"Fermi Bubbles," which might appear as a void in visible light in spiral galaxies. is the term used by Richard Carrigan at the Fermi National Accelerator Laboratory in his work on the search for cosmic-scale artifacts like Dyson spheres or Kardashev civilizations. A Fermi bubble would grow as the civilization creating it colonized space, according to Carrigan.
As Carl Sagan
observed, the time to colonize an individual system is small compared
to the travel time between stars. A civilization, believes Carrigan,
could engulf its galaxy on a time scale comparable to the rotation
period of the galaxy, or every 225–250 million years, and perhaps
shorter.
Searching for signatures of cosmic-scale archaeological artifacts
such as Dyson spheres or Kardashev civilizations is an interesting
alternative to conventional SETI.
Uncovering such an artifact does not require the intentional
transmission of a signal on the part of the original civilization.
This type of search is called interstellar archaeology or sometimes
cosmic archaeology. The detection of intelligence elsewhere in the
Universe with interstellar archaeology or SETI would have broad
implications for science. The constraints of the anthropic principle,
for example, would have to be loosened if a different type of
intelligence was discovered elsewhere.
A variety of interstellar archaeology signatures could include
non-natural planetary atmospheric constituents, stellar doping with
isotopes of nuclear wastes, Dyson spheres, as well as signatures of
stellar and galactic-scale engineering.
The concept of a Fermi bubble due to interstellar migration grew out
of the the discussion of galactic signatures. These potential
interstellar archaeological signatures are classified using the
Kardashev scale, developed by Nikolai Kardashev, who divided
civilizations into those harvesting all the energy of a planet, of a
star, and of a galaxy. With few exceptions interstellar archaeological
signatures are clouded and beyond current technological capabilities.
However SETI for so-called cultural transmissions and planetary atmosphere signatures are currently under way.
According to the Kardashev scale, radio SETI might be a type 0
civilization. A type I civilization would utilize the energy available
from a planet. Signals from exosolar planetary atmospheres fall roughly
in this category. A Dyson Sphere, a star cloaked in broken up planetary
material, would be an example of type II.
Another example would be some
sort of engineering of the stellar burning process suggested by Martin
Beech. A civilization using all of the energy of a galaxy would be type
III.
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. For the
case of the Sun with the planetoids at the radius of the Earth the temperature would be approximately 300 ºK.
Many of the earlier searches for Dyson Spheres have looked for
so-called partial Dyson Spheres where the loose shell only partially
obscures the star. The Dyson Sphere investigation at Fermilab looks for
so-called pure Dyson Spheres as well as partial Dyson Spheres.
Studying the M51 Whirlpool galaxy (image above), Carrigan says a
rough qualitative estimate shows there are no unexplained ‘Fermi
bubbles’ at the level of 5 percent of the M51 galactic area. The quest
is tricky because spiral galaxy structure includes natural voids — even
if a void in visible light with infrared enhancement were traced, it
would be hard to regard it as anything other than natural.
The distribution of galaxies on a plot of galactic optical brightness
or luminosity versus the maximum rotation velocity or radius of the
galaxy follows a fairly consistent pattern. Cases lying below the
typical galactic trend line reflect visible light that has been absorbed
and emitted somewhere else in the electromagnetic spectrum.
Looking elsewhere, ynthetic or unnatural constituents in an exoplanet
atmosphere could show a sign of ETI. The fingerprints of life, or
biosignatures, are hard to find with conventional methods, but advances
for eample by the ESO's
VLT team in Chile team have pioneered a new approach that is more
sensitive. Rather than just looking at how bright the reflected light is
in different colours, they also look at the polarisation of the light,
an approach called spectropolarimetry.
"The light from a distant exoplanet is overwhelmed by the glare of
the host star, so it's very difficult to analyse — a bit like trying to
study a grain of dust beside a powerful light bulb," says Stefano
Bagnulo of Armagh Observatory, Northern Ireland. "But the light
reflected by a planet is polarised, while the light from the host star
is not. So polarimetric techniques help us to pick out the faint
reflected light of an exoplanet from the dazzling starlight."
In the attempt to identify Dyson spheres, their use would greatly
expand the useful area for activities for any culture that could build
them, absorbing most or all visible light and re-radiating the energy of
the star at lower temperatures. Various searches for infrared excesses
around visible stars –hoping to target a partial Dyson sphere, perhaps a
ring — have been attempted, but with no luck from the searches of
several thousand stars. Even a pure Dyson sphere, completely surrounding
its star, is not definitive because there are natural objects that
mimic it, especially since dust clouds surround stars as they are born
and as they die.
Carrigan used data from the IRAS spacecraft’s database of low
resolution spectra, discarding objects that had been previously well
categorized and narrowing the sample to sixteen sources that he calls
‘mildly interesting.’
Only three had relatively low spectral
statistical fluctuations. All of the sixteen sources have some feature
which clouds their identification as a Dyson sphere.
The search suggests that there are few if any even mildly interesting candidates within several hundred light years of Earth.
Carrigan observes that "a Dyson sphere does not require intent to
communicate on the part of a civilization. The current detection reach
is comparable to a SETI search. However there is a problem of
confounding signatures from mimics such as carbon stars. Searches for
potential Dyson spheres would be sharpened by developing more realistic
pictures of construction scenarios including such factors as time to
build and approaches to stability… "Finally it would be interesting to
consider how stellar evolution might stimulate the necessity of such
large scale structures with a view to looking at candidate objects in
the later stage of evolution along the main sequence."
When we search for the type of structures or effects, the
"signatures," interstellar archaeology, we acknowledge that they demand
technologies so far beyond our own that their construction seems all but
miraculous.
"We can look for Dyson spheres," Carrigan says, for example, "but
scarcely imagine how a culture could build at this scale. But these are
limitations of our own state of development, and they don’t keep us from
extrapolating to what civilizations far older than our own might be
capable of developing."
The image at the top of the page shows a Gamma-ray emission detected by
Fermi LAT that fills bubbles of hot gas created by the most massive
stars in Cygnus X. The turbulence and shock waves produced by these
stars make it more difficult for high-energy cosmic rays to traverse the
region. When the particles strike gas nuclei or photons of starlight,
gamma rays result.
Image Credit: NASA/DOE/Fermi LAT Collaboration/I. A. Grenier and L. Tibaldo
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