The End of Everything (Astrophysically Speaking)

by Katie Mack

Eschatology—or more specifically, the end of the world—provides a way for many of the world’s religions to contextualize the lessons of theology and to drive home their meaning with overwhelming force.

Perhaps the promise of a final judgment serves to somehow make up for the unfortunate fact that our imperfect, unfair, arbitrary physical world cannot be relied upon to make existence good and worthwhile for those who live right. In the same way a novel can be redeemed or retroactively ruined by its concluding chapter, many religious philosophies seem to need the world to end, and to end “justly,” for it to have had meaning in the first place.

Secular stories of the end, on the other hand, run the gamut from a nihilist view that nothing matters at all (and that nothingness ultimately prevails) to the heady notion of eternal recurrence, where everything that has happened will happen again, in exactly the same way, forever.

We are a species poised between an awareness of our ultimate insignificance and an ability to reach far beyond our mundane lives, into the void, to solve the most fundamental mysteries of the cosmos.

It is said that astronauts returning from space carry with them a changed perspective on the world, the “overview effect,” in which, having seen the Earth from above, they can fully perceive how fragile our little oasis is and how unified we ought to be as a species, as perhaps the only thinking beings in the cosmos.

Acknowledging an ultimate end gives us context, meaning, even hope, and allows us, paradoxically, to step back from our petty day-to-day concerns and simultaneously live more fully in the moment. Maybe this can be the meaning we seek.

Uniting cosmology and particle physics has already allowed us to measure the basic shape of spacetime, take an inventory of the components of reality, and peer back through time to an era before the existence of stars and galaxies in order to trace our origins, not just as living beings, but as matter itself.

cosmology refers to the study of the universe as a whole, from beginning to end, including its components, its evolution over time, and the fundamental physics governing it. In astrophysics, a cosmologist is

anyone who studies really distant things, because (1) that means looking at quite a lot of universe and (2) in astronomy, faraway things are also far in the past, since the light that reaches us from them has been traveling for a long time—sometimes billions of years.

phenomenology—the space between the development of new theories and the part where they’re actually tested.

To a cosmologist, the past is not some unreachable lost realm. It’s an actual place, an observable region of the cosmos, and it’s where we spend most of our workday.

Everything you see is in the past, as far as you’re concerned. If you look up at the Moon, you’re seeing a little over a second ago. The Sun is more than eight minutes in the past. And the stars you see in the night sky are deep in the past, from just a few years to millennia.

cosmological principle. Simply stated, it’s the idea that for all practical purposes, the universe is basically the same everywhere.

So when we look at a galaxy a billion light-years away, and see it as it was a billion years ago, in a universe that was a billion years younger than our universe is here and now, we can be pretty confident that the conditions here a billion years ago would have been fairly similar.

The thing we’re watching is essentially fully in the past, but the “now” for that exploded star is unobservable to us, and we won’t receive any knowledge of it for millions of years, which makes it, to us, not “now,” but the future.

spacetime—a kind of all-encompassing universal grid in which space is three axes and time is a fourth—we can just think of the past and the future as distant points on the same fabric, stretching across the cosmos from its infancy to its end.

And it didn’t happen at a single point, but at every point. Every point in space in the universe today—a spot on the edge of a distant galaxy, a piece of intergalactic space just as far in the other direction, the room in which you were born—every one of these points was, at the beginning of time, close enough to touch, and at that same first moment, rapidly tearing away from one another.

we can see that distances between galaxies are getting larger over time—which

and perhaps probable, that the universe is infinite in size. Which means that it was infinite at the beginning too. Just much denser.

Every point in space is the center of its own sphere of ever deepening time, bounded by a shell of fire.

The cosmic microwave background, or CMB, went on to become one of the most important tools we have for studying the history of the universe. It’s hard to overstate its significance, both as an astronomical data set and as a technological achievement. We can now collect, analyze, and map the glow of the hot early cosmos. The first thing it tells us is: the hypothesis that the early universe was one big inferno, glowing with heat, is completely confirmed.

blackbody curve

But the CMB, right in the middle, gives us a solid anchor to extrapolate from in each direction, and now we have a compelling narrative for how the universe evolved over time, starting from the first billionth of a billionth of a billionth of a second and arriving at today, 13.8 billion years later.

Only, a singularity doesn’t have to be a point—it could just be an infinitely dense state of an infinitely large universe.

So the singularity is one hypothesis for what might have started everything off, but we can’t really be sure.

