The hope that humanity will one day expand beyond its solar system, drift in space on self-supporting vessels, and explore other planets is almost universal. The theme occupies a central place in science fiction. We know that one day the earth will be consumed by the sun as the latter becomes a red giant. And so a vague if cautious optimism is maintained about science’s ability to help us escape our doomed home. This was the general gist of Neil deGrasse Tyson’s tweet that “Asteroids are nature’s way of asking: ‘How’s that space program coming along?’”
However, it’s quite likely that our cosmos will eventually not support life of any kind. We may survive billions of years, but after so many trillions, the very laws of physics will turn against us. However, there is now renewed speculation that, ultimately, our future may not be so bleak.
Here’s an account of what modern astronomy has to tell us about the fate of the cosmos.
The shape and expansion of the universe
The fate of the universe largely depends on its shape. We either live in a closed, open, or flat universe. The density of the universe will determine this. Matter warps space and time, and so the shape of the universe depends on its volume.
The expansion of the universe is also tied to its mass. More mass equals more gravity, and gravity works to pull everything together, stopping expansion.
A closed universe has enough mass to stop expansion (or would if dark energy didn’t exist). The geometry of space in a closed universe is spherical like a balloon. If there were no dark energy, gravity could pull all the matter in the cosmos back together in a “Big Crunch,” which some hoped would set off another Big Bang. Most of us have heard this theory. A cyclical universe would be nice, since it be the most straightforward (though not the only) way to allow for earth’s eternal recurrence.
To the best of our astronomical knowledge, however, this will not happen. As NASA has reported, “We now know (as of 2013) that the universe is flat with only a 0.4% margin of error.” Open and flat universes continue to expand forever. In addition, our universe does have dark energy (or something with similar repulsive effects).
And so our universe will continue to expand forever.
Earth’s end and the lonely supercluster
Now that we’ve established the infinite and expanding nature of our universe, let’s see what’s in store for it.
In about 4 billion years, the Milky Way will merge with its closest neighboring galaxy, Andromeda. The stars in these galaxies are so far apart that it is unlikely that they will collide with each other when this happens. However, in only about 3.75 billion years, the Earth will be charred by the heat of its expanding sun.
Our galaxy is in a cluster of about 54 other (mostly smaller) galaxies, creatively named the Local Group. All of these galaxies are expected to converge in about a trillion years. A trillion years after that, everything beyond the Local Supercluster will be invisible (the Local Supercluster is a cluster of clusters that includes the cluster we call home). According to Hubble’s Law, the further away a celestial body is from some point of reference, the faster it will be moving from that point of reference. Things will be moving so quickly away from our vantage point in the Milky Way that they will be completely invisible.
The death of stars
A universe that will last forever means that stars will eventually die off, and hence no life sustained from stellar activity will exist.
Stars burn by fusing atoms in their core. Normal stars, such as our sun, burn hydrogen. Hydrogen is the lightest kind of atom. When hydrogen fuses in a star’s core, it creates a heavier element: helium. To make up for a slight loss of mass in this process, energy is released.
But here’s the problem: the heavier the elements get, the harder it is to fuse them. Most stars, like our sun, just aren’t massive enough to create conditions where elements heavier than carbon or oxygen can form. And once you get up to elements heavier than iron, it actually takes energy to fuse atoms together. So iron and everything heavier can’t be used as fuel in nuclear fusion.
Now if stars shine by fusing lighter elements into heavy ones, then eventually there won’t be any light elements left. Stars will run out of fuel. There won’t be enough hydrogen left, since it will be used up.
This would happen about one hundred trillion years from now. However, most of the mass recycled for star fuel will be unavailable “only” a trillion years from now. This is because fusion fodder will by then be locked up in other celestial bodies: brown dwarfs, white dwarfs, neutron stars, black holes, and planets.
The end result is the same: the stars will be snuffed out as galaxies darken.
Ten quadrillion (1019) years in the future, galaxies will begin dissolving like dandelions.
When celestial objects pass by each other, they exchange kinetic energy through the gravitational force they exert on one another. To give an example, Jupiter slingshots a lot of meteors that get too close to it, launching them back into interstellar space. NASA has used this principle to add velocity to crafts like the Voyager 1, as did the guys in that cheesy movie Armageddon.
So, whenever a large body gives a smaller body more speed through its gravitational force, it loses some kinetic energy.
This phenomenon, applied to a huge scale and timeline, will end up meaning that most stars and planets will be ejected from their galactic nests. The ones that remain will be slowed down by giving away their energy. They will, therefore, succumb to the gravitational pull of the supermassive black holes at the centers of their galaxies. Hence, no more galaxies. What we once understood to be a teeming universe will be nothing more than black holes and drifters in the dark.
A wilderness of black holes
In 1040 years, the primary objects in the universe will be black holes. A black hole occurs when an object is so dense that the spacetime around it is too warped for anything, even light, to escape. The point at which objects can enter a black hole but no longer escape is known as its event horizon.
