Strangest star in the universe
Black holes might be collapsing stars that are exploding in slow motion
Continually collapsing under extreme gravity and holding onto space-time's secrets. Meet the stars born from black holes
What happens inside a black hole? That is a question that has long plagued astronomers, with numerous theories put forward, and numerous problems. Black holes have a gravitational pull so strong that nothing, not even light, can escape. This leads to a problem known as the ‘information paradox’ where information could disappear forever inside a black hole, something that doesn’t hold up against our laws of physics. But an emerging theory put forward a few years ago proposes an unusual solution to this problem: that black holes are not what we think, and instead contain an object known as a
‘Planck star’ – collapsing stars rebounding in slow motion that, over time, emerge from view.
The idea of Planck stars was proposed in a paper by Carlo Rovelli from the University of Marseille in France and Francesca Vidotto, then of Radboud University Nijmegen in the Netherlands, in 2014. The two astronomers suggested that at the core of the black hole
there could exist a tiny object – a Planck star – that stores all information about infalling material into the black hole. As the black hole evaporates and its gravitational boundary – known as the event horizon – shrinks, it would eventually meet this Planck star, exploding in a violent event and allowing information to then escape into space, supposedly solving the information paradox.
Planck stars would form in much the same way a black hole does. A black hole forms when a very massive star runs out of fuel at the end of its life. With no outward pressure to counteract the incoming force of gravity on the star, it collapses into a singularity and produces a black hole. However, Rovelli and Vidotto argue, that might not be the end of the story. They suggest that a black hole is the process of a massive star exploding like a supernova, with its event horizon gradually shrinking over very long scales.
“The black hole forms in the sense that you have the creation of a horizon,” says Vidotto. “But on the inside, the collapse keeps on happening, and at a certain point you have new forces of quantum origin that balance the contraction. So instead of having a contraction that goes on forever and creates a singularity, you have instead these new forces that trigger a new phase that could be an expanding phase. The object that at a certain point corresponds to this maximum contraction is what we call a Planck star.”
Eventually this event horizon reaches the middle of the black hole, the singularity, where the remains of the original star have been squashed into a tiny point less than a trillionth of a trillionth of a
metre in size, known as the Planck length – the smallest possible length in physics. Once the event horizon reaches this point, the infalling event horizon bounces out again, and the star unleashes its matter into space. It’s an event that should be fairly quick, but the intense gravity of it leads to something called time dilation, and from our point of view, everything is moving slower than it actually is. Thus, when we look at a Planck star, we are seeing the process of a massive star collapsing and rebounding essentially “in slow motion”, says Vidotto. From our point of view the entire process takes billions of years, depending on the size of the black hole.
Stefano Liberati from the Scuola Internazionale Superiore di Studi Avanzati (SISSA) in Italy compares the idea to the 2014 film Interstellar. In the film the protagonists visit a planet orbiting near a black hole, Gargantua, where time runs more slowly owing to the its immense gravitational pull. “That is basically the time dilation due to the gravitational redshift, which is a phenomenon which we know is there – it’s predicted by general relativity,” he says. “The idea is that the bounce [of the Planck star] is very fast if you are running the clock on the collapsing star. But seen from outside, there is a huge gravitational time dilation.”
A Planck star is a wholly unusual and unstable object, being condensed into the size of an atom. Planck star theory builds upon the idea that black holes have an event horizon and a point of near infinite mass and density at their cores, known as a singularity. The information paradox posits that it is impossible to ever know anything about this singularity, owing to the immense forces involved and the fact that nothing can escape. But if a black hole were actually a Planck star in the process of collapsing and rebounding, it could solve this paradox.
This is similar to one of the proposed theories for the end of the universe, known as the Big Crunch. Our universe is now known to be expanding at an accelerating rate, but scientists once thought the expansion might start to slow, eventually leading the universe to collapse. This would result in the opposite of a Big Bang, known as the Big Crunch or Big Bounce, where all matter in the universe would be condensed to a singularity. Once the