The Size of the Universe

The speed of light is one of the most important and fundamental properties of our universe; it’s used in a number of ways, for distance measurement, interplanetary communications, and in various mathematical calculations. And that’s just the start of it.

The speed at which light travels through a vacuum—186,282 miles (299,792 kilometers) per second—is static and unchanging. If you remove that variable, the very foundation of modern physics crumbles. This occurs for a number of reasons, but to sum the general rule, “nothing in the universe can travel faster than the speed of light.”

As you can imagine, some confusion arises when one considers the fact that the universe is not 13.8 billion light-years across—a number that corresponds with the age of the universe. By current estimates, it’s quite a bit larger, measuring in at a staggering size of some 93 billion light-years across. And that’s just what we can see. What we can’t may go on forever.

So how can the universe be 93 billion light-years across, if it is only 13.8 billion years old, and nothing can travel faster than light?

Understanding Redshift

Before you can understand why the universe’s size is so much greater than its age, it is important to understand just how light works.

Sir Isaac Newton was unquestionably one of the greatest minds to ever live; In addition to “inventing” calculus, he was the first scientist to truly understand the essence of light. And what happens when you break it into its constituent parts?

Light Dispersion Through a PrismFor starters, his research revealed that black is the absence of color, conversely white light—like that which comes from the Sun and other stars—is a combination of every color. Looking at them through a prism, one can see the corresponding elements their light represents, which can then be used to help determine the composition, temperature, and even where the object is in its evolution.

In more ways than one, his work revolutionized physics, and paved the way for all of the greats, like Niels Bohr, Max Planck, and (of course) Albert Einstein.

For the purposes of this discussion, the most relevant scientist to build off Newton’s work went by the name of Christian Doppler, who came to prominence hundreds of years after Newton’s death.

If you aren’t familiar with his work, Doppler discovered something that is, today, called the Doppler effect. This process explains why some light from cosmic sources tends towards the red end of the electromagnetic spectrum and others tend towards the blue end.

Absorption Lines in Spectra of Distant Galaxies
Absorption Lines in Spectra of Distant Galaxies (Credit: Graphic: Chano Birkelind, Niels Bohr Institute)

In simple terms, the doppler effect notes how the wavelength of light is shifted based on the direction that the source is moving, like whether something is coming toward us or moving away.

Specifically, light waves will be stretched if the source is moving away from the observer, thus appearing red (the longer wavelength), and  the light waves will be compressed if it’s heading towards the observer, thus appearing blue (the shorter wavelength).

Along the way, a game changer presented itself. Ultimately, almost all galaxies appeared to to be shifted toward a longer wavelength, which means that they look red (like they are moving away from us). But most everything is not only moving away from us, we see that this redshift increases, meaning that objects are moving away from us faster and faster.

This led us to the discovery that the universe is not stationary, as some believed it to be—it’s actually expanding!

Understanding The Expansion of the Universe

Here’s where things get sticky. Our observations of redshift revealed that objects three times more distant are moving three times faster relative to nearby galaxies—the farther we look into space, the faster the galaxies are moving, so fast, they easily surpass the speed of light at these vast distances. However, as previously stated, the speed of light is the universal speed limit. So how can this be?

First note that, while there is an edge of what we can see, the actual universe extends much farther than we can comprehend.

This edge is called the “observable universe.” In it, there are:

  • 10 million superclusters
  • 25 billion galaxy groups
  • 350 billion large galaxies
  • 7 trillion dwarf galaxies
  • and 30 billion trillion (3×10²²) stars

If all of this was tucked in 13.7 billion light-years of spacetime, the universe would seem pretty packed. However, one of the most common misconceptions about the universe is that its size should be equal to its age in years (taking into account the area light covers in one year). The first problem with this assumption comes back to the first few moments that followed the big bang.

You see, when the universe first “popped” into existence approximately 13.75 billion years ago, spacetime itself began expanding at speeds greater than that of the speed of light. This period, called inflation, is integral in explaining much more than the universe’s size, but things like the homogeneous nature of space on a large-scale, and the conditions that existed during the first epoch.

The Inflation Model
The Inflation Model via Nature Magazine)

Basically, the universe transitioned from an infinitely dense and hot state into a vast area teeming with protons and neutrons—particles that eventually came together and forged the building blocks of all matter—within moments. After the initial inflation died down, the expansion slowed. Now, objects are being pulled apart by a mysterious forced called “dark energy.”

Through means that haven’t yet been ascertained, this expansion does appear to be happening faster than the speed of light, BUT that doesn’t mean probably what you think it does.

I’m afraid the confusion stems from a basic misinterpretation of relativity itself. You see, it says that OBJECTS can not travel faster than the speed of light through spacetime. It doesn’t, however, have anything to say about spacetime itself. So, to summarize, the size of space does not conflict with basic physics.

This is because the galaxies themselves (and any other objects in space) aren’t breaking any laws, because they aren’t traveling through space faster than light (at least, not in the traditional sense). Rather, every portion of space is expanding and stretching. It’s not even that the edges are flying outwards, but that spacetime itself—the area between galaxies, stars, planets, you and I—is stretching.

In short: Spacetime is expanding and pushing matter apart. Matter is not really traveling through spacetime.

As an interesting aside, unfortunately, the expansion has some bleak implications for the future of the universe. Assuming the expansion continues indefinitely (and doesn’t slow), the horizon of the visible universe will gradually shrink, until a point comes where objects will simply be too far apart for light to ever reach another galaxy.

For that matter, much of what we see now was originally much closer. Thanks to the expansion, these objects have been carried off, some galaxies and other objects have been red-shifted out of existence (or out of our view, anyways). In fact, the most distant galaxies are among the oldest things in the universe, having formed when the universe was just millions of years old. It’s likely a majority of these no longer exist, or are located in a completely different section of the cosmos today.

Additional reporting by Jaime Trosper.