Image credit: NASA/WMAP
What is the cosmological constant? How is the expansion of the universe related to quantum fields? How does a vacuum energy produce accelerated expansion? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Let's say you fall off ladder. Don't worry. You won't get hurt in this mental game in this scenario. It it's it's just for pretend, but you're gonna fall off a ladder. As you fall off the ladder, what do you feel?
You feel the pull of gravity. You for a moment, you're hanging in midair. You feel that sensation of free fall. The gravity of the Earth is accelerating you down, and then, bam, you actually slam into the ground and get hurt. Ask yourself, what would happen if that ladder were, I don't know, on a beach next to the ocean?
You would get hurt. You know, the sand might cushion your fall, but you would still fall. You'd still hit the ground with the exact same speed. What if you took that exact same ladder and brought it up to the top of Mount Everest? Like, now you're in base camp.
You're all the way at the top. You're at the summit, the highest point on Earth. You put that ladder up. You climb up the ladder so you're the tallest thing in the world, and then you fall off the ladder and you hit the ground right there, the peak of Mount Everest. I'm not gonna ask why you would do that, but it this it's for the purposes of this metaphor.
When you fall the distance off the ladder, when you hit the ground, whether it's on the top of Mount Everest or on a nice warm sandy beach, you're gonna have feel the exact same pull of gravity. You're gonna have the exact same brief feeling of weightlessness, and you're gonna have the exact same smack when you hit the ground. Why are they the same? Because gravity cares about differences. It doesn't matter where the ladder is.
What matters is that you fell off the ladder and hit the ground at the base of the ladder. Your your sea level, your absolute distance from some reference point, doesn't matter. What matters is the relative difference. And this is gravity. Gravity is a story about differences.
It's true with good old fashioned Newtonian mechanics, and it's true with the new hotness that is Einstein's general relativity. If you're calculating gravitational interactions on the surface of the Earth, like an apple falling from a tree, you falling from a ladder, you can add a constant number to the mathematics, to the equations, like the height of the ground, the sea level of the ground that you're about to hit, and all the results come out the same. For once, we have something comforting where the mathematics are representing something in our everyday experience. We know it we know it doesn't matter where you put the ladder. What matters is the distance that you fall.
That's not gonna be true later in the episode. When we go from Newtonian to Einsteinian to general relativity, General Relativity also allows you to add a constant number to the equations that doesn't really affect anything. This number that you just put into the equations represents, for lack of a better way to phrase it, and this is going to be a common theme throughout the episode, it represents the mass of empty space. If you have a box of nothing, no particles, no radiation, nothing, zilch, zero, just nothing, 100% pure rarefied vacuum, you can ask a question. How much does it weigh?
How much does empty space weigh? And if you've ever heard the phrase, how many angels can dance on the head of a pin, we're basically going down the same road except for science, and we're gonna get a really surprising result at the end. This episode, of course, is about dark energy, and I'm gonna talk about our attempts to solve the mysterious nature of dark energy, which is the name we give to the accelerated expansion of the universe, and if you haven't listened to the episode from a year ago where I introduced the topic, you should now. And the ultimate answer to that, what is dark energy, appears to be answering a different question, which is how much does empty space weigh? It could be zero.
Empty space could weigh nothing at all. A vacuum could be a vacuum. It could be 10. It could be 10 to the minus 10 or 10 to the 10. It could be 10 to a thousand or 10 to a minus thousand.
It could be 57. It could be 57 and a half. When you just start out with the raw equations of general relativity, you don't know how much empty space weighs, how much mass or energy is in the vacuum, the empty space time itself. It's a constant, and it doesn't affect the dynamics. If you're surrounded by empty space and there's mass associated with empty space, then, you know, there's some empty space right in front of you, so you'll feel a gravitational pull right in front of you.
But there's also some empty space right behind you, so you'll feel a gravitational pull right behind you, and those two will balance out. Same with left to right, same with up and down. It won't affect you because it's affecting you equally everywhere. So it doesn't change your dynamics and your motion. But if you were to put a Jupiter there in front of you, then you would notice the Jupiter because the jupiter sticks out from the background like a ladder sitting on top of a beach or sitting on top of Mount Everest, and you would feel the gravitational attraction of Jupiter because it's something special.
