Updated Prognosis: The Universe May End 10¹⁰²² Years Sooner Than We Thought

Previous estimates put the universe’s end at around 10¹¹⁰⁰ years, or 1 followed by 1,100 zeros. The new estimate places the decay of the universe at 10⁷⁸ years, or 1 followed by 78 zeros. That’s a huge difference in terms of expected lifespan. If you wanted to put it in human terms, it’s like being told you should be expected to live for 50 more years, only to find out you have less than a trillionth of a trillionth of a second before your imminent demise. Fortunately though, we’re talking in incredibly long timescales, and there’s plenty of time left to go.

So why the big difference in estimates? The team of three Dutch scientists looked at Hawking radiation, first proposed by famed British physicist Stephen Hawking. This is the idea that black holes slowly lose mass through thermal radiation. 

“Due to a combination of the properties of quantum mechanics and gravity, the curved space-time surrounding a black hole emits a small amount of heat,” NASA explains. “For black holes that are not accreting mass to make up for this energy loss, Hawking radiation causes the event horizon to shrink over time, and such a black hole will eventually evaporate in a flash of energetic particles and gamma rays. Stellar-mass black holes would take dozens of times the present age of the universe to evaporate, and supermassive black holes would take even longer.”

This radiation occurs as virtual particles and their antiparticle pairs pop into existence near the event horizon, with one particle falling in and the other escaping, as Hawking radiation. In a previous paper the team of scientists, from Radboud University in the Netherlands, found that new particles can be created far from the horizon of a black hole, essentially a new kind of radiation which may need to be taken into account. 

“We show that far beyond a black hole the curvature of spacetime plays a big role in creating radiation. The particles are already separated there by the tidal forces of the gravitational field,” Walter D. van Suijlekom explained in a statement at the time.

Previously it had been assumed that Hawking radiation was only possible at the event horizon, whereas this study appeared to show it was unnecessary.

“That means that objects without an event horizon, such as the remnants of dead stars and other large objects in the universe, also have this sort of radiation,” Heino Falcke added. “And, after a very long period, that would lead to everything in the universe eventually evaporating, just like black holes. This changes not only our understanding of Hawking radiation but also our view of the universe and its future.”

After that study, the team received a lot of questions. In particular, people wanted to know how long it would take before other objects – such as neutron stars – would take to decay, assuming this work is correct. That’s where the prognosis gets bleak.

Looking at objects, from the human body to supermassive black holes and dark matter halos, the team found that neutron stars and stellar-mass black holes take around the same amount of time to evaporate at around 10⁶⁷ years, which is a little unexpected given the stronger gravitational fields of black holes.

“But black holes have no surface,” co-author and postdoctoral researcher Michael Wondrak explained in a statement. “They reabsorb some of their own radiation which inhibits the process.”

Looking at white dwarfs – the final remnants of Sun-like stars, previously thought to be the longest-living objects in the universe, slowly decaying over absurd timescales – the team found that they evaporate in 10⁷⁸ years rather than the previously predicted 10¹¹⁰⁰ years.

“So the ultimate end of the universe comes much sooner than expected,” lead author Heino Falcke said, “but fortunately it still takes a very long time.”

One interesting prediction of this work is that we may be able to detect the fossil stellar remnants of previous universes, given a number of assumptions.

“The maximum age of neutron stars, τ ∼ 10⁶⁸ yr, is comparable to that of low-mass stellar black holes. White dwarfs, supermassive black holes, and dark matter supercluster halos evaporate on longer, but also finite timescales. Neutron stars and white dwarfs decay similarly to black holes, ending in an explosive event when they become unstable,” the team explains in their paper.

“This sets a general upper limit for the lifetime of matter in the universe, which in general is much longer than the Hubble–Lemaître time, although primordial objects with densities above ρmax ≈ 3×10⁵³ g/cm³ should have dissolved by now. As a consequence, fossil stellar remnants from a previous universe could be present in our current universe only if the recurrence time of star forming universes is smaller than about ∼ 10⁶⁸ years.”

At a certain point, neutron stars would reach a critical mass, after which they would explode, but because of the long timescales involved and other uncertainties the team is not confident that such a fossil would be found.

“If present, fossil neutron stars would now be growing by accretion from the intergalactic medium and the cosmic microwave background rather than shrinking, unless some instability would make them undergo such a phase transition after all,” the researchers add.

“Given the long timescales, we do not expect any neutron stars formed in our current universe to undergo such an evolution. One could speculate that fossil neutron stars from a previous universe might still be around and be near the critical mass. This is only possible if inflation does not prevent universes from occupying the same phase space.”

For fun, the team also calculated how long it would take objects like the human body to decay as well as the Moon, at around 10⁹⁰ years, though of course for these bodies there are other factors which will make them disappear a lot sooner than that.

“By asking these kinds of questions and looking at extreme cases, we want to better understand the theory,” van Suijlekom added, “and perhaps one day, we [will] unravel the mystery of Hawking radiation.”

The study has been accepted by the Journal of Cosmology and Astroparticle Physics, and is posted on pre-print server arXiv.