The Cosmic Web: Mysterious Architecture of the Universe

Author: J. Richard Gott
Publisher: Princeton University Press, 2016 (available February)
B_2_Gott_Cosmic

Professor of Astrophysics J. Richard Gott was among the first cosmologists to propose that the structure of our universe is like a sponge made up of clusters of galaxies intricately connected by filaments of galaxies — a magnificent structure now called the “cosmic web” and mapped extensively by teams of astronomers. Here is his gripping insider’s account of how a generation of undaunted theorists and observers solved the mystery of the architecture of our cosmos.

Drawing on Gott’s own experiences working at the frontiers of science with many of today’s leading cosmologists, The Cosmic Web shows how ambitious telescope surveys such as the Sloan Digital Sky Survey are transforming our understanding of the cosmos, and how the cosmic web holds vital clues to the origins of the universe and the next trillion years that lie ahead.

All text and images courtesy of the publisher.

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Cosmic background: 51 years ago, an accidental discovery sparked a big bang in astrophysics

SPIDER

The balloon-borne spacecraft, SPIDER, prior to launch. PHOTO BY ZIGMUND KERMISH

ON NEW YEAR’S DAY 2015, A BALLOON-BORNE SPACECRAFT ascended above Antarctica and snapped crisp photos of space, unobscured by the humidity of Earth’s atmosphere. Meanwhile, a telescope located 4,000 miles to the north, in the desolate Chilean desert, scanned roughly half of the visible sky.

By air and by land, physicists have staked out the best vantage points on the globe, not for stargazing, but for peering between the stars at the thermal traces of the Big Bang.

Spread nearly evenly across the universe is a sea of invisible radiation called the Cosmic Microwave Background (CMB) that keeps space a chilly 2.7 degrees above absolute zero. The now-cold CMB, however, is a remnant of a much hotter, more violent cosmic epoch. About 13.8 billion years ago, immediately after the Big Bang, the universe was filled with a hot gas of ionized particles and radiation. As space expanded, the waves of radiation were stretched and diluted into their current low-energy state. The boiling plasma has since cooled and clumped into galaxies, stars, planets and human beings, all drifting through the faint afterimage of the first flash.

The prediction, discovery and study of the CMB 50 years ago comprise a story that is deeply intertwined with several generations of faculty at the Princeton physics department. The story continues today as University researchers probe the microwave background with the goal of understanding the past and future of our cosmos.

The discovery of the background radiation was a serendipitous one. In 1964, Bell Laboratories technicians Robert Wilson and Arno Penzias racked their brains for an explanation of the noisy signal recorded by their radio antenna. When it turned out that the “noise” was actually radiation from the CMB, the two engineers found themselves unexpectedly pulled into the growing field of modern cosmology. The detection of the CMB earned them the Nobel Prize.

Yet the discovery wouldn’t have been possible without the work of physicists at Princeton, 40 miles down the road from Bell Labs. Back then, the branch of science known as cosmology was ignored by most serious researchers. The physics community viewed the origin and development of the universe as dead-end topics, yet a few at Princeton had dared to tackle it.

At the time, P. James Peebles was a physics postdoctoral researcher at Princeton. “When I started working in this field, everyone was saying, ‘There’s no evidence. Why are you studying this?’” said Peebles, who today is the Albert Einstein Professor of Science, Emeritus. Instead, the mainstream focus was particle physics, which studies the subatomic particles that make up the universe.

Guyot Hall

David Wilkinson and Peter Roll used this experimental setup on the roof of Guyot Hall, which housed the Department of Geology (now Geosciences), to search for the CMB, at Bob Dicke’s suggestion. Wilkinson is holding a screwdriver, and Roll is almost obscured by the instrument. Photo by Robert Matthews circa 1964-65

Two Princeton professors, John Wheeler and Peebles’ mentor, Robert Dicke, decided that research on the cosmic scale should not be neglected. Since 1915, when Einstein developed the theory of general relativity to explain the behavior of large objects in space, hardly any further research had been done on gravity or the structure of the universe. This was due in part to respect for Einstein’s picture of the cosmos, and in part to the difficulty of devising fruitful experiments. “In the mid-1950s, Bob started a serious program of laboratory and extraterrestrial tests for general relativity,” said Peebles. “John started a school for the theoretical study of the subject. These changes marked a renaissance.”

