Star witness

(Photography by John Zich)

Star witness

Wendy Freedman calculated when the universe began. Now she wants to see it happen.

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Spring/15

Wendy Freedman grew up in Northern Ontario and has early memories of dark skies filled with stars. “It never occurred to me when I was young, though, that I would end up a professional astronomer,” says Freedman. “That happened in university.” Now an acclaimed observational cosmologist, her career was built on peering into the dark skies with ever-advancing technology.

Freedman joined the Department of Astronomy and Astrophysics as a University Professor this past September, following 30 years at the Carnegie Observatories in Pasadena, California—starting as a postdoctoral fellow in 1984, becoming the first woman on the observatories’ permanent scientific staff in 1987, and becoming the Crawford H. Greenewalt Director in 2003.

She also has chaired the board of directors of the Giant Magellan Telescope (GMT) Organization since its 2003 inception. The GMT, expected to reach completion in 2021 at the Las Campanas Observatory in Chile, “is on schedule to be the first of the next generation of big telescopes on the air.” One of the most powerful telescopes ever built (see “When Stars Align”), the GMT will have seven mirrors, forming a segmented but incredibly accurate surface 80 feet across.

Astronomers will use the GMT to collect light from the earliest objects in the universe. “There’s a spectrograph on this telescope that will allow us to take hundreds or maybe in some cases thousands of spectra, where you disperse the light of the faintest and the most distant galaxies,” says Freedman. Looking farther out also means looking further back in time, and astronomers will get to watch galaxies forming. “We’ll actually be able to see that directly rather than just surmise.”

Freedman’s own research relies on the ability to look as far out and back as possible. She co-led the Hubble Space Telescope Key Project, using the telescope launched in 1990 to measure distances to other galaxies for the first time. “We set out to measure the current expansion rate of the universe—the Hubble constant,” says Freedman, “one of the most important parameters in cosmology that sets the age and size scale of the entire observable universe.”

The project began in the mid-’80s and concluded in 2001, when the team determined the universe to be 13.7 billion years old, with a 10 percent uncertainty. Now she’s leading the Chicago Carnegie Hubble Project, using the Spitzer Space Telescope, the Hubble Space Telescope, and the Chile-based Magellan telescopes to reduce that uncertainty to just a few percentage points.

To determine expansion rate, explains Freedman, “you need both a distance and a velocity.” Edwin Hubble, SB 1910, PhD 1917, discovered in 1929 that there was a relationship between the two. “It’s the slope of that correlation that we measure,” Freedman says.

Velocity can be determined mathematically by measuring cosmological redshift—when an astronomical object’s spectrum, like the light from a star, shifts into longer, redder wavelengths as it moves farther away, carried by expanding space. It’s similar to the Doppler effect, when an object’s motion changes its observed wavelength.

Distance can be measured by several methods, and with increasing accuracy as telescopes become more powerful and incorporate new detectors. The anchor of the distance scale, stellar parallax, uses an observational effect and simple high school geometry to measure distances to stars within our galaxy (see right). But Freedman’s work requires the ability to measure much greater distances.

When observing stars far outside the Milky Way, astronomers must consider the difference between brightness (how much light we detect on Earth) and luminosity (how much light an object emits from its surface). Are they seeing a nearby dim star or a far-off bright one?

The Hubble Key Project measured Cepheids, stars with pulsating atmospheres that follow a period-luminosity relation, varying in brightness at regular intervals directly related to how much light they emit. More luminous Cepheids have longer intervals, or periods. Astronomers compare the luminosities of Cepheids to their periods to determine distance using another principle—the inverse square law for light.

When Cepheids become too faint because they’re too far away, “we use supernovae,” says Freedman—“really bright explosions of stars at the end of their lifetime.” Type Ia supernovae are exploding white dwarf stars, which all reach about the same luminosity at the peak of their explosion and follow a dimming curve. Similar to Cepheids, distance is measured by comparing luminosity to how fast the supernovae dim with time.

Freedman’s current projects measure both Cepheids and supernovae. The Chicago Carnegie Hubble Project makes new observations of Cepheids to continue refining the universe’s current expansion rate, she says, “but we will tie into the nearby sample of supernovae, which we’re observing with the Carnegie Supernova Project.”

The supernova project, which Freedman cofounded in 2004, uses the du Pont, Swope, and Magellan telescopes at Las Campanas Observatory in Chile to measure objects farther out in the universe, and therefore calculate historical expansion rates. By comparing past rates to the current local expansion rate, Freedman can study the universe’s acceleration—which in turn contributes to the study of dark energy, the hypothetical explanation for cosmic acceleration.

When astronomers discovered in the late 1990s that the universe was accelerating, most cosmologists had expected the opposite—that the universe was decelerating. Although evidence for acceleration was compelling, “there was still a question of whether something in the universe was making the supernovae appear dimmer,” says Freedman, such as dust particles in the regions between stars, which can absorb radiation and cause errors in expansion calculations.

The success and credibility of future experiments on acceleration and dark energy rely on the most accurate distance measurements possible. The Carnegie Supernova Project uses infrared spectroscopy to obtain such accuracy—dust doesn’t affect infrared light as much as visible radiation, Freedman says. Her team uses spectroscopy to study supernovae chemical composition as well, which also could affect the visible part of the spectrum.

Although Freedman’s research focuses mostly on the expansion and acceleration of the universe, she is also interested in the possibility of discovering new physics. “Since Galileo turned a telescope to the sky in 1609, every time there’s been a jump in capabilities or that next generation of telescopes, we’ve made discoveries, without exception,” says Freedman. “One of the most interesting and exciting things is what we just don’t know.” The GMT is poised to answer questions astronomers never thought to ask.