Hubble telescope to tackle the big questions

Baltimore Sun

With time running out for the Hubble Space Telescope, astronomers in Baltimore and around the world are gearing up for the biggest research project ever mounted on the orbiting observatory.

Later this year, astronomers from dozens of institutions will begin gathering images of more than 250,000 of the most distant galaxies in the universe. They will seek answers to some of astronomy's biggest questions - queries that go to the origins of the universe itself.

There is a sense of urgency to the effort. Nearly 20 years after it was launched into orbit, the Hubble telescope is working better than ever, but the most recent repair mission was its last. A breakdown could hobble or end the scientific work at any time.

"What's really crucial now is to look forward to the fact that we won't have Hubble in a few years," said Sandra Faber, chair of astronomy at the University of California Santa Cruz and a principle investigator on the new project.

"We don't want [future] generations to say, 'Those guys were so stupid. ... They never really tackled the big questions.' We don't want to look stupid," she said during a recent visit to the Space Telescope Science Institute in Baltimore, which manages Hubble science.

The scientists, more than 90 of them, have been awarded an unprecedented amount of Hubble use for the work. It works out to more than 900 orbits. That's 3 1/2 months, or roughly 10 percent of Hubble's total observing time over three years.

The data they gather, combined with pictures and information gathered by other observatories, is expected to be mined long after the Hubble is retired, as researchers work to piece together the early evolution of the first stars and galaxies, and of the expanding universe itself.

The mammoth undertaking is part of the Hubble Multi-Cycle Treasury Program. It's being launched now because the space telescope, in many respects, is in the prime of its life.

Last May, astronauts replaced its failing gyroscopes and batteries, and installed the Hubble's most powerful workhorse camera ever, the Wide Field Camera 3 (WFC3, pronounced Whiff-see 3). They also repaired the Advanced Camera for Surveys (ACS), which is ideally suited for cataloging objects at vast distances, such as a peculiar kind of exploding star known as a Type Ia supernova.

But while the telescope is working better than ever, last May's house call was NASA's final servicing mission to the Hubble. The shuttle fleet is being retired.

Some of the scientific questions that remain are "ridiculously important, big ones," Faber said. "That's why they haven't been done up to now."

The committee that reviews and approves requests for observing time on the Hubble asked astronomers for proposals that could "accomplish multiple scientific goals that would be a lasting legacy of the telescope, that would have an enduring use," said Henry Ferguson of the Space Telescope Science Institute.

Scientists responded with 39 proposals, seeking 15 times more telescope time than was available. The committee selected three, one of which merged Faber's ideas with a similar proposal submitted by Ferguson.

Their combined objectives are ambitious, Ferguson said. They include "trying to get a handle on how dark energy is behaving, how the acceleration of the universe happened [and] studying the earliest galaxies."

Faber said project scientists will seek their answers amid galaxies in five regions of the sky, areas that astronomers have named Hubble Deep Field, Ultra Deep Field, GOODS, COSMOS and Extended Groth Strip.

The target areas were chosen because they are outside the plane of our own spiral Milky Way galaxy, and therefore relatively empty of nearby stars. Each is a kind of an open window to the farthest reaches of the universe.

These are the fields "in which extra-galactic astronomy can really concentrate and pound away," Faber said. "We have established the touchstone fields that will carry us possibly for centuries, certainly decades."

Ferguson calls them "deep watering holes," where the Hubble, Spitzer, Chandra and Herschel space telescopes have gathered data in a variety of wavelengths of light - information that will complement and support Hubble's new observations.

As important as they have become, these star fields constitute just a tiny fragment of the sky, barely an eighth of the area covered by a full moon. But they contain hundreds of thousands of far-off galaxies, a critical factor for astronomers.

"You need hundreds of thousands, if not millions of objects - galaxies and stars - to be able to do the statistical tests that will say whether we're really understanding how galaxies evolved, how the universe is expanding," Ferguson said.

That's because galaxies change far too slowly to allow astronomers to follow a single example as they might a single child, taking pictures as it's born and grows. They can only see snapshots of hundreds of thousands of galaxies, showing what each one looked like at just one moment in its evolution.

Puzzling out how individual galaxies formed and changed over time then becomes a math problem. Researchers must take a sort of census of the characteristics of thousands of actual galaxies as they were 9 billion to 13 billion years ago, and compare it with theoretical models of how different kinds of galaxies might have developed over time.

Some models will be shown to be wrong, Ferguson said, but "usually it's the case that none of the models work, so you're guiding the models by what you find." With luck, astronomers might get answers to such nagging questions as: Were the first stars that formed gathered together by the gravity of black holes? Or, did black holes form later from the in-falling matter at the center of the first galaxies? Or could both be true? How did star formation rates in galaxies change over time?

Fifteen orbits are being dedicated to resolving another, nagging question about Type Ia supernovas. Astronomers have assumed that these peculiar exploding stars are all alike and equally bright when seen up close.

They then estimate their distance by measuring how much dimmer they appear than nearby examples. It's like estimating distances by the brightness of 100-watt bulbs placed 100 yards, a mile and five miles away.

The supposed uniformity of Type Ia supernovas has made them the best available yardstick for measuring the acceleration in the expansion of the universe that seems to have begun about 6 billion years ago.

"The big worry about [Type Ia] supernovas is that they might not be perfect standard candles," Ferguson said. The physics suggests that these stars should be changing over time, so perhaps they don't all explode with the same brilliance.

"If WFC3 is able to push out to a high enough distance ... we can test whether these are good standard candles," Ferguson said. "We've been trusting that they are."

"Now it's 'trust but verify,' " Faber added.

Getting a better handle on Type Ia supernovas will also help astronomers figure out how dark energy works.

Dark energy is a concept astronomers invented to explain the acceleration in the expansion of the universe. It was first reported in 1998 in a paper by Adam Riess, a Johns Hopkins University astrophysicist. But no one really knows what it is or how it throttled up the expansion of the universe.

In the end, Ferguson and Faber expect that their project will establish a data trove that will yield more answers - providing high-resolution images from the farthest reaches of time and space that will nourish research for decades to come.

Already, Faber said, the Hubble's vast archive of images is "very rich; people use them for new projects all the time." In fact, she said, "People are now proposing more projects with the archive than they are with new observations."

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