In 2011, an analysis of data from a NASA Fermi Gamma-Ray Space Telescope
turned up massive, previously unseen galactic structures. A group of
astrophysicists located two massive bubbles of plasma, now know as
"Fermi Bubbles," each extending tens of thousands of light-years,
emitting high-energy radiation above and below the plane of the galaxy.
The structure spans more than half of the visible sky, from the
constellation Virgo to the constellation Grus, and it may be millions of
years old.
Now, more recently, in 2013, astrophysicists Dan Hooper of Fermi National Accelerator Laboratory and Tracy Slatyer at Princeton University,
have published a study suggesting that a massive outflow of charged
particles from Fermi bubbles, as they are known, outflows of charged
particles (gamma rays) traveling at nearly a third the speed of light from the center of the Milky Way galaxy,
may be partly due to collisions between dark matter particles that
result in their annihilation, and the subsequent creation of the
building blocks of visible matter—charged particles that appear as two
lobes or "bubbles," above and below the center of the Milky Way Galaxy.
Another possibility includes a particle jet from the supermassive
black hole at the galactic center. In many other galaxies, astronomers
see fast particle jets powered by matter falling toward a central black
hole. While there is no evidence the Milky Way’s black hole has such a
jet today, it may have in the past. The bubbles also may have formed as a
result of gas outflows from a burst of star formation, perhaps the one
that produced many massive star clusters in the Milky Way’s center
several million years ago.
“In other galaxies, we see that starbursts can drive enormous gas
outflows,” said David Spergel, a scientist at Princeton University in
New Jersey. “Whatever the energy source behind these huge bubbles may
be, it is connected to many deep questions in astrophysics.”
According to Fulai Guo and William G. Mathews of the University of
California at Santa Cruz: “The Fermi bubbles provide plausible evidence
for a recent powerful AGN jet activity in our Galaxy, shedding new
insights into the origin of the halo CR population and the channel
through which massive black holes in disk galaxies release feedback
energy during their growth.”
From its orbital perch hundreds of kilometers above Earth's surface,
Fermi has charted the location of gamma-ray sources with its Large Area
Telescope (LAT). Where the gamma rays originate from is not always
clear; the foreground of Fermi's view was clouded with emission from
events such as cosmic rays striking dust in the Milky Way's disk.
"Fermi Bubbles" was coined by Richard Carrigan at the Fermi National
Accelerator Laboratory in his work on the search for artifacts like Dyson spheres
or Kardashev civilizations. A "Fermi bubble," according to Carrigan,
would grow as the civilization creating it colonized space. Carrigan
notes that, as Carl Sagan
and others have observed, that the time to colonize an individual
system is small compared to the travel time between stars. A
civilization could engulf its galaxy on a time scale comparable to the
rotation period of the Milky Way, or every 225–250 million years, and
perhaps, fewer. According to Carrigan, of the 11,224 potential sources
of low range emissions identified that might be a manifestation of Dyson
spheres in the Milky Way, only 16 showed strong potential.
James Annis,a member of Experimental Astrophysics Group at Fermilab,
has suggested that elliptical galaxies, which exhibit little structure,
might be a more likely place to look for Fermi bubbles than spiral
galaxies. Annis examined existing distributions for spiral and elliptic
galaxies and looked for sources below the normal trend lines where more
than 75% of the visible light would have been absorbed. But no
candidates were found in his sample of 137 galaxies. From this Annis
inferred a very low probability of a Type III civilization appearing that would be found using this search methodology.
In 1960 Dyson suggested that an advanced civilization inhabiting a
solar system might break up the planets into very small planetoids or
pebbles to form a loose shell that would collect all the light coming
from the star. The shell of planetoids would vastly increase the
available "habitable" area and absorb all of the visible light. The
stellar energy would be reradiated at a much lower temperature.
If the visible light was totally absorbed by the planetoids a pure
Dyson Sphere signature would be an infrared object with luminosity
equivalent to the hidden star and a blackbody distribution with a
temperature corresponding to the radius of the planetoid swarm.
More information: Two Emission Mechanisms in the Fermi Bubbles: A
Possible Signal of Annihilating Dark Matter, arXiv:1302.6589
[astro-ph.HE] arxiv.org/abs/1302.6589; ' The Fermi Bubbles. I. Possible
Evidence for Recent AGN Jet Activity in the Galaxy
http://arxiv.org/abs/1103.0055.
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