Transformative discoveries in cosmology are unfolding at Princeton as researchers explore the universe’s large-scale structure, including galaxies, dark matter, and dark energy.
Professor of Astrophysics Neta Bahcall explained to The Daily Princetonian that her team researches the universe when it was “less than a billion years old … to understand how different structures formed in the universe … and figure out what happened thirteen billion years ago when galaxies first formed.”
Bahcall began her work in physics in Israel. When she was growing up, Israeli universities did not offer astronomy degrees, but she loved physics and math in high school and later majored in them at Hebrew University. With her husband, she moved to the California Institute of Technology, where she completed her PhD in nuclear astrophysics.
Working with various astronomers and professors, including William Fowler and Martin Schmidt, she learned about quasars, extremely bright and distant galaxy cores that contain massive black holes. She was fascinated by the questions these scientists asked, especially regarding dark matter quasars and galaxies.
“I thought it was so interesting to figure out what the universe contains,” she said. “From there I moved into astrophysics, dark metal galaxies, quasars, and then I moved to Princeton and continued in that direction.”
When Bahcall began her research, the standard cosmological model was the Standard Cold Dark Matter (SCDM) model, which posited that the universe had to have enough matter to stop its expansion. Using different methods, her research determined that SCDM was incorrect—thereby corroborating the theory that the universe will continue to expand infinitely. She and her team found that there was only around 25–30 percent of the critical density of dark matter that would be necessary to stop expansion.
Bahcall gave a talk in January 2025 at the American Astronomical Society, where she was awarded the Henry Norris Russell Prize for her “central contributions to determining the average density of matter in the universe and establishing the concordance model of cosmology.” The concordance model is called ΛCDM, which stands for dark energy cold dark matter, and is now the accepted cosmological model. Dark matter does not emit, absorb, reflect, or scatter light, and is poorly understood by scientists.
Bahcall’s team is currently working to understand how dark matter is distributed in the universe.
“We can see most of the normal matter: the stars… the light in the galaxies… and we can measure how it’s distributed. We know the big clusters of galaxies contain a lot of dark matter, but how is the dark matter distributed?” Bahcall asked.
Her team measures the dark matter’s gravitational impact on the system by either giving them velocities or through gravitational lensing, which occurs when a massive celestial body causes the path of light around it to curve. Using data from the Sloan Digital Sky Survey that measures the mass from lensing, they compare the distribution of the entire mass with the underlying distribution of the light they can measure.
“In the field of cosmology, of understanding the universe, there are several big questions that we want to understand,” Bahcall explained. “To understand their properties better, or from what we can get from the data about the nature of dark matter, because we, at the end, will need to detect the dark matter particles that make the dark matter.”
Her team finds the dark matter particle and using astronomy, attains as much information as possible about the properties of the dark matter and its distribution. “We have to use as much as we can from the astronomical observations to help understand the physics of these properties. What is a dark metal particle? What is dark energy? How do galaxies form and evolve with time?” she said.
Bahcall reflected on her journey to answer these questions and is excited about the speed at which the field is growing.
“In 50 years, I cannot even imagine what we will know then, and maybe some of what we think today we know will be totally reversed then,” she said. “That’s how science goes.”
Aanya Kasera is an assistant Podcast editor who also writes about research for the ‘Prince.’
Please send any corrections to corrections[at]dailyprincetonian.com.
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