| From: VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 05:13:01.16
To: [email protected]
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Subj: [ASTRO] Astronomers, Using New Method, Find Evidence for the Normal-Matter Component of the "Dark Matter" in the Universe
Astronomers, Using New Method, Find Evidence for the Normal-Matter
Component of the "Dark Matter" in the Universe
For years, scientists have been unable to account for all of the material
they believe would have been needed to form the cosmos billions of years
ago. But now two Johns Hopkins astrophysicists may have found much of the
"missing matter" by using a new method to study the early universe.
Their new analytical method is detailed in a scientific paper to be
published on April 20 in the Astrophysical Journal. The paper was written by
astrophysicists Arthur F. Davidsen and HongGuang Bi.
"I have been very excited about this recent work," said David Schramm, a
University of Chicago astrophysicist involved in similar research. "A
long-standing problem in cosmology is, 'Where is all the normal matter?'
Stars and galaxies do not add up to as much normal matter as we feel must be
there from our analyses of nuclear processes that took place in the early
universe. Davidsen and Bi appear to have found the normal matter out between
the galaxies. Furthermore, the amount they find is completely consistent
with the amount we expected to be there from our nuclear physics arguments,
so the whole picture holds together remarkably well."
The dark-matter problem can be summarized like this: the universe is made of
visible matter and so-called dark matter. Visible matter is seen in the form
of stars and galaxies, which emit light and other forms of radiation. Dark
matter has not been seen directly, but it is inferred to exist from the
gravitational effects it appears to exert on the visible matter.
Dark matter itself appears to come in at least two varieties. One component
is made of ordinary "baryonic" matter, the same stuff that makes up all the
visible matter in the universe. Baryons are ordinary matter particles like
protons and neutrons. But astronomers have not been able to account for all
the baryonic matter that is thought to exist, based on their studies of the
nuclear reactions that occurred during the Big Bang. The visible stars and
galaxies contain only a small fraction of the total amount of such ordinary
baryonic matter believed to exist. The other component of the dark matter is
widely believed to be some sort of exotic particle that does not emit or
absorb light.
The analysis reported in the Hopkins paper suggests that the missing
baryonic matter has been found. It was spread throughout intergalactic space
in the form of a very diffuse gas of hydrogen and helium atoms whose
presence is detected through its effects on light passing through it. These
findings don't address the nature and amount of the exotic type of dark
matter, which scientists believe makes up a majority of all matter in the
universe.
Astronomers had thought that the primordial medium of gases that existed in
the early universe was contained in individual "clouds," with nearly empty
space in between. But the Johns Hopkins astronomers have found evidence that
the gases were not arranged that way. Using their method, Davidsen and Bi
propose that the early universe contained a "continuous medium" of hydrogen
and helium gases, with regions of higher and lower density blending together
smoothly.
Although other scientists are using powerful supercomputers to make similar
calculations about the evolution of the universe, the Johns Hopkins
scientist have devised a method that requires only "fairly simple analytical
equations," Davidsen said. They used their analytical method to explain data
from observations made by other astronomers over the past 20 years.
Astronomers have detected the primordial hydrogen gas by using spectrographs
to analyze light emitted by very distant objects called quasars. Astronomers
find places in the sky where there are no galaxies, to get a clear line of
sight to a quasar. As the light from the quasar shines through space, it
also shines through the gas, like a headlight through fog. The quasar is so
far away that the light now reaching earth is from a time when the universe
was roughly one-quarter its present age, about 10 billion years ago.
But intense radiation from quasars and early galaxies has ionized much of
the gas, stripping away electrons from the atoms and making the gas largely
invisible to detection by spectroscopy. So astronomers are only detecting a
small portion of the gas.
"The gas is so highly ionized that we are seeing only the tail of the dog,"
Davidsen said. "It's a big dog but we are only seeing the tail. If we had a
theory that told us exactly what dog it is, based on what the tail looks
like, then we could say something. That's what we have now -- a theory that
connects the tail to the dog. We now believe we can say how much
intergalactic gas, baryonic material, there must have been."
Astronomers believe that the simplest elements, hydrogen, helium and
deuterium, were created in the Big Bang. Those simple elements formed stars,
in which the more complex elements were manufactured. Exploding stars later
released those more complex elements.
But how did the hydrogen and helium come together to form stars in the first
place? Astronomers believe that concentrations of the exotic form of dark
matter formed gravity "wells" that attracted the gases, beginning the
process of star and galaxy formation. The Johns Hopkins astronomers have
used their method to see that process going on in the universe about 10
billion years ago, Davidsen said.
"Although a small fraction of baryons had by then managed to condense into
stars, galaxies, and quasars, it now appears that most of them were still
spread throughout intergalactic space, in the form of very diffuse hydrogen
and helium gas that was ionized by the ultraviolet radiation of the
quasars," Davidsen said.
The method was inspired by previous findings with the Hopkins Ultraviolet
Telescope, which was operated from the cargo bay of a space shuttle in 1995.
HUT observations of the primordial helium yielded data that contradicted the
theory that the primordial gases were contained only in discrete clouds.
"The missing baryons used to be one of the so-called `dark matter problems,'
but this matter is no longer dark, thanks to the work of Davidsen and Bi,"
Schramm said.
