Dark Energy >>> A manifestation of the Dark Mother

[Guest guest]

Along these lines, I just had to share this news item, for those of

you who might have missed it.

 

The thing is, astrophysicists are now suggesting that a "highway" of

dark matter ripped from the dwarf galaxy Sagittarius (which is being

consumed by the Milky Way) is currently streaming right through the

Earth.

 

The story has been tossed around the media for about a week now, but

this article from Science Daily (link below) is about the clearest

and most informative piece I've seen for the lay reader:

 

HIGHWAY OF DARK MATTER STREAMING THROUGH EARTH?

 

March 24, 2003 - Particles called "WIMPs", speeding at 670,000 mph

on a "highway" through space may be raining onto Earth – a

phenomenon that might prove the existence of the "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 – 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 a 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

DAMA's 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."

 

Source: Science Daily

URL: http://www.sciencedaily.com/releases/2004/03/040322082706.htm