It's something extra. Now this number, the weight of empty space, has a fancy name, of course. We call it the cosmological constant because it's constant. It's just a number, and it's spacey, I guess. So, you know, you put those together cosmological constant, and actually because Einstein himself was the first person to stick it in to general relativity.
But why bother sticking in? Why should we care about the weight of empty space if I just went through the motions of how it doesn't matter at all? Well, I need to be more specific. It doesn't matter locally. It won't affect your motion through the solar system.
It won't affect the motion of the galaxy and our local group and clusters of galaxies. It doesn't matter there. But we're talking about cosmology here. We're just talking about the whole stinking universe. And the universe cares how much the vacuum weighs because general relativity is the connection between mass and energy and the bending of space, which we have explored in exhaustive detail a few episodes back.
Mass and energy tells spacetime how to move. Bending of spacetime tells mass and energy how to move. And figuring out the entire history of the universe is actually and this is frightening if you think about it too much one of the simplest, most straightforward applications of general relativity around. It actually boils down to a relatively simple set of equations. And it's a super straightforward process.
You add up all the stuff in the universe gas, dust, dark matter, termites, radio waves, missing socks, everything, you tally it all up, and that stuff tells the universe how to bend, what shape it is. And when it comes to cosmology, it's telling space how to expand, how quickly to expand, or slowly to expand, or change its direction and start contracting instead of expanding. Like I said, that's it. That's it. That's the entire game of cosmology.
Easy peasy lemon squeezy. And when I say add up all the stuff, I really do mean all the stuff. If empty space, the pure vacuum, has a mass to it, has an energy associated with it, then it gets counted in the total and it will affect the evolution of the universe. So even though it doesn't matter here, it does matter out there. So what could this mysterious number be now that we actually have to worry about it?
It's it's a quantity that we can't ignore because it is going to affect the evolution of the universe, because everything affects the evolution of the universe. Your first instinct might be to make it zero. So you know what? Yes, you can add it to the equations, but it really doesn't matter. And we can it really is zero.
We can just go back to ignoring it, pretend it never happened, and just go on with our lives. But Einstein found that if you set this cosmological constant, which at the time you had no idea how to even, like, predict the value of this number, so he said, okay, let's just make it zero for simplicity's sake. You find a slight hiccup. You find that the universe wants to move. That if you have a universe with no cosmological constant, it's just made of matter and radiation, the universe wants to move.
It's not stable. And this is the ultimate answer. When people ask, like, why does the universe expand? It's because that's what the universe wants to do. The natural state of our universe is to be in motion either expanding or contracting.
Why we happen to see an expanding rather than contracting universe, that's another show. But the universe wants to move. The universe wants to expand. This is its natural state. A perfectly static, balanced universe that doesn't move is unnatural and unstable, a slight hiccup.
A little cough. If you imagine everything froze and the and the expansion of the universe froze in this moment and everyone held their breath, don't move, don't move, as soon as one person just does that one, like, or, like, starts giggling or something, then that's it. It sets off a chain reaction that sets the universe back in motion. And at the time when Einstein was developing general relativity, we thought the universe was static. All the observations we had said we live in a static, unmoving, unchanging universe, and then his general relativity equations came out and said, oh, by the way, the universe should be expanding or contracting.
He's like, that's not right. So he inserted into the equations. He took this cosmological constant, which he had the freedom to do because there is no reason to prefer one number over another. He picked a number to counteract the natural motion of the universe and make it static. And then Edwin Hubble came along and pointed out that, yeah, we live in an expanding universe.
And I guess when no one was looking, Einstein went in and erased the cosmological and he's like, just kidding, guys. Just kidding. No. No. No.
I wasn't serious about the whole time. He actually called it his greatest blunder because he had a chance. He had a chance. Years before the discovery of the expanding universe, he had the chance to predict that we live in a dynamic expanding universe. But he shied away from it, which you know what?
He had good evidence to do so. And it's just a shame. You know? If Einstein had called it, he, you know, he could've won the Nobel Prize. He'd be world famous.
