What is faster, the speed of light or the speed the universe is expanding

 

Credit: NASA, Goddard Space Flight Center

The speed of light in a vacuum is the ultimate speed limit that nothing can surpass, according to the law that most people are aware of regarding the universe. Being a large particle, you can only get close to the speed of light; you can never reach it, let alone surpass it. You have no other option if you are massless; you can only travel through spacetime at the speed of light in a vacuum or a slower speed in a medium. You move more slowly through time the faster you go through space, and vice versa. Since these facts are the foundational idea upon which relativity is predicated, they cannot be avoided.

Nevertheless, when we gaze at far-off things in the universe, they appear to contradict our common sense and reasoning.  We are certain that the universe is precisely 13.8 billion years old based on a number of accurate observations.  Even if we sent a signal at the speed of light today, we could never reach galaxies farther than about 18 billion light-years away. The most distant galaxy we have seen so far is currently 32 billion light-years away, and the most distant light we have seen corresponds to a point that is currently 46.1 billion light-years away.

All of this, however, just deviates from our innate ideas of how things should act; it does not violate the rules of relativity or the speed of light.  Here are some facts concerning the speed of light and the expanding universe that everyone should be aware of.

Understanding the Basics of Special Relativity:

I can follow an object's movement and see how its position changes over time when I observe it. I can note the time I see it as well as its observed position when I see it. I can then determine its velocity by applying the definition, which states that it is a change in distance divided by a change in time. I must thus make sure that the velocity I obtain never surpasses the speed of light, whether I am staring at a huge or massless object, as it would be against the laws of relativity. Although it isn't always the case, this is true in the majority of our shared experiences. Specifically, all of this involves an assumption that we hardly ever consider, let alone declare.

The aforementioned assumption? That area is level, straight, and static. This takes place in Euclidean space, which is the kind of space that comes to mind when we consider our three-dimensional universe.

Einstein’s General Relativity:

To put it another way, most of us are aware of the fundamental idea of special relativity—that nothing can travel faster than light—but we are unaware that special relativity cannot adequately capture the nature of the actual universe.  Rather, we must consider that the universe is supported by a dynamical fabric of spacetime, and that the principles of special relativity only apply to the motion of objects through that spacetime.

The ways that the fabric of space deviates from this idealized, flat, three-dimensional grid, where each succeeding minute is represented by a universally applicable clock, are not captured in our common conception.  Rather, we must acknowledge that spacetime evolves according to Einstein's General Relativity, which governs our Universe.

Expanding Universe and the Hubble Constant:

Nothing can travel through space more quickly than light, but what about the ways that space itself changes?  You've probably heard that the universe is expanding and that the Hubble constant, which measures the rate at which space itself expands, has been determined.  From all of our measurements and observations, we can be positive that the current rate of expansion is exactly between 66 and 74 km/s/Mpc, or kilometers per second per megaparsec. We have even measured that rate with precision.

 However, what does the expansion of space mean?

Raisin Bread Analogy:

I prefer to use the "raisin bread" paradigm to conceptualize the expanding universe. Consider a ball of dough that has raisins scattered throughout it. Imagine now that the dough expands in all directions as it leavens. (If you'd want, you might even picture this taking place in a zero-gravity setting, such as the International Space Station.) What do you think the other raisins would do if you put your finger down on one?

As the dough between them grows, the raisins that are closest to you will seem to travel gently away from you.  Because there is more dough between you and the farther-off raisins than the closer ones, the farther-off raisins will seem to be going away more fast.  Even farther-off raidins will seem to be getting farther away faster and faster.

Energy Density and Dark Energy:

The expansion rate decreases as the Universe expands because it is reliant on the total amount of "stuff" in a given volume of space.  The density of matter and radiation decreases as the Universe expands and its volume rises because matter and radiation are composed of a constant number of particles.  Because the energy of radiation is determined by its wavelength, which extends as the Universe expands, radiation loses energy at a somewhat quicker rate than the density of matter.

However, the "dough" itself has a limited, positive, non-zero energy density in every part of space, and this energy density doesn't change as the Universe expands.  The energy of the "dough" (or space) itself stays constant while the matter and radiation densities decrease; this is what we see as dark energy.

Redshift and the Expanding Universe:

The light from a far-off galaxy is always visible to us as it is at the moment of its arrival.  This indicates that the emitted light undergoes a variety of combined effects, including the difference in gravitational potential between its source and destination, the motion of the emitting and absorbing objects in their respective local spaces, and the cumulative effects of the universe's expansion, which cause the light's wavelength to be stretched.

Fortunately, the initial portion is typically quite tiny.  The second component, referred to as unusual velocity, can vary from a few hundred to several thousand kilometers per second.  However, the impact of cosmic expansion makes up the third section.  It is always the main influence at distances greater than roughly 100 megaparsecs.

Challenges of Measurement:

Our inability to accurately gauge the speed of a far-off object is one of our challenges.  Assuming we know or can determine how inherently brilliant or large it is, we can use a variety of proxies to measure its distance, such as how bright or faint it is or how large or small it appears on the sky.  Its redshift, or how the light is "shifted" from how it would be if we were in the exact place and under the exact circumstances where the light was emitted, can also be measured.

But rather than measuring a specific speed, we're measuring the combined effects of motions and the expansion of the universe.  What we mean when we say that "the Universe is accelerating" is actually the opposite of what you might think. If you watch the same object as the Universe expands, it will not only continue to get farther and farther away from you, but the light it emits will also continue to show an ever-increasing redshift, giving the impression that it is accelerating away from you.

In actuality, however, the redshift results from space expansion rather than the galaxy rapidly accelerating away from you.  Living in a universe dominated by dark energy means that the expansion rate is, if we were to measure it over time, still decreasing and will eventually asymptote to a finite, positive, and non-zero value.

Understanding Cosmic Distance:

A cosmic snapshot, or "God's eye view," of the state of affairs at this specific moment in time—when the light from these far-off objects arrives—is always taken while discussing the distance to an object in the expanding Universe.  In the distant past, some 13.8 billion years after the Big Bang, we know that we are seeing these objects not as they are now, but rather as they were when they generated the light that we see today.

We don't ask how far away the object was from us when it emitted the light we see today, nor do we question how long the light has been traveling when we ask, "How far away is this object?"  Rather, we want to know how far away the object is from us at this precise moment, assuming we could somehow "freeze" the universe's expansion at this moment.

About 32 billion light-years away, the furthest known galaxy, GN-z11, released its now-arriving light 13.4 billion years ago.  We would be able to see 46.1 billion light-years away if we could see back in time to the Big Bang. The future visibility limit is the distance to the farthest object whose light has not yet reached us but will in the future, which is currently ~61 billion light-years away.

But it 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 move through the universe, but it will never reach us because of how relentlessly space will expand.  Every unbound thing becomes farther and farther away with every second that goes by, and objects that were formerly within reach cross that threshold to become permanently inaccessible.  In an expanding universe, nothing travels faster than light, which is both advantageous and disadvantageous.  All but the nearest galaxies might remain inaccessible to us indefinitely unless we can find a way around this.