The following is an excerpt from The End Of Everything (Astrophysically Speaking) by Katie Mack.

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The End Of Everything (Astrophysically Speaking)

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As galaxies get closer together and merge more frequently, galaxies across the sky will burst with the blue light of new stars, and giant jets of particles and radiation will rip through the intergalactic gas. New planets might be born along with those new stars, and perhaps some will have time to develop life, though the terrifying frequency of supernovae in this chaotic, collapsing universe might irradiate the new planets clean. The violence of the gravitational interactions between galaxies and between central supermassive black holes will increase, flinging stars out of their own galaxies to end up caught in the gravity of others. But even at this point, collisions of individual stars will be rare, and they will remain so until very late in the game. The destruction of stars comes about through another process, one that also ensures, with great finality, the destruction of any planetary life that might still be lingering on.

Here’s how.

The expansion of the universe as it is occurring today does more than just stretch out the light of distant galaxies. It also stretches out and dilutes the afterglow of the Big Bang itself. One of the strongest pieces of evidence for the Big Bang is the fact that we can actually see it, simply by looking far enough away. What we see, specifically, is a dim glow, coming from all directions, of light produced in the universe’s infancy. That dim glow is actually a direct view of parts of the universe that are so far away that, from our perspective, they are still on fire—they’re still experiencing the hot early stage of the universe’s existence, when every part of the cosmos was hot and dense and opaque with roiling plasma, like the inside of a star. The light from that long-burned-out fire has been traveling to us all this time, and, from sufficiently distant points, has just now arrived.

The reason we experience this as a low-energy, diffuse background (the cosmic microwave background) is that the expansion of the universe has stretched out and separated the individual photons to the point that they’re now merely a bit of faint static. And the fact that they show up as microwaves is due to extreme redshifting. The expansion of the universe can do a lot, including taking the heat of an unimaginable inferno and diluting and stretching it out until it’s just a faint microwave hum we might experience only as a tiny bit of static on an old-fashioned analog TV.

If the expansion of the universe reverses, this diffusion of radiation does too. Suddenly the cosmic microwave background, that innocuous low-energy buzz, is blueshifting, rapidly increasing in energy and intensity everywhere, and heading toward very uncomfortable levels.

But that’s still not what kills the stars.

It turns out that there is something that can create more high-energy radiation than concentrating the afterglow from space itself being on fire. As the universe has evolved over time, it has taken what was, at the very beginning of the cosmos, a fairly uniform collection of gas and plasma and used gravity to collect that gas into stars and black holes (and other minor things like planets and people, but for the purpose of this discussion we can ignore those). Those stars have been shining for billions of years, sending their radiation out into the void to be dispersed, but not to disappear. Even the black holes have had their chance to shine, producing X-rays as the matter falling into them heats up and creates high-energy particle jets. The radiation produced by stars and black holes is even hotter than the final stages of the Big Bang, and when the universe recollapses, all that energy gets condensed too. So rather than being a nicely symmetric process of expansion and cooling followed by coalescence and heating, the collapse is actually much worse. If you’re ever asked to choose between being at a random point in space just after the Big Bang, or just before the Big Crunch, choose the former. (To quote the legendary D:Ream, “things can only get better.”) The collected radiation from stars and high-energy particle jets, when suddenly condensed and blueshifted to even higher energies by the collapse, will be so intense it will begin to ignite the surfaces of stars long before the stars themselves collide. Nuclear explosions tear through stellar atmospheres, ripping apart the stars and filling space with hot plasma.

The interesting question becomes not “Will anything survive?” but “Can a collapsing universe bounce back and start again?”

At this point, things are really very bad. No planet that survived this long could possibly exist un-incinerated when stars themselves are exploded by background light. From here, the intensity of the universe’s radiation becomes so high that it can be compared to the central regions of active galactic nuclei, the places where high-energy particles and gamma rays shoot away from supermassive black holes with so much power they make jets of radiation a thousand light-years long. What happens to matter in an environment like that, after it’s reduced to its component particles, is uncertain. A collapsing universe will, in the final stages, reach densities and temperatures beyond what we can produce in a laboratory or describe with known particle theories. The interesting question becomes not “Will anything survive?” (because by this point it is very clear that the answer to that is a straightforward No), but “Can a collapsing universe bounce back and start again?”

Cyclic universes that go from Bang to Crunch and back again forever have a certain appeal in their tidiness. Rather than a beginning from nothing and catastrophic, final end, a cycling universe can in principle bounce along in time arbitrarily far in each direction, with endless recycling and no waste.

Of course, like everything in the universe, it turns out to be significantly more complicated. Based purely on Einstein’s theory of gravity, general relativity, any universe with a sufficient amount of matter has a set trajectory. It starts with a singularity (an infinitely dense state of spacetime) and ends with a singularity. There isn’t really a mechanism in general relativity to transition from an end-singularity to a beginning one, however. And there is reason to believe that none of our physical theories, general relativity included, can describe the conditions of anything close to that kind of density. We have a pretty good understanding of how gravity works on large scales, and for relatively (ha!) weak gravitational fields, but we have no idea how it works on extremely small scales. And the kinds of field strengths you’d encounter when the entire observable universe is collapsing into a subatomic dot are all kinds of incalculable. We can be fairly confident that for that particular situation, quantum mechanics should become important and do something to make a mess of things, but we honestly don’t know what.

Another problem with a bouncing Crunch-Bang universe is the question of what makes it through the bounce. Does anything survive from one cycle to another? The asymmetry I mentioned between an expanding young universe and a collapsing old one, in terms of the radiation field, is actually potentially very problematic here, as it implies that the universe gets (in a precise, physically meaningful sense) messier with every cycle. That makes the cyclic universe less appealing from the standpoint of some very important physical principles that we’ll discuss in later chapters, and it’s certainly more difficult to fit into a nice neat reduce-reuse-recycle ecology.

Excerpted from The End of Everything: (Astrophysically Speaking) by Katie Mack. Copyright © 2020 by Katie Mack. Excerpted by permission of Scribner, an imprint of Simon & Schuster, Inc.