In the latest episode of their continuing efforts to embrace and understand the dark side of creation, astronomers sifting data from a new satellite say they have discerned the existence of a mysterious haze of high-energy particles surrounding the center of the Milky Way galaxy.
Nobody knows where the particles came from, and the five astronomers who posted their results online on Monday did not offer a formal opinion. But one tantalizing prospect, they admit, is that the particles are the decayed remains of the long-sought dark matter that constitutes 25 percent of the universe.
“Obviously we wouldn’t be doing this if we didn’t think it could be dark matter,” said one of the authors, Douglas Finkbeiner, of the Harvard-Smithsonian Center for Astrophysics.
If true, it would mean that astronomy has finally entered the realm of seeing the unseeable.
The identity of this dark matter, presumably exotic elementary particles left over from the Big Bang, is one of the biggest mysteries in physics. Other experts, however, say it is far too soon to draw such far-reaching conclusions based on signals from the confused and energetic environs of the galactic center.
“In my opinion, they are skating on very thin ice,” said Eliott Bloom of Stanford, a member of the team running NASA’s Fermi Gamma-Ray Space Telescope, which recorded the signals.
And indeed, promising signals of dark matter from an alphabet soup of cosmic observing satellites and balloons have popped up in recent months and then disappeared.
Nevertheless, the new paper - by Gregory Dobler, of the Kavli Institute for Theoretical Physics in Santa Barbara, California, and four other physicists from Harvard and New York universities - has created a buzz among astrophysicists and is sure to be discussed next week at a conference in Washington on results from the Fermi Gamma-Ray Space Telescope.
At issue is the origin of a haze of gamma rays surrounding the center of our galaxy, which does not appear connected to any normal astrophysical cause but matches up with a puzzling cloud of radio waves, a “microwave haze,” discovered previously by NASA’s WMAP satellite around the center. Both the gamma rays and the microwaves, Dr. Dobler and his colleagues argue, could be caused by the same thing: a cloud of energetic electrons.
The electrons could, in turn, be the result of decaying dark matter, but that, they said, is an argument they will make in a future paper.
In an e-mail message, one of the authors, Neal Weiner, of N.Y.U., explained, “It seemed it was important to have a less controversial first paper,” just establishing the electron cloud.
Gordon Kane, a particle physicist at the University of Michigan, called the present paper a solid result, adding, “They have demonstrated they know how to use the Fermi data.”
But Dr. Bloom said the authors were going too fast. “The galactic center is the Hells Kitchen of astrophysical forces,” he said, borrowing a phrase from a recent talk by his French colleague Johann Coehn-Tanugi of Laboratoire de Physique Theorique et Astrophysique, and the University Montpellier 2.
Separating the potential dark matter signal from the astronomical background sources could take years, said Dr. Bloom, especially as the new Fermi satellite adds to the list.
The center of the galaxy, being the center of the local cosmos, is filled with all kinds of high-energy objects, like a giant black hole millions of times more massive than the Sun, the whirly-gig beams of pulsars, exploding stars and their remains. Cosmological calculations indicate that it should also be full of dark matter particles, whose clumps form the gravitational scaffolding for the thin film of visible matter in the universe. According to many models of dark matter, such particles, when they meet, can annihilate one another in a flash of energy, which would add to the cacophony from the center.
So astronomers have had their antennae out looking for suspicious signals from the sky. One of them was the microwave haze discovered earlier by the WMAP satellite. Dr. Finkbeiner and his colleagues had argued that the haze could be produced by energetic electrons and positrons - their antimatter opposites - whirling around in the Milky Way’s magnetic field.
If such electrons existed, they should also produce gamma rays when starlight bounced off them and, thus, show up in the Fermi data. As they approached the data, said Dr. Finkbeiner, “we had some trepidation.”
Indeed, when all the known sources of gamma rays were erased from Fermi observations of the Milky Way center, they were left with a blob of radiation, about 12,000 light-years across, matching the microwave haze exactly.
The existence of this new gamma ray haze, they say in their paper, settles the question of the origin of the microwave haze, although others like Dr. Bloom disagree. And the question of whether and how these clouds are connected to dark matter is yet to be fought out.
In a model of dark matter recently proposed by Dr. Weiner and others, there is a “dark force” as well as a dark particle; when the dark particles crash into one another, they first produce the carriers of this force, which then decay after thousands or millions of years into the electrons and positrons creating these observed clouds.
Dr. Kane favors a simpler view - namely that dark matter particles decay directly into gamma rays.
Dr. Finkbeiner said Dr. Kane’s predictions also fit the gamma ray cloud very well. “Kane’s prediction is freakishly similar to what we are seeing.” But, he admitted, a simple adjustment in models of what kinds of electrons are produced by pulsars could also change the results.
Dr. Bloom said that with all the confusion and uncertainty, it was easy to be misled. “Depending on what the part of the elephant you’re feeling,” he said, “you can have very different models that fit part of what you’re looking at.”
“We need more data and time to make this work,” he said about Fermi and the quest for dark matter. “It is not something that will come easily.”
