In a previous article, Alon Retter argued the big bang was a white hole – a time-reversed black hole, the possible existence of which has been brought to popular attention by Carlo Rovelli. But the big bang and the white hole have crucial differences in their structures, which creates more questions than answers, writes Geraint F. Lewis.

Where did our universe come from? What gave birth to all the matter that formed into stars, planets, and people? Astronomers tell us that our cosmos was born in a fiery event almost fourteen billion years ago and has been expanding ever since, but while this Big Bang picture accurately explains the universe's evolution, the ultimate origin remains mysterious. A radical suggestion is that our universe is a white hole, the mirror image of a black hole, but is this physically reasonable?

To answer this, we will need to understand what a white hole is. To do this, we need to understand a little about their more famous alter egos, black holes.

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The universe is full of incredible things, but black holes are perhaps the most incredible. With matter crushed down to infinite density and a gravitational pull that prevents anything from escaping, black holes have been found across the universe. Supermassive black holes, billions of times the mass of the Sun, have been identified in the hearts of galaxies, while smaller black holes are known to roam between the stars. In the last decade, new gravitational wave detectors have revealed the unmistakable signature of merging black holes deep into the cosmos.

Mathematically, black holes are part of Einstein’s general theory of relativity, originally derived in Karl Schwarzschild’s explorations of relativity in 1916. However, black holes remained a theoretical curiosity until the 1960s when astronomical evidence finally convinced us that black holes truly existed.

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A word of caution, however, as while the mathematics of relativity permits white holes, whether they physically exist remains unanswered. No conclusive evidence points to white holes existing in our universe.

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Time in relativity is complicated and the flow of time, which seems so natural to us, is not defined and needs to be imposed. In Schwarzschild’s original mathematical formulation, the flow of time ensured that gravity was attractive and that mass fell into the hole. However, given the ambiguity of time’s flow, time-reversed solutions are equally mathematically valid. The time-flipped version of a black hole is a white hole and instead of drawing matter in, white holes expel material into the universe.

Just like a black hole, at the heart of a white hole sits a singularity, a point of infinite density. And just like a black hole, a white hole is surrounded by an event horizon, but instead of preventing anything from escaping, this horizon is a one-way barrier to anything trying to enter the white hole. A word of caution, however, as while the mathematics of relativity permits white holes, whether they physically exist remains unanswered. No conclusive evidence points to white holes existing in our universe.

So white holes possess a singularity and spew matter into the universe. The Big Bang began with singularity resulting in matter spewing into the universe. Superficially, a white hole and the Big Bang appear to be similar, so maybe they are the same thing? Maybe our universe actually is a white hole, and we are living inside? Alas, a look below the surface shows that this similarity is nothing more than skin-deep.

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The Big Bang, however, is not matter pouring into pre-existing space, but the expansion of space itself.

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There are major differences between the mathematical structure of a white hole and the Big Bang. A white hole exists within an infinitely large universe, and matter is thrown into the universe like fragments from an explosion. The Big Bang, however, is not matter pouring into pre-existing space, but the expansion of space itself. Unlike the white hole, which has a central location from where all matter originates, our expanding universe does not possess a unique centre. Every point of space within a Big Bang universe can equally claim to have been at the origin when it all began.

We have used our current cosmological mathematics for almost a century, and they have yielded accurate predictions for our view through telescopes. The expansion is revealed through the redshift of cosmological sources, as well as the evolution of stars and galaxies over billions of years. The brightness of distant exploding stars has shown that the expansion is accelerating, and the cosmos is dominated by a strange dark energy. There are still mysteries left to solve, such as the nature of dark energy and the process that brought the universe into being, but the mathematics of the Big Bang have provided the underlying framework of modern cosmology.

How does the notion of us being inside a white hole fare in explaining the observed universe? Essentially it doesn’t as there is no real mathematical white hole cosmology, only an idea or a hypothesis at best. There are no predictions about what we should see through our telescopes or measure about the universe. Without any of this, the notion that our universe is actually a white hole is not even a scientific theory, and cannot be compared to the successes of the Big Bang.

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We can be quite certain that the white hole, the time reverse picture of a Schwarzschild black hole, simply does not represent the universe around us, which is seen to be expanding and evolving.

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The picture of a white hole playing a role in the formation of the universe remains alluring, and the notion does raise its head every so often. The mathematical picture extracted from Schwarzschild’s insights is not sufficient to represent the cosmos, so additional flexibility is added. Instead of matter pouring out, perhaps it comes in a single intense burst. Perhaps the formation of black holes is tied to the formation of white hole universes through some quantum mechanical interactions? What if the universe is a projection of a white hole in a higher-dimensional cosmos?

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So, is the universe a white hole? We can be quite certain that the white hole, the time reverse picture of a Schwarzschild black hole, simply does not represent the universe around us, which is seen to be expanding and evolving. What of the potential extensions, such as the inclusion of quantum mechanics to tie black holes and white holes? These hypotheses are too underdeveloped to be held up to nature and tested scientifically. At this stage, we should not simply rule them out, but clearly they need a lot of scientific development if they are ever to truly challenge the supremacy of the Big Bang.

As a final takeaway message, the idea that the universe is a white hole is cool and can grab the headlines, and it might, just might, have a kernel of truth about it,  but I wouldn’t bet on it!