Johns Hopkins Magazine - February 1995 Issue

Science & Technology

Blink on, blink off, etc.

Space telescope helps pin down universe's vital stats

An international team of astronomers using the Hubble Space Telescope (HST) has marched a huge step toward one of the telescope's main goals--determining the age of the universe- -and a Hopkins graduate student has played a starring role.

The huge step is a newly precise estimate of the Hubble constant, the rate at which the universe has been expanding since the Big Bang. The Hubble constant is one of the most critical numbers in cosmology because knowing how fast the universe is expanding allows astronomers to determine how long it has been expanding: the age of the universe.

The Hubble constant is the ratio of two numbers: how fast a given object is moving away from Earth and the distance between the object and Earth. The more distant that object, the more reliable the result. Astronomers can easily determine the first number. The stumbling block has been the second. But the HST can take those measurements because it gives a clearer view of distant objects, especially since being outfitted with a new Wide-Field Planetary Camera (WFPC-2).

The distant objects used for this study are pulsating stars called Cepheid variable stars, which blink on and off in periods ranging from one to 70 days. They are also known as "cosmic yardsticks" because their brightness and rate of pulsation can be used to measure their distance from Earth.

The Hopkins research team searched for Cepheids in a series of images of remote galaxy M100 taken with WFPC-2. In 12 sessions over a two-month period, the camera had photographed a field in M100 containing 100,000 identifiable stars. If there were any Cepheids in the field, in some images they would show up as bright spots (when pulsing on), while in others they would disappear (when pulsing off).

It was Hopkins graduate student Laura Ferrarese, working closely with professor of physics and astronomy Holland Ford, who had the painstaking job of picking out Cepheid stars from among the thousands of stars in each image. Using a computer program that highlights variations in brightness over time, she narrowed the list down to 62 Cepheids. The team then used the 20 Cepheids with the best data to estimate a value for the distance from Earth to M100, and thus to calculate the Hubble constant.

Before Space Telescope made it possible to peer at the universe from above the Earth's atmosphere, only a handful of Cepheids in distant galaxies had ever been spotted. "If you said to someone three to four years ago, 'How nice it would be to find 62 Cepheids in M100,' they'd say, 'It's crazy,'" says Ferrarese.

"It was extraordinarily exasperating to observe Cepheids from the ground," adds Ford. Astronomers rarely got enough observing time at ground-based telescopes to see a star during both its "blink-on" and "blink-off." Turbulence and other effects in the atmosphere also change the apparent size and location of a star. "Space Telescope is always in focus," says Ford. "It gives just superb images every time in picture after picture. It's a Cepheid machine par excellence."

For more than 60 years, astronomers have struggled to pin down a value for the Hubble constant, which until recently, they put between 50 and 100 kilometers per second per megaparsec (a megaparsec is 3.26 million light-years). The team's conclusions, which they report in the October 27 Nature, indicate the universe is expanding at a rate of 80 km/s/Mpc, give or take 17 km/s/Mpc. That rate, in turn, means the universe is 12 to 18 billion years old.

But a spooky quandary emerges, because a lot of astronomical evidence indicates our galaxy's oldest stars are about 16 billion years old. If the universe is closer to the lower end of its estimated age range, say 13 billion years, it is 3 billion years younger than its oldest stars. How can that be?

Several theories could account for the discrepancy. First, in addition to the universe's expansion rate, its density also factors into calculating its age. Astronomers are still debating whether we live in a "light" or relatively dense universe, and the various estimates allow enough wiggle room in the calculations to make the universe older than the stars.

Still another possibility, says Ferrarese, is that "our understanding of cosmology is not complete yet. There's some debate as to whether the Hubble constant is really a constant. It could be that there are 'bubbles' in the universe so that you experience a different rate of expansion depending on where you are. In the next few years, I think our understanding will change quite a bit."

She and her colleagues may one day provide the data to rewrite the theories. They have been awarded 300 hours of observation time to measure distances to Cepheid stars in another 20 galaxies.

Ford will also be spending more time with another HST project. In late December, he received a call from NASA saying his team's new camera design for the telescope had won over stiff competition. The camera, for now called the Hubble Advanced Camera for Exploration (HACE), will be eight times more sensitive in certain wavelengths, allowing astronomers to survey regions of the sky 15 times faster than they currently can, says Ford. He will lead a 15-member Hopkins team developing the camera. Scheduled for installation aboard Space Telescope in 1999, the $30-million camera will be built in Boulder, Colorado, in conjunction with Ball Aerospace Corporation. --MH

How babies learn -- and the rest of us, too

Odd but true: When you hear something in your left ear only, the stimulus goes to both hemispheres of the brain. "But it gets to the right side faster, and gets processed more thoroughly on that side," says Marie Balaban. Conversely, the right ear feeds first to the left hemisphere. The assistant professor of psychology has used these facts to investigate mental development in 8- to 9-month-old infants.

Babies can't tell Balaban what they hear, of course. She figures that out by equipping the baby with headphones, so she can pipe sound into one ear only. Then she uses a technique devised by researchers in the '80s: Working with one baby at a time, you familiarize the tots with a simple six-tone melody (call it X). But sometimes you play them a different six-tone melody (Y), then trigger some mechanized toy like a hip-hopping bunny. Soon, upon hearing the second melody, most babies expect the bunny to hop, and will turn their heads to see it. The head turn shows that they recognize the second tune as different from bunnyless X. Though infants are easy to distract, some are correct 90 percent of the time.

