Say, for the sake of argument, somewhere out there exists a farmer ant, whose cozy anthill sits on the surface of an inflated balloon.
Its world is two-dimensional. The only known directions are left, right, forward, and backward, obviating the meaning of up and down. Its working day consists of traversing a relatively short path to milk some honeydew-producing aphids. This is how our ant friend spends her time, living a simple but enriching life.
But one day it begins to perceive a potential break in its peaceful existence. Its daily walk seems to be taking longer than usual. Usually, it would take the ant eight minutes on foot from its house to reach the desired point. But now it seems that, with each new awakening of the sun, a minute is added to its travel time. As such, a path that used to take eight minutes became one of nine, then ten, eleven, twelve, and so on. Moreover, the ant notices that the time it takes to get to other familiar places has also increased. Feeling sure that its pace is not slowing down, the ant decides to investigate what is happening.
It is through its observations that the ant arrives at the following conjecture:
Both the trail to the aphids and the paths to other locations have increased. None of these is moving away from the ant, since they have a fixed distance with respect to the rubber of the balloon. Therefore, the ant concludes that the ground under its feet is somehow expanding.
Such a deduction seems VERY strange to the ant. It has walked around its world many times and never noticed the existence of some edge or something “outside” of the balloon where this expansion could take place. But the fact is that the expansion is happening and, just as in the ant universe, our Universe also exhibits this phenomenon.
In our Universe, the distance between us and remote galaxies is increasing. Astronomers generally say that distant galaxies are “receding”; that is to say, that they are “moving away” from us. But galaxies are not travelling through the fabric of space-time. Even though galaxies move randomly within their respective agglomerates, galactic clusters are practically at rest. Consequently, what expands is the space between these galaxies and us. And this is how the microwave background radiation spreads through the Universe, defining a universal frame of reference, analogous to the rubber of a balloon, and allowing us to measure movement.
Now, the interesting thing behind this inflated balloon analogy is that it is one of the best known among the public to explain the cosmological model of the Big Bang. Under this particular iteration, each element serves as an allegory: the ant is the human race, the anthill Earth and the balloon the Universe. Sadly, this analogy has been stretched far beyond its initial intent, facilitating the fabrication of erroneous hypotheses. Among these are:
- Our Universe has a finite size and a non-zero curvature.
- There are higher dimensions in which our spatially three-dimensional Universe is embedded.
- “Something” inside the Universe is causing its expansion.
As for the first assumption, we must understand it with a certain degree of humility and admit that we don’t know. We assume that the Universe is not finite and that its curvature is not different from zero, but only time and physics will determine the truth behind this supposition. Subsequently, thanks to Einstein’s theory of general relativity, we know that space is dynamic and that it can shrink and curve without the need to be embedded in a higher-dimensional space. In this sense, the universe is autonomous, able to do without a center from which to expand and an empty space out of itself.
Thus, we finally arrive at the last point. In my experience, this one tends to appear more frequently than the other two. After all, this is where we enter the realm of dark energy and dark matter, two poorly understood terms that are often used interchangeably as buzzwords to specify the “something” that expands the Universe.
To begin with, we must establish that dark energy and dark matter are NOT the cause of the expansion of the Universe. Dark energy is a hypothetical form of energy that exerts a negative and repulsive pressure while behaving as the opposite of gravity. It was hypothesised to account for observational properties of distant supernovae that show that the Universe is in an accelerated period of expansion. As such, dark energy is what’s behind the acceleration of the Universe’s expansion.
But what about dark matter? When talking about the subject in physics, dark matter is understood as a non-luminous material that is postulated to exist in space and whose capacity allows it to take any of several forms. Though it can move, its interaction with gravitation means dark matter possesses a mass different from zero, rendering it unable to travel at and/or exceed the speed of light. Thereby, dark matter is used to explain strange galactic rotation curves, making it responsible for the way galaxies are organized on grand scales.
So why are both, our Universe and the ant’s, expanding?
This is the result of what happened 13.8 billion years ago. What we see today are the echoes of a cosmological event so magnificent and essential to the existence of everything we know: the Big Bang.
Both we and the ant were not present at that time. It is only through the remnants “hidden in plain sight” that we know part of the story behind it. We are unaware of why things happened the way they did, but that encourages us to keep uncovering every detail. After all, just like the inquisitive ant, we yearn to know the mysteries and wonders of our great Universe. One guess at a time.