While everyone else was thinking small, Dicke and Wheeler were thinking big. More specifically, Dicke was thinking about the Big Bang, a concept that dated back to the 1920s, when it was first observed that the universe is expanding. Yet extrapolating the current expansion back in time to a tiny, hot, dense state from which it all began was not a widely accepted leap. Peebles said: “Until the ’60s, the evidence that this is what happened was minimal. It was still just an idea, popular in some circles, detested in others.”

Dicke took the Big Bang theory from guesswork firmly into the realm of empirical physics when he proposed the CMB as evidence for a hot, dense beginning. Peebles recalls how Dicke almost casually set the course for his career and that of his peers: “He persuaded Dave Wilkinson and Peter Roll [Princeton physics faculty members] to build a device called a Dicke radiometer to look for this radiation, and he told me with a wave of his hand, ‘Why don’t you go think about the theory.’ And I’ve been doing it ever since.”

By 1970, the scientific community had accepted that the CMB had the properties that made it undeniable evidence for the Big Bang. Physicists then shifted their attention to more detailed scrutiny of the remnant radiation as a way of deepening our understanding of the birth of the universe, its expansion and its fate.

One area of scrutiny is whether the universe went through a period of rapid expansion, or inflation, after the Big Bang. To look for signs of inflation and to map the CMB in our region of space, NASA in partnership with Princeton and other universities launched the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, named postmortem in honor of Wilkinson’s contribution to experimental cosmology.

The inflationary model predicts a particular pattern to the fluctuations of the CMB. When WMAP released its first set of results in 2003, they neatly matched the predictions of inflation. Among the many Princeton researchers who played significant roles in WMAP were Lyman Page, the James S. McDonnell Distinguished University Professor in Physics; Norman Jarosik, senior research physicist; and David Spergel, the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation. Thanks to WMAP, Spergel said, “We have a coherent cosmological model that fits all the data.”

Since then, several other projects, including most recently the European Space Agency’s Planck space telescope, have mapped the CMB and provided evidence for inflation. But scientists are looking for additional evidence in the form of long undulations — called gravitational waves — stretching across the fabric of space. The remnants of these waves could be detected as a faint pattern in the CMB known as B-mode polarization. A reported detection of gravitational waves earlier this year from another project, BICEP2, created a stir in the astrophysics community but turned out to be an artifact of interstellar dust.

Detecting the remnants of gravitational waves is one of the goals of the Atacama Cosmology Telescope (ACT), an international project funded by the National Science Foundation and led by Princeton’s Suzanne Staggs, the Henry DeWolf Smyth Professor of Physics. The team includes Lyman Page, the James S. McDonnell Distinguished University Professor of Physics; Spergel, the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation; and many colleagues at collaborating institutions.

The data collected during the flight of the balloon-borne SPIDER mission in Antarctica — funded by NASA, the National Science Foundation, the David and Lucile Packard Foundation, and the Natural Sciences and Engineering Research Council of Canada; and led by Assistant Professor of Physics William Jones — could also reveal evidence of these waves in the CMB.

The search for gravitational waves is just one of the ways in which the CMB provides opportunities for studying the early universe. The ACT collaboration is also looking for evidence of dark energy, a mysterious force that is speeding up the expansion of the universe, and answers to even bigger questions about the cosmological model. It is clear that the CMB is an important tool for the foreseeable future of cosmology. However faint, it illuminates the distant past, which in turn illuminates the future.

