The universe is approaching the midpoint of its 33-billion-year lifespan, a Cornell physicist calculates with new data from dark-energy observatories. After expanding to its peak size about 11 billion years from now, it will begin to contract - snapping back like a rubber band to a single point at the end.
Henry Tye, the Horace White Professor of Physics Emeritus in the College of Arts and Sciences, reached this conclusion after adding new data to a model involving the "cosmological constant" - a factor introduced more than a century ago by Albert Einstein and used by cosmologists in recent years to predict the future of our universe.
"For the last 20 years, people believed that the cosmological constant is positive, and the universe will expand forever," Tye said. "The new data seem to indicate that the cosmological constant is negative, and that the universe will end in a big crunch."
Tye is the corresponding author of "The Lifespan of our Universe," published Sept. 18 in the Journal of Cosmology and Astroparticle Physics.
The universe is 13.8 billion years old and expanding. According to current models, Tye wrote, its two simplest fates are either: It will continue its present expansion forever, if the cosmological constant is positive; or, if the cosmological constant is negative, it will reach a maximum size, then contract, eventually collapsing to zero.
The latter is the conclusion Tye reached with his recent calculation.
"This big crunch defines the end of the universe," Tye wrote. He determined from the model that the big crunch will happen about 20 billion years from now.
The big news this year is the reports by the Dark Energy Survey (DES) in Chile and the Dark Energy Spectroscopic Instrument (DESI) in Arizona this spring. Tye said these two observatories, one in the southern hemisphere and one in the northern, are in good accord with each other. The whole idea of the dark energy survey of these two groups is to see whether dark energy - 68% of the mass and energy in the universe - really comes from a pure cosmological constant. They found that the universe is not just dominated by a cosmological constant dark energy. The dark energy actually has something else going on.
Tye and his collaborators proposed in the paper a hypothetical particle of very low mass that behaved like a cosmological constant early in the life of the universe but does not anymore. This simple model fits the data well but tips the underlying cosmological constant into negative territory.
"People have said before that if the cosmological constant is negative, then the universe will collapse eventually. That's not new," Tye said. "However, here the model tells you when the universe collapses and how it collapses."
There are more observations to come, Tye said. Hundreds of scientists are measuring dark energy by observing millions of galaxies and the distance between galaxies, gathering even more accurate data to feed into the model. DESI will continue observations for another year, and observations are ongoing or will begin soon at several others, including the Zwicky Transient Facility in San Diego; the European Euclid space telescope; NASA's recently launched SPHEREx mission; and the Vera C. Rubin Observatory (named after Vera Rubin, M.S. '51).
Tye finds it encouraging that the lifespan of the universe can be quantitatively determined. Knowing both the beginning and the end of the universe provides greater understanding of the universe, the goal of cosmology.
"For any life, you want to know how life begins and how life ends - the end points," he said. "For our universe, it's also interesting to know, does it have a beginning? In the 1960s, we learned that it has a beginning. Then the next question is, 'Does it have an end?' For many years, many people thought it would just go on forever. It's good to know that, if the data holds up, the universe will have an end."
Tye's co-authors are his former Hong Kong University of Science and Technology doctoral students Hoang Nhan Luu and Yu-Cheng Qiu.
Kate Blackwood is a writer for the College of Arts and Sciences.
