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Years ago, when I first moved to New York and was a bright optimistic young person, I would travel around the city via subway. Every day I entered and left the same entrance to the same underground train stop. One day somebody dropped a bag of cheap glitter stars right by the exit. These stars were different colors and different shapes. They started as a glittering clump at the top of the stairs but thanks to foot traffic and weather, they quickly got everywhere. Blocks away one would come upon glistening stars dotted along the sidewalk.
Then the stars began to fade and deteriorate. Their shininess wore off in the rain. Their arms broke as people walked on them, but every once in a while you would see one that had caught in a protected location and survived. Eventually there were no glitter stars left at all, but I suppose that the stuff they are made out of—bits of plastic and metal foil—is still out there in a landfill, or running down through drains into the ocean, or just blowing in the wind.
I mention all of this because it is a poignant metaphor for the currently projected fate of the expanding universe. The ultimate destiny of the universe was once a problem relegated to theologians and mystics (and crazy people). However, when cosmologists fathomed how the universe began–with the big bang 13 plus billion years ago–the question of how the universe would end became a legitimate scientific topic. Edwin Hubble’s discovery that the universe was expanding provided an ominous hint as to the ending.
The way the universe will end is contingent on whether the momentum of the universe’s expansion is greater than the force of gravity (which in turn depends on the amount of matter in the universe and the density of that matter). The current scientific consensus–based on carefully considered estimates of the universe’s mass density and on the most recently observed rate of cosmic expansion—is that the universe will continue to expand indefinitely.
Physicists, astronomers, and cosmologists therefore now project a rather glum fate for everything that exists. The death of the universe is divided into 4 stages summarized below:
Stelliferous: (Now to 100 trillion years into the future) This is an era teaming with stars: great masses of matter coalesce together into stellar masses and begin fusing together and releasing all sorts of energy as they do so. Bit by bit though the brightest stars will wink out and only long-lived red dwarfs will remain.
Degenerate (100 trillion to 1037 years in the future) After even the red dwarfs fade into darkness, most of the universe’s mass will remain in the dying husks of stars, or in the remains of more exotic stellar death (black holes left by the destruction of super massive stars, neutron stars, and white dwarfs). Electromagnetic energy in the Degenerate era will be generated by particle annihilation and proton decay rather than stellar fusion.
Black Hole Era (1038 to 10100 years into the future) After protons all decay away from the universe, the only large objects creating energy will be black holes which will themselves slowly evaporate into exotic matter over a vast expanse of empty time.
Dark Era (10100 years into the future to eternity) Protons and black holes will be gone and only the effluvia radiation from their passing will still exists in the universe. All that remains will be photons of immense wavelength, neutrinos, positrons, and electrons–sad burned-out remnants which lack any order or meaning. The universe will be incredibly vast, a horrible dark cold broken thing and it will remain that way forever.
So there you have it. The fate of the universe is to slowly freeze and decay over trillions and trillions of years and then dissipate away into dark nothingness over eternity.
Of course 10100 years is a very long time indeed. There is still plenty of time for beach parties and flower gardening, but this is not the end I would have wanted for anything I esteem as much as the universe. There isn’t much I can do about it though, except to hope that some unknown aspect of the cosmos provides a more spectacular and fiery end or that Vishnu intervenes. Or maybe there is a multiverse in which our cosmos floats like an abstract bubble: in the past, every time we thought we understood the universe it turned out that we were only looking at a small part of a greater whole. It wouldn’t surprise me if something still eludes us.