–By Takim Williams

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Telescopes take the universe’s temperature

Two telescope projects are measuring cosmic microwave background radiation with the goal of understanding more about the universe’s early history. The telescopes (pictured) are located on a peak in the Atacama Desert in Chile. (Image courtesy of ACT Collaboration)

Two telescope projects are measuring cosmic microwave background radiation with the goal of understanding more about the universe’s early history. The telescopes (pictured) are located on a peak in the Atacama Desert in Chile. (Image courtesy of ACT Collaboration)

Two telescopes on a Chilean mountaintop are poised to tell us much about the universe in its infancy. They are surveying the faint temperature fluctuations left over from the explosive birth of the universe, with the goal of piecing together its early history and understanding how clusters of galaxies evolved.

The telescopes are measuring these temperature fluctuations, known as cosmic microwave background radiation or CMB for short, from their perch 17,000 feet above sea level in Chile’s desolate Atacama Desert, where a dry atmosphere permits radiation to reach Earth with relatively little attenuation. In contrast to backyard telescopes that help us see visible light from stars and planets, these telescopes collect invisible microwave radiation.

Lyman Page

Lyman Page

These invisible waves are mostly uniform but contain slight differences in intensity and polarization that hold a wealth of information for cosmologists, said Lyman Page, the Henry De Wolf Smyth Professor of Physics. Page and Professor of Physics Suzanne Staggs co-lead two telescope projects, the Atacama Cosmology Telescope (ACT) and the Atacama B-mode Search telescope (ABS), which are funded by the National Science Foundation.

“If you imagine the temperature perturbations as a distant mountain range, the peaks and valleys correspond to the temperature variations,” Page said. “By looking at the patterns — the spacing between peaks, and whether they are narrow or fat — we are able to answer questions about the composition and evolution of the universe,” Page said.

ACT, which is about 18 feet across and looks like a giant metal bowl, has already made new discoveries, and confirmed and extended the findings of other CMB surveys, including two space-based telescopes, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck mission. A new, more sensitive receiver is currently being mounted on the ACT telescope, which is a collaborative effort with David Spergel, Princeton’s Charles A. Young Professor of Astronomy on the Class of 1897 Foundation, along with researchers at the University of Pennsylvania, National Institutes of Standards and Technology, the University of British Columbia, and 10 other institutions contributing significantly to the instruments and analysis.

Suzanne Staggs

Suzanne Staggs

The CMB originated in the hot plasma soon after the Big Bang, which cosmologists consider to be the birth of the universe. As the universe expanded, the radiation propagated, carrying the secrets of the early universe with it. One of the questions is why the CMB on opposite sides of the universe is so similar in temperature. The leading explanation of this observation is the inflation model, which posits that the universe underwent a rapid acceleration of its expansion just after the Big Bang.

This is where the lower-resolution, second telescope comes in. Co-led by Staggs, the ABS is looking for signs of inflation. “Inflation should produce gravitational waves which create patterns in the CMB called ‘B modes,’” said Staggs. B modes are extremely faint — to measure them requires an instrument that can detect temperature changes of just billionths of a degree. To obtain these sensitivities, ABS mirrors, which are relatively small at about two feet across, sit inside a cryogenically cooled barrel.

The two telescopes can be operated remotely, but require frequent trips to the Chilean peak, which often include Princeton students and postdocs. The team at Princeton includes Senior Research Physicist Norm Jarosik, Associate Research Scholar Jonathan Sievers, postdoctoral researchers Matthew Hasselfield, Rénee Hložek, Akito Kusaka and Laura Newburgh, and graduate students Farzan Beroz, Kang Hoon (Steve) Choi, Emily Grace, Colin Hill, Shuay-Pwu (Patty) Ho, Christine Pappas, Lucas Parker, Blake Sherwin, Sara Simon, Katerina Visnjic and Sophie Zhang.