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| From: VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 05:09:58.70
To: [email protected]
CC:
Subj: [ASTRO] The Mystery Of The Missing Mass in the Universe: Could it be Neutrinos?
The Mystery Of The Missing Mass in the Universe: Could it be Neutrinos?
The elusive neutrino may not be disappearing at all, but simply
changing its "flavor"
Like the Cheshire cat, the elusive neutrino particle can appear
and disappear, seemingly at will. And like the smile on the face
of the Cheshire cat, the neutrino may be a mere wisp, or have
actual substance.
But evidence is growing that this ghostly subatomic particle
actually has a corpus. If that is so, the particle may not have
been disappearing at all, but simply cloaking itself with another
form.
"There are still many puzzles," says University of Washington
physics professor Kenneth Young. "But experiments are giving us
something much stronger than hints that neutrinos have mass."
Young is reporting on the international quest to unravel the
nature of the neutrino at the spring meeting of the American
Physical Society and the American Association of Physics Teachers
in Washington, D.C.
Young and his colleagues are preparing to release the first year's
results from a gargantuan Japanese laboratory designed to probe
the mysteries of the neutrino. It is called Super-Kamiokande, and
it detects the particles in a massive tank, containing 50,000
metric tons of water, buried more than a half-mile deep in a mine
outside Kamioka in the Japanese Alps.
The University of Washington is one of nine institutions
supporting the $100 million laboratory, which began its research
last April.
Based on the first 100 days of research in Japan, Young says there
are tantalizing hints that not only does the neutrino have mass
and can change its form (or what researchers call "flavor"), but
it may also be more abundant at night than during the day, and
more plentiful during certain times of the year.
The importance of the neutrino is far more than an intellectual
exercise. A puzzle of astrophysics is that much of the universe --
perhaps 90 percent -- is seemingly hidden from view. Researchers
postulate that much of this so-called dark matter is actually
composed of neutrinos, which are clearly abundant in nature. If
the neutrino has mass, then it could be part of "the omnipresent
dark matter," says Young.
To date, the results from Japan are largely "the inference of
statistics," he notes. It will take another two years to provide
evidence that the particle may have mass. Along the way,
researchers also hope to solve the myth-like question of how a
subatomic particle can suddenly disappear, then reappear.
Super-Kam (as physicists have dubbed the laboratory) tracks
neutrinos from two sources, the sun and the Earth's atmosphere,
where they are created from the reaction of proton bombardment.
Every day, says Young, the laboratory registers about one million
particle reactions in the water. Most of these reactions are the
result of background radiation, such as that produced by rocks
surrounding the water tank. Only 30 reactions are separated out as
solar neutrinos, and just 10 are identified as atmospheric
neutrinos. Because the neutrinos are so shadowy they cannot be
tracked directly, but are registered through their collisions with
atoms in the ultra-pure water, which is constantly filtered to
remove dust and debris.
Although Super-Kam's measurement of solar neutrinos striking the
Earth confirms previous experiments, the central mystery still
remains: theoretical predictions of the sun's emission of
neutrinos calls for twice as many solar neutrinos as are being
recorded. The suggestion that previous experiments have simply had
a low efficiency in measuring solar neutrinos is discounted by
researchers. They have proved their case by firing electrons into
the water, and accurately counting their numbers.
So where are the missing neutrinos? The answer, says Young, could
be that the neutrinos are there all along, but are changing
flavor. Specifically, the laboratory tracks just one type of
neutrino, called the electron neutrino. But there are also two
other types: the muon neutrino (muons are massive electrons) and
the tau neutrino (tau particles are very heavy). Each of these is
called a different flavor of neutrino.
Says Young: "The question is, do these neutrinos stay with a
particular flavor all their lives, or do they change? If they do
indeed have mass, it is possible they can change so that the
electron neutrino becomes the muon or the tau neutrino? In the
case of the solar neutrino we would not be able to see it anymore,
because this experiment is at an energy level that can only see
the electron neutrino."
This changing form from one type of neutrino to another is called
oscillation, and it could partially explain why so many solar
neutrinos appear to be missing. But there may also be other
explanations, says Young. One is "the suspicion" that there may be
a detectable drop in electron neutrino population during the day,
and an increase at night. During the day, the neutrinos have only
to pass through the Earth's atmosphere and the mine rock face to
reach Super-Kam. But at night the neutrinos pass completely
through the Earth, because the sun is below the horizon.
"Oscillation may be far greater if the electron neutrinos have to
go through the Earth's core," Young theorizes. In fact, this
effect could be so great that just another year of results from
Super-Kam "could be enough to make a definitive statement."
The electron neutrino population, he says, may also be greater at
certain times of the year. That is because of the Earth's
elliptical orbit, which changes its distance from the sun by 5
percent during the year.
Until Super-Kam can provide statistical evidence for these
theories over the next year or two, the greatest frustration of
neutrino hunting may lie with the researchers themselves. Says
Young: "They can invent theories faster than they can improve
their measurements."
###
The U.S. Super-Kamiokande home page is at
http://www.phys.washington.edu/~superk/
A cutaway sketch of the laboratory is at
http://www.phys.washington.edu/~young/superk/drafts/sk_build01_k.jpg
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