In this series of posts, we sit down with a few of the keynote speakers of the 245th AAS meeting to learn more about them and their research. You can see a full schedule of their talks here, and read our other interviews here!
One of the most profound discoveries in cosmology over the past few decades is that the universe’s rate of expansion is accelerating. When speaking with Dr. Bahcall about her life’s work, it is interesting to note how our knowledge of cosmology itself also seems to be accelerating in its expansion. In describing the world of astronomy she entered as a graduate student compared to today, Dr. Bahcall says, “Astronomy has grown so big and wide. It was not like this in the early days when I started in the [19]60s and 70s. It was much smaller. We barely knew there was even dark matter. We didn’t know very much. To think about how much we have grown in astronomy and cosmology. It’s unbelievable.”

Dr. Bahcall will present the Henry Norris Russell Lectureship to the American Astronomical Society, awarded to an astronomer for “a lifetime of eminence in astronomical research,” in recognition of “her central contributions to determining the average density of matter in the universe and establishing the concordance model of cosmology, her dedication to astronomical education, and her exemplary service to the community.”Dr. Bahcall was born in Israel and began her career there studying nuclear physics. She graduated with her bachelor’s degree in physics and mathematics from Hebrew University. She was then working on her Ph.D. in Physics at the Weizmann Institute when her future husband, John Bahcall (who was an esteemed astrophysicist in his own right and namesake of the AAS John Bahcall Public Policy Fellowship), visited from the California Institute of Technology. They met and married soon after, and she moved to CalTech and studied under the esteemed nuclear and astrophysicist William Fowler. After completing her Ph.D., she and John moved to Princeton in 1970, where they spent the rest of their careers. Dr. Bahcall described her time at CalTech as the perfect place to transition from nuclear physics to astrophysics. “At that time, there was no astronomy in Israel, but nuclear physics was booming. At CalTech, there was a lot of astronomy. I started talking with and listening to talks about astrophysics in general. There was Maarten Schmidt with quasars. There was [Fritz] Zwicky who talked about dark matter. And that’s how I got interested in those areas.”
Professor Bahcall would spend much of her career teasing out facets of what we now refer to as \(\Lambda\)CDM cosmology, which is our currently accepted cosmological model (for a crash course on \(\Lambda\)CDM cosmology, check out Astrobite’s recently released guide to \(\Lambda\)CDM). In general relativity, matter/energy curves spacetime, and so the total amount of matter/energy in the universe affects the overall shape of the universe. The universe is flat instead of curved if there is exactly the right amount, called the ‘critical density.’ This ratio of matter/energy in the universe relative to the critical density is one of the key parameters of most cosmological models, called \(\Omega\). This $\latex \Omega$ can be divided into its constituent pieces: how much of the total matter/energy is energy (\(\Omega_{\Lambda}\), today known as dark energy), dark matter, and regular baryonic matter (collectively \(\Omega_{m}\)). Radiation (i.e., photons) and curvature itself (think of the stored energy of a bent ruler that wants to snap back to flat) could also contribute to this total \(\Omega\), but \(\Lambda\)CDM currently believes those contributions are negligible.
Dr. Bahcall entered the field when dark matter was not yet accepted. She recounts the state of the field in the early 1980s: “The suggestion for inflation came from Alan Guth in 1980. Standard Inflation required a flat universe. And the flat universe at that time was believed to be \(\Omega_{m}\) equals one. Nobody thought about \(\Lambda\). It became known as the standard cold dark matter (SCDM), where \(\Omega_{m} = 1\) (Not the total \(\Omega = 1\)), We then used different methods to test what \(\Omega_{m}\) is.”
In the 1980’s/90’s Dr. Bahcall and her team made a discovery that would set the stage for a low \(\Omega_{m}\) by studying galaxy clusters. “The galaxy correlation function was known. We thought, let’s see how clusters are clustered. We found very unexpected results; we found that the cluster correlation function – which measures how clusters are clustered in space – was 20 times stronger than the galaxy correlation function.” Once this discrepancy in correlation functions was discovered, they began investigating what values of each of the \(\Omega\)’s could give rise to it. What they concluded is that \(\Omega_{m}\) could only be a small part of the total \(\Omega\), only about ~30% (\(\Omega_{m} \approx 0.3\)). But if this was the case, did anything constitute the rest, or was the universe Open, not Flat? That opened the door for other CDM models. One of Dr. Bahcall’s most well-known works provides an overview of observations and implications for these different CDM models in what she calls the cosmic triangle, showing that \(\Lambda\)CDM was the model that best fits all the available data. Around the same time, observations of type Ia supernovae would provide convincing evidence for the astronomical community to begin accepting \(\Lambda\)CDM as the CDM cosmology version matching observational evidence.
Beyond her groundbreaking research, Dr. Bahcall exudes a love for her late husband, John, and her family. “John and I had an incredible life together, a wonderful family, and luckily, wonderful careers for both of us.” Her advice to graduate students contemplating balancing academic careers and families is heartfelt. “Family was always the most important thing for us. We would not sacrifice anything about the family for work. The important thing for a couple is understanding what’s important to the other and supporting that. John was enormously encouraging and supportive of everything that I did, and I was very supportive of everything he did.”
To learn more about the development of \(\Lambda\)CDM, attend Dr. Bahcall’s Henry Norris Russell Lecture at 11:40 AM – 12:30 PM EST on Tuesday, January 14th!
Astrobite edited by: Jessie Thwaites