtual universe in such microscopic detail, the researchers were able to study the structure of dark matter haloes ranging in mass from that of the Earth to a big galaxy cluster.
“Surprisingly, we find that haloes of all sizes have a very similar internal structure, i.e., they are extremely dense at the center, become increasingly spread out, and have smaller clumps orbiting in their outer regions,” said Wang. “Without a measure scale it was almost impossible to tell an image of a dark matter halo of a massive galaxy from one whose mass is a fraction of the Sun.”
Particles of dark matter can collide near the centers of haloes, and may, according to some theories, annihilate in a burst of energetic (gamma) radiation.
“By zooming in on these relatively tiny dark matter haloes, we can calculate the amount of radiation expected to come from different sized haloes,” said Frenk. Most of this radiation would be emitted by dark matter haloes too small to contain stars and future gamma-ray observatories might be able to detect these emissions, making these small objects individually or collectively “visible”.
Harbor a Substantial Fraction of the Universe’s Dark Matter
“We expect that small dark matter haloes would be extremely numerous, containing a substantial fraction of all the dark matter in the universe, said co-author Simon White, of the Max Planck Institute of Astrophysics, Germany, “but they would remain mostly dark throughout cosmic history because stars and galaxies grow only in haloes more than a million times as massive as the Sun. Our research sheds light on these small haloes as we seek to learn more about what dark matter is and the role it plays in the evolution of the universe.”