There’s also the question of what was “before” the singularity. That question might be, depending on who you ask, incoherent nonsense (because the singularity was the beginning of time as well as space, so it had no “before”), or one of the most crucial questions in cosmology (because the singularity might have been the end point of a previous phase of a cyclic universe: one that goes from Big Bang to Big Crunch and back again forever).

But at extreme densities you have to contend with both, and they don’t work well together, at all. Extreme gravity involves well-defined massive objects that warp space and alter the flow of time; quantum mechanics allows particles to pass through solid walls or exist only as fuzzy probability clouds. The fundamental incompatibility of our theories of the very massive and the very small is one of the things that hints at the direction we should go in creating new, more complete theories. But it is also rather inconvenient when we’re trying to explain the very early universe.

To sum up where we’ve gotten so far: there may have been a singularity. If there was, it was immediately followed by an era we can’t really say much about called the Planck Time.

In some ways, this is the ultimate goal of theoretical physics: to find a way to take all the complicated messy stuff we see around us and rearrange it into something pretty and compact and simple that just looks complicated because of our weird low-energy perspective.

There’s a general belief among physicists that sometime around the Planck Time, gravity was somehow unified with the other forces (along with the dragons or whatever else was happening then). But, as we discussed before, general relativity and particle physics don’t like to work together in their current form, and so we’ve made even less progress toward a TOE than a GUT.

The problem with the standard picture of the early universe is that it doesn’t include a situation in which two distant parts of the universe could interact and come to an agreement about a temperature. If we take two points on opposite sides of the sky and work out how far apart they are now, and how far apart they were at the very beginning, 13.8 billion years ago, it becomes clear that there was never a time in the history of the universe when they were close enough that light beams could have traveled back and forth between them to carry out the equilibrium process. A beam of light that started at one of those points at the beginning of the universe would not have had time even in 13.8 billion years to cover the distance necessary to get to the other. They are, and always have been, outside each other’s horizons: unable to communicate in any way.

The basic idea is that there was a time in the early universe, after the singularity but before the end of the primordial fireball stage, when the universe was expanding astonishingly fast.

Figure 6: Cosmic timeline. The size of the observable universe increased rapidly during inflation, just after the very beginning. The universe has been expanding (at a slower rate) ever since. Labeled here are some of the important moments in the history of the cosmos.

Not only were the forces of nature

operating under different laws, the universe contained a different mix of particles, and temperatures were so high that bound states of quarks could not exist in a stable form. Quarks and gluons bounced around freely in a hot roiling mix called a quark-gluon plasma—kind of analogous to the inside of a fire, but nuclear.

This “quark era” lasted until the universe reached the ripe old a...

As an aside, the fact that pretty much all the hydrogen in the universe was produced in the first few minutes means that a pretty large fraction of what you and I are made of has been hanging around the universe in one form or another for almost as long as the universe has been here. You may have heard that “we are made

of stardust” (or “star stuff” if you’re Sagan), and this is absolutely true if we measure by mass.

All the heavier elements in your body—oxygen, carbon, nitrogen, calcium, etc.—were produced later, either in the centers ...

This is what we see when we look at the cosmic microwave background: the moment that delineates the end of the Hot Big Bang, and the transition to a universe in which space is dark and silent and light travels through it.

It’s not really dark, but rather invisible: seemingly unwilling to interact with light in any way.

In about four billion years, Andromeda and our own Milky Way galaxy will collide, creating a brilliant light show.

And from our perspective, all the galaxies farther out than our near neighbors are careening away from us as fast as they can. It’s not us; it’s cosmology.

Redshift or blueshift is now one of the most straightforward things to observe about any source of light in the universe, provided a spectrum is taken and it has any recognizable line patterns at all.

a galaxy’s apparent speed is directly proportional to its distance.

The crucial part for us here is the connection between redshift and distance. It means that we can look at a distant galaxy, measure the redshift, and determine from that exactly how distant the galaxy is. (With some technical caveats.)

If we measure a galaxy’s redshift, we know how quickly it’s receding from us, and we can use the Hubble-Lemaître Law to get its distance.

“High redshift” is long ago when the universe was young; “low redshift” is more recent.

the very beginning of the universe, if we could see it, would have a redshift of infinity.

Starting with some very basic and reasonable physics assumptions, there appear to be only three possibilities for the future of an expanding universe, and they are all fairly direct analogs to what can happen to a ball thrown into the air.