Eternal silence: Notes on the future of the universe

The eternal silence of these infinite spaces fills me with dread.

—Pascal, Pensee 201

Nothing inspires Pascalian dread like thinking about the long-term fate of the universe.  I was going to present the discussion below in my cosmology class, but I ran out of time and the semester ended.  It occurs to me that some of my readers here might find it entertaining.

For the rest of this post, we’re going to assume the standard (“LambdaCDM”) model of cosmology, and the current understanding of the laws of physics (standard model + general relativity).  We’ll be pushing these theories past the realm where they’ve been tested, so what I have to say will be somewhat speculative.  With that warning, let us proceed to consider the long-term fate of our universe and the objects in it.  For more information, I recommend the excellent review article “A Dying Universe” by Adams and Laughlin.

First, the fate of nearby objects.  The sun has about five billion more years as a main sequence star.  Its luminosity will slowly climb through this time, and in less than a billion years, the Earth will become uninhabitable.  The oceans will boil, and all life will die.  After five billion years, the sun will expand enormously and become a red giant, swallowing the planets Mercury and Venus in the process.  (Whether or not the Earth will be absorbed by the sun or just skim the surface and live on as a charred rock is currently uncertain.)  Then the outer (about) half of the sun’s mass will be blown off, while the core will settle down as a white dwarf.

Five billion years is also roughly when we expect the Milky Way and Andromeda galaxies (currently on what appears to be a collision course) to collide.  Even if this passing turns out to be a near miss, the Milky Way, Andromeda, and (soon enough) all the smaller galaxies in the Local Group will collide and merge into a single galaxy because of a process called dynamical friction (an effective drag force created by these galaxies moving through a dark matter background).

According to the LCDM model, the universe will expand forever.  Indeed, due to the “anti-gravity” like effect of the dark energy’s negative pressure, the expansion of the universe will continue to accelerate, becoming exponential.  By 100 billion years, the universe will be expanding so fast that light won’t be able to travel from one galaxy/cluster to another anymore.  (Each gravitationally bound object will be outside the others’ horizon, as cosmologists put it.)  The merged Milky Way-Andromeda galaxy will be totally isolated.

The current setup of stars and galaxies won’t continue forever, though, most importantly because the universe is running out of gas to make stars with.  Already, most of the gas in the Milky Way’s galactic plane is in stars, not gas.  Some of this gas is returned to the interstellar medium when a star dies, but a significant chunk of the star’s mass (from the core) gets trapped in a compact object:  a white dwarf, neutron star, or black hole.  In 10^12 — 10^14 years, the gas will be depleted from galaxies, and star formation will stop.  The last generation of stars will live for a while (the smaller-mass stars have very long lifetimes), but one by one, every star in the sky will go out.  By 10^14–10^15 years, the mass of the galaxy will mostly be in cold, dark white dwarfs.

The galaxy itself won’t stay together forever.  The more massive stellar remnants will fall into the central black hole.  Most of the stellar remnants, though, will be flung out of the galaxy through a process called dynamical relaxation (basically the cummulative effect of little tugs that the stars’ gravitational pulls on each other give their orbits).  After around 10^20 years, galaxies will have evaporated, leaving only their supermassive central black holes and their dark matter halos behind.

White dwarfs, neutron stars, and black holes are extremely stable objects, so we must really push our knowledge of physics to ask what eventually happens to them.  If dark matter is made of WIMPs, then the halo should annihilate in around 10^22 years.  If the proton decays on a very long timescale (something not observed but expected by GUT theories), this will destroy the white dwarfs in 10^39 – 10^200 years (depending on what you assume for the decay rate).  Black holes eventually evaporate due to Hawking radiation (although, again, this is not a process that’s been actually observed).  This will do in all the universe’s black holes in 10^65-10^100 years (depends on the mass of the hole–smaller ones evaporate first).  The final burst of radiation as a black hole disappears will be the last bright event in the history of the universe.  After this, the universe will be nothing but an extremely diffuse gas of photons, neutrinos, electrons, and positrons–dark and lifeless, for all eternity.

Of course, changes in our understanding of the current constituents of the universe will change these projections.  For example, since we don’t know what the dark energy actually is, it may be a bit presumptuous to assume that it will always continue to act on the cosmic expansion the way it is doing now.  Similar statements should be made about the dark matter.

When we look at the lifetime of the universe on a logarithmic scale, it appears that we exist at a special time when life is possible.

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