"Is it true that the universe is faster than light? A Simple Explanation of Why the Theory of Relativity Remains Intact"

Why "Faster-than-Light Expansion of the Universe" Doesn't Betray Einstein

"The Universe is Expanding Faster than Light"

When you come across such a statement on your timeline, many people might think:

"Wait, is the theory of relativity over?"

However, to conclude, Einstein is not betrayed at all.
In the latest cosmology, it is understood that the "expansion speed of the entire universe" exceeding the speed of light is rather considered "normal."Phys.org

So, what is the mechanism behind this? Let's organize the surprising and misunderstood reactions on social media based on the explanatory article "Yes, the universe can expand faster than light" published on Phys.org.Phys.org

The Speed Limit of Light is a "Local Rule"

What the theory of relativity prohibits is "an object moving faster than light when observed from the same location."
Whether it's a rocket or a spaceship, the speed at which it whizzes past must always be below the speed of light.

What is important here is the distinction between

• the speed at which "an object moves through space" (local)

• and the speed at which "space itself expands or contracts" (global).

They are different things.

In general relativity, it doesn't matter how fast space itself expands.
The image is that it's the rocket that gets caught for speeding, but if "the road itself" stretches on its own, there's no penalty.

The expansion of the universe is precisely this phenomenon of "the road (space) stretching."
Galaxies are essentially "riding" on it, and as the universe expands, they move away as a result.

The Universe is 13.77 Billion Years Old, Yet the Observable Universe is "45 Billion Light-Years in Radius"

Let's look at the numbers and get even more confused.

• Age of the Universe: Approximately 13.77 billion years

• The farthest distance we can "currently see": Approximately 45 billion light-yearsPhys.org

"Wait, if only about 13.8 billion years have passed, how can we see 45 billion light-years away?"
This gap is the evidence that "the expansion of the universe exceeds the speed of light."

The point is thatthe universe continues to expand even while light is traveling.

1. A galaxy born right after the Big Bang emits light

2. That light travels towards Earth over billions of years

3. Meanwhile, space continues to stretch

4. By the time the light finally reaches us "now," that galaxy has moved much farther away than it was back then

As a result,the "current location" of the farthest galaxy we can see nowis much farther than the distance the light traveled.
In cosmology, this distance is called the "particle horizon," and the current value is approximately 45 billion light-years.Phys.org

Beyond the Hubble Distance, Galaxies Recede "Apparently" Faster than Light

The expansion speed of the universe increases with distance.

This is known as Hubble's Law,

expressed in a simple relation where the farther a galaxy is, the greater its recession speed (the speed at which it moves away from us).

Extending this equation, there is a point at a certain distance where "recession speed = speed of light."
This point is called the Hubble Distance, and in the standard model of modern cosmology, ΛCDM, it is around 13.77 billion light-years.Phys.org

• Galaxies closer than the Hubble Distance
  → Recession speed is below the speed of light

• Galaxies farther than the Hubble Distance
  → Recession speed exceeds the speed of light (apparently)

You might think, "Wait, aren't they moving faster than light?" but it's important to note that

it's not the galaxies themselves moving, but the "space between us and the galaxies."

Therefore, it does not contradict the theory of relativity, which imposes local speed limits.

The Cosmic Event Horizon, Beyond Which "Light Emitted Now" Will Never Reach Us

Even more complicated is the concept of the "cosmic event horizon."Phys.org

• Particle Horizon... The farthest distance "light has reached so far" (about 45 billion light-years)

• Event Horizon... The boundary where "light emitted at this very moment will never reach us in the future"

According to the Phys.org article, the event horizon is currently located at approximately 17 billion light-years.Phys.org

Light emitted "now" from galaxies beyond this distance will never reach us,no matter how long we wait.This is because the universe is expanding at an accelerating rate, causing regions that recede faster than light to increase.

Though it has a similar name to the horizon of a black hole, here we are talking about the "horizon of the entire universe."
It is the boundary beyond which information will never reach us due to the expansion of the universe.

In 100 Billion Years, Only the "Local Group of Galaxies" Will Be Visible

Under the ΛCDM model, it is believed that the universe will continue to be dominated by dark energy and will keep expanding at an accelerating rate.Phys.org

The future vision is rather lonely.

• The event horizon will eventually hit a ceiling at around 60 billion light-years.

• Meanwhile, light coming from distant galaxies will continue to redshift, stretching its wavelength until it becomes unobservable.Phys.org

According to the explanation on Phys.org,in about 100 billion years, the entire universe except for the local group of galaxies (a small group including the Milky Way and Andromeda galaxies) will disappear from view.Phys.org

Even if a civilization remains in the universe at that time, they might

mistakenly believe that "only this group of galaxies has existed in a quiet universe."

It will become much more challenging to arrive at the Big Bang cosmology than it is now.

Reactions to the "Faster-than-Light Universe" on Social Media

The article titled "Yes, the universe can expand faster than light" sparked significant reactions on social media, partly due to its provocative title (here, we reconstruct typical reactions).

1. Confusion of the "Relativity is Over" Type

"If the universe is faster than light, did Einstein lose?"
"Physics has changed its settings again..."

Such comments quickly appeared on X (formerly Twitter) and Threads.
In reality, as mentioned earlier, the "faster-than-light" expansion of the universe is consistent with the theory of relativity.
In fact, it is general relativity itself that makes this strange phenomenon possible.

2. Dreams and Reality of "So Can We Do Warp Drive?"

"If we can move space itself, can't we realize SF warp navigation?"
"If we can locally replicate what the universe is doing on a large scale..."

If the universe is expanding space itself, the idea of "can't humans do the same on a smaller scale?" is natural.
In fact, in the world of theoretical physics, there are studies dealing with "warp bubbles" like the Alcubierre Drive, and there are papers stating that "in principle, it does not contradict relativity."

However, in the current models, the technical and theoretical hurdles remain extremely high, such as requiring enormous negative energy.
On social media,

"Even if it's theoretically okay, it's like winning the lottery first prize ten times in a row."

Voices of calm skepticism were prominent.

3. Philosophical Mode of "What's Beyond the Edge of the Universe?"

"What and how much is spreading beyond the invisible region?"
"How do they see us from beyond the event horizon?"

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