The observable universe, the part of the universe that we can see, is approximately 93 billion light years in diameter. This means that the distance between us and the edge of the observable universe is about 46.5 billion light years in any direction.
What is the “observable” universe?
Now, some of you may be wondering, what is the observable universe and how does this answer the original question? The observable universe is the only part of the universe that we have access to; beyond it is more of the same, but we cannot see it because light has not had enough time to reach us from those great distances.
Not only this, but the universe is expanding, which adds a further complication to the definition of “observable.” Any question about the size of the universe can really only be answered in terms of the observable universe, at least with any hope of accuracy.
Before we delve into a discussion of how the size of the universe was determined, we have to talk about some basic assumptions in cosmology. First of all, it is important to know that the speed of light in a vacuum is constant. This is key to the definition of cosmic distances; one light year is the distance light can travel in one year.
Secondly, the idea that the universe is expanding is crucial to determining the size of the universe. As was discussed in our look into dark energy, Edwin Hubble determined that distant galaxies are moving away from us. His observations and ones that followed lead to the realization that the universe is expanding.
The last thing to know is that the rate at which the universe is expanding is increasing. Originally discovered in the late 20th century, scientists realized that the universe isn’t just expanding, it’s accelerating its growth.
What is the distance to the edge of the universe?
Now that we have the tools to solve the problem, how do we determine the size of the universe? In order to understand the size of the universe, its critical to distinguish between the concepts of proper and comoving distances.
The proper distance between two objects in space is a measurement of their separation at some instant in time. This is the definition of distance that you are familiar with, but there is a slight complication. Since light travels at a finite speed, we can never know the actual distance to a moving object in the instant we observe it. This is because light takes time to travel from a source to us, and in this time, the source may have moved. This means that the proper distance of an object can never be observed, but it can be calculated.
The comoving distance between two objects is one that takes the expansion of space out of the equation. In a static universe, where nothing is in motion, the comoving distance and the proper distance would be the same, since there would be no expansion to account for. In our universe, however, the relationship between the constant comoving distance and the proper distance is a quantity known as the scale factor. This value changes over time to account for the variable rate of expansion of the universe. Currently, this scale factor is equal to 1. Therefore, right now, the comoving distance and proper distance are the same.
If you are confused, that is completely understandable! They are both hard to visualize; the comoving distance is a purely cosmological measure of distance, and the proper distance is impossible to actually see. The really confusing part is that the distances to everything we can see, from the Moon to the most distant galaxy, are facades of the actual distance. As mentioned previously, it has to do with the speed of light in combination with the expansion of space. To clear up this ambiguity, we will look at a brief example.
The most distant objects we can see are about 13.4 billion light years away. Except they aren’t actually; they are much farther away. The light from those objects traveled for 13.4 billion years, which in a static universe would mean the proper distance of the objects was 13.4 billion light years, since they could not have moved as the light traveled to us.
However, given that our universe is not static, the light left those objects when they were much closer to us, and that light took 13.4 billion years to reach us through continuously expanding space. In other words, the light left when the objects were much closer to us than they are now, but the space through which it traveled expanded to 13.4 billion light years across as it traveled through it. This gives us the illusion that the objects are 13.4 billion light years away.
In reality, these objects are much farther away: about 32 billion light years according to the latest cosmological models. Their proper distance when the light left them was much smaller than 13.4 billion light years and now it is 32 billion light years.
Therefore, the distance to the edge of the universe appears to be approximately 13.8 billion light years, but its proper distance is actually about 46.5 billion light years.
How are these different types of distance determined?
To put these varying definitions of distance into more concrete terms, it would be as if someone threw a ball at you, and as they threw it, the space between you and the person throwing it grew. As it grows, the ball takes longer to reach you, and the person ends up being much farther away from you than when they threw the ball.
The only problem with this analogy when compared to the situation with the most distant objects is that the speed of light is the cosmic speed limit. It is the speed at which information is transmitted to us, and nothing can go faster (as far as we know).
Therefore, it would be as if the person throwing the ball was invisible and we could only determine where they were based off of the ball. If we didn’t realize space was expanding, we would assume the person was much closer than they really were, just like the illusion created by the most distant objects in the observable universe.
