Parallel Worlds (41 page)

Einstein lenses
can also be used as an independent method to measure the amount of MACHOs in
the universe (which consist of ordinary matter like dead stars, brown dwarfs,
and dust clouds). In 1986, Bohdan Paczynski of Princeton realized that if
MACHOs passed in front of a star, they would magnify its brightness and create
a second image.

In the early
1990s, several teams of scientists (such as the French EROS, the
American-Australian MACHO, and the Polish-American OGLE) applied this method to
the center of the Milky Way galaxy and found more than five hundred
microlensing events (more than expected, because some of this matter consisted
of low-mass stars and not true MACHOs). This same method can be used to find
extrasolar planets orbiting other stars. Since a planet would exert a tiny but
noticeable gravitational effect on the mother star's light, Einstein lens- ing
can in principle detect them. Already, this method has identified a handful of
candidates for extrasolar planets, some of them near the center of the Milky
Way.

Even Hubble's
constant and the cosmological constant can be measured using Einstein lenses.
Hubble's constant is measured by making a subtle observation. Quasars brighten
and dim with time; one might expect that double quasars, being images of the
same object, would oscillate at the same rate. Actually, these twin quasars do
not quite oscillate in unison. Using the known distribution of matter,
astronomers can calculate the time delay divided by the total time it took
light to reach Earth. By measuring the time delay in the brightening of the
double quasars, one can then calculate its distance from Earth. Knowing its
redshift, one can then calculate the Hubble constant. (This method was applied
to the quasar Q0957+561, which was found to be roughly 14 billion light-years
from Earth. Since then, the Hubble constant has been computed by analyzing
seven other quasars. Within error bars, these calculations agree with known
results. What is interesting is that this method is totally independent of the
brightness of stars, such as Cepheids and type Ia supernovae, which gives an
independent check on the results.)

The cosmological
constant, which may hold the key to the future of our universe, can also be
measured by this method. The calculation is a bit crude, but it is also in
agreement with other methods. Since the total volume of the universe was
smaller billions of years ago, the probability of finding quasars that will
form an Einstein lens was also greater in the past. Thus, by measuring the
number of double quasars at different times in the evolution for the universe,
one can roughly calculate the total volume of the universe and hence the
cosmological constant, which is helping to drive the universe's expansion. In
1998, astronomers at the Harvard-Smithsonian Center for Astrophysics made the
first crude estimate of the cosmological constant and concluded that it
probably made up no more than 62 percent of the total matter/energy content of
the universe. (The actual WMAP result is 73 percent.)

DARK MATTER IN YOUR LIVING ROOM

Dark matter, if
it does pervade the universe, does not solely exist in the cold vacuum of
space. In fact, it should also be found in your living room. Today, a number
of research teams are racing to see who will be the first to snare the first
particle of dark matter in the laboratory. The stakes are high; the team that
is capable of capturing a particle of dark matter darting through their
detectors will be first to detect a new form of matter in two thousand years.

The central idea
behind these experiments is to have a large block of pure material (such as
sodium iodide, aluminum oxide, freon, germanium, or silicon), in which
particles of dark matter may interact. Occasionally, a particle of dark matter
may collide with the nucleus of an atom and cause a characteristic decay
pattern. By photographing the tracks of the particles involved in this decay,
scientists can then confirm the presence of dark matter.

Experimenters
are cautiously optimistic, since the sensitivity of their equipment gives them
the best opportunity yet to observe dark matter. Our solar system orbits around
the black hole at the center of the Milky Way galaxy at 220 kilometers per
second. As a result, our planet is passing through a considerable amount of
dark matter. Physicists estimate that a billion dark matter particles flow
through every square meter of our world every second, including through our
bodies.

