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A gamma-ray intensity map of the region of the galactic plane isolating the dark matter halo. | Photo credit: Tomonori Totani, University of Tokyo
Thanks to NASA’s Fermi Gamma-Ray Space Telescope, scientists may have “seen” dark matter for the first time. If so, it would be the first direct evidence of the universe’s most mysterious substance.
Dark matter was theorized in 1933 by astronomer Fritz Zwicky, who discovered that the visible galaxies of the Coma cluster lacked the necessary gravitational influence to prevent this cluster from flying apart. Then, in the 1970s, astronomer Vera Rubin and colleagues found that the outer edges of spiral galaxies rotate at the same speed as their centers, which would only be possible if the majority of these galaxies’ mass were not concentrated in their centers but rather more widely dispersed. These are not direct observations of dark matter, Of course, but conclusions drawn from dark matter’s interactions with gravity, as well as gravity’s influence on ordinary matter and light. Nevertheless, based on these findings, astronomers have now calculated that all large galaxies are embedded in giant dark matter halos that extend far beyond the limits of visible matter in galaxies (such as galactic halos of stars).
It is estimated that the particles of this mysterious substance outnumber the particles that make up everyday matter by a ratio of five to one. This means that everything we see around us every day – stars, planets, moons, our bodies, the cat next door, etc. – only makes up 15% of the matter in the universe, while dark matter makes up the remaining 85%. Adding to the mystery of dark matter is the fact that it does not emit, absorb or reflect light because it interacts only weakly or not at all with electromagnetic radiation. Therefore, it is virtually invisible in all wavelengths of light – or at least we thought it was.
There is a possibility that would cause dark matter to produce light. If dark matter particles “annihilate” when they meet and interact with each other, much like matter and its counterpart antimatter, then it should produce a shower of particles, including photons from gamma rays that, while invisible to our eyes, could be “seen” by sensitive gamma-ray space telescopes. One of the proposed “self-annihilating” particles that are theorized to comprise dark matter are so-called “weakly interacting massive particles” or “Sissies.”
A team of researchers led by Tomonori Totani of the University of Tokyo’s Department of Astronomy pointed the Fermi spacecraft toward the regions of the Milky Way where dark matter is thought to accumulate, namely the center of our galaxy, and looked for this telltale gamma-ray signature.
Well, Totani believes we’ve finally found that signature.
Gamma ray intensity map with no components other than the halo, spanning about 100 degrees toward the galactic center. The horizontal gray bar in the central area corresponds to the area of the galactic plane that was excluded from the analysis to avoid strong astrophysical radiation. | Photo credit: Tomonori Totani, University of Tokyo
“We discovered gamma rays with a photon energy of 20 gigaelectron volts (or 20 billion electron volts, an extremely large amount of energy) propagating in a halo-like structure toward the center of the Milky Way,” Totani said. “The gamma ray emission component closely matches the shape expected from the dark matter halo.”
And that’s not the only close game. The energy signature of these gamma rays is broadly consistent with those predicted to be produced by the annihilation of colliding WIMPs, whose mass is about 500 times greater than that of a proton, the common particles of matter at the heart of atoms. Totani points out that there are no other astronomical phenomena that could easily explain the gamma rays observed by Fermi.
“If true, to my knowledge it would be the first time humanity has ‘seen’ dark matter. And it turns out that dark matter is a new particle that is not included in the current Standard Model of particle physics,” Totani said. “This represents a major development in astronomy and physics.”
While Totani is confident that what he and his colleagues have discovered is the signature of dark matter WIMPs annihilating each other in the heart of the planet Milky WayThe scientific community at large will need more solid evidence before closing the book on this nearly century-old mystery.
“This could be achieved as more data is collected, and if so, it would provide even stronger evidence that the gamma rays come from dark matter,” Totani added.
The team’s research was published Tuesday (November 25) in the Journal of Cosmology and Astroparticle Physics.
