Download The All-Seeing, All-Magnifying Eye

Gerald Fitzgerald
February 8, 2003
THE ALL-SEEING, ALL-MAGNIFYING EYE
Einstein was a real busybody, a man with his fingers in everything. When he wasn’t working
out the speed of light, he was designing atom bombs, playing around with the nature of
subatomic particles, and when he wasn’t doing that, he was theorizing ways to see further
into deep space. Never mind that this was very early in the 20th Century and there was no
way to perform most of his experiments!
One of the predictions of Einstein’s General Theory of Relativity was that powerful
gravitational fields would be strong enough to bend light waves. It wasn’t until recently that
this phenomenon was observed in nature, specifically, during a solar eclipse, when
astronomers observed that light from stars passing very close to the Sun was slightly bent, so
that they appeared slightly out of position.
Einstein believed it was possible for any celestial object to bend light, such as a distant
galaxy, and that under certain conditions, you might be able to observe multiple images of a
single source, using a phenomenon called a “gravitational lens.” But Einstein delayed
publishing his ideas for over twenty years, because he felt that these multiple images would
be impossible to see with the limited technology of his time.
It wasn’t until the advent of powerful radio telescopes and CCD optical detectors that
astronomers were able to see much further – to resolutions far beyond Einstein’s capabilities.
Today, dozens of gravitational lenses have been observed, offering a wealth of new data for
exploring the universe. The study of gravitational lenses is an important (albeit bizarre) part
of the future of astronomy and astrophysics.
Who’s Doing the Research?
One major project is called “CASTLeS,” or, the CfA-Arizona Space Telescope Lens Survey.
It is made up of scientists led by Brian McLeod of the Harvard-Smithsonian Center for
Astrophysics (CfA), with colleagues from the CfA and the University of Arizona (UA). The
members of the CASTLeS team include Emilio Falco (CfA), Christopher Impey (UA),
Christopher Kochanek (CfA), Joseph Lehar (CfA), Hans-Walter Rix (UA), and Chien Peng
(UA).
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A normal view of the Smithsonian Institution's Castle
on the National Mall.
The Smithsonian's Castle as it would appear if a black
hole with the mass of the planet Saturn lay between the
viewer and the Castle.
Other studies include CLASS (the Cosmic Lens All-Sky Survey) and JVAS (Jodrell/VLA
Astrometric Survey). These are surveys of flat-spectrum radio sources designed to identify
gravitational lens candidates.
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The Cosmic Lens All-Sky Survey (CLASS), is an international (USA, UK and Netherlands)
collaborative project to map more than 10,000 radio sources in order to create the largest and
best-studied statistical sample of radio-loud gravitationally-lensed systems. CLASS is aimed
at identifying lenses where multiple images are formed from compact flat-spectrum radio
sources.
Suspected lenses are followed up by higher-resolution VLBI and MERLIN studies. JVAS
and its associated analysis was completed in 1992, leading to the discovery of five new
gravitational lenses. CLASS is a much larger survey, undertaken by groups at Jodrell Bank
in England, Caltech, the University of Pennsylvania, Dwingeloo (Netherlands) and the
University of Groningen (Netherlands). CLASS is in the final stages of analysis and followup
of suspected lenses.
VLBI stands for “Very Long Baseline Interferometry” and MERLIN stands for “MultiElement Radio Linked Interferometer Network.” Both projects are conducted by Jodrell
Bank Observatory and the University of Manchester, and involve the coordination of several
radio telescopes distributed around Great Britain.
The results will be used to learn more about the properties of distant galaxies, and to
determine the “Hubble Constant” (the rate at which the Universe is expanding). Personally, I
don’t know what we will do with the knowledge once we get it, but I suppose it will be
satisfying to know how long our molecules have existed. After all, why should God have all
the fun!
How Does It Work?
Simply put, a Gravitational Lens occurs when the gravitational field of a very massive object,
such as a black hole or neutron star, bends the light coming from a much more distant
background source, usually a quasar or a galaxy. The lens effect is produced by the
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This picture of gravitational lens 0024+1654 was taken
October 14, 1994 with the Wide Field Planetary Camera-2
aboard the Hubble Space Telescope.
tremendous gravitational field that bends the light to magnify, brighten and distort the image
of the more distant object. How distorted the image becomes and how many copies are made
depends upon the alignment between the foreground cluster and the more distant galaxy,
which is behind the cluster.