We're talking about him to this day, but but he missed that boat. He missed that boat, and, he's gonna be irrelevant, I guess. As a side note, I should mention that even if you introduce a cosmological constant like Einstein did to try to force the universe to be static, Later cosmologists did some analyses and figured that the universe can basically never be stable. Even with a cosmological constant, the preferred state, its lowest energy configuration is to be in motion, to be expanding or contracting. But back to the story.
So the cosmological constant as a concept was dropped, like, okay, maybe it really is zero. We mean it this time. And in the decades, it would come up every once in a while like, hey, everyone. Maybe it's not zero. And then we're like, nope.
Nope. No. It's zero. It's zero. And this kind of went back and forth all the way until the late nineteen nineties when we found that the expansion of the universe is accelerating.
That's right. It's not just getting bigger. It's getting bigger and bigger, faster and faster every single day. And I know I introduced this topic a year ago, and a year ago I said soon I would talk about the nature of dark energy, And I guess one year is soon in Ask a Spaceman time. I could make these episodes faster if I had some help from Patreon.
Patreon dot com slash p m sutter is how you support this show. Go to patreon.com/pmsutter because you can help keep these shows going. I can't really promise I'll make them faster, but they'll be able to keep going, which sometimes is more important than being faster. Didn't we learn a lesson from the tortoise? Anyway, once we discovered that the expansion of the universe is accelerating, we had to cast them out for an explanation.
Like, okay. Now we have an observational mystery. Who's the culprit? Who's doing this? Who's making the universe accelerate in its expansion?
In any time your theory, in this case general relativity, doesn't match up with observations, you can blame the theory. Maybe Einstein was wrong. Or you can blame the observations. Maybe we don't understand the universe. In the case of general relativity, which is what we're doing using to apply to this problem, either we don't understand gravity, the equations of general relativity are incomplete on cosmological scales, or we don't understand the ingredients of the universe.
Does this story sound familiar? We had the exact same discussion about dark matter. And as far as we can tell, which we've checked, general relativity is correct. That's a story for another day. I'd I'd love to do a a podcast entirely about why we still think general relativity is correct in the face of things like dark energy.
The short answer is all alternatives to general relativity fail, and they fail pretty bad. And so general relativity is less left standing. So I wanna focus on the most likely solution to dark energy, our best explanation for what is this accelerated expansion of the universe. And I should say most likely is a very relative term because we basically don't understand dark energy. And you might have guessed because I introed this episode by talking about the cosmological constant, that's where we're gonna head down for our discussion of dark energy.
We're gonna resurrect Einstein's cosmological constant, bring it back from the dead, brush it off, give it a new coat of paint, and call it dark energy. Remember, the cosmological constant is just a number that you can add to the equations of general relativity. You're free. Like being able to set the sea level for the scenario of you falling off a ladder, there's this constant that you can add that describes the the weight of the vacuum. And even though I can say it's the weight of the vacuum, it's it's that doesn't really have, like, a strong physical significance or meaning.
Like, I could just say, like, oh, yeah. Empty space weighs a little bit. And you'd be like, really? Like, what causes that weight of empty space? There's nothing in general relativity that causes empty space to have weight.
Weight. There's nothing there's no other place in the equation where you can say, oh, yeah, yeah, yeah, the vacuum has a weight to it because of x, y, z. Instead, you have to turn to other kinds of physics to give you some sense of what's going on. Thankfully, we have more than Einstein on the case. We have Zel'dovich.
Yakov Zel'dovich, perhaps the greatest physicist that you've never heard of. Nuclear physics pioneer, magnetism pioneer, black holes pioneer, cosmology pioneer, the works, an absolute genius. Why haven't you heard about him? Because he was the Soviet scientist and his access to the West and Western scientists was, let's say, highly restricted and controlled. So he would work and work and work and work, and then every few years he would be allowed to go to a conference or have some visitors, and he'll be all like, by the way, since the last time we chatted, I totally revolutionized your field.
Anyway, gotta go. Here's some notes. Since the fall of the Soviet Union in the early nineties, he's beginning more and more deserved recognition, but, anyway, this is not Zeledovich's story, but feel free to ask. I'd love to tell his. But he did make a key insight.
Remember the very old episode I did about vacuum energy and quantum fields? How each particle, like an electron or top quark or a neutrino or a photon, has its own quantum field associated with it. And the quantum field completely soaks all of space time. In fact, in this quantum field theory picture, the quantum fields are the most important physical objects, and it's when you excite the field or ring it like a bell or pinch it off, that's when you get something that you call a particle. Good times.
