If there’s one universal rule everyone knows, it’s that nothing can surpass the speed of light in a vacuum. Massive particles not only can’t exceed that speed but can’t even reach it, only approach it. If you’re massless, your only option is to move at the speed of light in a vacuum or slower in other mediums. The faster you move through space, the slower you experience time, and vice versa. This is a fundamental principle of relativity, leaving no room for exceptions.

Yet, when we observe distant objects in the Universe, they seem to challenge our common sense. Precise observations show that the Universe is 13.8 billion years old. The farthest galaxy we’ve detected is 32 billion light-years away, and the most distant light comes from a point 46.1 billion light-years away. Any galaxy more than 18 billion light-years away is unreachable, even if we sent a light-speed signal today.

However, this doesn’t violate the speed of light or relativity laws; it only contradicts our intuition about how things should behave. Here’s what everyone should understand about the expanding Universe and the speed of light.

The phrase “nothing can travel faster than the speed of light” is true, but what does it actually mean? Most people interpret it like this:

This understanding holds in most cases but doesn’t apply universally. It assumes space is flat, uncurved, and unchanging—like in Euclidean space, the kind we usually imagine. But the real Universe can’t be accurately described this way. Instead, it follows the rules of General Relativity, which considers space-time as dynamic. The key points are:

When we say “nothing moves faster than light,” we mean nothing moves faster than light through space. It says nothing about how space itself behaves.

Space doesn’t expand at a speed. We live in an expanding Universe, and scientists have measured the expansion rate—the Hubble constant. The present rate is between 66 and 74 km/s/Mpc. For every megaparsec (3.26 million light-years) away a distant object is, it seems to move away at 66-74 km/s.

But this is tricky because the object isn’t really moving through space at that speed; it’s the space between us and the object that’s expanding. So, when objects are far enough away, they can seem to move away faster than the speed of light, but that doesn’t break relativity—it’s the space between us that’s expanding.

A helpful analogy is the “raisin bread” model. Imagine dough with raisins inside. As the dough rises, it expands, and the raisins move farther apart. The raisins (galaxies) aren’t moving themselves; it’s the dough (space) expanding between them.

The expansion rate depends on the amount of “stuff” in space. As the Universe expands, the density of matter and radiation drops, but the energy in space itself (dark energy) remains constant, dominating the Universe’s future.

When we observe distant galaxies, we see light as it reaches us now. The light has been stretched by the expanding Universe, a process called redshift. The farther away the object, the more its light is redshifted. This isn’t because the object is moving away faster than light, but because space is expanding. Over time, the redshift increases, making it seem like galaxies are accelerating away, but this is just the result of space expanding.

The farthest galaxy, GN-z11, emitted its light 13.4 billion years ago and is now 32 billion light-years away. Some distant objects beyond 18 billion light-years are forever beyond our reach because space expands too quickly for their light to ever reach us. Nothing moves faster than light, even in an expanding Universe, and this might mean most galaxies will remain unreachable forever unless we find a way to overcome this barrier.