00:00Scientists estimate that there could be a gigantic onion somewhere in space.
00:05Not a real onion, of course, but something even more amazing.
00:09Stars stacked in successive layers, one on top of the other.
00:14These captivating objects, called gravastars, could be close relatives of black holes.
00:19Even more, they could offer essential keys to understanding some of the biggest enigmas of the universe,
00:26such as dark energy or even other dimensions.
00:31Let's start with gravity.
00:32More than a century ago, Albert Einstein revolutionized our understanding of this force.
00:37Contrary to what one might think, it is not limited to attracting objects to the Earth
00:42or dropping apples on Newton's head.
00:45Much more extraordinary, it deforms the very pattern of space-time.
00:51Imagine that you drop a heavy ball, like a bowling ball, in the middle of your bed.
00:56What happens to the sheets?
00:58They sink and stretch around the ball.
01:03Now, if you roll a small ball on this bed, it will no longer follow a straight line.
01:08On the contrary, it will start spinning around the ball, getting closer and closer,
01:13as if the ball was pulling it irresistibly.
01:16This is gravity.
01:19But instead of sheets, it is the very fabric of space-time that is concerned.
01:24Massive objects such as planets and stars create hollows in it.
01:28And the moons, planets, and even light follow these deformations.
01:35Gravity is the curvature that determines their trajectory.
01:39In this case, you can consider black holes as the ultimate bowling balls,
01:44the heaviest there are.
01:46Gravity is so intense that it compresses them to the extreme,
01:50to the point of becoming tiny,
01:52so small, to tell the truth, that it could hold in your hand while possessing the mass of several suns.
01:58Some even contain the equivalent of more than 40 billion solar masses.
02:03It seems absurd, but these objects actually create hollows without a bottom in space-time.
02:09At measured gravity, the region around them, called the event horizon, is terrifying.
02:15It is a point of no return.
02:17Once something enters it, including light, it is immediately drawn inside, unable to escape.
02:24This is precisely why they appear as black holes.
02:28The gravastars, on the other hand, are even more unique.
02:32Just like black holes, they are probably extremely compact.
02:36However, instead of being infinitely dense gravity wells,
02:39they could hold in their hearts something incredible,
02:43dark energy.
02:46Dark energy is one of the greatest mysteries of the cosmos.
02:51It is an invisible and enigmatic force
02:54that seems to constantly accelerate the expansion of the universe.
02:59It acts as the exact opposite of gravity.
03:01While this last time to gather matter and slow down the cosmic expansion,
03:06dark energy tries to push back the limits of the universe, making it ever larger.
03:11So far, dark energy has taken over,
03:14without which we would be trapped in a contracting universe.
03:17But if we can explain gravity with simple analogies,
03:21what is really dark energy?
03:23A force, an energetic field, or something else?
03:27We have no idea.
03:29Scientists have observed its effects and know that it is omnipresent in the universe,
03:34but they are still unable to define its true nature.
03:38If, however, it was imprisoned in the gravastars, we could finally discover the truth.
03:44This name is the contraction of gravitational vacuum star.
03:48These were imagined in 2006 by two physics professors,
03:51Paweł Mazur and Emil Motola.
03:54Their reflection was based on possible alternatives to black holes
03:58when a massive star collapses on itself.
04:01This is how they proposed this new hypothesis.
04:04Imagine that gravity pulls everything to the center, compressing the massive stars.
04:09The more they shrink, the denser they become, and their gravity intensifies.
04:13Beyond a certain threshold, they cross a critical limit and turn into black holes.
04:19But what if a dark energy was present inside to counteract this collapse?
04:25At the heart of a gravastar, a region filled with false voids or dark energy
04:30could exert pressure outward, opposing gravity.
04:35This evokes the image of the irresistible force encountering an immutable object.
04:41Thanks to this opposition, the core of the star would not collapse into a black hole.
04:47This may seem crazy.
04:50We do not even know what dark energy is.
04:53So how could it fill anything?
04:56However, even if its exact nature escapes scientists,
05:00they have reliable mathematical models to analyze its behavior in the universe.
05:05So they decided to test this theory.
05:09And this is where things get even stranger.
05:12A new hypothesis suggests that gravastars could not be simple isolated objects,
05:17but stack them inside each other.
05:20Each would form a layer with its own dark internal energy bubble,
05:25enveloped by a thin layer of matter.
05:27The outer shell would contain a smaller gravastar,
05:31which would house an even smaller one, and so on.
05:35It is comparable to a series of balloons.
05:38The pressure of the air in each balloon is stable and evenly distributed.
05:43None of them deflates or explodes under the pressure of the others.
05:48In the case of gravastars, the air corresponds to the dark energy,
05:52and the outer shell represents the matter surrounding them.
05:56This seems insane, but makes the idea more apprehensible.
06:00A single gravastar could have a very thin envelope,
06:04but the thicker the layers, the more likely their stability becomes.
06:10And finally, the most fascinating part,
06:13is that it could potentially solve one of the greatest mysteries of our world,
06:17the origin of the universes.
06:20When a star collapses to become a gravastar,
06:23the matter could, in theory, implode through its center and create a new dimension,
06:29thus linking these objects to the Big Bang.
06:32Some researchers go further by speculating that dark energy
06:35could represent the energy exchanged between our universe and a child universe born from a gravastar.
06:42Naturally, all this remains purely speculative for the moment.
06:47But where are all these gravastars, and how could we verify their existence?
06:52Physicists are not sure.
06:54It is more a theoretical hypothesis than a concrete discovery.
06:58The two professors at the origin of this idea
07:00simply tried to imagine what would happen if gravity and dark energy acted together.
07:06For the moment, LIGO, the large observatory that detects undulations in space-time,
07:12has not provided clear evidence of the existence of gravastars.
07:16Another problem is their stability, which would only be possible under precise conditions.
07:21For example, a rotation too fast or an envelope too thick
07:25could cause oscillations or a progressive disintegration,
07:28even if this could take billions of years.
07:33In addition, even if we have not yet detected them,
07:36we must remember that the dark holes themselves have followed a similar path.
07:41When Einstein published his revolutionary theory of general relativity,
07:45Karl Schwarzschild used these equations to demonstrate that an extremely strong gravity
07:51could potentially prevent light from escaping, thus creating a kind of hole in space.
07:58Einstein himself considered this idea too strange to be plausible.
08:02It took us many years to confirm the existence of dark holes,
08:06and it was only recently that we took the very first picture of one of them,
08:10the blurry image of the supermassive black hole Sagittarius A-star,
08:15located in the center of the Milky Way, which went around the world.
08:19This is why scientists take gravastars very seriously.
08:23Mathematical calculations being correct, these objects are theoretically possible in our reality.
08:30It remains only to verify if they really exist somewhere.
08:34Interesting fact, they would probably look like classic black holes,
08:39emitting high-energy radiation by consuming matter.
08:43They could even produce Hawking radiations,
08:46a type of theoretical energy that escapes black holes.
08:50In other words, it would be almost impossible to distinguish them from black holes.
08:55However, if their envelope is transparent to light,
08:58gravastars could bend it in a slightly different way.
09:03By closely observing the way light is deflected around these mysterious objects,
09:07we could perhaps perceive a variation.
09:11And even if gravastars do not exist in our universe,
09:14they will nevertheless have made a precious contribution to science,
09:18by helping us to better understand gravity, black holes and the limits of relativity.
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