Good times, and they're gonna play a starring role here. Back in the sixties, Zel'dovich realized that the quantum vacuum fields that are at the essence of quantum field theory in our understanding of how particles and forces work, the fields are Einstein's cosmological constant. The weight of the vacuum that Einstein identified as a number that he had to pull out of the hat or other places is caused by the fundamental quantum fields that soak all of space time. There it is. It's a physical explanation.
It's a real thing that we can sink our teeth into. We know about quantum fields. We understand them. We manipulate them. We have the technology to deal with quantum fields.
We can slot these in to general relativity to give us a cosmological constant and understand dark energy. Except not. Except it totally, totally, totally breaks down. Before I get into why it breaks down, I want to answer a couple of burning questions that I think think you might have. And the answers to the question are not easy at all, and I apologize in advance.
I'm gonna do my best. They're super simple question. It's this is always the best thing when really simple questions lead to deep insights into the nature of reality. Here's two questions. If the cosmological constant is the weight of the vacuum, if it's caused by these quantum fields, then as the universe expands, you know, the universe is getting bigger and bigger every day, that means you're getting more and more dark energy every day because the density remains constant, but the volume of the universe keeps going up.
So you're getting more and more stuff in our universe, and we call this stuff dark energy. And every single day, there's more dark energy than there was the day before. Doesn't this violate conservation of energy or something I thought energy couldn't be created and destroyed, and here I am in the story talking about how energy is being created all the time? The second question is, how in the world does a cosmological constant slash vacuum energy, whatever you want to call it, called Steve or Cassandra for I don't care, how does it make the universe accelerate in expansion? What's the mechanism there?
What's the driving force? How how does it put the foot to the gas pedal to really get this universe going? First question first. I always enjoy personally when someone when I read about someone, you know, talking about dark energy or other some other physics concept and brings up violation of conservation of energy. Like, hey.
Hey. Doesn't this violate conservation of energy? No. It doesn't because how we predicted the existence of dark energy or the cosmological constant is from our physical theories. Our physical theories have conservation of energy baked into them from the start.
General relativity is just a very, very fancy way of stating conservation of energy and conservation of momentum in gravitating systems. Quantum field theory, conservation of energy, conservation of momentum in quantum particle systems. These are our ways of expressing these fundamental concepts. So it's impossible for something like dark energy to violate conservation of energy because it's built into the equations that predict its very existence. Let me explain how.
The conservation of energy that you learn about in high school or college or even grad school isn't the whole picture. Conservation of energy applies in static closed systems. It's only applicable to a limited number of cases when the system you're studying doesn't change with time. The universe is changing with time. This was a big deal when we discover it, so conservation of energy as you might know it doesn't strictly apply.
It has to get modified from its familiar form, from its familiar statement. And in that modified form, you could have things like energy being added to the universe. It's just no biggie. We're replacing intuitions here on Ask a Spaceman. That's what we do.
That said, so one answer is just to chuck your entire notion of conservation of energy because it doesn't apply in the way you think it does to the whole entire universe. The other way is to to kinda talk about different forms of energy. So let me give you an example because this is a case where the mathematics are incredibly simple, but the words we need to use to describe it are very, very long and complicated. You can find certain definitions of energy, quote, unquote, to paint a pit prettier picture if you wanted to if you wanted to. It's not universally, useful.
It's not entirely applicable to all systems. We can paint this picture to help wrap our minds around the cosmological scenario. And I want you to imagine the most ridiculous tug of war you've ever seen. On one side, you have a bulldozer, something that's like, it's impossible to to pull it back. Okay?
It's just big and hulking and mechanical. Instead of a a normal rope, you have the world's best stretchiest bungee cord. Like, this thing can stretch like nobody's missing. It could stretch to infinity if you worked hard enough. It's just really great bungee cord.
And on the other end, fighting against the bulldozer with the stretchy bungee cord is, I don't know, the world a world's strongest man competition. You know, a bunch of a bunch of beefy dudes who are gonna try to fight this bulldozer, pull the opposite direction with this infinitely stretchable bungee cord. And it's a group effort. It's a group effort. It's not one on one.
