A team of British astronomers have managed to glean some secrets about the mysterious dark matter that inhabits the galaxies. Dark matter is warmer than expected and may in fact be composed of primordial particles called WIMPs (weakly interacting massive particles).
Using several large telescopes around the globe, including the Very Large Telescope in Chile, the team determined that the 12 dwarf galaxies they studied contain about 400 times more dark matter than regular matter. In addition, the team found that the dark matter was moving faster — in other words was hotter — than expected.
The speed of the subatomic particles also sets parameters on the volume they can occupy. They need more room than predicted.
“It looks like you cannot ever pack it smaller than about 300 parsecs – 1,000 light-years; this stuff will not let you. That tells you a speed actually – about 9km/s – at which the dark matter particles are moving because they are moving too fast to be compressed into a smaller scale,” Prof Gerry Gilmore of the Cambridge Institute of Astronomy told the BBC.
In other words, if the dark matter particles act like an ideal gas, their average kinetic energy (temperature) creates enough pressure to combat the gravitational force pulling them together.
Since the 12 dwarf galaxies all orbit the Milky Way, our own galaxy, the team’s measurements indicate the Milky Way is heavier than previously thought.
The team has not published its findings in a scientific journal, so the only details available so far about the project are in the lay media. Scientific discoveries are normally subjected to peer review before public announcements are made, so we need to regard the Cambridge team’s announcements as only tentative.
Even so, they may have managed to deduce more properties of the mysterious dark matter than anyone has to date.
One of the great mysteries of cosmology is the mass of the universe. As it turns out, observable matter — stars, gas clouds, planets, etc. — does not contribute enough mass, and thus gravity, to account for its observable behavior. Stars revolving around the cores of galaxies, for example, appear to move more quickly than the laws of physics predict. Since we are reluctant to throw out laws that otherwise work perfectly well, cosmologists have suggested there is a form of matter than cannot be observed directly, dark matter, that is influencing the behavior of the matter we can see.
Careful measurements of stellar velocities in galaxies, and the degree to which galaxies warp the space around them (and thus the light passing nearby), has supported the dark matter hypothesis. Astronomers have thus been able to get some idea of the mass of dark matter, but little idea of its other properties.
Cosmologists hypothesize that dark matter is left over from the Big Bang, and must be some kind of “exotic” matter. One possible candidate for this exotic remnant is the WIMP, a particle that responds only to gravity and the weak nuclear force. Such particles do not respond to the other two fundamental forces, the strong nuclear force and electromagnetism (light), so detecting them directly is quite a challenge. Efforts are underway to detect WIMPs by their very occasional interactions with regular matter.
One question regarding dark matter has been its overall temperature, or kinetic energy. The currently favored idea has been that dark matter is slow, so it is cold dark matter, but other hypotheses suggest it can be hot or warm. The Cambridge results would favor the warm hypothesis.
One primary effect of dark matter would be to influence the expansion of the universe from the Big Bang, which astronomer Edwin Hubble first detected in the late 1920s. If the universe has enough mass, the combined gravitation of that matter will either eventually slow the expansion to a dead stop or will reverse the expansion into a Big Crunch. Getting a handle on the amount of dark matter and its properties will help cosmologists determine the ultimate future of the universe.
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