The End of Everything (Astrophysically Speaking)

by Katie Mack

Even with the most pessimistic estimates, any Big Crunch event can only occur many billions of years in the future—our universe has been around for 13.8 billion years and with respect to the possibility of future collapse, it is definitely no more than middle-aged.

The leading idea is that dark matter is some kind of as yet undiscovered fundamental particle that has mass (and therefore gravity) but doesn’t have anything to do with electromagnetism or the strong nuclear force.

But even when dark matter was properly accounted for, it was difficult to determine whether the density of matter in space was on one side or the other of the “critical density” that defined the border between a recollapsing universe and an eternally expanding one.

The fact that the discovery indicates that we are almost certainly safe from a fiery death in a Big Crunch has turned out to be cold comfort.IX The alternative to recollapse is eternal expansion, which, like immortality, only sounds good until you really think about it. On the bright side, we’re not doomed to perish in an apocalyptic cosmic inferno. On the, well, dark side, the most likely fate for our universe turns out to be, in its own way, much more upsetting.

That deceleration parameter—the one measuring how quickly the expansion rate was slowing down—was negative. The expansion wasn’t slowing down at all. It was speeding up.

Throw the ball incredibly inhumanly fast and it might just escape

the Earth’s pull and drift out in space forever, always slowing: a universe perfectly balanced between expansion and gravity.

Throwing it even faster than that means it’ll escape and just coast forever, approaching a constant speed as the gravity of Earth becomes less and less of an influence: that’s like a universe that keeps expanding forever, not having anywhere near enough matt...

Figure 10: Open, closed, and flat universes and their evolution over time. The diagrams above indicate the shape of space for three different cosmic models. In an open universe, parallel light beams diverge over time. In a closed universe, they converge. In a flat one, they remain parallel. The different geometries correspond to different fates of the cosmos as shown in the graph. In the closed case, there is enough gravity to cause the cosmos to recollapse, whereas in the open case, the expansion wins out and the universe expands forever. A perfectly balanced flat universe continues expanding but always slowing in its expansion rate. However, if a universe contains dark energy, its expansion can accelerate (while the geometry of space remains flat).

Accelerated expansion meant the cosmological constant had to be revived, with the only small mercy being that it was by then far too late for Einstein to say, “I told you so.”

dark energy

an evolving (i.e., nonconstant) dark energy is often called quintessence

(One downside of the quintessence hypothesis is that it’s theoretically possible for a dark energy that changes over time to violently destroy the universe. For instance, if whatever is accelerating the expansion now turns around, it could cause the universe to stop and recollapse, bringing us back to a Big Crunch after all. Fortunately, that looks very unlikely, though we can’t entirely rule it out.)

dark energy is a cosmological constant: an unchanging property of spacetime that has only recently (i.e., in the last few billion years) come to dominate the evolution of the universe.

A cosmological-constant-induced apocalypse is a slow and agonizing one, marked by increasing isolation, inexorable decay, and an eons-long fade into darkness. In some sense, it doesn’t end the universe exactly, but rather ends everything in it, and renders it null and void.

Figure 11: The Hubble radius now and in the future. As the expansion of the universe accelerates, galaxies that are currently inside our Hubble radius will be outside it. Eventually, no galaxies outside our Local Group will be visible.

“disordered motion of particles or energy.”

The more disordered a system, the higher its entropy.

You can’t hide entropy in black holes, because they have entropy of their own. Which means they have a temperature (they create heat). Which means they are not black at all.

The point of this diversion was to say that we can safely assume that when facing the Heat Death, black holes do indeed evaporate away, leaving nothing but a bit of radiation spreading out through an increasingly empty universe. I hope that helps.

Every time a star burns out or a particle decays or a black hole evaporates, it converts more matter into free radiation, which spreads through the universe as heat: pure disordered energy.

I should just reiterate here that the arrow of time and the Second Law of Thermodynamics are so integral to the functioning of the universe that if there’s no way for entropy to go up, nothing can happen. It is no longer possible for any organized structures to exist, for any evolution to happen, for any meaningful processes of any kind to occur.

all, but in a universe that is in an eternal expansion and contains only a cosmological constant, there’s a lot of time and space for waiting around, and even extremely low-probability events happen.

Poincaré recurrence. If you have an infinite amount of time to work with, any state the system can be in is a state it WILL be in again, an infinite number of times, with a recurrence time determined by how rare or special that configuration is.

In one rather arresting example, physicists Anthony Aguirre, Sean Carroll, and Matthew Johnson once calculated that if you were willing to wait something like a trillion trillion times the age of the universe, you could watch an entire piano spontaneously assemble itself in a seemingly empty box.

