The more we learn about dark matter the greater its mystery.
Four recent astronomical observations have cast doubt on scientists’ prior claims and assumptions.
Physicists believe that 83% of all the
matter in our universe is invisible and very different from the matter
in our bodies, in planets, in stars, or in anything else we can see. We
call this strange stuff “dark matter.” It emits no light, it casts no
shadow, and its only effect is to exert an immense gravitational pull on
everything that we do see. Dark matter may be the greatest
mystery in physics; we have almost no solid evidence to indicate what it
is. The most popular guess is that dark matter consists of WIMPs,
Weakly Interacting Massive Particles, which sounds much more
professional than AGAC (Ain’t Got A Clue).
This is what the recent observations tell us…
Dark Matter Is Heavy
Three years of observations by the Fermi
Gamma-ray Space Telescope have set a new stringent limit on the WIMP
mass — it must be greater than 40 GeV, about the mass of a calcium atom.
This is not an unreasonable value. Particles of normal matter have
masses up to 170 GeV. But this new limit does contradict three
experiments done deep below Earth’s surface that reported faint
indications of dark matter with masses from 7 to 12 GeV.
Physicists from Brown University and the
Fermi-LAT collaboration searched for gamma rays coming from seven dwarf
galaxies from the annihilation of WIMPs with anti-WIMPs. The selected
dwarf galaxies appear to have lots of dark matter and lack any
high-energy activity due to normal matter, such as supernovae. Because
the normal matter in these galaxies generates so few gamma rays (what
physicists call “low background noise”), these researchers were able to
set the best limit yet on the WIMP mass.
No Dark Matter In Our Backyard
European astronomers report finding
almost no dark matter within 80,000 trillion miles of Earth. If true,
this may doom all Earth-based laboratory experiments designed to catch
and analyze dark matter particles.
Prior measurements of the velocities of
stars near the edge of our galaxy, the Milky Way, indicated that our
galaxy is embedded in a vast halo of dark matter. These stars are moving
so fast that the gravity of all the galaxy’s visible matter isn’t
enough to keep them from flying off into the great beyond. But, recent
measurements of the movements of over 400 stars near our Sun tell a
different story. Using observations from La Sillia in Chile and other
telescopes, astronomers computed the amount of mass within 13,000
light-years of our Sun. That amount matched quite precisely the total
mass of the normal matter (stars, dust and gas) observed in this region,
leaving no extra mass to attribute to dark matter.
Prior estimates of the amount of dark
matter in the Milky Way and assumptions about the halo shape, predict
700 grams of dark matter in each volume the size of Earth. But, when
astronomers subtracted the amount of normal matter from the amount of
total matter, the amount left for dark matter was 0±70 grams. Note again
that this new measurement applies only to the vicinity of our Sun.
A possible explanation is that the dark matter halo has a radically different shape than predicted by present theories.
There are over 50 major Earth-bound
experiments currently searching for dark matter. They all assume Earth
is passing through the Milky Way’s vast cloud of dark matter and that
eventually some dark matter particles will hit our detectors. If Earth
is indeed in a hole in that dark matter cloud, we would have nothing
here to detect.
Dark Matter Not Centered In Galaxy Centers
Recent Hubble Space Telescope
observations confound yet another popular assumption: that dark matter
clumps at the center of galaxies. Since dark matter is very massive,
some expected it to be pulled into galactic cores — the heaviest things
usually fall to the bottom of a gravitational potential.
Apparently that doesn’t happen.
This study was also done on nearby dwarf
galaxies: Fornax and Sculptor. These galaxies contain only about 1% as
many stars as our Milky Way. But up to 99% of the mass of a dwarf galaxy
may be in the form of dark matter. The researchers measured the motions
of 2000 stars and used these to compute the distribution of dark
matter. They found no substantial peaking at the galaxy centers; the
dark matter seems uniformly spread over a vast area. This will send a
lot of theorists back to their whiteboards.
Dark Matter Might be Sticky After All
Another Hubble Space Telescope study
seems to contradict yet more common wisdom about dark matter. These data
show that dark matter might have some interaction other than just
gravity - that perhaps it can stick to itself.
Prior analysis, based on the spectacular
image of the Bullet Cluster, shown below, indicated that dark matter
does not interact with normal matter, neutral or charged, and does not
interact with itself, except via gravity. In this image, two galaxy
clusters, each with 1000 trillion stars, passed through one another 150
million years ago. Here the computed dark matter halos are shown in
purple. The key point is that each halo passed through galaxies, clouds
of charged normal matter (shown in pink), and even the other dark matter
halo with no apparent effect.
Yet in the new Hubble image of another
galaxy cluster collision, shown below, a very different picture emerges.
In this image, the computed dark matter locations, shown in teal, lag
behind their clusters. This may indicate a substantial dark matter
interaction, possibly even that the dark matter halos stuck to one
another.
There might be alternative explanations
for any individual celestial image — our universe is so vast that there
are many strange events to ponder.
For those seeking definitive answers,
these recent developments may be discouraging. They show that much of
the little that we thought we knew about dark matter is either wrong or
needs major revision.
But for those who enjoy exploring the mystery of our universe, dark matter has gotten a lot more interesting.
Best Regards,
Robert
June 5,2012
Note: Previous newsletters can be found on my website.
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