What is the end of time?
In 1 billion years, the Sun's brightness will increase by 10%, the Earth will become a second Mars, and in 5 billion years, the hydrogen inside the Sun will be depleted, and the whole will begin to expand into a red giant, and all celestial bodies within the orbit of Mars will be swallowed.
Six billion years later, the sun, which has turned into a planetary nebula, has only a white dwarf core left in the central region, and only a few outermost gas giants are now left in the eight planets of the solar system.
In 100 trillion years, all the stars in the universe will be extinguished, all the black holes will evaporate, the nascent nebula is no longer enough to collapse into new stars, and the whole universe will only be left with creepy darkness and cold, the heat that originated at the moment of the Big Bang will completely disappear at this moment, and the white dwarf that once illuminated the last moments of the universe has gradually cooled down into a black dwarf that does not emit light.
However, the universe is still accelerating under the drive of dark energy, the residual density of matter is constantly expanding, after the Great Wall of the universe breaks, galaxies fall apart, and finally all atoms in the universe will be torn apart, the entropy increase will also reach the maximum, and the entire universe will completely disappear at the level of thermodynamics and physics.
However, Hawking's former friend, Nobel laureate in physics Penrose, believes that our universe is likely to be a continuous cycle of time and space, the starting point of the Big Bang 13.82 billion years ago is actually the end of the last round of the universe, and the universe will not continue to accelerate expansion, at some point in the future, our universe will change from accelerated expansion to deceleration expansion, then stop expanding, and finally begin to collapse under the action of gravity.
When the time comes, all the galaxies in the universe will begin to get closer to the earth, the density of matter in the universe will become larger and larger, galaxies will collide with each other, stars will be born in large quantities, and finally all the matter in the universe will be compressed to a singularity state by the force of the universe collapse, and there will be a new big bang in the universe.
Judging from the current observations, there are about 2 trillion galaxies in the observable universe with the Earth as the center radius of 46.5 billion light years, 98% of which are far away from the Earth because of the speed of light, and will forever lose their causal relationship with the Earth, that is to say, the light emitted by those galaxies will never fly to the Earth.
But if the universe begins to shrink in the future, these galaxies will return to a meaningful state, and the alien civilizations that may inhabit those galaxies will also become closer and closer to the earth, and then we will use radio telescopes to search in the universe, and there is a high probability that we will receive a lot of radio signals, which are caused by the closer and closer distances between galaxies.
However, from the perspective of time scale, compared with the life span of stars of tens of billions of years, the history of human beings and even the entire human civilization of 10,000 years is really a flick of a finger, which leads to the fact that we have no time to witness any phenomena in the universe in our lifetime, and what the Hubble and Webb telescopes see is only the residual image of the universe billions of years ago, tens of billions of years ago, and we will never see the real-time universe.
Overall
Whether it is the end of time or the end of space, we are all outside the light cone, maybe at this moment somewhere in the universe has begun to decay in vacuum, the whole universe is disintegrating at the speed of light, but for human civilization, we actually only exist for a moment compared to the universe.