What Will Be the Universe's Final Significant Event?

The answer is Hawking radiation, through which the last black holes will evaporate.

Hawking radiation can be traced to quantum mechanics. It occurs near the event horizon of black holes, where pairs of virtual particles are spontaneously created.

Sometimes, one of these particles falls into the black hole while the other escapes, resulting in a net loss of mass for the black hole and the emission of radiation. This process gradually causes black holes to lose mass and eventually evaporate over immense periods of time.

A long time

The time it takes for a black hole to evaporate depends on its size, with smaller black holes evaporating more quickly than larger ones. According to Hawking’s calculations, a typical black hole with the mass of the sun would take about 10⁶⁷ years to evaporate. 1 followed by 67 zeros. To put it into perspective, the universe's age is estimated to be about 13.8 billion years.

As black holes continue to lose mass, the energy they emit will eventually lead to the universe's hypothesized “heat death,” which is the most prevailing model of its fate.

In this state, the universe will reach a stage of maximum entropy and thermodynamic equilibrium, meaning no more heat or work can be extracted. Thus, the universe will become essentially a cold, lifeless void.

“nothing”

But it is nothing, void less?

We have energy left over from evaporating black holes that leave over quantum fluctuations, so something still exists even after the last black holes evaporate.

A Kerfuffle

There is an ongoing kerfuffle about whether quantum fluctuations can genuinely be defined as "nothing." Can they? While some attempt to redefine 'nothingness,' others object, arguing that it seems strange to assert that the quantum "void" is truly nothing, given that it is, in fact, something—albeit a wee amount. Still, it is something.

Moreover, it operates under the governance of quantum laws.

Condede

The one thing that the objectors concede regarding nothingness is that nothing significant will occur once the last black holes have evaporated. It’s a scenario of ultimate stillness, where the universe reaches a state of equilibrium devoid of the cosmic forces that once defined its existence.

It'll be the last significant event of the universe.