These black holes will eventually evaporate in the process of Hawking Radiation. Hawking Radiation occurs because of quantum activity near a black hole’s event horizon. Though this is a very complex process, a simple way of thinking about it is to imagine particle / anti-particle pairs emerging from and converging back into nothingness on quantum scales all throughout space.
Near the event horizons of black holes, the anti-particle of these pairs gets carried over the edge of the horizon, allowing the positive particle to escape. The escaping positive particle is radiated into space. The negative anti-particle, still “looking” for a bit of positive matter to converge with, eats away a little at the black hole’s mass.
This process will take upwards of 10100 years, ending in some spectacular explosions (well, it would be spectacular if there were any spectators around, which there won’t be.)
By the way, 10100 is a really face-smackingly huge number. The mass of an electron is 1090 times smaller than the mass of the entire universe. So take that difference to the power of ten, and now you have 10100.
Proton decay—unraveling the world’s fabric
It’s likely that protons decay. As you know, the proton is the essential component of the nucleus of an atom. If protons decay, any object consisting of atoms will no longer be possible after a certain point in time. So I spend more time on this topic, but it’s probably the most important aspect of our universe’s future, as far as we’re concerned.
Obviously, atoms are made of protons, neutrons, and electrons. In turn, protons and neutrons are made of quarks. Quarks come in different kinds; some kinds have a positive charge and some have a negative charge.
Remember those little anti-particle / particle pairs I mentioned in the last section? Which arise throughout space on quantum scales? This is part of a phenomenon known as quantum fluctuation.
Given a known amount of time, the amount of energy in a given space may is uncertain, which allows for these particle pairs to pop up. These pairs pop up on the smallest possible scales, briefly violate the law of energy conservation, but then annihilate each other (except in the aforementioned case of Hawking Radiation).
According to these same basic principles of quantum fluctuation, things called “virtual black holes” also pop up. These are really, really tiny black holes.
I think it’s commonly supposed that black holes are big. Actually, anything, including Planet Earth, could be squished down enough to become a black hole. It’s just a matter of density.
When protons in atoms come across a virtual black hole, which in theory they should, these black holes ‘suck in’ quarks, evaporate them into Hawking Radiation, and then leave the remaining proton unstable. Protons normally don’t decay into anything lighter, but after the value of their charge is altered, this is no longer the case.
The end result is that atoms will cease to exist, if the theories are correct.
Death by banality
After 10100 to 10200 years, all normal atoms will be gone.
In this period, the universe is mostly made up of dark matter, photons, electrons, positrons (which are anti-electrons), and neutrinos.
It is assumed that the universe will now reach a very low energy level. In order for energy to be transferred between any two physical systems, there actually has to be an energy difference between those systems.
Will heat death occur?
The second law of thermodynamics states that the total entropy of the universe will never decrease. Entropy is a measure of disorder, information loss, and progresses towards uniform distribution of matter and energy. Such uniform distribution is thermodynamic equilibrium.
Many think that the universe will continue to approach thermodynamic equilibrium, a state of maximal entropy. This will eventually entail that no physical work or information processing will be possible, including any kind of life. This would be as close to death as an infinite universe could approach, and is reminiscent of the Greek concept of chaos: a formless void.
Many physicists, however, think that assumptions about the thermodynamic behavior of the universe are speculative and premature.
But what could combat such a well-established force such as that described by the second law of thermodynamics? One “loophole” is to be found in the Poincaré recurrence theorem. Recall that the total entropy in the universe continually increases. This means that the farther back in time we go, the less entropy there will be. But this theorem states that systems like that of thermodynamics will, due to mathematical and quantum mechanical laws, actually get to a point where the system’s state is very much like its initial state. The timeline for this to occur is astonishingly huge. But as Schopenhauer said, “it is all the same to us when we are dead whether three months or ten thousand years pass away in the world of consciousness.”
Here’s another reason heat death might not be our ultimate fate. Physicists such as Seth Shostak suggest that “Quantum mechanical fluctuations [could] produce the cosmos” by instigating the/a big bang. In low energy states characteristic of a universe approaching thermodynamic equilibrium, the laws of quantum mechanics will begin to make big differences. So during this extremely low-energy period, quantum tunneling or loop quantum gravity may spark another big bang.
The bottom line
While we can make predictions about the future of our sun, galaxy, and universe with a fair amount of confidence even up the black hole era, things become increasingly speculative the farther into the future we go. As Neil deGrasse Tyson wrote, “Most scientific claims made on the frontier will ultimately be disproved, due primarily to bad or incomplete data.”
The issue of whether or not protons will decay is not universally agreed upon, and things become less and less certain when we get into the issues of “big rips” and heat death versus future Big Bangs.
We know the universe will not carry on forever as it is now, as Galileo believed. But the question that now matters most to us is still not answered: will the universe’s death be final, or will it rise again from its own ashes?
As to which future is in store for us, we will have to wait for more information to materialize, which will take some time.
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Kyle York is a writer from the Colorado Springs area.
