Key Takeaways:

  • Nothing, not even massless particles, can travel faster than the speed of light in a vacuum.
  • The universe is expanding, but this expansion is not a speed through space. It’s the stretching of space itself.
  • Distant objects appear to recede faster than light due to the expansion of space, not their actual motion. This is measured by the redshift of their light.
  • CWhen we look at faraway objects, we are seeing them in the past, not the present, due to the travel time of light and the expansion of space.
  • There’s a limit to how far we can see in the universe. Objects beyond 18 billion light-years will forever be out of reach due to the accelerating expansion.

The speed of light in a vacuum is the ultimate speed limit that nothing can surpass, if there is one law of the universe that most people are aware of. Not only can you never get there, but as a massive particle, all you can do is get close to the speed of light. If you’re massless, there’s nothing you can do but move through spacetime at the speed of light in a vacuum or a slower speed in a medium. The faster your motion through space, the slower your motion through time, and vice versa. These facts cannot be avoided, since they form the basis of the theory of relativity.

Nevertheless, the far-off objects in the universe appear to defy common sense when it comes to reasoning. We have conclusively determined that the universe is precisely 13.8 billion years old based on a number of exact observations. Even if we were to send a signal at the speed of light today, we could never reach galaxies farther away than about 18 billion light-years. The furthest galaxy we have seen thus far is currently 32 billion light-years away. The furthest light we see corresponds to a point currently 46.1 billion light-years away.

However, none of this violates the laws of relativity or the speed of light; rather, it only defies our innate beliefs about how things should operate. Here are some facts about the speed of light and the expanding universe that everyone should be aware of.

Instead of an empty, blank, three-dimensional grid, putting a mass down causes what would have been ‘straight’ lines to instead become curved by a specific amount. In General Relativity, we treat space and time as continuous, but all forms of energy, including but not limited to mass, contribute to spacetime curvature. In addition, the distances between unbound objects evolve with time, owing to the expansion of the universe. (Credit: Christopher Vitale of Networkologies and the Pratt Institute.)

What “nothing can travel faster than the speed of light” actually means

Nothing can move faster than the speed of light, it is true. What does that really mean, though? When most people hear it, they typically consider the following:

  • I can follow an object’s movement and see how its position changes over time when I observe it.
  • I can note its observed position and the time I observe it when I see it.
  • I can then calculate its velocity by applying the definition of velocity, which is a change in distance divided by a change in time.
  • Because of this, I must always ensure that the velocity I observe when looking at an object—massive or massless—never surpasses the speed of light in order to avoid breaking the laws of relativity.

While this is true for the majority of our shared experiences, it isn’t always the case. Specifically, all of this involves an assumption that we hardly ever consider, much less make.

The assumption in question? That area is level, straight, and unchanging. This takes place in Euclidean space, which is the kind of space that typically comes to mind when considering our three-dimensional universe. The majority of us imagine overlaying a three-dimensional “grid” over everything we see and attempting to represent locations and times using four coordinates—one for each of the dimensions of time, space, x, and y.

Given enough time, light that was emitted by a distant object will arrive at our eyes, even in an expanding universe. However, if a distant galaxy’s recession speed reaches and remains above the speed of light, we can never reach it, even if we can receive light from its distant past. (Credit: Larry McNish/RASC Calgary)

To put it another way, most people are aware of the fundamental idea behind special relativity—that nothing can move faster than light—but they are unaware of the fact that special relativity is unable to accurately represent the nature of the real universe. Rather, we must consider that the universe is supported by a dynamical fabric of spacetime, and that the laws of special relativity only apply to the motion of objects through that spacetime.

Our shared understanding leaves out the ways in which the structure of space deviates from this idealized, flat, three-dimensional grid, in which every new moment is defined by a clock that is applicable to all situations. Rather, we must acknowledge that spacetime evolves according to Einstein’s General Relativity, which governs our universe.

Specifically, the laws of relativity apply to objects as they move through space, not to space itself. Space itself can expand or contract, be positively or negatively curved, and not just flat.

In other words, the statement that “nothing can move faster than light” actually refers to “nothing can move faster than light through space,” but the way in which objects move across space reveals nothing about the future evolution of space. If not, all we can say is that in spacetime, nothing moves faster than light in relation to another object at the same place, or event.

