I've always loved Robert Frost's razor-sharp poem, written in 1920, called "Fire and Ice":
Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
How the world will end has fascinated people for as long as we've been able to think about the question. Various mythologies created their own pictures of the universe's swan song -- the best-known of which is the Norse tale of Ragnarök, when the forces of good (the Æsir, Vanir, and their allies) teamed up against the forces of evil (the Jötnar, trolls, and various Bad Guys like Surtr, the trolls, Midgard's Serpent, Níðhöggr, and, of course, Loki). Interestingly, in the Norse conception of things, good and evil were pretty evenly matched, and they more or less destroyed each other; only a few on either side survived, along with enough humans to repopulate the devastated world.
Once we started to take a more rational view of things, scientists naturally brought their knowledge to bear on the same question. After figuring out about stellar mechanics, we've become fairly certain that the Earth will meet its end when the Sun runs out of hydrogen fuel, swells up into a red giant -- at which point it's likely the Earth's orbit will be inside the radius of the Sun -- then ultimately jettisons its outer atmosphere to become a white dwarf.
But what about the universe as a whole?
When I was in school, just about everyone (well, just about everyone who understood science, anyhow) accepted that the universe had begun at the Big Bang. The mechanism for what caused it, and what (if anything) had come before it, was unknown then and is still unknown now; but once it occurred, space expanded dramatically, carrying matter and energy with it, an outward motion that is still discernible in the red shift of distant galaxies. But would that expansion go on forever? I think the first time I ran into a considered answer to the question was in Carl Sagan's Cosmos, where he explained that the ultimate fate of the universe depended on its mass. If the overall mass of the universe was above a particular quantity, its gravity would be sufficient to halt the expansion, ultimately sending everything hurtling backward into a "Big Crunch." Below that critical quantity -- the expansion would slow continuously but would nevertheless keep going, spreading everything out until it was a uniform, thin, cold gas, a fate that goes by the cheery name "the Heat Death of the Universe."
But it turned out the picture wasn't even that simple. In 1998, Adam Riess and others discovered the baffling fact that the universe wasn't slowing at all, so neither of the above scenarios seemed to be right. Data from distant galaxies showed -- and it has since been confirmed over and over -- that the universe's expansion is accelerating. The existence of a repulsive force powering the expansion was proposed, and nicknamed dark energy, but how that could possibly work was (and is) unknown.
Then they found out that dark energy comprises just shy of three-quarters of the universe's total mass-energy. Physicists had a huge conundrum to explain.
It also led to another possibility for the universe's fate, and one that's even more dire than the Heat Death. If the amount of dark energy per unit volume of space is constant -- which it appeared to be -- then the relative proportion of dark energy will increase over time, because conventional matter and energy is thinning out as space expands (and dark energy is not). As this happens, the relative strength of the dark energy repulsion will eventually increase to the point that it overwhelms all other forces, including electromagnetism and the nuclear forces -- tearing matter up into a soup of fundamental particles.
The "Big Rip."
Confused yet? Because the reason all this comes up is that there's just been another discovery, this one by DESI (the Dark Energy Spectroscopic Instrument) indicating fairly strongly that the force of dark energy has been decreasing over time. I say "fairly strongly" because at the moment the data sets this is based on range from 2.8 to 4.2 sigma (this is an indicator of how strongly the data supports the claim; for reference, 3 sigma represents a 0.3% possibility that the data is a statistical fluke, and 5 sigma is considered the threshold for breaking out the champagne). So it appears that although the quantity of dark energy per unit volume of space is constant, the strength of the dark energy force is less now than it was in the early universe.
So what does this mean about the fate of the universe? Will it be, in Frost's terms, fire or ice? A bang or a whimper? We don't know. The first thing is to figure out what the hell dark energy actually is, and how it works, and -- if the DESI results hold up -- why it seems to be diminishing.
All I can say is the cosmologists have a lot of explaining to do.
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