Emma is one of my regular guest bloggers. I feel really thrilled about the possibility to post one educating post of this great blogger once a month over the next couple of months. Thank you so much, Emma, for sharing these great posts with us! If you would like to check out the previous guest posts, this amazing blogger wrote for me, head over here.
Imagine that for your entire life, you have been chained to the wall of a dark cavern. Behind you lies a flame, and between you and the flame parade objects that cast shadows onto a wall in your field of view. This is all you can see of your world: the two-dimensional shadows of objects you only dream of seeing. It is your only reality. Your shackles prevent you from seeing the true world, a realm with one additional dimension to the world you know, a dimension complex and very capable of explaining all that you see.
This allegory was first told by Plato, and he may have been onto something. We now know that the universe was created in the Big Bang, when a singularity—an infinitely dense point—exploded into the universe we know and is still expanding today. But now scientists suggest that we can trace the beginning of the universe back to a time before the Big Bang, to an era with an additional dimension of space. The universe we see appears in three dimensions of space and one of time, a geometry we refer to as the “three-dimensional universe.” But what if our three-dimensional universe is merely a shadow of a world with four spacial dimensions?
Black holes are formed when a star of large enough mass becomes too heavy to support itself in its own gravitational field, and it collapses, or implodes. It becomes a singularity, an infinitely dense point at which the gravitational field is infinite. In our three-dimensional universe, a three-dimensional black hole forms with a two-dimensional event horizon, beyond which the gravitational field is so strong that escape for anything, even light, becomes impossible—hence the term “black hole.” But what would happen if, in a four-dimensional universe, a four-dimensional star imploded? Such a star would collapse into a four-dimensional black hole with a three-dimensional event horizon.
Scientists propose that the Big Bang singularity was a four-dimensional black hole, and that it is now shielded from our sight by a three-dimensional event horizon that is rapidly expanding. That idea turns the Big Bang into a cosmic mirage, and as crazy as it sounds, it provides a better explanation for our observations of our universe than we have so far. For example, even if we do know that our universe started with the Big Bang, we have little idea of what caused the Big Bang. What would cause a singularity to explode, and then expand?
There is another question, a question scientists have pondered for decades. If the universe is expanding from a single point, and that expansion will continue to accelerate exponentially, then what explains the near flatness and high uniformity of the cosmos? Using only our concept of the three-dimensional universe, there is no logical explanation for why the cosmos shouldn’t be spread out unevenly—clustered near the center of the universe and spread farther apart towards the edges.
If our universe was born from a black hole in a four-dimensional universe, then the four-dimensional universe could have existed for an infinitely long time in the past, and any hot and cold spots would have had plenty of time to come to equilibrium. The four-dimensional universe would be smooth, and our three-dimensional universe would inherit this smoothness.
I know it sounds crazy. It sounds crazy to me too, and I can barely wrap my head around it. But we may even be able to test it using the cosmic microwave background radiation (or CBM), which first alerted us to the universal expansion. Additionally, if the four-dimensional black hole is spinning, which would be common, then our three-dimensional universe may not look the same in all directions. The large-scale structure of our universe would appear slightly different in different directions. Astronomers may also be able to find this directionality by studying subtle variations in the CBM sky.
The four-dimensional universe theory answers questions that we have not yet answered using our three-dimensional perspective. But it opens up still more mysteries, such as one most pressing question: Where did our universe’s parent universe come from?
We continue to explore.