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月度暗物质的背景补充阅读
Virtually everything astronomers known about objects
outside the solar system is based on the detection of
photons-quanta of electromagnetic radiation. Yet there
is another form of radiation that permeates the universe:
(5) neutrinos. With (as its name implies) no electric charge,
and negligible mass, the neutrino interacts with other
particles so rarely that a neutrino can cross the entire
universe, even traversing substantial aggregations of
matter, without being absorbed or even deflected. Neu-
(10)
trinos can thus escape from regions of space where light
and other kinds of electromagnetic radiation
are blocked
by matter. Furthermore, neutrinos carry with them
information about the site and circumstances of their
production: therefore, the detection of cosmic neutrinos
(15)
could provide new information about a wide variety of
cosmic phenomena and about the history of the uni-
verse.
But how can scientists detect a particle that interacts
so infrequently with other matter? Twenty-five years
(20)
passed between Pauli’s hypothesis that the neutrino
existed and its actual detection: since then virtually all
research with neutrinos has been with neutrinos created
artificially in large particle accelerators and studied
under neutrino microscopes. But a neutrino telescope,
(25) capable of detecting cosmic neutrinos, is difficult to co-
nstruct. No apparatus can detect neutrinos unless it is
extremely massive, because great mass is synonymous
with huge numbers of nucleons (neutrons and protons),
and the more massive the detector, the greater the pro-
(30) bability of one of its nucleon’s reacting with a neutrino.
In addition, the apparatus must be sufficiently shielded
from the interfering effects of other particles.
Fortunately, a group of astrophysicists has proposed
a means of detecting cosmic neutrinos by harnessing the
(35) mass of the ocean. Named DUMAND, for Deep Under-
water Muon and Neutrino Detector, the project calls for
placing an array of light sensors at a depth of five kilo-
meters under the ocean surface. The detecting medium is
the seawater itself: when a neutrino interacts with a
(40)particle in an atom of seawater. the result is a cascade of
electrically charged particles and a flash of light that can
be detected by the sensors. The five kilometers of sea-
water above the sensors will shield them from the interf-
ering effects of other high-energy particles raining down
(45) through the atmosphere.
The strongest motivation for the DUMAND project
is that it will exploit an important source of information
about the universe. The extension of astronomy from
visible light to radio waves to x-rays and gamma rays
(50) never failed to lead to the discovery of unusual
objects
such as radio galaxies, quasars, and pulsars. Each of
these discoveries came as a surprise. Neutrino astronomy
will doubtless bring its own share of surprises.
1. Which of the following titles best summarizes the
passage as a whole?
(A) At the Threshold of Neutrino Astronomy
(B) Neutrinos and the History of the Universe
(C) The Creation and Study of Neutrinos
(D) The DUMAND System and How It Works
(E) The Properties of the Neutrino
2. With which of the following statements regarding
neutrino astronomy would the author be most likely
to agree?
(A) Neutrino astronomy will supersede all present
forms of astronomy.
(B) Neutrino astronomy will be abandoned if the
DUMAND project fails.
(C) Neutrino astronomy can be expected to lead to
major breakthroughs in astronomy.
(D) Neutrino astronomy will disclose phenomena that
will be more surprising than past discoveries.
(E) Neutrino astronomy will always be characterized
by a large time lag between hypothesis and
experimental confirmation.
3. In the last paragraph, the author describes the
development of astronomy in order to
(A) suggest that the potential findings of neutrino
astronomy can be seen as part of a series of
astronomical successes
(B) illustrate the role of surprise in scientific discovery
(C) demonstrate the effectiveness of the DUMAND
apparatus in detecting neutrinos
(D) name some cosmic phenomena that neutrino
astronomy will illuminate
(E) contrast the motivation of earlier astronomers with
that of the astrophysicists working on the
DUMAND project
4.According to the passage, one advantage that neutrinos
have for studies in astronomy is that they
(A) have been detected for the last twenty-five years
(B) possess a variable electric charge
(C) are usually extremely massive
(D) carry information about their history with them
(E) are very similar to other electromagnetic particles
5. According to the passage, the primary use of the
apparatus mentioned in lines 24-32 would be to
(A) increase the mass of a neutrino
(B) interpret the information neutrinos carry with them
(C) study the internal structure of a neutrino
(D) see neutrinos in distant regions of space
(E) detect the presence of cosmic neutrinos
6. The passage states that interactions between neutrinos
and other matter are
(A) rare
(B) artificial
(C) undetectable
(D) unpredictable
(E) hazardous
7. The passage mentions which of the following as a
reason that neutrinos are hard to detect?