We're gonna assemble all the world's strongest men. And when you know, just everyone. We're just gonna put everyone in. If you've got a muscle, you can participate, and we're gonna put you up against this formidable bulldozer. So the expansion of the universe is the bulldozer.
It's this inevitable thing. The universe is getting bigger with time. Spacetime itself is stretching out, bringing more and more spacetime every day that goes by. Our rope is stretching out. In the dudes, the world's strongest men are dark energy.
The bulldozer pulls, the rope stretches. There's more rope. So say you had four guys all lined up as close as they can get fighting against the bulldozer. The bulldozer pulls, there's more rope. You can add another dude.
You can put him at the end. Now you got five dudes pulling, pulling, pulling, rope stretches. Add another dude. Six dudes, seven dudes, eight dudes. As the universe expands, we can add more dark energy to it.
There's more dudes getting in on the rope trying to fight the bulldozer. It looks like energy is not conserved. Where are these dudes coming from? How did you get so many? We started with five.
Now we have six. Now we have seven. Now we have a thousand. As long as we can keep extending the rope, we're getting even more dudes. There are more dudes on the rope than the population of the planet.
Like like, where are you coming up with these people? But they're pulling, aren't they? They're trying they're resisting the expansion of the universe. They would prefer the universe not expand. They're pulling.
They're pulling as hard as they can. They have tension. They are resisting that expansion. They are doing work against the direction of the stretch. The bulldozers pull in one way, but they're doing tug of war, so they're pulling in the opposite direction.
They have attention. And in physics, in physics, work done opposite the direction of movement, if you're applying a force opposite the direction of movement, if they're bulldozers stretching one way and you're pulling the other, you're doing work with a negative sign attached to it. It's it's how we have to define things in physics so everything is consistent and you're applying a force in the opposite direction. You're applying attention that is work with a minus sign that is energy with a minus sign attached to it. Energy with a minus sign attached to it.
A negative energy and its imbalance. For all the energy that you add to the system by adding another dude and he has mass, you're taking some energy out in the form of negative work. It's always in balance. The positive energy of adding dudes is balanced by the negative energy of adding tension to the bungee cord that is opposing the expansion. So in that view, if you want to define energies that way, which you're totally legit allowed to do, everything's in balance in the net energy expended by the universe is zero.
Some is added by dark energy, by extra dark energy being added to the universe, but then some is taken away because dark energy is trying to resist expansion. And that's where the acceleration comes in in the most non intuitive way because we're about to seriously go off the rails, folks. This is a slippery concept. I'm gonna do my best. No promises.
Just hold on. Dark energy wants to resist expansion. It has energy just like mass. If I have a a big galaxy, a galaxy is gonna try to fight the expansion of the universe with its gravity. The gravitational attraction between components of the universe, between matter and dark energy, wants to fight the expansion.
But that desire is its own undoing. It accidentally leads to accelerated expansion. General relativity is matter and energy telling space time how to bend. When it comes to cosmology, this bending is manifested by the expansion of the universe and specifically the expansion rate. How quickly or slowly the expansion of the universe occurs is determined by the contents of the universe.
All forms of energy must count. Mass density? Check. Energy density? Check.
Radiation? Check. Tension? Check. Tension is a kind of energy and must be counted.
Almost all things slow down the expansion of the universe. Gravitational attraction of matter wants to slow down the expansion of the universe. The energy attraction of dark energy wants to slow down the expansion of the universe. But the tension in dark energy, the negative energy that it's adding, that it's coming with its very existence, is very strong. And in fact, even though the energy's balanced this is the craziest thing, but I swear it's backed up by mathematics the energy's balanced.
So you get the positive energy from the existence of dark energy and the negative energy from its tension trying to resist the expansion. Those are in perfect balance. But when you look at their effects on the expansion of the universe, the tension totally dominates. The tension is way, way, way, way stronger than anything else. We don't see this with matter or radiation because the tension caused by matter and radiation is basically zero.
There's just the straight gravitational attraction of the matter in the radiation, their energy, their presence with dark energy, this high high tension, this negative pull, this negative energy dominates by a huge amount. Normally we don't care about the energy contained in the tension of a rope. This is something beyond our normal experience which is why it's so hard to find metaphors for it. This is something that the mathematics are very clear about, but the language is not. And we just have to soak in that for a little bit.