Edwin Hubble’s original plot of galaxy distances versus redshift (left), establishing the expanding universe, versus a more modern counterpart from approximately 70 years later (right). In agreement with both observation and theory, the universe is expanding. (Credit: E. Hubble; R. Kirshner, PNAS, 2004)

Space doesn’t expand at a speed

Thus, nothing travels through space more quickly than light, but what about the ways in which space itself changes? You’ve probably heard that the universe is expanding and that the Hubble constant, which represents the rate at which space itself expands, has been measured. The current rate of expansion is precisely between 66 and 74 km/s/Mpc, or kilometers per second per megaparsec, according to all the measurements and observations we have made. We have even measured that rate very precisely.

But what does it mean that space is expanding?

A distant and unbound object will appear to be regressing from us at a speed of 66–74 km/s for every megaparsec, or 3.26 million light-years, that it is from us. Something moving away from us at a distance of 20 Mpc should be seen to be traveling at 1320–1480 km/s; at a distance of 5000 Mpc, it should be traveling at ~330,000–370,000 km/s.

But there are two reasons why this is unclear. One, this is the result of the space between objects expanding; it isn’t really traveling through space at that speed. Furthermore, given that light travels at a speed of 299,792 km/s, the hypothetical object located approximately 5000 Mpc away appears to be departing from us at a faster rate than light.

expanding universe
The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. The farther away any two raisin are from one another, the greater the observed redshift will be by time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations, and has been consistent with what’s been known all the way back since the 1920s. (Credit: NASA/WMAP Science Team.)

I like to use the “raisin bread” model to conceptualize the expanding universe. Consider that you have a ball of dough that is covered in raisins. Imagine now that the dough expands in every direction as it leavens. (If you’d like, you can also picture this taking place in an environment with no gravity, such as the International Space Station.)

  • As the dough between them expands, the raisins that are closest to you will appear to move slowly away from you.
  • Raisins that are farther away will appear to be moving away more quickly, as there’s more dough between them and you than the closer raisins.
  • Even further away raisins will appear to be getting farther away faster and faster.

In this analogy, the dough represents the expanding universe, and the raisins represent galaxies or bound groups/clusters of galaxies. In this instance, however, the dough that stands in for the fabric of space is invisible and cannot be directly tracked; it also does not become less dense with the expansion of the Universe; rather, it merely serves as a “stage” for the raisins, or galaxies, to live on.

While matter and radiation become less dense as the Universe expands owing to its increasing volume, dark energy is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant. (Credit: E. Siegel/Beyond the Galaxy)

The total amount of “stuff” in a given volume of space determines the expansion rate, so as the Universe expands, the rate of expansion decreases due to dilution. Since matter and radiation consist of a finite number of particles, their densities decrease as the universe’s volume grows and expands. Since the energy of radiation is determined by its wavelength, which stretches as the universe expands, radiation loses energy at a slightly faster rate than the density of matter.

However, the “dough” itself has a finite, positive, non-zero amount of energy in every area of space, and that energy density doesn’t change as the universe expands. Dark energy is the result of the energy of the “dough” (or space) itself remaining constant while the densities of matter and radiation decrease. We can conclude with confidence that the energy budget of the Universe was dominated by radiation for the first few thousand years, followed by matter for the next few billion years, and finally by dark energy for the remaining few billion years in our real Universe, which contains all three of these. Dark energy will, as far as we can tell, always rule the universe.

The expected fates of the Universe (top three illustrations) all correspond to a Universe where the matter and energy combined fight against the initial expansion rate. In our observed Universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained. All of these Universes are governed by the Friedmann equations, which relate the expansion of the Universe to the various types of matter and energy present within it. (Credit: E. Siegel/Beyond the Galaxy)

This is where things get tricky. Every time we gaze upon a far-off galaxy, we are witnessing its light in its current state—that of arrival. This means that a variety of combined effects are experienced by the light that was emitted:

  • the difference in the motion of the emitting object through its space and the motion of the absorbing object through its local space;
  • the gravitational potential from the point of emission to the destination
  • the cumulative effects of the universe’s expansion, which lengthen the wavelength of light

Fortunately, the first portion is typically quite little. The peculiar velocity, which can vary from hundreds to several thousand kilometers per second, is the second component.

expanding universe
This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. (Credit: Rob Knop.)