(A) Their pervasiveness in the universe
(B) Their ability to escape from different regions of
space
(C) Their inability to penetrate dense matter
(D) The similarity of their structure to that of nucleons
(E) The infrequency of their interaction with other
matter
8. According to the passage, the interaction of a neutrino
with other matter can produce
(A) particles that are neutral and massive
(B) a form of radiation that permeates the universe
(C) inaccurate information about the site and
circumstances of the neutrino’s production
(D) charged particles and light
(E) a situation in which light and other forms of
electromagnetic radiation are blocked
9. According to the passage, one of the methods used to
establish the properties of neutrinos was
(A) detection of photons
(B) observation of the interaction of neutrinos with
gamma rays
(C) observation of neutrinos that were artificially
created
(D) measurement of neutrinos that interacted with
particles of seawater
(E) experiments with electromagnetic radiation
文章一
The findings, published in Science, solve a longstanding problem of the widely accepted model -- Cold Dark Matter（暗物质） cosmology（宇宙论） -- which suggests there is much more dark matter in the central regions of galaxies than actual scientific observations suggest.
"This standard model has been hugely successful on the largest of scales--those above a few million light-years--but suffers from several persistent difficulties in predicting the internal properties of galaxies," says Sergey Mashchenko, research associate in the Department of Physics & Astronomy at McMaster University
. "One of the most troublesome issues concerns the mysterious dark matter that dominates the mass of most galaxies."
Supercomputer（超型计算机） cosmological simulations prove that indeed, this problem can be resolved. Researchers modeled the formation of a dwarf galaxy to illustrate the very violent processes galaxies suffer at their births, a process in which dense gas clouds in the galaxy form massive stars, which, at the ends of their lives, blow up（爆炸） as supernovae（超新星）.
"These huge explosions push the interstellar gas（星际气体） clouds back and forth in the centre of the galaxy," says Mashchenko, the lead author of the study. "Our high-resolution model did extremely accurate simulations, showing that this 'sloshing'（晃动） effect -- similar to water in a bathtub-- kicks most of the dark matter out of the centre of the galaxy."
Cosmologists have largely discounted the role interstellar gas has played in the formation of galaxies and this new research, says Mashchenko, will force scientists to think in new terms and could lead to a better understanding of dark matter.
The simulations reported in the research paper were carried out on the Shared Hierarchical Academic Research Computing Network
文章二
Dark matter is one of the greatest mysteries in modern astronomy. Scientists use the term as an umbrella definition for all the invisible "heavy stuff" in the universe. Astronomers currently believe that there are two components to dark matter. One part of dark matter is made up of exotic（外来的） materials, different from the ordinary particles that make up the familiar world around us. The other part consists of dark celestial bodies（天体） -- like planets, black holes, or failed stars -- which do not produce light or are too faint to detect from Earth.
Astronomers suspect that about most of our galaxy's and universe' weight comes from dark matter. For almost a century, they scoured our Milky Way（银河） galaxy for both exotic dark matter and dark celestial bodiesin hopes of accounting for the missing weight.
Now, research is showing that NASA's Spitzer Space Telescope may be able to play an important role in identifying the "invisible" celestial bodies that are weighing our galaxy down.
MACHOs
Astronomers refer to dark celestial objects like undetected planets, black holes, and failed stars, as massive compact halo objects, or MACHOs, because they hide in the far reaches, or halo, of galaxies.
