The Big Bang may have actually been the explosive rebound of a collapsing black hole – one which our entire Universe could still be inside of.
That’s the claim of a new study led by Enrique Gaztañaga at the University of Portsmouth. The paper suggests the Big Bang was actually a ‘Big Bounce’, when matter falling into a giant black hole compressed, then rebounded and expanded outwards to create the Universe.
“In other words, our entire observable Universe could be the inside of a black hole formed in a larger Universe,” Gaztañaga told BBC Science Focus.
Trapped in the event horizon
The research, recently published in Physical Review D, re-examined what happens when a dense, extremely large cloud of gas collapses under its own gravity.
Rather than the traditional view that it will form an infinitely dense point (known as a singularity), the study proposes that the in-rushing matter instead reaches a certain point before bouncing off itself.
This rebound results in a rapid expansion that closely resembles what cosmologists believe happened after the Big Bang, meaning our reality could be trapped within the event horizon of a black hole.
This ‘Black Hole Universe model’ would help explain several key problems with the current, mainstream understanding of cosmology, known as the standard model.
The standard model only works if there was a period of inflation, where the entire cosmos rapidly expanded a fraction of a second after the Big Bang. It also requires ‘dark energy’, a mysterious substance that’s pushing the cosmos apart, to explain why the Universe’s more recent expansion appears to be accelerating.
“But we don’t know what either of these components actually are,” said Gaztañaga. “In contrast, both periods of rapid expansion emerge naturally in the Black Hole Universe model as consequences of the geometry and dynamics of the bounce.
“One reason this model is compelling is its simplicity: it explains cosmic expansion, inflation, and dark energy using only gravity and quantum mechanics – no extra assumptions or unknown ingredients.”
The Black Hole Universe model is not without its own problems. For instance, it still gives no insight into what dark matter is. We know the invisible substance is spread throughout the Universe, as we can see it holding galaxies together, but astronomers have struggled to find what it’s made of.
“It’s possible that some forms of dark matter are connected to relics from the collapsing phase of our Universe, but more work is needed to explore that idea,” said Gaztañaga.
The entire Universe could be locked within the event horizon of a black hole – Credit: Getty Images
If the Universe began inside a black hole, that would mean we could still be inside one, which is itself inside a larger, wider universe. It could even be that some of the black holes we see around us each have their own mini-cosmos, complete with their own mini-black holes.
“You can think of this like a nested structure – black holes inside black holes, like Russian dolls, said Gaztañaga.
That doesn’t necessarily mean every one of the untold trillions of black holes in our Universe has its own miniature version of the cosmos inside it, as the size of the black hole determines how much time the tiny creation has to develop.
“Large black holes (like ours) allow structure to form – galaxies, stars, planets – while smaller ones lead to universes that expand or collapse too quickly for anything interesting to happen,” said Gaztañaga.
“That’s important, because gravitational collapse predicts far more small black holes than large ones. The fact that we live in one of the rare, very large cases might not be a coincidence – it’s the only kind of Black Hole Universe where observers like us could exist.”
The idea of a Black Hole Universe came about when Gaztañaga and his team took a new approach to looking at how our world began.
“Instead of assuming the Universe began with an unexplained ‘Bang’, we reverse the approach: we start with matter collapsing into a black hole,” he explains.
It all comes down to what’s known as the quantum exclusion principle. In short, this means that two identical particles don’t like to be doing the same thing, at the same time, in the same place.
Because of this, there’s a limit on how densely matter can be packed before the particles can’t cram in any closer without violating the quantum exclusion principle.
It’s one of the reasons that stars like white dwarfs don’t just collapse under their own weight.
“Inside a black hole, the exclusion principle still applies,” said Gaztañaga. “It prevents matter from collapsing to a point [a singularity]. Instead, it slows the collapse, halts it at high density, and causes a bounce, avoiding the singularity altogether.”
Relic black holes
While the idea that the Universe started with a Big Bang works perfectly on paper, cosmologists won’t know if the idea is correct until they test it.
Fortunately, the theory makes several predictions about what our Universe should look like, which astronomers can use to test the theory.
“For example, it predicts the Universe is slightly curved – positively curved like a sphere – not exactly flat,” said Gaztañaga.
The first direct visual evidence of a black hole (the one at the centre of the elliptical galaxy Messier 87 in the Virgo constellation) was captured by the Event Horizon Telescope in April 2017. – Photo credit: EHT Collaboration
While most efforts to measure the Universe’s curvature have found it to be flat, there could be a subtle bending that they haven’t been sensitive enough to measure. This is why the European Space Agency’s Euclid spacecraft is currently making the most accurate measurement of cosmic curvature ever taken, though it’s not due to finish until 2030.
“It also predicts the existence of relic black holes and relic neutron stars – objects formed in the collapsing phase that survive through the bounce and might still be around today,” said Gaztañaga.
These would have influenced how the galaxies and stars grew over time. It could be possible to detect the signature of these relics in our current view of the Universe and discover whether we really are living inside a black hole.
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