So, how do we determine how far the light from any object has traveled? The short answer is by using the Doppler effect. This effect, which you might be familiar with, is what explains the change in the pitch of the siren on an emergency vehicle as it drives past you.
As it speeds towards you, its pitch increases, and as it goes away from you, the pitch drops. This is due to the velocity of the vehicle causing a shift in the frequency of the sound wave. Motion towards you causes the frequency to increase and motion away causes it to decrease. The same effect happens with light.
A distant galaxy that has some sort of recessional velocity due to the expansion of the universe will emit light that gets redshifted as it travels to Earth. This term “redshifted” refers to the decrease in the frequency of the light as caused by the Doppler effect. Since red is the lowest frequency of visible light and blue is the highest, we use the terms redshifted and blueshifted to describe the change in frequency.
But, how do we know the frequency is shifted? Using a spectrograph, which breaks down the constituent wavelengths in light to reveal the composition of a source, scientists can find a specific signature of an element and match it to those in a distant object.
They measure the frequencies of the same element in the laboratory, where it is at rest with respect to the spectrograph. Then, when they compare this measurement to the one taken from the distant object, they can determine how fast the object is moving away from us.
Using this information, in combination with Hubble’s law, which tells us that the distance to an object and its recessional velocity are directly related, we can determine how far the light had to travel, and, from this, calculate the age of the observed object.
In fact, you might be wondering why there is such a large difference between the proper distance of the most distant visible objects and the edge of the universe as compared their apparent distances. This is due to the Doppler effect; for instance, an object 13.2 billion light years away is much less redshifted than one 13.3 billion light years away.
What is beyond the edge of the observable universe?
Perhaps the most mind-boggling implication of current cosmological models is that there are objects in the universe that we will never be able to see. We know that the proper distance to the edge of the observable universe is about 46.5 billion light years. We also know that it is rapidly expanding. In fact, objects that are currently beyond 13.8 billion light years (give or take a few million) away right now have a recessional velocity greater than the speed of light.
Wait a minute. Isn’t the speed of light supposed to be the cosmic speed limit? Yes. It still is; these objects aren’t moving away from us at that speed, its just that the space between us and them is expanding so fast that it appears that they are.
Try not to mistake traditional translational velocity with the expansion of space; this is critical to understanding the “mind-boggling” part. In other words, there is a difference between an object being propelled to velocities at or above the speed of light and the distance between two objects increasing at a similar rate due to the expansion of space. The former violates the laws of physics, the latter does not.
Objects that are so far away that space is expanding faster than the speed of light lie outside of what is called the Hubble sphere. This sphere has a radius of 13.8 billion light years, which exactly corresponds with the age of the universe.
As mentioned before, many objects, particularly those that appear near the edge of the sphere, actually now lie outside the sphere. That is, their proper distance is much farther than the edge of the sphere. It’s just that the light from their distant pasts lies inside of it.
So, objects that lie outside the Hubble sphere reside in space that is expanding at a superluminal rate (faster than light). The crazy part is that light leaving these objects right now will eventually fall within the Hubble sphere and then be visible from our part of the universe (after traveling a long time, of course).
Eventually, when the universe expands to a radius of about 62 billion light years, we will reach what is called the cosmic event horizon. This means that light emitted from objects beyond that distance will never fall inside of the Hubble sphere. This is because the space between the object and the edge of the sphere will be expanding too fast for it to reach the edge. In other words, we will never be able to see these objects. Consider yourself sufficiently “mind-boggled.”
An even more intense consequence of the expansion of the universe is that at some distant point in the future, even the closest objects to us will be so distant that they will be moving away faster than light. This means that eventually, all of our galactic neighbors may be invisible to us (assuming there is still an “us” at that point). Not to worry, of course; according to our current understanding of cosmology and dark energy, this won’t happen for a long time.
Clearly, understanding the size of the universe is not as simple as you might have originally thought. The universe is full of unintuitive and mysterious phenomena, and the edge of it is no exception. In dealing with the immense size and time scales that come up frequently in cosmology, it is no wonder that we find ourselves as a species to be, at best, a small and insignificant part of something much grander.