The idea that the universe is expanding at speeds exceeding light seems to break the ultimate speed limit. It doesn’t. But it does make defining distance in space a lot trickier than just measuring how long light took to arrive.
Space keeps growing while photons are in transit. A galaxy emits a ray of light. That photon races toward our telescope. Meanwhile, the fabric of space between them stretches. By the time the light hits the lens, the source galaxy is much farther away than when the light started its journey. This isn’t just theoretical. It’s why astronomers have to rely on models to figure out where things are now.
The light shows us the past. Not the present.
To map the current location of these distant objects, we use a cosmological framework. Specifically, the Lambda-CDM (LCDM) model. It accounts for dark matter and dark energy. We know dark matter plays a role (see separate episode). We know dark energy drives the acceleration (another episode). Alternative theories exist, but they rarely shift the core mechanics of cosmic expansion enough to matter for this specific question.
Defining the Observable Universe’s Edge
Let’s look at the numbers. The universe is about 13.77 billion years old. Light has had that much time to travel. So, naively, you’d think the edge of what we can see is 13.77 billion light years away.
You’d be wrong.
Because space has been expanding the whole time the light was traveling, the object that emitted that oldest light is now roughly 45 billion light years distant. This boundary is called the particle horizon. Sometimes cosmologists call it the comoving horizon. Sometimes they just use “cosmological horizon.” The name changes with the mood of the astrophysicist, not the physics.
45 billion. Way bigger than 13.77.
Does this mean information is traveling faster than light? No. It means space itself is expanding. And space isn’t bound by the same rules as matter.
The speed of light is a local speed limit. It governs how fast objects move through space. It does not limit how fast space itself can expand. You will never see a rocket fly past your window faster than c. But if you watch two distant galaxies, they can recede from each other at any speed. They are not moving through space in the traditional sense. The space between them is simply growing.
Why Redshift Matters More Than Speed
How do we know this is happening? We measure redshift.
When a galaxy moves away from us, its light stretches into the redder, longer wavelengths of the electromagnetic spectrum. Edwin Hubble used this exact phenomenon to discover that the universe is expanding. He saw that more distant galaxies had higher redshifts.
Higher redshift means faster recession.
In an expanding universe, distance and speed are linked. The farther away an object is, the more space there is between you and it. More space means more expansion. And more expansion means higher velocity.
There is a specific tipping point. It’s called the Hubble distance. It sits at about 13.77 billion lightyears from Earth. Objects at this distance are receding from us at the speed of light.
Beyond that point? They are moving faster than light.
Is this cheating? No. It’s how special relativity works on a cosmic scale. Locally, nothing beats light. Globally? Space can do whatever it wants.
The Hiding Boundary: The Event Horizon
So we can see galaxies receding faster than light. That seems paradoxical. But there’s a catch.
We only see those distant, superluminally receding galaxies because the light we’re catching now was emitted billions of years ago. Back then, they were much closer. They were within the reach of our telescopes.
There is a hard limit to how much we can see. It’s not the particle horizon (the 45-billion-light-year mark). It’s something closer and more final.
The cosmological event horizon.
Located about 17 billion lightyears away, this boundary separates galaxies we can see today from those we will never see, no matter how long we wait for new light to arrive. Light emitted from beyond this line right now will never reach us. The expansion is too fast.
Dark energy is making this limit more aggressive.
The accelerating expansion driven by dark energy ensures that the event horizon will stabilize. It will grow slightly, eventually leveling off at roughly 60 billion lightyears. But beyond that? Darkness.
Most distant galaxies will redshift out of existence. Their light will stretch into wavelengths so long and dim that they become undetectable.
This isn’t speculation. It’s a prediction based on current data. In about 100 billion years, if humans (or whatever comes after us) are still looking up, every galaxy outside our Local Group will have vanished. The universe will look empty. Just our own neighborhood in a sea of nothing.
It’s a lonely future for astronomers. But it’s consistent with the physics we have. The universe expands. Space stretches. And some things are lost to the dark, not because they died, but because they got too far, too fast.
And that’s just how it is.
