The icy fate of the universe

(Somber music)

Frost poem reading: 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.  

Daniel Leonard: That’s the opening to Robert Frost’s famous poem “Fire and Ice.” While Mr. Frost may favor fire, astronomers are now quite convinced that the world — or, rather, the entire universe — will end in ice.

(Pensive music) 

They call this theory the “heat death of the universe,” or the “Big Freeze.” But why do astronomers believe this? I turned to my Philadelphia-based correspondent to discuss that question.

Theresa Leonard: My name is Mary Theresa Leonard, and I was born in Emmitsburg, Maryland in 1926, which makes me… really old. But that’s okay.

Daniel Leonard: Theresa Leonard is also my grandmother. Theresa is knowledgeable about many things, but — like many of you listening — she doesn’t really keep up with the field of cosmology. So I was curious what her best guess would be about the ultimate fate of the universe. 

[To Theresa:] So actually my first question is, do you know how scientists think the universe began?

Theresa Leonard: Uh… Big Bang.

Daniel Leonard: The Big Bang theory, of course, is the widely-accepted theory that the universe began in a tiny, compressed state, then exploded outwards and began rapidly expanding. 

[To Theresa:] But are you familiar with how scientists think the universe will end?  

Theresa Leonard: Well, I hope not soon…

Daniel Leonard: No, thankfully, uh, not soon. But, do you think it’ll end in another big, hot explosion? Like how it began? Or, might it end in a cold, dark state?

Theresa Leonard: A dark space. I think that’s what’s gonna happen. Just because I think that would be better…

Daniel Leonard: Yeah?

Theresa Leonard: For the end of the world. To just fade off.

(Pensive music returns) 

Daniel Leonard: Well, my grandma happens to be in luck. Currently, the consensus among scientists is that the universe will end, not with a bang (Gong sound), but with a whimper, as it slowly fades into ice-cold darkness. I sat down with a cosmology expert to get a better understanding of this theory.

Abhishek Maniyar: Rather than a theory, I would — at this point, I would call it a prediction coming out of what we think our universe is made up of.

Daniel Leonard: That’s Abhishek Maniyar, a postdoctoral researcher in cosmology at New York University. 

Abhishek Maniyar: 68% of the universe is made up of something called dark energy. And it’s because it’s the most dominant component of the universe, and its basic property is pushing things away from each other, this is what is causing the expansion of the universe to accelerate.

Daniel Leonard: In other words, because of this mysterious, widespread dark energy, the entire universe is expanding at a faster and faster rate.

Abhishek Maniyar: And if you just keep on extrapolating this, everything will start going away from each other, the stars will eventually run out of fuel. And new stars are not going to form. Because how do stars form? They form from colliding gaseous clouds, but these clouds are now so far away from each other, they are never going to collide. The only objects that would be left would be the black holes, and black holes evaporate as well.

Daniel Leonard: So, old stars are dying, new stars aren’t being born — and, essentially, the entire universe is heading towards disorder. Abhishek told me that that trend towards disorder is actually a fundamental law of thermodynamics. Picture a sugar cube dropped into a cup of water. 

(Splash sound)

Whereas the sugar cube was once an orderly structure, it’ll almost immediately head towards disorder, with the sugar molecules randomly mixing in with the surrounding water molecules. The same is true of the entire universe. Right now, the universe’s energy is organized into little orderly pockets of stars, planets and black holes separated by vast distances of nothingness. But as the stars and black holes die out, their energy will eventually spread out evenly across the universe — like sugar molecules in a glass of water.

Abhishek Maniyar: So what’s going to happen eventually is like, everything will be at the same temperature. When the Big Freeze finally happens, everything will be at equilibrium, all the stuff in the universe. And that’s one more reason that you cannot really get stuff done. Because if you want to get stuff done, you need to have some difference in the temperature, in a sense.

Daniel Leonard: So the universe’s limited energy is going to get spread out evenly across the vastness of space. Once that happens, and everything is equally cold, all the action in the universe will grind to a halt. 

(Music slows to a stop) 

That’s why they call it “the Big Freeze.”

Theresa Leonard: Okay… I think that’s better than getting hotter and hotter. If I had a choice of being too cold or too hot, I would take too cold.

Daniel Leonard: I asked Abhishek what that final universe would look like. 

Abhishek Maniyar: It’s all dark, there is nothing to look at, because all the visible matter has evaporated.

(Wind sound)

Like, the stars, stars have no fuel left, they have all died. There are no new stars forming so there is no new light. Black holes… they would evaporate by that time as well. And then supermassive black holes — which are like the last remaining objects in the universe — they would eventually evaporate as well. So there is no light left in the universe. It’s all dark, and empty, and cold.

Daniel Leonard [to Abhishek]: Wow. And in roughly how many years is this projected to happen?

Abhishek Maniyar: One of the numbers I’ve seen somewhere is 10 to the power of 150 or 160… So yeah, 150, 160 zeros after ten.

Daniel Leonard: That number is a rough estimate for when all the supermassive black holes in the universe will have faded away — though many studies put that figure slightly lower, at 10 to the power of 100 years. But, even after that, the universe will still exist as a cold soup of particles, until even these particles reach a state of equilibrium many, many years later. That’s all trillions, upon trillions, upon trillions of years from now — well after our own Sun burns out in 5 to 10 billion years. So, naturally, it’s highly unlikely any humans will still be around for the Heat Death.

Abhishek Maniyar: No matter what happens, we are not going to be there to see it. So, I think, yeah, that doesn’t make me sad, but that still makes me curious, let’s say. There are many, many other things — small, small things — that make me sad. So the universe doesn’t. (Laughs)

(Somber music returns)

Daniel Leonard [to Theresa]: And how does the heat death theory make you feel, grandma? Sad?

Theresa Leonard: No. I don’t think we’re gonna make it. Yeah, you can’t worry about what’s going to happen way down the line.

Daniel Leonard: Returning to the wise words of Robert Frost…

Frost poem reading: I think I know enough of hate / To know that for destruction ice / Is also great / And would suffice.

Daniel Leonard: For Scienceline, I’m Daniel Leonard.

(Music fades)