However, cosmic expansion’s effect makes up the third section. It is always the dominant effect at distances greater than roughly 100 megaparsecs. The Universe’s expansion is all that matters on the largest cosmic scales. It’s crucial to understand that space expands at a frequency, or a speed per unit of distance; there is no intrinsic speed to the expansion. It becomes less clear when conveyed as a number of kilometers per second per megaparsec because “kilometers” and “megaparsecs” are both measures of distance that cancel out when converted from one to the other.

It is true that light from far-off objects gets redshifted, but this isn’t caused by anything expanding or receding more quickly than light. We are the ones who impose a “speed” on space because that is what we are used to; space just expands.

Friedmann equation
Whatever the expansion rate is today, combined with whatever forms of matter and energy exist within your universe, will determine how redshift and distance are related for extragalactic objects in our universe. (Credit: Ned Wright/Betoule et al. (2014))

What’s actually accelerating in our accelerating Universe?

One of our challenges is that we are unable to determine the speed of an object that is far away. If we know or can determine how intrinsically bright or large it is, we can use a variety of proxies to measure its distance, such as how big or small it appears in the sky. Its redshift, or how the light is “shifted” from how it would be if we were in the exact same place and experiencing the exact same circumstances as the light was emitted, is another property that we can measure. Because we are accustomed to seeing waves shift as a result of the Doppler effect (for example, sound waves), we frequently interpret that shift as a recession.

We are, however, measuring the combined effects of motions as well as the expansion of the universe rather than an actual speed. When we say “the Universe is accelerating,” what we really mean—and this is completely unintuitive—is that if you watch the same object as the Universe expands, it will not only continue to get farther and farther away from you, but it will also continue to display an ever-increasing redshift in the light that it gives off, giving the impression that it is accelerating away from you.

The redshift, however, is actually caused by the expansion of space rather than the galaxy moving faster and faster away from you. Living in a universe dominated by dark energy means that, if we were to measure the expansion rate over time, it would eventually asymptote to a finite, positive, and non-zero value. The expansion rate is currently decreasing.

The size of our visible Universe (yellow), along with the amount we can reach (magenta). The limit of the visible Universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years. However, beyond about 18 billion light-years, we can never access a galaxy even if we traveled towards it at the speed of light. (Credit: Andrew Z. Colvin and Frederic Michel, Wikimedia Commons; Annotations: E. Siegel)

So what determines “distance” in an expanding Universe?

In the expanding universe, when we discuss an object’s distance, we are always taking a cosmic snapshot, or a kind of “God’s eye view,” of the state of affairs at the specific moment when light from these far-off objects arrives. We know that we are witnessing these objects not as they are right now, 13.8 billion years after the Big Bang, but rather as they were when they first began to emit the light that we see today.

But when we talk about, “how far away is this object,” we’re not asking how far away it was from us when it emitted the light we’re now seeing, and we aren’t asking how long the light has been in transit. Rather, we are asking how far away the object is from us at this precise moment if we could somehow “freeze” the expansion of the Universe. The farthest known galaxy, GN-z11, is roughly 32 billion light-years away and released its now-arriving light 13.4 billion years ago. If we could see all the way back to the instant of the Big Bang, we’d be seeing 46.1 billion light-years away, and if we wanted to know the most distant object whose light hasn’t yet reached us, but will someday, that’s presently a distance of ~61 billion light-years away: the future visibility limit.

But that doesn’t mean you can get there just because you can see it. Any object that is currently more than 18 billion light-years away from us will continue to emit light, and that light will travel throughout the universe, but it will never be able to reach us because of how quickly space will expand. Every instant that elapses causes every unbound object to get further and further away, and objects that were once within reach to cross that boundary and become permanently out of reach. In an expanding universe, nothing moves faster than light, which is both a benefit and a drawback. If we don’t find a way around this, all galaxies other than the closest ones might always be out of our reach.

4.6 5 votes
Article Rating
Notify of

Inline Feedbacks
View all comments