Even when the melodic shift is subtle, infants can tell the difference, Balaban found. In some of her experiments, for example, she played two somewhat similar tunes:

Tune Y preserved the original melody's contour, its overall shape of ups and downs. Nevertheless, when listening to Y with their right ear, babies looked for the bunny correctly more than half the time, on average. Their left hemispheres already had good acuity for detail. They did less well when hearing Y with the left ear.

But when what was altered was the melody's basic shape, as in Tune Z,the left ear did significantly better: Babies correctly looked for the bunny 52 percent of the time, as opposed to 33 percent in their right-ear tests. Even in children this young, the right hemisphere appears to be specialized for processing global, holistic attributes of melody, and probably much else.

"We think the specialized hemispheres have to do with perceptual processing of the world," Balaban explains, "including the learning of speech." She believes that perception of phonemes, such as hearing that "Puh" is not the same as "Buh," may be handled by that detail meister the left hemisphere, whereas large-scale contour shifting, such as changes in intonation, may be handled by the right hemisphere.

The psychologist cautions against oversimplifying. After all, both hemispheres can perform both functions. Yet the specialization, though subtle, is real. "The question is not do these differences exist," she says, "but why?"

"I don't think we know," is her answer. "You could argue that the right hemisphere is somehow preferentially involved in attention. You're supposed to notice the approach of large objects, or a change in background noise. But we also need fine-grained analysis, to perceive speech detail," and for that we need a different mechanism.

In Balaban's next study, she will investigate the way infants respond to different facial expressions. Faces also have a global aspect, she points out. It is not so much features as their relationship that makes a face or expression distinctive. --EH

HUT II set for take-off

Few of us get to repeat a moment of past glory. In March, if all goes as planned, Hopkins astronomer Sam Durrance will get that chance, when he and the Hopkins Ultraviolet Telescope (HUT) take their second voyage into space aboard the space shuttle.

Designed and built by Hopkins astrophysicists and engineers, HUT is the only telescope that provides a sharp view of the far ultraviolet portion of the electromagnetic spectrum, particularly in the largely unexplored region between 900 and 1,200-angstroms. From spectra collected during a nine-day flight aboard the space shuttle Columbia back in December 1990, astronomers reaped a tome of published research papers, on subjects ranging from white dwarfs to the universe's missing mass.

Since then, NASA smoothed out glitches in the technology for operating HUT and the two other telescopes that will accompany it. Working closely with Durrance, NASA devoted many hours to troubleshooting a quirky telescope pointing system, which the Hopkins astronaut had to operate by hand on his first voyage. HUT itself has received major improvements. The telescope's 36-inch mirror and spectrograph grating were coated with silicon carbide, a new, more reflective substance. (The mirror focuses ultraviolet light from celestial objects into the spectrograph grating, which breaks up the light into a spectrum, a sort of fingerprint of a star or planet.) "HUT II is a factor of at least three times more sensitive than HUT I was," says Bill Blair, associate research professor in physics and astronomy. It will take clearer spectra and see fainter objects.

Though launch dates are always tentative until they happen, as of press time the space shuttle Endeavour carrying HUT II was scheduled to be launched March 2. Until then, HUT astronomers are losing sleep from nervousness. Any one of a million little things could hinder, maybe even scrap, the mission. Nevertheless, the astronomers are hoping for another bumper crop of scientific finds. --MH

Moon rather like a very young Earth, Clementine reveals

Clementine, a four-foot spacecraft developed as part of the federal "Star Wars" program, circled the moon for two months last winter, making the most comprehensive survey ever of the lunar surface. The Apollo missions had learned much about several small areas on the "front" of the moon, near the equator. Clementine, however, tracked gravity and bounced lasers off both front and back, North Pole to South.

Thinking we understood the moon from Apollo's data, says geophysicist Maria Zuber, is like "understanding" the Earth from a visit to Beverly Hills. An associate professor of Earth and Planetary Sciences at Hopkins, Zuber led a team from Hopkins and the Goddard Space Flight Center that recently published a first analysis of the Clementine data in the December 16 Science.

Until now, most scientists believed the lunar terrain to be gently rolling. "People thought the moon was like a billiard ball," says Zuber, because it was still warm when it was bombarded with large debris some 4.5 billion years ago. "And warm rock acts like molasses." So you'd think any craters and peaks made by those early collisions would have oozed into gentle undulations. Not so, according to the Clementine data.

What the mission revealed:

This basin is adjacent to the planet's "highest high," says Zuber, a mass of material pushed up perhaps five miles by the sideways impact that created the basin. Conceivably, the impact could even have spun off a planetary satellite, rather as the moon itself was formed. "And it's tempting us to say that impacts played a really profound role in shaping the moon."

One beauty of Clementine's results, for many earthlings, will be the light they may shed on the blue planet. Scientists believe the moon was formed from a collision between Earth and another planet-sized body some 4.5 billion years ago. Today, without the shifting plates, volcanic action, wind, water, or chemical weathering that occur on Earth, the moon is shaped much as it was when its formative bombardment stopped 3.8 billion years ago--that is, it is rather like the very young Earth, whose rock it shares.

"Now we can go back to Earth and do a better job of figuring out its history," says Zuber. "The Earth has lost its early record. The moon has not."

As for the $75 million Clementine, after malfunctioning en route to an asteroid, she is locked in Earth orbit, providing data on radiation. --EH

Written by Elise Hancock and Melissa Hendricks

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