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Feb. 22, 1999: In recent years astronomers have come
to realize that the Universe is somewhere between 12 and 18 billion
years old. They arrive at this estimate by measuring how fast
the Universe is expanding due to the Big Bang and whether the
expansion is accelerating or decelerating. By tracing the cosmos
back in time to an era when the entire Universe was contained
in a single point, they can estimate the time elapsed since the
Big Bang.  Right: This image combines a view of the Caltech
radio millimeter interferometer and an artist's concept of the
Chandra X-ray Observatory with a radio/X-ray map of the "Sunyaev-Zeldovich
Effect." Unfortunately for cosmologists, who would like to know exactly when the Big Bang happened, it's difficult to measure precisely how fast the Universe is expanding and how the rate of expansion has changed since the Big Bang. Traditional methods lead to rather large uncertainties in the final answer. Now two astronomers, Dr. Marshall Joy (NASA/MSFC) and Dr. John Carlstrom (University of Chicago), may have a new way to tackle the problem. For the past 7 years Joy, Carlstrom, and their colleagues have used radio interferometers to probe tiny fluctuations in the Cosmic Microwave Background Radiation (or "CMBR"). By combining their radio-wavelength images with data from NASA's Chandra X-ray Observatory they hope to open a new window on the history of the Universe. "We could be on the brink of answering some important cosmological questions," says Joy, "but we need more data that only Chandra can provide." |
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 The Universe is filled with
conglomerations of galaxies called clusters that are millions
of light years across, consisting of hundreds or thousands of
galaxies held together by gravity. Mostly clusters have atmospheres
of very hot gas that we can see because of the X-rays they emit.
Sunyaev and Zeldovich realized that something interesting happens
when a CMBR photon passes through such a cluster. There is a
good chance that it will collide with one of the electrons in
the hot atmosphere. In the process, some photons would gain energy
while others would lose energy. At microwave radio frequencies,
they predicted, the intensity of the CMBR would appear to be
depleted in the direction of the cluster because the photons
would be "scattered" to other frequencies outside the
microwave frequency band. This process is called the Sunyaev-Zeldovich
Effect.  Left: The Coma Cluster of Galaxies
is one of the densest clusters known - it contains thousands
of galaxies. Almost every object in the above photograph is a
galaxy. The hot atmosphere of the Coma Cluster is visible in
images
made at X-ray wavelengths. The effect they predicted was very small, but also very important. Observations of Sunyaev-Zeldovich fluctuations in the CMBR could potentially determine the age of the Universe and also reveal answers to important questions about the cosmos, like "How big is it?", "How fast is it expanding?", and "Will it collapse on itself in the Big Crunch?" To measure the age of the Universe by means of the Sunyaev-Zeldovich effect, it is necessary to have high-quality X-ray observations of a cluster's atmosphere and corresponding radio images of the CMBR in the direction of the cluster. One without the other is just an interesting observation, but together they address some of the most important questions in science. |
| In 1992 Carlstrom, a radio astronomer, and Joy, an X-ray astronomer, decided the time was right to join forces and tackle the problem. According to Dr. Joy, "One of the key science drivers for the Chandra X-ray Observatory is the Sunyaev-Zeldovich effect. Chandra has the collecting area and the sensitivity to high energy X-rays to make high quality images of the hot cluster atmospheres. If we could get radio maps of the same clusters and measure the size of the Sunyaev-Zeldovich effect, we could begin to do some interesting cosmology." |
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Left: The Berkeley-Illinois-Maryland
Interferometer at Hat Creek, Calif. (Links to 720x481-pixel,
186K JPG.) The Owens
Valley Millimeter Interferometer, located near Bishop, CA,
is pictured in the graphic near the top of this story. Another
image of the Owens Valley array may be viewed here.