Nearly a decade ago, a team of astronomers scoured the halo（晕轮） of our Milky Way galaxy for dark celestial bodies. They called their project MACHO, after the dark bodies they were searching for -- and with ground-based telescopes（望远镜） managed to indirectly sense the gravitational（重力的） presence of 17 of them. However, technology at the time was not sensitive enough to tell the scientists what the objects were.
In a follow up survey, graduate student Nitya Kallivayalil of the Harvard-Smithsonian Center for Astrophysics（天体物理学）, Boston, Mass., used Spitzer's supersensitive infrared（红外线的） eyes to zoom in（放大）on two of the previously detected MACHOs and identify them as puny（弱的） stars called M-dwarfs. These stars are about 4,000 times dimmer than our Sun and cannot be seen from Earth with the naked eye. This was the first time any of the previously detected sources had ever been identified.
"This is a really exciting discovery," said Kallivayalil, whose paper on the discovery was published in the December 2006 issue of Astrophysical Journal Letters.
Although MACHOs are invisible to most telescopes, the MACHO team originally located them through a technique called "gravitational microlensing." Everything in the universe with mass has gravity. The more massive an object is, the more gravity it wields. Gravity is helpful for identifying invisible MACHOs, because it warps(偏差，使不正常) the space surrounding that object.
When the MACHO moves in front of a distant star, light from the star travels through the warped space and becomes magnified（放大）. By looking for magnified starlight, astronomers can sense a MACHO.
Once the location of a MACHO is known, astronomers can use Spitzer's infrared eyes detect heat coming off of the object and identify it. Kallivayalil used Spitzer to identify MACHOs known to astronomers as LMC- 5 and LMC-20 as small, dim M-dwarf stars.
"It is important to know what may constitute 'MACHOs', and what their contribution is to the dark matter in the Milky Way," says Kallivayalil, who is currently in the process of analyzing Spitzer's observations of the other 15 previously detected MACHO sources.
Weighing the Milky Way
There are two ways commonly used by astronomers to calculate the weight of a galaxy. One way is to look at the galaxy's brightness and convert it into mass, or weight. The other way is to track the movements of the galaxy's stars.
Everything in the universe moves. In our own Milky Way galaxy, our Earth moves around the Sun, and the Sun moves around the galaxy's center. By measuring how fast stars at the edge of the galaxy move, astronomers can calculate the Milky Way's weight. The faster the outer stars move, the heavier the galaxy is.
Problems began to show up when twentieth century astronomers compared the results of these two calculations for multiple galaxies -- they noticed that the numbers consistently did not match. The brightness calculation showed that the galaxies were lighter than the movement of its stars indicated.
Since both these techniques have been proven successful for determining an object's weight, astronomers concluded that there must be more to a galax y than just the bright objects. They referred to the invisible stuff as "dark matter."
Rotating spiral galaxies（螺旋星云）are not the only places dark matter makes its presence felt. Over the last two decades astronomers explored the question "If there really is dark matter, what are all the impacts it would have on the universe?" The circumstantial evidence（旁证）accumulated clearly points to dark matter as the solution for the mysterious motion of galaxies.
The clearest example is gravitational lensing12. Think about the transparency of dark matter, and the analogy with the glass patio（院子） door again. Light passes through glass, but glass bends（弯曲） the path of the light through refraction（折射）. Dark matter lets light pass through; could it also bend the light as it passes? Yes.
[tr][/tr][td][/td]
The fix Einstein proposed for the law of gravity, the fix motivated by the mysterious motion of Mercury, also predicts that all mass should bend light. One of the early confirmations of Einstein's theory came during a total solar eclipse（日食） in 1919 in which the Sun bent the light of a star. Dark matter in a galaxy and, by extension, in a cluster of galaxies, should bend light passing through the galaxy or cluster. The curved arcs in the picture to the right are very distant galaxies whose image has been distorted by dark matter in a relatively nearby galaxy cluster. Dozens of examples like this demonstrate, completely independent of the motion of galaxies, that dark matter pervades
（遍及） space.
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发表于 2012-3-6 12:35:54
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