You might have heard of the Big Bang at the beginning of our universe. But what caused the Big Bang? Did anything exist before the Big Bang? If our universe is expanding, what is it expanding into? Could there be other universes out there, floating in nothing?
Image credit Universe Today
Scientists did not think much about these questions until recently. First everything that exists was compressed into a space smaller than the head of a pin. Then it began to expand, and did it really matter what triggered it when we won’t ever have a chance to observe it directly?
Big Bang May Have Created Multiverse With Similarities to Poorly Made Raisin Bread
This could so easily become the domain of “God of the Gaps” arguments if it weren’t for some cosmologists who have taken up the matter in recent years. They noticed some discrepancies between the original Big Bang theory and the current state of scientific observations. For instance, the Big Bang did not explain the relatively even distribution of galaxies throughout the observable universe. If the universe was a loaf of raisin bread and galaxies were the raisins, any cosmic baker would simply not have had enough time to stir the dough long enough to evenly distribute the raisins. While galaxy clusters do exist, the original Big Bang theory says that those clusters should be larger and farther apart than they actually are.
One of the more credible theories has it that inflation began before the Big Bang instead of afterward. In the 2014 book Our Mathematical Universe: My Quest for the Ultimate Nature of Reality, MIT physicist Max Tegmark argues, “If we take inflation seriously, then we need to start correcting people claiming that inflation happened shortly after our Big Bang, because it happened before it, creating it.”
The critical point appears to be exactly what is meant by “Big Bang”.
One camp that includes Tegmark argues that the Big Bang refers to the superheated, superdense state of matter that expanded and cooled into the universe as we know it. It isn’t meant to explain how the universe began any more than evolution was meant to explain how “not life” can become “life”. It may not even have been a “bang” in the same way that an exploding balloon might go bang, but a gradual expansion that acted more like rapidly rising dough.
The initial expansion may not even have been very even and there was no “kill switch” that could stop that expansion all at once. As a result, some of the results of the Big Bang may have expanded more than others. Our universe could actually be part of a multiverse created by a cosmic dough that formed pockets, bubbles, and an uneven distribution of ingredients as some parts rose faster than others. If our “observable universe” could be expanded to include the multiverse, we may still notice that the raisins are evenly distributed in our part of the loaf, but are actually quite unevenly distributed throughout the loaf. We may notice that there are no raisins at all in some parts. There may even be other loafs of raisin bread that we can’t observe directly with the tools we have available.
What Caused The Big Bang?
Lack of direct evidence on what actually caused the miniature proto-universe to start expanding has not stopped some cosmologists from speculating. Cosmologist Andrei Linde proposed that the answer could be a derivative of elementary particle theories called chaotic inflation, in which pockets of inflating space can emerge in an already-existing universe that is less orderly than the observable universe that we are used to.
Other theorists say that the universe can “bounce” — contract to a point of maximum density and then start expanding again. Variations of this theory proposes that our universe is actually floating in a theoretical 11-dimensional space derived from string theory and the “bounce” might be triggered by a collision with a neighboring universe. Others believe that the extra dimensions and unobservable other universes are not necessary for a “bounce”.
The Energy Problem
The amount of energy that the original Big Bang model implies has also caused headaches for cosmologists. To continue inflating, the universe would essentially have to continually create the energy that provides the repulsive force that powers its expansion. However, the law of conservation of energy dictates that there is no such thing as a free lunch. More energy simply cannot be created out of nothing. Or can it?
Cosmologist Alan Guth, who is currently employed by the Massachusetts Institute of Technology, and a Soviet scientist named Alexei Starobinsky may have independently conceived the answer. If one starts with nothing, it may be possible to create equal amounts of positive energy and negative energy. Gravity is an example of negative energy that acts as an attractive force. The repulsive energy driving inflation is positive energy.
Caltech’s Sean Carroll proposes that the answer may lie in the concepts of increasing entropy and time as a “one-way trip”. The laws of physics dictate that it should be possible to move “both ways” along the X, Y, or Z axis, so why shouldn’t the same be true for time? The discovery of the second law of thermodynamics was one early clue. Entropy will always increase even if there are some temporary pockets of order. This means there must have been a time when entropy was very low in our universe – something that could not be explained by inflation or a Big Bang. But what if our universe was a function of an environment with an extremely high level of entropy, such as empty space?
Quantum theory states that empty space is the final equilibrium state for a once active universe. Empty space is more likely to possess some form of disorder than a universe that has a predictable structure or even a dense “hot soup” structure. As one might guess, empty space can be pretty boring, but it also allows for interesting events like the creation of a baby universe.
The baby universe would essentially be pinched off from the empty space universe to form a new pocket of space and time in which entropy is low but starting to increase locally. This does not violate the second law of thermodynamics because it adds to the still-increasing entropy in the now-mostly-empty mega-universe.
Some cosmologists suggest that the apparent one-way trip through time is a function of increasing complexity or forces like gravity. This suggests that time is an emergent property that depends on one or more of a universe’s other properties to exist. Others suggest that time is a fundamental property that can exist independently.
Still others suggest that cosmologists have essentially jumped the shark and left the arena of empirical science. Cosmologists answer that they are beginning to pick up some data that can answer some unresolved questions surrounding the Big Bang and the origins of our universe. A team that includes Xingang Chen, Avi Loeb, and Zhong-Zi Xianyu from Harvard recommended searching for faint signals from a brief period before the Big Bang in which high-energy fields could create different patterns in the cosmic structure by oscillating at regular time intervals. This could provide clues on whether the universe was expanding or contracting in the brief period before the Big Bang.
However, scientists can be faked out. In 2014, the BICEP2 telescope at the South Pole picked up a polarization pattern in cosmic microwaves that could have ruled out the “collision between two universes” theory, but it turned out to be a signal from ordinary dust in our galaxy, for instance.
Will we ever know for sure what might have existed before the Big Bang or what might have triggered it? It might sound unlikely with our current level of technology and capacity for observing the universe. Humans are limited by our ability to only perceive three spatial dimensions plus time instead of the theoretical “additional” dimensions proposed by concepts like string theory. Like most of our tools, our scientific instruments are built to enhance our existing capabilities. However, this does not stop cosmologists from dreaming up hypotheses regarding what might have triggered the Big Bang and possibly proposing ways to test those hypotheses.