Highway fo WIMPS May Solve Cosmic Mystery – Debris from Gobbled-up Galaxy Could Be “Smoking Gun” for Dark Matter
March 22, 2004 WIMPs speeding at 670,000 mph on a “highway” in space may
be raining onto Earth — a phenomenon that might prove the existence of
“dark matter” that makes up most our galaxy and one-fourth of the
universe, says a study co-authored by a University of Utah physicist.
Many researchers have long suspected that dark matter may be made of WIMPS
or Weakly Interacting Massive Particles, which are theorized subatomic
particles. More than 20 groups of physicists worldwide are building or have
built devices to detect them.
Scientists who run a WIMP detector named DAMA (Dark Matter) in Italy
claimed in 1998 that the underground device sensed WIMPs reaching Earth from
an unseen halo of dark matter surrounding our Milky Way galaxy. The claim
was doubted by scientists who run other WIMP detectors, which are designed
differently than DAMA and have not found WIMPs.
The new study –published in the March 19 issue of the journal Physical
Review Letters– advises how the DAMA scientists might prove their claim.
“We’re suggesting a way to check if what DAMA claimed to have seen are
really WIMPs,” says study co-author Paolo Gondolo, an assistant professor of
physics at the University of Utah. “This is about finding out what 90
percent of our galaxy is made of.”
Gondolo and colleagues say that in addition to the WIMPs pouring into our
Milky Way galaxy from the surrounding halo, a dark matter “highway” of WIMPS
may be raining onto our solar system after flying out of Sagittarius, a
dwarf galaxy that slowly is being gobbled up and torn apart by gravity from
the Milky Way.
The combination of the Milky Way WIMPS and those from the Sagittarius dwarf
galaxy should produce a distinct pattern in the Italian data that “would be
a smoking gun for WIMP detection,” the new study says.
Gondolo conducted the research with physicist Katherine Freese and graduate
student Matthew Lewis of the University of Michigan, and astronomer Heidi Jo
Newberg of Rensselaer Polytechnic Institute in Troy, N.Y.
The Dark Side of the Universe
Scientists realized a few decades ago that the motions of galaxies within
the universe could not be explained by the gravitational pull of visible
galaxies, stars and gases. For a long time, scientists said that 10 percent
of the universe was visible matter and 90 percent was unseen dark matter
filling the voids among stars and galaxies.
In recent years, however, astronomers determined that the universe and its
galaxies were flying apart at an accelerating rate, a phenomenon consistent
with the existence of an anti-gravitational force known as “dark energy.”
Gondolo says scientists now believe the universe is about 5 percent visible
matter, 25 percent dark matter and 70 percent dark energy.
Unlike dark matter, which is subject to gravity, dark energy is not pulled
into our galaxy, so the Milky Way is about 10 percent matter and 90 percent
dark matter, Gondolo says.
The spinning motion of the flattened, spiral disk-shaped Milky Way is too
fast to be explained merely by the gravity of its visible stars and gases,
so scientists believe it is surrounded by a much larger “halo” –actually a
flattened sphere” that contains some stars but mostly unseen dark matter.
Over the years, numerous theories were proposed as to the nature of the
dark matter: from dim brown dwarf stars that never ignited to the
whimsically named MACHOs (Massive Compact Halo Objects) and subatomic WIMPs.
Gondolo says WIMPs and other subatomic particles called axions now are
considered the most likely candidates to be dark matter.
The DAMA detector, located at Italy’s Gran Sasso National Laboratory, is
run by an international collaboration of physicists led by the University of
Rome. The DAMA group announced in 1998 that it found evidence for WIMPS.
Because DAMA is underground, overlying rock filters out particles created
when cosmic rays hit Earth’s atmosphere and produce showers of smaller
particles. WIMPs are “weakly interacting” particles, so they pass through
Earth. But they can hit sodium iodide crystals inside DAMA, causing flashes
of light and making sodium or iodine ions recoil.
If WIMPs do exist, they flow toward our solar system from the halo around
our galaxy. As the Earth orbits around the sun, it sometimes moves
“upstream” against the flow of oncoming WIMPs, and sometimes moves with the
flow. The DAMA scientists believe this explains the up-and-down pattern in
the number of particles detected by DAMA, and supports the assertion those
particles are WIMPs.
Other physicists, however, remain unconvinced. Their detectors, which use
germanium as a sensor instead of sodium iodide, should be equally sensitive,
but have not “seen” WIMPs. They argue the annual fluctuation in the number
of particles detected by DAMA may be caused by seasonal changes in the
atmosphere, the DAMA detector or DAMA’s environment, so that the particles
have not been proven to be WIMPs.
The New Study: A Solution from Sagittarius?
The visible Milky Way is vast, about 100,000 light years across, or about
588 million billion miles (588 times 10 to the 15th power). For eons, the
Milky Way has been absorbing and tearing apart the Sagittarius dwarf galaxy,
which is one-tenth the Milky Way’s diameter.
Newberg and other astronomers recently discovered two arc-shaped “tails” or
streams of stars flowing from Sagittarius. The streams are believed to also
contain WIMPs ñ if they exist. Our solar system sits in one of these
streams, which Gondolo and Freese describe as a possible “dark matter
‘highway’ raining down upon the solar system.”
In the new study, Gondolo and colleagues suggest how the combination of
WIMPs from the Milky Way’s halo and from the Sagittarius stream would
register on the DAMA detector:
— The dates of the maximum and minimum number of WIMPs detected by DAMA
would shift when dark matter from Sagittarius is considered. That is because
the Sagittarius WIMPs hit Earth from a different angle than Milky Way halo
WIMPS, changing the dates when the most and the fewest WIMPs hit Earth and
thus DAMA. Gondolo says the peak should be May 25 instead of June 2 if
Sagittarius WIMPs and halo WIMPs both hit Earth. DAMA found the maximum was
May 21, plus or minus 22 days.
— The “smoking gun” that would prove WIMPS exist is more complicated to
explain. When particles hit sodium iodide in DAMA, the ions recoil in
proportion to the mass and speed of the incoming particle. Gondolo says
WIMPs from the Milky Way halo move at speeds of zero to 600 kilometers per
second (1.34 million mph), with an average speed of 220 kilometers per
second (about 492,000 mph). WIMPs in the Sagittarius stream or highway all
move at 300 kilometers per second (about 671,000 mph). When the recoil
energies of the two kinds of WIMPs are combined and plotted on a graph,
there should be a steep “step” or drop in the number of collisions with
higher recoil energies, reflecting the fact that Sagittarius WIMPs do not
exceed 671,000 mph.
If DAMA scientists find that “step” in their data, it should be the smoking
gun to prove dark matter exists in the form of WIMPs, Gondolo says.
“This would be a corroboration of their result,” he adds. “As way to check
if they really have seen WIMPs, they could look for the specific signature
of WIMPs in the Sagittarius stream.”
Scientists at DAMA are aware of the new study and are rechecking their data
to determine if it contains the evidence that could prove the detector found
WIMPs. The process could take months, and it will take a few years for newer
detectors to confirm the finding, Gondolo says.
He and his colleagues suspect other detectors have not found WIMPs because
the particles may be lighter and smaller than expected, so germanium does
not recoil much when hit by an incoming WIMP, while DAMAís ions have
measurable recoil.
Gondolo says he studies dark matter because “I want to know what the
universe is made of. I was unsatisfied when I learned most of the universe
is not made of atoms.”