Although we live
in a "dark matter wind" that blows through our solar system,
experiments to detect dark matter in the laboratory have been exceedingly
difficult to perform because dark matter particles interact so weakly with
ordinary matter. For example, scientists would expect to find anywhere from
0.01 to 10 events per year occurring within a single kilogram of material in
the lab. In other words, you would have to carefully watch large quantities of
this material over a period of many years to see events consistent with dark
matter collisions.

So far,
experiments with acronyms like UKDMC in the United Kingdom; ROSEBUD in
Canfranc, Spain; SIMPLE in Rustrel, France; and Edelweiss in Frejus, France,
have not yet detected any such events. An experiment called DAMA, outside Rome,
created a stir in 1999 when scientists reportedly sighted dark matter
particles. Because DAMA uses 100 kilograms of sodium iodide, it is the largest
detector in the world. However, when the other detectors tried to reproduce
DAMA's result, they found nothing, casting doubt on the DAMA findings.

Physicist David
B. Cline notes, "If the detectors do register and verify a signal, it
would go down as one of the great accomplishments of the twenty-first century
. . . The greatest mystery in modern astrophysics may soon be solved."

If dark matter
is found soon, as many physicists hope, it might give support to supersymmetry
(and possibly, over time, to super- string theory) without the use of atom
smashers.

SUSY (SUPERSYMMETRIC) DARK MATTER

A quick look at
the particles predicted by supersymmetry shows that there are several likely
candidates that can explain dark matter. One is the neutralino, a family of
particles which contains the super- partner of the photon. Theoretically, the
neutralino seems to fit the data. Not only is it neutral in charge, and hence
invisible, and also massive (so it is affected only by gravity) but it is also
stable. (This is because it has the lowest mass of any particle in its family
and hence cannot decay to any lower state.) Last, and perhaps most important,
the universe should be full of neutralinos, which would make them ideal
candidates for dark matter.

Neutralinos have
one great advantage: they might solve the mystery of why dark matter makes up
23 percent of the matter/energy content of the universe while hydrogen and
helium make up only a paltry 4 percent.

Recall that when
the universe was 380,000 years old, the temperature dropped until atoms were
no longer ripped apart by collisions caused by the intense heat of the big
bang. At that time, the expanding fireball began to cool, condense, and form
stable, whole atoms. The abundance of atoms today dates back roughly to that time
period. The lesson is that the abundance of matter in the universe dates back
to the time when the universe had cooled enough so that matter could be stable.

This same
argument can be used to calculate the abundance of neutralinos. Shortly after
the big bang, the temperature was so blistering hot that even neutralinos were
destroyed by collisions. But as the universe cooled, at a certain time the
temperature dropped enough so that neutralinos could form without being
destroyed. The abundance of neutralinos dates back to this early era. When we
do this calculation, we find that the abundance of neutralinos is much larger
than atoms, and in fact approximately corresponds to the actual abundance of
dark matter today. Supersymmetric particles, therefore, can explain the reason
why dark matter is overwhelmingly abundant throughout the universe.

SLOAN SKY SURVEY

Although many of
the advances in the twenty-first century will be made in instrumentation
involving satellites, this does not mean that research in earthbound optical
and radio telescopes has been set aside. In fact, the impact of the digital
revolution has changed the way optical and radio telescopes are utilized,
making possible statistical analyses of hundreds of thousands of galaxies.
Telescope technology is now having a sudden second lease on life as a result of
this new technology.

Historically,
astronomers have fought over the limited amount of time they were permitted to
use the world's biggest telescopes. They jealously guarded their precious time
on these instruments and spent many hours toiling in cold, damp rooms
throughout the night. Such an antiquated observation method was highly
inefficient and often sparked bitter feuds among astronomers who felt slighted
by the "priesthood" monopolizing time on the telescope. All this is
changing with the coming of the Internet and high-speed computing.

