Black Hole's Evil Twin - Gravastars Explained

What's this? Your reminder to get the perfect gift this holiday season. There might be an object so indestructible, extreme and brutal that it could kill black holes.

Gravastars! Cosmic soap bubbles filled with pure energy and with a shell made of the weirdest material that's possible in nature. What are they?

What do they look like? And are they just a theoretical fever dream or will they change our understanding of the universe forever? [The birth of the most extreme objects in the universe] Very massive stars die in the most dramatic way possible: a supernova.

We've explained this process in detail before, but, in a nutshell, in less than a second their cores collapse, crushed under their extreme gravity. The star's shell rushes in, bounces against the collapsing core, and explodes, shining brighter than whole galaxies. Depending on how massive the star was, there are two possible outcomes.

Either the core compresses into a superdense neutron star or it kind of breaks reality and collapses into a singularity, an infinitely dense point with no size or dimensions at all. A place where the laws of the universe stop making sense and time and space are reversed. A black hole.

Gravastars are a third, even weirder option. Instead of collapsing into an infinitely dense point, the core is kind of ground down, like a rock pulverised to dust by a cosmic hydraulic press. Atoms and particles are crushed so hard that they transform into pure energy.

A sort of mini-universe, if you want, and, just like our universe, this bubble violently wants to expand and grow. In a fraction of a second, the bubble smashes into the collapsing star around it. The unspeakable mass of the star collapsing under its own gravity meets the titanic violence of the expanding energy bubble.

Like an ancient God hammering on its anvil, matter is trapped between an immovable object and an unstoppable force, forging a new kind of material that we've never seen before but that we know is physically possible. And then it suddenly stops. A gravastar is born.

What does it look like? [Cosmic soap bubbles] Just like black holes, a gravastar can have any mass, but a typical one would be about the size of the London metropolitan area and as massive as 10 Suns. The shell of the gravastar is utterly dark and the coldest thing in the universe: only a billionth of a degree above absolute zero.

If we look at it in deep infrared, even the cosmic microwave background glows bright in comparison. How can anything made of matter be that cold? Don't all atoms jiggle back and forth?

The thing is, the shell's not made out of atoms. It's made from an entirely new, unique and extreme matter that doesn't have a name yet and that's at the very limit of what's physically possible in nature. Actually, the shell is so incredibly thin that atoms seem truly gigantic next to it.

And yet, despite being ultra thin because it's been forged by two impossibly extreme forces, the shell is incredibly tight. So tight that if you wanted to stretch the whole shell by just one meter, you'd need the energy of an entire supernova. What about the inside?

Well, it only gets weirder. The interior of a gravastar is perfectly simple because it's sort of empty. Completely empty.

A perfect vacuum without a single atom, particle or wave. But despite being as empty as it gets, this vacuum is boiling with the most primitive and fundamental kind of energy in the universe. We need a detour to explain how any of this makes sense.

[The fundamental nothingness at the core of it all] The inside of a gravastar breaks our brains a bit because it's a sort of supercondensed nothingness. What does this even mean? We'll have to simplify and use metaphors to make sense of what scientists measure and calculate.

According to our current understanding of physics, particles like quarks, electrons, photons and so on are not really solid objects but sort of waves in an ocean. In our human world, you can't have waves without water and in the smallest world you also can't have particle waves without some kind of underlying omnipresent cosmic fluid. This fluid is the vacuum, what we perceive as nothingness.

It's the fundamental ocean at the bottom of reality. The waves of this vacuum ocean are the particles that make up you and everything else. But even when there are no waves or particles traveling through it, the fluid is still there.

And like any fluid we know, it has inherent energy. Vacuum fluid is everywhere in the universe. The room you're in is 99.

98% vacuum between the air particles bouncing around. Between the trillions of particles making up your cells there's vacuum. But it's different inside a gravastar.

When our star collapsed and condensed so violently, it was as if the universe took a cosmic pump and compressed as much vacuum fluid as physics allows. Into a kind of superdense nothingness. As said before, even without any waves the nothingness vacuum ocean of the universe has energy.

But the superdense vacuum inside a gravastar has almost a billion trillion trillion trillion times more energy per cubic centimeter than the vacuum outside the star. This is an unbelievable amount of energy and mass in a tiny space, just like, you may have guessed it, black holes. This intensity compressed vacuum ocean can't be compressed any further.

It's at the absolute physical limit of anything that can be squeezed together without breaking physics like black holes do. The ocean would love to stop being so tight. It wants to stretch out and flow back into the ocean that surrounds the star.

But it's trapped in the safest prison possible, the shell, which itself is right at the edge of the physical limit of any material possible. An eternal stalemate between two limits of the universe. Let's leave this world of metaphors and get back to our world that feels more real.

In our world, gravastars are perfectly black, eternal objects with borderline insane amounts of mass. Because they're so cold, dark and massive from the outside, gravastars look and behave exactly like black holes. Both massively curve space around them and create all the fun effects we love black holes for, from trapping mass and light in accretion disks or slowing down time as you get closer.

For details, we've made one or two videos on black holes before. If you fell into a gravastar, you'd be extremely dead before you even hit the surface, ripped apart and ground down by the gravitational forces. And once your scattered remains touch the shell, the atoms you were once made of would probably breakdown and dissolve completely, only to be converted into the vacuum energy of the interior, making the gravastar an infinitesimal bit bigger and an infinitesimal bit more massive.

OK, this was fun and all, but what exactly is the point? Isn't this just another video of wild scientific speculation just for the sake of it? [The point] Black holes were suggested more than a century ago as an abstract solution to equations of gravity.

For more than 50 years they were considered mathematically valid but too absurd to be real. Few believed they actually existed. But scientists kept working on paper and looking at weird things.

And then we saw stars being thrown around by invisible titans. We saw light stretching around invisible gaps in the sky. And as our technology and theories improved, we even sort of took a picture of them.

We have evidence for them and they fit our theories, and nowadays it's kind of common sense to accept them as real. Black holes are extremely elegant and fascinating but they also created a lot of questions that have traumatized physicists for decades. Singularities literally break our best understanding of physics.

They seem to delete information which shouldn't be possible. Gravastars are a relatively new idea without any of those problems. They don't need singularities that break physics or delete information.

They solve the puzzles of black holes. But they too create new problems. Like weird exotic matter for their incredibly cold and tight shell.

Superdense nothing to make a supermassive empty core. But just like black holes, they do work on paper and fit what we see in the sky. So are they real?

And will we ever know? Actually, there is a way to find out. Black holes have an event horizon while gravastars have a physical shell made of matter, which means that they behave very differently when they smash into each other.

The collision of two objects as massive as they are, creates huge amounts of gravitational waves, ripples in spacetime that travel at the speed of light. You can think of them as the music of cosmic cataclysms. The collision of two black holes should sound like a bass drum, a deep thumb that stops quickly.

But two gravastars colliding should sound like a gong, leaving subtle echoes behind. Scientists are listening for these echoes in the music of the cosmos, Unfortunately, black holes and gravastars are surrounded by such strong gravity that it swamps most of the music. It's like trying to tell two instruments apart through a thick wall of concrete.

You need very sharp technology for that. While we've made incredible progress in the last few years, we're not quite there yet. So this is where we'll end to this story.

Gravastars have the potential to answer some of the biggest problems in physics. Or they're just another idea for our discard pile. But this is why we do science.

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