Carlstrom and Joy have since been granted months of observing time at two radio observatories: the Caltech Millimeter Array at the Owens Valley Radio Observatory (OVRO), and the Berkeley-Illinois-Maryland Array (BIMA) at Hat Creek, CA. Both radio telescopes are "synthesis interferometers," which means that they are able to make images at radio wavelengths much like familiar optical photographs. The BIMA and OVRO arrays were designed to work at radio wavelengths around 3 millimeters, but Carlstrom and Joy expected the Sunyaev-Zeldovich effect to be greatest at a longer wavelength, around 1cm. "We had to build our own receivers for 1 cm" explained Dr. Joy. "Because the S-Z effect is so small everything has to work well, the receivers need to be very stable and low noise, and we are constantly making improvements to increase our sensitivity. The millimeter arrays are really ideal for this work because at 1 cm their fields of view are large enough to include the entire cluster, and the surface accuracy of the antennas is very high." |
| So far Carlstrom, Joy and their collaborators
have detected Sunyaev-Zeldovich fluctuations around 27 clusters
of galaxies. Typically, the deficit in the CMBR is only 0.05%
of the cosmic microwave background intensity. Detecting these
small perturbations requires lots of observing time and painstaking
data reduction. A sample of their data is pictured right, where
an X-ray image and radio map of the galaxy cluster A2218 are
superimposed. Right: The image shows a map of the sky around the galaxy cluster Abell 2218. The contours are 28.5 GHz radio data obtained by Carlstrom et al using the BIMA interferometer. They represent levels of constant negative intensity where there is a deficit of CMBR photons caused by the Sunyaev-Zeldovich effect. The colored areas are X-ray emission from hot gas imaged by the ROSAT PSPC instrument. The radio hole is deepest where the emission from the hot gas is most intense. |
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Life, the Universe, and EverythingRadio images of Sunyaev-Zeldovich holes are one piece of a great cosmological puzzle. To complete the picture Joy and Carlstrom need X-ray data from the Chandra X-ray Observatory. "X-ray emission depends on the cluster gas density, and so does the SZED effect, but the dependence is not the same," explains Dr. Joy. "Doubling the density of the cluster gas will simply double the size of the Sunyaev-Zeldovich effect, but it will roughly quadruple the X-ray emission." The differing sensitivity of x-rays and radio waves to gas density means that scientists can measure the size and mass of a cluster by combining x-ray and radio images. |
By measuring the size of a galaxy cluster, astronomers
can directly estimate the age of the Universe. Suppose that Sunyaev-Zeldovich
measurements show the cluster to be 5 million light years in
diameter, and the cluster subtends an angle in the sky of 0.03
degrees. The cluster must then be 10 billion light years away
to appear so small. Once the distance to the cluster is known,
the next step is to measure the recession velocity of
galaxies inside the cluster. Galaxies throughout the cosmos appear
to be moving away from us because the whole universe is expanding
-- that's the basic idea behind Big Bang cosmology. The doppler
shift of spectral lines in starlight reveals how fast a galaxy
is receding. Once the recession velocity and the distance of
a galaxy are known it is straightforward to calculate the Hubble
Constant (see the sidebar). The reciprocal of the Hubble Constant
(i.e., "1" divided by "H") is the approximate
age of the universe.  Above: Hubble's original data showing that galaxies recede faster the further away they are. The slope of the line in the plot is called the Hubble Constant. Click for a larger image. Joy and Carlstrom plan to combine X-ray data from Chandra with their existing radio images to estimate the absolute distances to a number of galaxy clusters. By combining their distance measurements with optical redshift data they can calculate the Hubble constant for each cluster. "There are various systematic errors in Sunyaev-Zeldovich estimates of Hubble's constant. But if we're careful, we can arrive at a new value for the age of the Universe that's truly independent of values obtained by other means," said Marshall Joy. |
In 1929 the astronomer
Edwin Hubble announced his discovery that galaxies everywhere
appeared to be moving away from us. The further the galaxy was
from Earth, the faster it was receding. Hubble's discovery was
the beginning of Big Bang cosmology, which postulates that the
entire universe is expanding due to an explosion around 12 billion
years ago. No matter where you are in the cosmos, the rest of
the universe appears to be receding, faster and faster as you
peer further and further away. |
Weighty Questions"We can also use Sunyaev-Zeldovich measurements to draw
some conclusions about the total mass in the Universe,"
said John Carlstrom. |
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