The Earth we live in is but a mere speck of dust in the grand scheme of things. In terms of our size, we are irrelevant to the universe, which is both regarded as infinitely large and still expanding. But does the universe really have no limits? If the universe is infinite, how can it be expanding?
When we say that the universe is expanding, it means that the distance between two points is ever increasing. However, current observations suggest that the universe is infinitely large and that it is also expanding not in terms of its size but in terms of the distance between two points.
It can be pretty confusing when you try to understand how something that is infinitely large is still expanding its size especially because expansion is usually related to something that is finite. However, as difficult as it is to understand, the universe is truly expanding and it is also infinite. We’ll try our best to make that concept easier to understand.
Can the universe be infinite?
Before we get to discuss more about the universe and whether or not it is infinite, let us try to make things simpler by defining what it is to be infinite so that we can start off this discussion on the right foot.
According to the Merriam-Webster dictionary, what it means to be infinite is: 1. extending indefinitely or endless; 2. immeasurably or inconceivably great or extensive; and 3. subject to no limitation or external determination. So, when we ask whether or not the universe is infinite, it should fall into any of those three definitions.
But, as we discuss more about how infinite the universe is, you have to understand that we live in a universe that is too large for our imaginations and our brains to comprehend. We already live on a large planet but you’d be surprised to know how small we really are in the grand scheme of things.
The Earth is the fifth-largest planet in our solar system but it pales in comparison to the sizes of the giant planets like Jupiter and Neptune, which are both many times larger than our planet. Meanwhile, the Sun itself is so large that it can fit over a million Earths in it. And our solar system itself is also big enough that it can have 42.7 million Suns in a two-dimensional plane.
But, get this, the solar system is only a small fraction of what makes up an entire galaxy. Our solar system belongs to the Milky Way galaxy, which is part of the Local Group of galaxies. In comparison, the Milky Way is so big that it is 890 billion times more massive than the Sun itself. So, if we were to say that a quarter (as in the US coin) measures the distance between the Sun and Neptune, then the Milky Way itself is the entire United States of America minus Alaska and Hawaii.
As big as the Milky Way is, it pales in comparison to the sizes of other known galaxies such as Andromeda, which is more than twice the size of the galaxy we belong to. Meanwhile, IC 1101 is the largest known galaxy and is about 50 times the Milky Way’s size, and has a mass that is 2,000 times that of our own galaxy. And, in a grander scheme, the observable universe (as in the limits of the universe we are capable of observing) is at least 930,000 times larger than the Milky Way.
So, all in all, it is needless to say that the universe is indeed large and is difficult to comprehend in terms of its overall size when you look at it on a personal level. But, does being so large mean that the universe is infinite? Well, yes and no.
We always live in a universe that has two possibilities. Either it is infinite or finite. We already talked about the definition of what it is to be infinite. Meanwhile, being finite is simply the opposite of being infinite.
It is without a doubt that the universe is so big and that the observable universe is about 93 billion light-years from end to end in terms of its diameter. So, scientists have come up with a hypothesis that it is much larger than what we can observe as there could be space outside the bubble that we can observe by using whatever technology we have today.
But the reason why we cannot truly measure how large the universe is can be related to how old the universe is. The big bang happened 13.8 billion years ago and the universe has been expanding ever since. Meanwhile, the heavenly objects that we are seeing emitting light (such as the faraway stars in the night sky) are about 46 billion light-years away. That means that we are seeing them as they were 13.8 billion years ago and not as they are right now.
So, in relation to that, we cannot truly measure the size of the universe because it is not yet old enough for the light of the other parts of the universe to travel to our location. In other words, for us to be able to observe more of the universe, we have to wait for the light of the other galaxies to travel closer to us as the universe grows older and older. Of course, our current technology still plays a huge role here.
In that sense, while there is no certainty as to whether or not the universe is infinite or finite, what we can be sure of is that we are not yet in the position to tell whether or not it has limits. All we can say is that it has been continuously expanding since the Big Bang.
How can we tell if the universe is expanding?
So, in relation to the last point we brought up (the expansion of the universe since the Big Bang), how were we able to tell that the universe is expanding?
Well, the simplest way to describe how they do it can be explained by how scientists have studied the wavelengths of light emitted by any of the observable objects in space such as the faraway stars situated in other solar systems and galaxies. They are able to tell the distance between us and a star by examining the change in position of lines in the electromagnetic spectrum from that object.
From there, it is important to discuss more about the Doppler effect, which basically discusses how frequencies in different waves such as sound waves tend to change as the object moves farther away from or closer to you. The sound doesn’t change at all but the frequency changes due to how it is moving away or closer to you. For instance, the sound waves of a guitar may change in frequency and wavelength when you move closer or farther away from the source of the sound but the sound itself doesn’t change.
Applying that to how astronomers are able to tell that the universe is expanding, the Doppler effect shows that certain observable objects in the universe are gradually changing in terms of the wavelength that their light is emitting. This was observed by Edwin Hubble in 1929, who observed that every galaxy he observed was moving away from the Earth. That means that the space between the Earth and those galaxies is only expanding and growing bigger.
How rapidly is the universe expanding?
Going back to how Edwin Hubble was able to notice that the galaxies he observed were moving away from us due to the changes in the wavelength of the light they are emitting, he was also able to come up with what we call the Hubble Constant, which is the ratio between the speed at which the galaxies are moving away from us and their relative distance to us.
The Hubble Constant explains that, for each additional megaparsec (which is 3.26 million light-years), galaxies move 500 kilometers per second faster. In short, the universe is expanding at a rate of 500 kilometers per second per megaparsec. But that was only based on Hubble’s observations, which happened nearly a century ago.
Over the years, our technology has been able to describe the speed at which the universe has been expanding much more accurately. In 2001, Wendy Freedman found that space is expanding at 72 kilometers per second per megaparsec. But, then again, other observations changed that theory as the European Planck mission suggested the number to be 67 while, in 2016, the measurement increased to 73.
There is still no certainty as to how fast the universe has been expanding but we can infer that our astronomers today are able to determine it at much more accurately than Hubble did nearly 100 years ago.
How can the universe be infinite and expanding?
So, if the universe is infinite, how can we explain the fact that it is still expanding? Well, that can be an idea that is pretty tricky to explain especially when you consider that there is still no certainty as to whether or not the universe truly is infinite.
Let us just look at this way regardless of whether the universe is infinite or not. When we say that something is expanding, we are not talking about how more matter is added to it so that it becomes bigger and more massive. Instead, we are talking about how the distance between the two points in the galaxy is increasing. In short, the universe is stretching and is not adding mass.
Imagine yourself baking raisin bread. Because bread naturally expands as it cooks, you would notice that the raisins that you placed on the bread separate from one another gradually without even adding more dough to the equation. That is basically what happens when the universe is expanding as it merely stretches out to create more separation between two different points.
So, if the universe is infinite, it is stretching out as far as it can go without any limits to how far it can stretch out. That is what it means for the universe to be both infinite and expanding. There is no container that will stop it from stretching in contrast to the case when you are baking bread because the limit of the expansion is constrained by the size of the bread pan or even the oven itself.