 |
| Composite image by NASA, University of Arizona The total mass of the “Bullet Cluster’s” individual galaxies adds up to far less than the mass of the cluster’s two clouds of hot x-ray emitting gas shown in red. The blue areas, which account for even more mass than the galaxies and x-ray gas combined, show the distribution of dark matter in the cluster |
Pity the poor physicist searching for dark matter, the exotic substance that accounts for roughly one-quarter of all the stuff in the cosmos, yet only interacts with the rest of the universe through gravity and the weak nuclear force. Hardly a week goes by, it seems, without a tantalizing new hint of a dark matter particle hovering at the threshold of statistical significance that eventually goes poof, dashing hopes yet again.
The search for dark matter involves a dizzying array of experiments, a veritable alphabet soup of acronyms, all using different techniques and technologies. This is how physicists look for something when they don’t know its precise properties. The problem is that although several experiments have detected possible hints of dark matter, the hints don’t agree with one another. Plot the color-coded results from various experiments onto a single graph, and it looks like abstract art.
Two years ago, Juan Collar of the University of Chicago was hopeful that dark matter was on the verge of being detected. But every subsequent new result seemed to point in a different direction. Small wonder that he opened a recent talk with a slide paraphrasing “The Big Lebowski”: “We are nihilists. We believe nothing.”
Ordinary visible matter — the planets, stars, galaxies and everything else that we see — makes up a mere 4.9 percent of all the matter in the universe. Most of the universe (68.3 percent) is made up of a form of energy dubbed dark energy, which is believed to be causing the expansion of the cosmos to accelerate. The remainder — roughly 26.8 percent of the universe — is made up of dark matter.
Physicists might not know precisely what the dark matter is, but they are confident that it exists. The notion made its debut in 1933, when Fritz Zwicky analyzed the velocities of galaxies in a certain cluster and concluded that the gravitational pull from visible matter alone could not prevent the speeding galaxies from escaping the cluster. Decades later, Vera Rubin and Kent Ford found further evidence of Zwicky’s “dark matter” in the stars orbiting the outskirts of spiral galaxies.
The search for dark matter involves a dizzying array of experiments, a veritable alphabet soup of acronyms, all using different techniques and technologies. This is how physicists look for something when they don’t know its precise properties. The problem is that although several experiments have detected possible hints of dark matter, the hints don’t agree with one another. Plot the color-coded results from various experiments onto a single graph, and it looks like abstract art.
Two years ago, Juan Collar of the University of Chicago was hopeful that dark matter was on the verge of being detected. But every subsequent new result seemed to point in a different direction. Small wonder that he opened a recent talk with a slide paraphrasing “The Big Lebowski”: “We are nihilists. We believe nothing.”
Ordinary visible matter — the planets, stars, galaxies and everything else that we see — makes up a mere 4.9 percent of all the matter in the universe. Most of the universe (68.3 percent) is made up of a form of energy dubbed dark energy, which is believed to be causing the expansion of the cosmos to accelerate. The remainder — roughly 26.8 percent of the universe — is made up of dark matter.
Physicists might not know precisely what the dark matter is, but they are confident that it exists. The notion made its debut in 1933, when Fritz Zwicky analyzed the velocities of galaxies in a certain cluster and concluded that the gravitational pull from visible matter alone could not prevent the speeding galaxies from escaping the cluster. Decades later, Vera Rubin and Kent Ford found further evidence of Zwicky’s “dark matter” in the stars orbiting the outskirts of spiral galaxies.
Simon Science Quanta Magazine: In the Hunt for Dark Matter, Promises to Keep?
No comments:
Post a Comment