From Cornell: Women in STEM – “The Space between Stars and Galaxies” Rachel E. Bean

Cornell Bloc

Cornell University

Jackie Swift

Rachel E. Bean

Mystery shrouds the birth of our universe. In a fraction of a second, the universe transformed from a size smaller than a subatomic proton through expanding exponentially faster than the speed of light, according to the Big Bang theory. At the heart of this event lies the explanation for all the constituents of the cosmos today. If the moment of the Big Bang can be understood, we may finally have the theory of everything that can reconcile the quantum Standard Model of particle physics with Einstein’s general theory of relativity, which holds that gravity is a result of the curvature of space and time.

Even the most powerful particle physics experiments on Earth don’t have enough clout to recreate the conditions in the early universe. We can find evidence of them, however, in the cosmic microwave background (CMB), which functions like a fossil remnant of that very early universe, says Rachel E. Bean, Astronomy.

CMB per ESA/Planck
CMB per ESA/Planck

“The CMB was made about 400 thousand years after the start of the universe,” she says. “It’s a pristine glimpse of what the universe was like at that instant, and buried inside that signal is a signature of what happened a trillionth of a second after the Big Bang.”

The Early Universe and the Cosmic Microwave Background

The CMB is a faint glow in the microwave wavelength that can be seen with telescopes that detect microwave radiation in the space between stars and galaxies. It has been traveling toward us for 13 billion years carrying information about those early moments. “At that time the universe was governed by quantum physics at a level that we don’t think we fully understand,” Bean says. “As we go back in time, the universe gets smaller, hotter and denser. At its very earliest instances, it was at temperatures and densities that we can never recreate on earth.”

In an effort to understand physics at those extreme properties, Bean looks for tiny temperature fluctuations in the CMB. These were generated a trillionth of a second after the Big Bang during a process called primordial inflation when the universe is thought to have expanded faster than the speed of light for a brief time. “We have to describe how quantum properties behave with gravity and space and time,” says Bean. “We don’t know how to do that. The only way we can try to figure this out is to look at these very early moments.” Bean hopes to connect these temperature fluctuations in the CMB to one of the potential theories—especially string theory—that are candidates to reconcile quantum mechanics and the general theory of relativity.

The CMB can also tell scientists about the effects of gravity on objects through time.

“The CMB has essentially seen everything that has been created since it was formed,” says Bean. “It traveled through the universe as it evolved, and as it did that it had the signatures of that history imbued upon it.”

Massive Galaxy Clusters, Dark Matter, and Dark Energy

Bean is interested in the information the CMB carries about its travels through the most massive objects in the universe: galaxy clusters. These are about a thousand times larger than our galaxy. As the CMB passes through a cluster, the heat of the cluster and its movement leaves a sort of Doppler shift on the frequency of the light from the CMB. “We can use the CMB as a motion detector for these clusters,” Bean explains. “We can see how fast they were moving when the CMB passed through them. This is useful because those clusters were moving because of the properties of gravity at that time.”

Bean will also be looking for evidence of the effects of dark matter and dark energy—two components of the universe that we cannot see. They do not emit light or absorb it, and none of our instruments can detect them. Scientists think that 95 percent of the matter in the universe is dark matter and dark energy, which change the properties of how gravity behaves. The only way to learn about the properties of these components is to look at their impact on astrophysical bodies such as galaxy clusters, Bean says. “By looking at how fast the galaxy clusters were moving in the past, we can test the properties of gravity and dark matter.”

Big Bold Telescopes, Up Soon

Astronomers will be able to do that in unparalleled detail over the next decade when four new telescopic surveys come online. These large-scale structure surveys will look at millions to billions of galaxies. They will either take multicolor images of them, revealing through color different physical properties, or they will record the galaxies’ spectra, the emission or absorption lines of light of particular wavelengths, which pinpoint the galaxies’ positions in space. “We’re going to be able to survey out billions of light years to be able to understand the structure of our universe with unprecedented precision,” Bean says.

Two of the telescopes will be ground-based and two will be space-based. Bean is the leader of an international collaboration of approximately 500 scientists—the LSST Dark Energy Science Collaboration—that will be using the data from one of the ground-based photometric imaging telescopes, the Large Synoptic Survey Telescope (LSST), which is an international venture led by two United States agencies, the National Science Foundation and the United States Department of Energy.

LSST/Camera, built at SLAC
LSST/Camera, built at SLAC
LSST Interior
LSST telescope, currently under construction at Cerro Pachón Chile
LSST telescope, currently under construction at Cerro Pachón Chile

The LSST will be commissioned in 2019 with the first surveys coming online in 2021. Bean is scrambling to prepare for the challenge of analyzing all the anticipated data. “It’s going to be like a massive gush,” Bean says. “If we’re able to analyze it properly, we will get orders of magnitude improvement in our understanding of the properties of the cosmos.”

The LSST will share the ground-based spotlight with another state-of-the-art telescope, the Dark Energy Spectroscopic Instrument (DESI), a United States Department of Energy initiative, which will come online in 2019.

LBL/DESI spectroscopic instrument on the Mayall 4-meter telescope at Kitt Peak National Observatory starting in 2018
“LBL/DESI spectroscopic instrument on the Mayall 4-meter telescope at Kitt Peak National Observatory starting in 2018

The final two telescopes, both space-based, will be launched in the next decade: the European Space Agency’s Euclid in 2021 and the National Aeronautics and Space Administration’s Wide Field Infrared Survey Telescope (WFIRST) in the mid-twenties.

ESA/Euclid spacecraft
ESA/Euclid spacecraft


Bean plans to take the new information on the nature of galaxies provided by the four surveys and combine it with data on the motions of galaxy clusters gleaned from the CMB. Altogether they should help her and other cosmologists uncover the true properties of dark matter and dark energy, “We’ll be able to test whether general relativity holds on a cosmic scale,” Bean says. “That’s really exciting!”

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