Prediction of when the Universe will end

 Physicists predict the universe will end in a big crunch              

A Cornell physicist proposes that the universe is only halfway through its 33-billion-year lifespan, and will one day reverse course. Based on new dark-energy data, Henry Tye’s model suggests the cosmos will stop expanding in about 11 billion years and ultimately collapse into a “big crunch,” ending in a single point. If recent discoveries that dark energy is evolving hold any water, our Universe will collapse under its own gravity on a finite timeline, new calculations suggest. Based on several recent dark energy results, a new model finds that the Universe has a lifespan of just 33.3 billion years. Since we are now 13.8 billion years after the Big Bang, this suggests that we have a smidge less than 20 billion years left. Dark-energy evidence suggests the universe will end in a “big crunch” roughly 20 billion years from now.

The findings suggest that the cosmos will continue expanding for roughly another 11 billion years before reversing course, contracting back into a single point in a dramatic “big crunch.” For another 11 billion years, the Universe will continue to expand, before coming to a halt and reversing direction, collapsing down to the hypothetical Big Crunch, say physicists Hoang Nhan Luu of Donostia International Physics Centre in Spain, Yu-Cheng Qiu of Shanghai Jiao Tong University in China and corresponding author Henry Tye of Cornell University in the US.  Most precise measure of Dark energy confirms universe won't tear apart. "For the last 20 years, people believed that the cosmological constant is positive, and the universe will expand forever," Tye says. "The new data seem to indicate that the cosmological constant is negative, and that the Universe will end in a Big Crunch."

Henry Tye, the Horace White Professor of Physics Emeritus in the College of Arts and Sciences, arrived at this conclusion after updating a theoretical model that incorporates the “cosmological constant,” a concept first proposed by Albert Einstein more than a century ago and widely used by modern cosmologists to describe the universe’s expansion. The cosmological constant Tye refers to is λ, introduced by Albert Einstein in his theory of general relativity to describe the expansion of the Universe. If the value of λ is positive, then it acts as a force that constantly pushes outward, contributing to the Universe's expansion. If λ is negative, it behaves like a constant pull that never fades, and can eventually stop and reverse the expansion. Recent observations hint that dark energy may be changing over time. In the new model, the authors' best-fit goes hand-in-hand with a small negative λ, though current data don't rule out that λ equals 0. Since a negative λ pulls inward, it would hinder rather than aid the Universe's expansion. Tye is the corresponding author of a recent study about the findings published in the Journal of Cosmology and Astroparticle Physics. 

The universe, now about 13.8 billion years old, continues to expand outward. According to Tye, the future depends on the value of the cosmological constant: if it is positive, expansion will continue indefinitely; if it is negative, the universe will eventually reach a maximum size before reversing direction and collapsing entirely. His calculations support the latter scenario, a future in which the cosmos contracts to zero, marking the ultimate end of space and time. Nevertheless, the Universe is indeed expanding, according to an overwhelming majority of evidence. But we can get to the observed behaviour of the Universe if we combine a small negative λ with an ultralight axion field which behaves like dark energy today. Axions are, we think, ultra-light particles that can also be thought of as a smooth, ghostly field throughout space, first proposed decades ago as a potential solution to some other problems in particle physics. In their new analysis, Tye and his colleagues describe the axion as a force which gives the Universe a gentle push outwards in the beginning, but that slowly eases over time.

“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. At the current time, the influence of the axion still reigns, pushing the Universe outward at an accelerating rate as gravity weakens between bodies stretching farther and farther apart, so the Universe is still accelerating today in this scenario.

Tye and his collaborators proposed in the paper a hypothetical particle of very low mass which 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.” In about 11 billion years, the axion's push will weaken sufficiently that the pull of negative λ will take over, bringing the Universe's outward expansion to a standstill at a maximum size of about 1.7 times its current size. Then, the Universe will start to contract again, whooshing down to a Big Crunch in just 8 billion years. It's a bit like riding a bike up a hill, with a tailwind pushing you: going up, as the tailwind eases, your ascent slows, then gently stalls at the crest before you head down the steeper side, gaining speed as you go.

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 on-going 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). According to the paper, the 'downhill' contraction is quicker because the axion's kinetic energy takes over, and rising densities strengthen gravity's pull, making the crunching phase shorter than the expansion phase. The Big Crunch is like the opposite of the Big Bang, where all the matter in the Universe smooshes back together into one infinitely dense singularity. It's important to note that this is far from a certainty, not a prediction, but one possible future if the recent hints can be validated. A lot more data-crunching will be needed to determine whether dark energy is, indeed, evolving. In addition, we still don't know what dark energy is; it may not be axions or axion-like particles at all, but something entirely different.

Nevertheless, the paper provides one potential answer to one of cosmology's biggest questions. "For any life, you want to know how life begins and how life ends – the end points," Tye says. "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 finds it encouraging that the lifespan of the universe can be quantitatively determined. Knowing both the beginning and the end of the universe provides a greater understanding of the universe, the goal of cosmology.