If the Big Bang is a state the universe has been in once, and the post–Heat Death universe is eternal (so eternal that, having lost the arrow of time, past and future are meaningless), there’s no reason a Big Bang can’t fluctuate out of the vacuum to start the universe anew.

If every state the universe has ever been in could be revisited through random fluctuations, that means this moment right now could happen again, exactly the same in every detail. Not only could it happen again, it could happen again infinitely many times.

The basic idea of this equilibrium version of de Sitter space is that the origin of our universe and everything that happens in it can be thought of as the result of random fluctuations out of an eternally expanding universe containing only a cosmological constant. From time to time, a universe fluctuates out of the heat bath into a very low entropy starting state, and then evolves forward (with increasing entropy) until it gets to its own Heat Death, decaying back into the background de Sitter universe. And from time to time, the fluctuation doesn’t produce a Big Bang, it just re-creates last Tuesday—specifically, that moment when you stubbed your toe on the kitchen table and spilled an entire cup of coffee on the floor. That moment. And every other moment of your life. And everyone else’s.

de Sitter Equilibrium hypothesis

The Boltzmann Brain problem is the assertion that this unfortunate brain, doomed to quantum-fluctuate back into the vacuum almost instantaneously after its creation, is so vastly more likely to occur than a whole universe that, if we want to use random fluctuations to build our universe, we have to accept that we’re much more likely to be just imagining the whole thing.

If dark energy is a cosmological constant, its defining feature is that the density of dark energy in any given part of space is constant over time, even as space expands. The expansion rate isn’t constant, just the density of the stuff itself, in any given volume of space.

Unless, that is, the dark energy is something more powerful than a cosmological constant.

If the pressure of some weird substance can be negative, it means that it can effectively cancel out the mass of the stuff, at least as regards its impact on the curving of spacetime. If you write down the pressure and density of dark energy in the form of a cosmological constant, in the appropriate units, the pressure is exactly the negative of the density.

It turns out that anything that has a value of w less than -1/3 gives you both negative pressure and accelerated expansion. But knowing the value of w could tell us whether dark energy is a true cosmological constant (w = -1 always), or some kind of dynamical dark energy whose influence on the universe might change over time.

For bodies within the Solar System, we can use lasers or radar (in addition to relationships between orbital times and distances) to measure distances. Distances to nearby stars can be measured with parallax, and Cepheid variable stars can help us determine distances within the Milky Way and some nearby galaxies. For more distant sources, we can use Type Ia supernovae.

The short version of the story is that the Higgs is a kind of energy field that pervades all of space and has interactions with other particles in a way that allows them to have mass. The Higgs boson has the same relationship to the Higgs field that the photon, the carrier of the electromagnetic force (and light), has to the electromagnetic field—it’s a localized “excitation” of something that pervades a larger space. The long version of the story has to do with electroweak theory, the theory that unites the weak nuclear force with electricity and magnetism, and how a process called “spontaneous symmetry breaking” separates those forces.

So if we can make black holes in the LHC, we have evidence for space having more dimensions than we thought.

For a while now, some physicists have suspected that the incongruous weakness of gravity might be forcing them to a similar conclusion. Maybe there’s nothing wrong with the strength of gravity. Maybe there’s something wrong with the universe that’s making gravity seem weaker than it really is. What could make gravity seem weak? The solution might end up being surprisingly mundane. It’s leaking. Into another dimension.

In the large extra dimensions scenario, there is another direction, or several, that we can’t access. All of the space part of our spacetime is limited to a 3D “brane”—think membrane—and a larger space extends outside it in some new direction (or directions) our limited human brains can only conceptualize mathematically.

In this scenario, particle physics and gravity still act fundamentally differently from each other, but not because of their inherent strength.

Concordance Model, or ΛCDM. In this picture, the universe has four basic components: radiation, regular matter, dark matter (specifically “cold” dark matter, CDM), and dark energy in the form of a cosmological constant (denoted in equations by the Greek letter lambda, Λ). The quantities of all these components are precisely measured, with the cosmological constant currently making up the largest slice of the cosmic pie. We have a good understanding of how these things have all varied over time as the universe has expanded, and we have an amazingly detailed description of the very early universe that includes a period of very rapid

tested theory of gravity, Einstein’s general relativity, which in the Concordance Model is taken to be completely correct. In this picture, because the cosmological constant is currently dominating the evolution of the cosmos, we can straightforwardly apply our understanding of gravity and the components of the universe to determine our cosmic evolution. Doing this leads us unambiguously to a Heat Death in the far future. And that’s that.