It is difficult to imagine a time, about 13.7 billion years ago, when the entire universe existed as a singularity.
When you think about it, an even more complex question arises: what existed shortly before the big bang occurred?
The question itself predates modern cosmology by at least 1600 years. The fourth-century theologian Saint Augustine wondered what existed before God created the universe. He concluded that the biblical phrase “In the beginning” implies that God had not done anything before. Moreover, Augustine argued that the world was not created by God at a specific time, but time and the universe were created at the same time.
In the early 20th century, Albert Einstein came to very similar conclusions with his theory of general relativity. Just consider the effect of mass on time. The huge mass of the planet distorts time, forcing a person to move on the surface of the Earth a little slower than a satellite in orbit. The difference is too small to notice, but time goes slower for a person standing next to a large boulder than for a person standing alone in a field.
Building on Einstein's work, the Belgian cosmologist Rev. Georges Lemaitre published an article in 1927 proposing that the universe began as a singularity and that the Big Bang led to its expansion.
Following this logic, the title of this article is fundamentally incorrect, time only appeared when this original feature expanded to its current size and shape.
Case is closed? Unfortunately no. In the decades after Einstein's death, the advent of quantum physics and a host of new theories have resurrected questions about the pre-Big Bang universe.
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Is our Universe bubbling from the previous one?
So, what if our universe is a product of another, older universe? Some astrophysicists suggest that this story is written in the relict radiation left over from the Big Bang: the cosmic microwave background (CMB).
This figure shows the cosmic microwave background - radiation left over from the Big Bang.
Astronomers first observed CMB in 1965, and this quickly created problems for the Big Bang theory - problems that were subsequently solved (for a while) in 1981 with inflation theory. This theory entails an extremely rapid expansion of the universe in the first few moments of its existence. The theory also takes into account fluctuations in temperature and density in the CMB, but dictates that these fluctuations should be uniform.
Recent mapping efforts actually suggest that the universe is one-sided, with more fluctuations in some areas than others. Some cosmologists see this observation as supporting evidence that our universe was "bubbling" from its parent universe, according to Caltech researcher Adrienne Ericchek.
In the theory of chaotic inflation, this concept goes even deeper: the endless development of inflationary bubbles, each of which becomes a universe, and each of these creates more inflationary bubbles in the immeasurable multiverse.
However, other models revolve around the formation of the singularity itself before the Big Bang. If you think of black holes as cosmic debris compactors, they are the prime candidates for all this primordial contraction, so our expanding universe could theoretically be an exit white hole from a black hole in another universe.
A white hole is hypothetical, and acts in the opposite way of a black hole, emitting serious energy and matter rather than sucking it in. Think of it like a space exhaust valve. Some scientists speculate that our universe may have been born inside a black hole, and each black hole in our own universe may also contain separate universes.
But some scientists believe that the universe began not with a Big Bang, but with a Big Rebound.
Big bounce
Long ago, medieval religious philosophers in India taught that the universe goes through an endless cycle of creation and destruction, in which it evolves from an undifferentiated mass into the complex reality we see around us, before destroying itself and starting over.
Some modern scholars have come up with an idea with striking parallels. Instead of a Big Bang, they believe, the universe expands and contracts in a cycle, bouncing back each time it contracts to a certain size. In the Big Bounce theory, each cycle would start with a small, smooth universe that wouldn't be as tiny as a singularity. It will gradually expand, becoming lumpy and deformed over time. Eventually, it will reach a point where it begins to collapse and gradually flatten out, shrinking to the size of the starting point. Then the cycle will start over.
For the Big Rebound idea to work, it must find a way around the singularity theorems developed by British physicists Roger Penrose and Stephen Hawking, who propose that the contracting universe will shrink to a singularity, such that a massive dying star will eventually condense to form a black hole. This Big Bounce model relies on the idea of negative energy counteracting gravity and reversing collapse so that the universe and spacetime will separate again and again.
These cycles of contraction and expansion will repeat themselves approximately once every trillion years.
A big bounce would have departed from the Western civilization's view of reality since St. Augustine, because he would have recognized that time did indeed exist before the universe as we know it.
But whether it's the Big Bang or the Big Bounce, the question of what existed before our present universe remains an open question. Perhaps nothing. Perhaps another universe or another version of our own. Perhaps a sea of universes, each with its own set of laws that dictate their physical reality.