The main goal of the CASTLeS project is to study the properties of all the known cases of
galaxies acting as gravitational lenses. In the example above, you will notice that there are
two images of each part of the building: one inside the circular ring, which has been turned
inside out, and one outside. The lensing effect causes each point in the original image to
appear twice in the distorted image: once in the outer part of the image; and, once again in
the inner part, but now upside down and mirror-reversed.
In the above photograph of Cluster 0024+1654, light from the distant galaxy bends as it
passes through the cluster, dividing the galaxy into five separate images. One image is near
the center of the photograph; the others are at 6:00, 7:00, 8:00, and 2:00. The light also has
distorted the galaxy's image from a normal spiral shape into a more arc-shaped object.
Astronomers are certain the blue-shaped objects are copies of the same galaxy because the
shapes are similar. The cluster is 5 billion light-years away in the constellation Pisces, and
the blue-shaped galaxy is about 2 times farther away.
Imagine looking at something the way it appeared ten billion years ago! That’s what you are
seeing when you look at these faraway galaxies through this “lens.”
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B0218+357 is an example of a typical JVAS (Jodrell/VLA
Astrometric Survey) double-image lens with an Einstein ring.
Several dozen such lenses have been mapped by radio telescope
(above) and photographed using the Hubble.
Though the gravitational light-bending process is not new, Hubble's high-resolution image
reveals structures within the blue-shaped galaxy that astronomers have never seen before.
Some of them are as small as 300 light-years across. The bits of white imbedded in the blue
galaxy represent young stars; the dark core inside the ring is dust, the material used to make
stars. This information, together with the blue color and unusual "lumpy" appearance,
suggests a young, star-making galaxy.
Sometimes I can imagine pointing a radio telescope at one of these “stellar nurseries” and
almost being able to hear the crying of babies being born. It’s overly poetic, I know, but
what can I say? Astronomy fires the imagination.
Finding Planets Using the “Flash” Method
Another extremely interesting application for Gravitational Lensing (and my personal
favorite) is the detection of extra-solar planets using the “Flash Method.” Here is how it
works:
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If a planet’s gravity deflects light in our direction, we see its star brighten as the deflected
light is concentrated toward us. This brightening can last from 15 minutes to a month,
depending upon the mass of the planet and how far it is from the star.
The brightening can be up to about one magnitude -- the star becomes about 3 times brighter.
So far, to date, finding this “flash” is the only technique (other than photometric transits or
pulsar-timing) by which we can detect terrestrial-type planets around Sunlike stars or close
double-star systems.
The brightening can be up to about one magnitude -- the star becomes about 3 times brighter.
So far, to date, finding this “flash” is the only technique (other than photometric transits or
pulsar-timing) by which we can detect terrestrial-type planets around Sunlike stars or close
double-star systems.
Gravitational lens experiments have been successful in determining how many small stars
there are compared to larger ones in our galaxy and other galaxies. However, determining
the same thing for planets is much more difficult because the star system must be at least
5000 light years away.
Another thing -- since the alignment will never repeat itself, all the data has to be gathered at
once by several observatories. A number of projects are currently looking at crowded star
fields to determine the mass distribution of stars in the Milky Way as well as the Large
Magellanic Cloud, Small Magellanic Cloud and the Omega Centauri globular cluster.
The “Einstein Cross” is a case in which a distant quasar
happened to be placed right behind a massive galaxy, and the
resulting gravitational effect causes the quasar to be seen four
times. Not only that, but stars in the foreground galaxy have been
seen to act as gravitational lenses as well. These stars make the
images change brightness relative to each other, and the changes
are visible on these two photographs of the Cross, taken about 3
years apart.
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When any project group spots a stellar brightening, a group known as PLANET steps into
gear and takes precise time measurements of the transits – measurements that are vital to
determining the masses of these extra-solar planets.
The “flash” method uses the planets themselves -- with their stars in the background -- as
telescopes that focus light toward us. It’s ingenious, and it’s cheap – and it’s going to teach
us a lot about what’s out there.
Oh, By the Way
Images from the CASTLeS project can be found at http://cfa-www.harvard.edu/castles/.
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