Today, many
telescopes are fully automated and can be programmed thousands of miles away
by astronomers located on different continents. The results of these massive
star surveys can be digitized and then placed on the Internet, where powerful
supercomputers can then analyze the data. One example of the power of this
digital method is SETI@home, a project based at the University of California at
Berkeley to analyze signals for signs of extraterrestrial intelligence. The
massive data from the Aricebo radio telescope in Puerto Rico is chopped up into
tiny digital pieces and then sent via the Internet to PCs around the world,
mainly to amateurs. A screen saver software program analyzes the data for
intelligent signals when the PC is not in use. Using this method, the research
group has constructed the largest computer network in the world, linking about
5 millions PCs from all points of the globe.

The most
prominent example of today's digital exploration of the universe is the Sloan
Sky Survey, which is the most ambitious survey of the night sky ever
undertaken. Like the earlier Palomar Sky Survey, which used old-fashioned
photographic plates stored in bulky volumes, the Sloan Sky Survey will create
an accurate map of the celestial objects in the sky. The survey has constructed
three- dimensional maps of distant galaxies in five colors, including the
redshift of over a million galaxies. The output of the Sloan Sky Survey is a
map of the large-scale structure of the universe several hundred times larger
than previous efforts. It will map in exquisite detail one quarter of the
entire sky and determine the position and brightness of 100 million celestial
objects. It will also determine the distance to more than a million galaxies
and about 100,000 quasars. The total information generated by the survey will
be 15 terabytes (a trillion bytes), which rivals the information stored within
the Library of Congress.

The heart of the
Sloan Survey is a 2.5-meter telescope based in southern New Mexico containing
one of the most advanced cameras ever produced. It contains thirty delicate
electronic light sensors, called CCDs (charge-coupled devices), each 2 inches
square, sealed in a vacuum. Each sensor, which is cooled down to -80 degrees C
by liquid nitrogen, contains 4 million picture elements. All the light collected
by the telescope can therefore be instantly digitized by the CCDs and then fed
directly into a computer for processing. For less than $20 million, the survey
creates a stunning picture of the universe at a cost of a hundredth of the
Hubble space telescope.

The survey then
puts some of this digitized data on the Internet, where astronomers all over
the world can pore over it. In this way, we can also harness the intellectual
potential of the world's scientists. In the past, all too often scientists in
the Third World were unable to get access to the latest telescopic data and
the latest journals. This was a tremendous waste of scientific talent. Now,
because of the Internet, they can download the data from sky surveys, read
articles as they appear on the Internet, and also publish articles on the Web
with the speed of light.

The Sloan Survey
is already changing the way astronomy is conducted, with new results based on
analyses of hundreds of thousands of galaxies, which would have been
prohibitive just a few years ago. For example, in May 2003, a team of
scientists from Spain, Germany, and the United States announced that they had
analyzed 250,000 galaxies for evidence of dark matter. Out of this huge number,
they focused on three thousand galaxies with star clusters orbiting around
them. By using Newton's laws of motion to analyze the motion of these satellites,
they calculated the amount of dark matter that must surround the central
galaxy. Already, these scientists have ruled out a rival theory. (An
alternative theory, first proposed in 1983, tried to explain the anomalous
orbits of stars in the galaxies by modifying Newton's laws themselves. Perhaps
dark matter did not really exist at all but was due to an error within Newton's
laws. The survey data cast doubt on this theory.)

In July 2003,
another team of scientists from Germany and the United States announced that
they had analyzed 120,000 nearby galaxies using the Sloan Survey to unravel the
relationship between galaxies and the black holes inside them. The question is:
which came first, the black hole or the galaxy that harbors them? The result
of this investigation indicates that galaxy and black hole formation are
intimately tied together, and that they probably were formed together. It
showed that, of the 120,000 galaxies analyzed in the survey, fully 20,000 of
them contain black holes that are still growing (unlike the black hole in the
Milky Way galaxy, which seems to be quiescent). The results show that galaxies
containing black holes that are still growing in size are much larger than the
Milky Way galaxy, and that they grow by swallowing up relatively cold gas from
the galaxy.