Expanding Universe

Have you ever wondered about the size of the Universe? Is the Universe infinite or has it started from a singular point and expanded over time? We can never be sure of the size of the Universe because we can observe only a limited portion of it, but we do know that the Universe is expanding! What does the expansion of the Universe actually mean, and what is the theory behind it? In this article, we will discuss the causes and effects of an expanding universe, and present you with a clever analogy to help you understand this theory much better!

The Expanding Universe Meaning

In an expanding universe, the distance between two different points in the universe increases with time. What this means exactly is best understood by utilising concrete analogies. Suppose space is a one-dimensional elastic string. Objects moving in one-dimensional space can move only forward or backwards. Imagine this elastic string is stretched with two objects on it, they will be moved apart, but the actual size of the objects on the string does not change.

Similarly, we can think of a two-dimensional space as an elastic plane stretching in all directions. As space expands, the distances between the objects increase, but the dimensions of the objects themselves do not change.

Fig. 1. The Expanding Universe can be thought of as a 2-dimensional space with a stretching elastic plane. As the space expands, the distances between the objects increase, but the dimensions of the objects do not change, Pixabay

Expanding Universe Theory

There is a lot of evidence to substantiate the theory of an expanding universe. One piece of important evidence was obtained by observing distant galaxies. Edwin Hubble observed in 1929 that distant celestial bodies in all directions, without exception, were moving away from the Earth. He found that as the distance from the Earth increased, the recessional velocity also increased! This observation helps validate the expanding universe model.

Calculations using cosmological models show that the points with great distances between them today were very close together 13.8 billion years ago. Two points in space that were once only 10m apart shortly after the big bang would now be separated by billions of light-years!

Fig. 2. Edwin Hubble showed that distant celestial bodies are moving away from the Earth. That observation confirmed the expanding universe theory, Creative Commons

Dark energy

The Big Bang created all the matter and energy in the universe. The galaxies were formed from these materials. Since the Big Bang, all galaxies have been accelerating away from each other. Under normal conditions, the rate of expansion should not increase but instead slow down due to gravity forcing the galaxies back towards each other and the universe should shrink. Like how a ball thrown into the air is slowed by gravity, reverses direction, and then falls back to the earth.

Measurements show that the expansion rate of the universe actually slowed until approximately 5 billion years ago and then bizarrely started to increase! The initial decrease in the expansion rate of the universe can be attributed to the effect of gravity. However, as the universe expands, the effect of the force of gravity decreases as objects in the universe move further apart.

The existence of dark energy has been suggested to explain this strange increase in the expansion rate of the universe. Unlike gravity, the effect of dark energy does not diminish as the universe expands. The universe, therefore, has an increasing expansion rate, as dark energy is causing the universe to accelerate in its expansion faster than the force of gravity can compact it.

Dark energy is a type of energy that is assumed to be the reason why galaxies get further away from each other and the Universe expands. Currently, even with advanced technology, we cannot see dark energy. We still do not understand what it even is. However, the majority of astronomers accept the existence of dark energy because although it is not visible to the naked eye, its effect on galaxies and the cosmos can still be observed!

Expanding Universe Model

Fig. 3. In the image, we see four scenarios for the expansion of the universe. The time passes by in the upward direction and the yellow square represents the present. NASA/ESA

Astronomers developed speculative simulations of the universe's future evolution before they had even understood dark energy or had a reliable estimate of how much matter there is in the cosmos. There were four scenarios as shown in the image above. In all four instances, the yellow square represents the present, and in each case, the Hubble constant is identical to its current value. The vertical direction is used to measure time. The growth rate slows down with time in the first two universes on the left, called decelerating universe.

When we look at the first decelerating model we see that the expansion will ultimately slow down, halt, and reverse, and finally implode. According to physicist John Wheeler, a big crunch means that matter, energy, space, and time would be squashed out of existence, they would all be gone. It is the opposite of the Big Bang, it is an implosion. It is the Big Crunch.

In addition, the coasting universe model states that the universe expands at a constant rate which is the Hubble constant. Last but not least, according to the accelerating universe model on the far right (which we currently have the most evidence for), the universe is expanding rapidly now and will do so forever.

Inflating balloon analogy

To better give an understanding of the expanding universe theory, let's look at the inflating balloon analogy!

Fig. 4. The inflating balloon analogy is helpful to explain the expanding universe theory. However, it is still not enough since there are many discrepancies due to the nature of the expansion of the balloon. Pixabay

Thanks to today's cosmological models and careful observations, we know that the Universe is accelerating in its expansion. This can cause people to question what the Universe is expanding into. Because that's the intuitive thing: If you inflate a balloon and expand it, it expands into the room it's in. If the universe is expanding too, it must be expanding into a room or space that contains it, right?

Imagine there are ants living on a balloon. The universe of these ants consists of 2 dimensions: They can go forward-backwards and right-left; however, they cannot exit or enter the surface of the balloon. If you inflate the balloon by blowing air into it, the distance between the two ants on the surface will increase steadily. If these ants were to observe each other, they would notice that they are moving further and further away from each other, and we, as outside observers, could observe and verify this expansion.

The critical point here is that while the ants on the surface, as a result of careful observation, can understand that their universe (the balloon's surface) is expanding and can calculate the rate of this expansion correctly, they would not easily be able to understand why. (Of course, this analogy isn't perfect as a balloon is still technically a 3-dimensional object expanding into 3-dimensional space, unlike the expanding universe.) Don't worry if this all seems a bit confusing, as not even the brightest scientists truly understand the science of cosmology.

Expanding Universe Effects

• As the Universe expands, the galaxies get more distant from each other. The faster a star or galaxy is moving relative to the observer, the greater its receding velocity. All other galaxies (except a select few which are close enough to the Milky Way, where the gravitational force is strong) are moving away from us.
• When the heat and energy are distributed evenly in the Universe because of expansion, the 'Big Chill' or 'Big Freeze' occurs. The universe's ultimate and final temperature will be barely over absolute zero, but this won't occur for trillions of years.
• If dark energy hastens the expansion of the universe, then the distance between clusters of galaxies will increase at a faster rate. Ancient incoming photons (including gamma rays) will be stretched to extremely long wavelengths and low energies by redshift, which would be impossible to detect by even the most sensitive equipment.
• As the universe expands, the number of galaxies capable of producing new stars will shrink. The supply of gas and dust required for star creation will become too spread out, resulting in less star formation over time. The universe will gradually and inevitably darken as older stars eventually die.