You don't usually care I'll say it again. You don't usually care about the energy stored in the tension of a rope, but this isn't a tug of war game. This is the whole entire universe. The whole entire universe does care about the tension stored in dark energy, doesn't care about its raw energy of its existence, which does go up every single day. It cares more about the tension that it's bringing to the game.
And with more dark energy, you get more tension, you get more negative energy, negative pressure, which gets counted in the ledgers as a repulsive force. Gravity can turn repulsive. Gravity can turn negative if you have some very curious scenarios. We are living in a very curious scenario where the tension in dark energy is overwhelming everything else in the universe. And since you're getting more and more of it every day as the universe expands, this causes the expansion to accelerate.
It's like compound interest. You're getting more and more in your bank account every single day, which generates even more the next day. I know. I I feel for you. This doesn't make a lot of intuitive sense.
There's not a lot of easy mental hooks to let us dig into how dark energy operates, but it's there. Now back to the story. If you remember the episodes about quantum field theory, it's very, very straightforward to calculate how much energy is in the vacuum. So we've identified dark energy as related to the vacuum energy of these quantum fields. We've identified that as the as Einstein's cosmological constant.
There is a reason why it causes the expansion of the universe to accelerate. Now how much? Can we actually predict it? We can measure it. We can measure it, but can we predict it?
Can we use our quantum field techniques to figure out and explain and solve dark energy? These vacuum fields, these quantum fields are always vibrating. There's a fundamental vibration, a ground level energy to the whole entire universe, and there's all sorts of random fluctuations and vibrations. There's little ones. There's big ones.
There's medium ones. So you all you have to do is add up all the tiny little fluctuations that are constantly happening around us in the vacuum of space time, and you'll get a number, and that number is infinity. Whoops. Technically, because we know how nature feels about infinities, technically that's not a problem. Just like gravity, all the interesting particle physics that we know and love occur on top of the vacuum energy.
If Mount Everest were infinity miles high, you could still fall off a ladder, I guess. That sounds really, really bad, and it's not exactly comfortable, but we've been able to work around it so we can make progress with quantum field theory. We're able to deal with those infinities. We have various shenanigans that we use to sweep the infinities under a rug so we can do calculations. It's not I don't wanna say it's not a big deal.
It's a big deal, but it doesn't really matter for in terms of being able to have a powerful predictive theory of nature. But it is very, very, very important if we want to identify the quantum fields as the cosmological cosmological constant because this is telling us that the cosmological constant is infinity, which would basically blow up the universe instantaneously. So that's that's out. Maybe it's not infinite. Maybe there's not an infinite amount of energy stored in the vacuum because there isn't.
And we're misunderstanding something, and we gotta play around with the math a bit. Let's say we'll add up our fluctuations in the vacuum, all our our little wibbles and wobbles. We'll add up all the energy, and we'll do some cutoff scale. Like, we won't count any vibrations, any fluctuations smaller than, say, the Planck scale because we don't understand the physics of the Planck scale anyway. So So let's just cut it off there, and we'll say that's it.
We'll say below that, anything smaller than a certain amount, a certain distance, other physics take o takes over, and we don't have to worry about quantum fields anymore. When you do that, you get a number. You don't get infinity. You get a finite number, which is great, and it's a 20 orders of magnitude larger than what we measure dark energy to actually be. Oh.
Here's the problem with dark energy. Our accelerated expansion that we observe is tiny. Yeah. It's a big deal, but it's tiny in magnitude. If it's caused by the weight of the vacuum itself, then empty space heart weighs hardly anything at all, less than one hydrogen atom per cubic meter.
That's like nothing. But when we take our most ignorant, simple, straightforward application of this wonderful apparatus known as quantum field theory and apply it to the problem of dark energy, we end up with 10 to 120 hydrogen atoms per cubic meter, which is way off. This is endlessly frustrating. Our first stab at taking a guess is way too big, and even more sophisticated stabs always end up with big, big numbers. What's going on?
You could. If you're you're just faced with a problem like, hey. You the simple application of the theory makes a number that's way too big. You're predicting dark energy to be way, way, way more powerful than it is. If you could come up with some fancy mechanism, like, okay.
