October 29, 2025

Looking Back to the Big Bang
Jo Dunkley
 British Astrophysicist, member of the Royal Society and Professor Physics at Princeton University

Jo Dunkley and George Bustin, introducer

​Minutes of the Eighth Meeting of the 84th Year

President George Bustin convened the meeting and Frances Slade led the invocation.
President Bustin welcomed new members elected at the previous meeting.
One hundred forty-three persons attended the session, including Stephen A. Pollard, guest of Mark Pollard; Dianne Schaffer, guest of Marlaine Lockheed; Peter Kann, guest of Stephen Lin; and Natalea Skvir, guest of Kassie Skvir.

George Bustin extended very special gratitude to the Jewish Center for agreeing to accommodate Old Guard meetings for most of the rest of our program year.

He then asked for some moments of silence for two Old Guard members, Graydon Vanderbilt and William Coleman, M.D., who recently passed away.

Benjamin Colbert read the minutes of the previous meeting.

George Bustin introduced the speaker, Jo Dunkley, Joseph Henry Professor of Physics and Astrophysical Sciences at Princeton University, who received her PhD in astrophysics from the University of Oxford. A cosmologist, she focuses her research on the evolution of the universe and has been awarded three major prizes for that work. In further recognition of her service to science, in 2019 she was named an Officer of the Order of the British Empire (OBE) and, just last year, was elected a Fellow of the Royal Society.

Professor Dunkley opened by commenting that she is pursuing three big questions: What are the ingredients of our universe? How did we come to be here? How did it all begin? She has now served on the astrophysics faculty of Princeton University for about a decade, attracted because many of the attempts to answer these questions have—and still are—being addressed here. Obviously, she said, to answer them we must be able to travel very far back in time.

Fortunately, the fact that light takes time to travel enables us to do so. We can calculate the distance between us and an astronomical body using basic trigonometry, then deduce the length of time it takes for light to reach us from that location. To illustrate, extend your arm, then view an upraised finger with first one eye, then the other. You will perceive a shift in that finger’s position. The line between those two positions across your nose demarks the base of a triangle from which you can calculate the length of the triangle’s other two sides. Astronomers use our planet, instead of the distance between their eyes, to define the base of such a triangle by observing the location of their target body and its background with observations from two separate locations a precisely known distance apart, or—when possible—from one site at greatly separate times, such as six months.

While it requires about 1.3 seconds for light from the moon to reach us and about eight minutes from the sun, light must travel four years from our nearest star to get here, and about a thousand years from stars in the constellation Orion’s belt. We are seeing them as they were in the past as measured by these “light years.” All are our neighbors here in our home galaxy—a revolving disk of 100 billion stars about 100,000 light-years across. We live about halfway out from the center. Obviously, as residents within it, we are unable to photograph our own galaxy from the outside, but when we view the Milky Way, we are looking at that immense disk from inside and across its center where most of its stars are concentrated.

When we look outside our galaxy with our newest telescopes, we discover that ours is but one out of 100 billion other similar galaxies in the cosmos, many also filled with 100 billion stars, many birthed far earlier in the life in the cosmos than we previously expected. They are mostly gathered in clusters and currently seem to be the building blocks of our universe. But is this all there is? Apparently not.

Back in the 1970s, the astronomer Vera Rubin systematically gauged the mass of galaxies distant from us by measuring the stretching and shrinking of their wavelengths as they revolved. We can get some understanding of how she did this by stretching and shrinking an expandable tape onto which a line of buttons has been attached. If we squint down the tape as we stretch or shrink it, we perceive the buttons farthest from us as moving away from us more rapidly than those nearest to us and astronomers can use that to estimate how quickly they are revolving. However, Rubin reported that she found that the galaxies she was observing were rotating far faster than that for which their estimated collective gravities could account. She surmised that they seemed to be embedded in some unknown “dark matter” we are unable to see and know almost nothing about other than its gravitational effects.

Has the cosmos always been like this? Einstein originally believed that the universe was static, but early in the 20th century, Edwin Hubble found that the universe is, in fact, expanding. Everything is moving away from everything else, much like raisins in a rising loaf of bread. If expanding, the universe had to have a beginning. If we mathematically wind the calculated rate of  its expansion back to its start, the “Big Bang” occurred about 14.5 billion years ago.

We do not yet know how it looked at this beginning  or immediately thereafter, but in 1964 mysterious microwave radiation was detected by researchers at the Bell Labs, not far from here, and astrophysicists on the Princeton faculty identified that finding as background microwave radiation from this presumed “Big Bang.”  Since 1964, we have found ways—again with the help of invaluable work here at Princeton—to search for clues about the look of our universe farther and farther back in time. We now have a map of what the cosmos looked like 500,000 years after the Big Bang. At that time, the universe was only a cloud of hydrogen and helium. No other elements existed—no stars, no galaxies. Nonetheless, there were some variations in temperature and bits of denser material appear in the soup of radiation produced by the Big Bang. They could well be the gravitational seeds that helped germinate the universe we see today. If the cosmos had just remained a uniform cloud of cosmic dust with none of these areas of denser materials and temperature variations, no cosmos as we know it today would have evolved and, of course, we would not be here. 

In response to later questions, Professor Dunkley said that attempts to understand the consequences of these variations in the background microwave radiation have prompted cosmologists for the first time to look into the possibility that quantum fluctuations may have played some role in generating these variations in temperature and clumping of denser material. A more recent discovery, using the Webb telescope, of “red dots” that some speculate look like young “black holes” attracting and concentrating material out of the surrounding cosmic soup has generated equal interest and excitement. But so far much remains unknown or unproven.
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“I want to know more about the universe in which I live,” Professor Dunkley confessed and, needlessly to say, she is pressing forward with her own research into the nature of that very early universe and how it evolved. The new Simon observatory in Chile’s very dry Atacama Desert provides her with a vital new tool she needs to pursue her explorations that far back in time. 

Respectfully submitted,
Ralph R. Widner