–By Catherine Zandonella

Jeremiah Ostriker named White House Champion of Change

Jeremiah Ostriker

Jeremiah Ostriker (Photo by Denise Applewhite)

Jeremiah Ostriker, the Charles. A. Young Professor of Astronomy on the Class of 1897 Foundation, Emeritus, was recognized as one of 13 White House Champions of Change during a ceremony at the White House for his contributions to theoretical astronomy, which include the use of large-scale numerical calculations to study interstellar medium, galaxies, quasars and cosmology. The honor celebrates those who use or develop technologies and tools to enhance open government and accelerate social progress.

Ostriker, who currently works in cosmology, was among the first to find evidence for dark matter in the universe. He also examines galaxy formation, black hole growth and quasars. In 2000, Ostriker was selected as a winner of the National Medal of Science — the nation’s highest scientific honor — by former President Bill Clinton.

Planck mission brings universe into sharp focus

Planck mission

Planck mission reveals universe (Photo courtesy of the European Space Agency)

Princeton researchers contributed extensively to the Planck space mission that earlier this year released the most accurate and detailed map ever made of the oldest light in the universe, revealing new information about its age, contents and origins.

The results suggest that the universe is expanding more slowly than scientists thought, and is 13.8 billion years old, about 100 million years older than previous estimates. The data also show there is less dark energy and more matter — both matter that we can see and invisible dark matter — in the universe than previously thought. Princeton researchers helped design and implement an essential scientific instrument on the European Space Agency’s Planck mission through the agency’s collaboration with NASA. Princeton astrophysicists worked with scientists from the United States, Europe and Canada to analyze the Planck data.

“The Planck satellite has expanded the knowledge we gained from previous missions,” said William Jones, a Princeton assistant professor of physics who was involved in the design of a detector that surveys light at high frequencies.

-By Catherine Zandonella

Detection of cosmic effect may bring universe’s formation into sharper focus

Cosmology

(Image courtesy of Sundeep Das, University of California-Berkeley)

The first observation of a cosmic effect theorized 40 years ago could provide astronomers with a more precise tool for understanding the forces behind the universe’s formation and growth, including the enigmatic phenomena of dark energy and dark matter.

A large research team from two major astronomy surveys reported in July 2012 that scientists detected the movement of distant galaxy clusters via the kinematic Sunyaev-Zel’dovich (kSZ) effect, which has never before been seen. The work, which appeared in the July 23 issue of the journal Physical Review Letters, was initiated at Princeton University by lead author Nick Hand, Class of 2011, as part of his senior thesis.

Proposed in 1972 by Russian physicists Rashid Sunyaev and Yakov Zel’dovich, the kSZ effect results when the hot gas in galaxy clusters distorts the cosmic microwave background radiation — which is the glow of the heat left over from the Big Bang — that fills our universe. Radiation passing through a galaxy cluster moving toward Earth appears hotter by a few millionths of a degree, while radiation passing through a cluster moving away appears slightly
cooler (see illustration).

Now that it has been detected, the kSZ effect could prove to be an exceptional tool for measuring the velocity of objects in the distant universe, the researchers report. It could provide insight into the strength of the gravitational forces pulling on galaxy clusters and other bodies and on the still-hypothetical dark energy and dark matter in the universe. The effect also can be used to trace the location of atoms in the nearby universe, which can reveal how galaxies form.

“One of the main advantages of the kSZ effect is that its magnitude is independent of a galaxy cluster’s distance from us, so we can measure the velocity of an object’s motion toward or away from Earth at much larger distances than we can now,” said Hand, who conducted the work with his senior thesis adviser David Spergel, the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation and chair of astrophysical sciences at Princeton.

The paper featured 58 collaborators from the Atacama Cosmology Telescope, which is supported primarily by the National Science Foundation (NSF), and the Baryon Oscillation Spectroscopic Survey (BOSS) project. BOSS is a part of the Sloan Digital Sky Survey III, which is supported by the NSF, the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science and participating institutions.