Maybe there's some physics here that we don't understand, which that much is obvious. This the fancy physics has to bring the number down. Like, you're way off base here. We gotta reel you back in to something more reasonable around the value that we're actually measuring in our universe. But usually in physics, when we devise some mechanism, when when one thing wants to go off a rails and then another effect, a competing effect comes in to squash it, it squashes it completely.
It brings it all the way to zero. Like, if I have some effect or, dynamo or something that just wants to keep growing and growing and growing and growing, and then I introduce some other physical mechanism, say, no. No. No. No.
You're not gonna do that. It's it's just gonna get rid of it. How do you get rid of something by a 20 orders of magnitude, but still leave a tiny, tiny bit left over? What process could possibly remove almost all the vacuum energy, but leave a tiny amount. I mean seriously, who does this?
Who does this? We call these kinds of problems fine tuning problems, and we don't like them. Neither should you. The answer to why dark energy is so small is we don't know. Dark energy presents one of the most profound mysteries in modern science.
No exaggeration. The answer is somewhere in particle physics, but we don't know where to go. But if it's so wrong, blatantly obviously wrong, that the vacuum energy is the cosmological constant in the explanation for dark energy as the cause of this accelerated expansion, why do we still toss it around if it's it's it's really bad? Because it's our best guess. Our really bad, horrible answer is the best answer we have.
It's the simplest explanation we have that fits the data. Our observations don't tell us a lot about dark energy. We know it exists for sure. We know it kicked on about five billion years ago. That's about the extent of it.
We're at a point now where we desperately need more measurements. Specifically, we're looking to see if it really does stay constant. Is it constant, constant, constant, or does it change a little bit in space or a little bit in time? Four billion years ago, was the universe experiencing a different value of the vacuum energy than we have today? We don't know.
And this bugs us too because there's something else that bugs us about dark energy. There's a lot of irritation here, and that's the universe right now is about 5% normal matter, 25% dark matter, and 70% dark energy. In the distant past, dark energy was essentially nothing. Matter and radiation dominated everything because the universe was small and hot and dense. In the distant future, dark energy will be everything because matter and radiation will be so diluted they don't matter at all.
It will just be 100% dark energy. Right now, dark energy and dark matter are, like, on the same playing field. Dark energy is totally winning, but dark matter is still in the game. Is this a coincidence? We don't like coincidences, do we?
Did we just happen to arise into consciousness and understand the great mysteries of the universe at this unique and precious time? Or is there something else going on? Is there more physics that this is telling us? Is this coincidence between dark matter and dark energy a hint that the universe has more in store for us? Is dark matter connected to dark energy somehow?
This is something called quintessence for fifth essence. Like, there's a fifth force of nature that only dark matter feels that feeds back onto dark energy, and so they track each other through cosmic time. Or if it really if dark energy really is constant, is it really constant? If so, what's making it have the value it has, and why do we see it the way we see it today? There are not a lot of answers in this episode.
There are not a lot of answers to dark energy. It's pretty hypothetical right now, our understanding of it. The observation I can't stress this enough. The observational existence of dark energy is for sure. Our explanation of it is wanting.
Maybe in five or ten years, I'll be able to do an episode on the answer. Thank you so much to all the people who asked about dark energy. I know it was a super exciting topic. Lots of people ask. So feel free to ask follow-up questions.
But I would like to thank Siri l, Oscar z, Olivia p, Solly f, Peter w, Scott, m, Mallett, Samuel, Chris Scott, Colin e, Dana r, Lindsey f, Rowan h, Robert r, Ozgur Gecko, and Sam b for all your questions about dark energy. And, again, if this topic if this episode sparked more questions, send those questions to me, and I'll do a follow-up. Maybe a year from now, but I'll do one. Don't forget your Patreon contributions keep this show going. I'd like to thank Robert r, Justin g, Kevin o, Justin r, Christy, and Helga B.
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Thank you so much for listening. Thanks for the contributions, the iTunes reviews, the telling your friends, the questions. Go to askaspaceman.com. You can email me at askaspacemangmail dot com, askaspaceman on Twitter and Facebook if you wanna send me more questions, and I'll see you next time for more complete knowledge of time and space. Space.