Our universe is going to die, no doubt about it. One of the most accepted models of the end of the universe is eternal expansion and eventual death by entropy. As the universe continues to expand, entropy increases until everything we know is gone. But what does life look like as the end approaches? That question has given rise to fascinating ideas about the universe and life itself.
No Stars Visible From Earth
In 150 billion years, the night sky from Earth will look very different. As the universe races to its heat death, space itself will start to expand faster than the speed of light. Many of us are aware of the idea that light speed is a hard limit on the speed of an object in the universe. However, that only applies to objects that are in space, not the fabric of space-time itself. This is a hard concept to wrap our minds around, but the fabric of space-time is already expanding faster than light. And in the far future, it will have strange implications.
As space itself is expanding faster than light, a cosmological horizon exists. Any object past the horizon would require us to have the ability to observe and record by detecting particles traveling faster than light. But no such particle exists. Once objects pass beyond our cosmological horizon, they are inaccessible to us. Any attempt to contact or interact with distant galaxies past the horizon requires us to have technology capable of traveling faster than the expansion of space itself. Right now, only a few objects are outside of our cosmological horizon. But as dark energy accelerates the expansion, everything will fall
What does that mean for Earth? Imagine looking up at the night sky in 150 billion years. The only things visible will be a few scattered stars that are within the cosmological horizon. Eventually, even those will go away. The night sky will go completely blank. An astronomer in the future will have no proof that there is any other object in the universe. All the stars and galaxies we see now will be completely out of telescope reach. For all we could see, our solar system would be the only thing left in our universe.
Our Sun Becomes A Black Dwarf
Right now, our universe has many different types of stars. Red dwarfs—cool stars that give off red light—are among the most common of these. Semantically related white dwarfs also fill the universe. These are stellar remnants of dead stars, made of degenerate matter, that are held together by quantum effects. Currently, astronomers consider white dwarfs to have essentially infinite life spans. The universe is just not old enough for them to have died out. But given enough time, even they will die and become exotic stars named black dwarfs.
Our Sun is on that path. In the distant future, our Sun will eject its outer layers and turn into a white dwarf star, staying in that state for billions of years. As the universe winds down, the white dwarf that was our Sun will start to cool. After 10100 years, it will cool down until its temperature is equal to the background microwave radiation, just a few degrees Kelvin above absolute zero.
When that happens, it will be a black dwarf. As this type of star is so cold, it is invisible to the human eye. Thus, anybody trying to find the Sun that used to give us life will find it impossible to see with optical systems. Instead, they will have to rely on detecting its gravitational effects. Most stars that we see in the night sky will become black dwarfs, but knowing that our warm Sun will devolve into a dark and cold stellar remnant is a little more personal.