No Galaxy Will Ever Truly Disappear, Even In A Universe With Dark Energy

As time progresses, galaxies beyond our local group will increasingly move away from us. Nonetheless, new galaxies will continually emerge.

The greater the distance of a galaxy from us in this ever-expanding Universe, the more rapidly it appears to be receding. Over time, each of these distant galaxies will not only drift farther away but will also seem to accelerate in their departure. In essence, the Universe is not only expanding; this expansion is happening at an accelerating pace. Recent findings over the last two decades have made it evident that dark energy is not only driving this acceleration but is also the most prevalent form of energy in our Universe.

Surprisingly, despite this phenomenon, we are currently able to observe a greater number of galaxies than at any other time in the 13.8 billion years since the hot Big Bang. Even more intriguing is that as time continues, the number of galaxies we can potentially observe will more than double as our cosmological journey progresses. Remarkably, even as galaxies recede at increasing speeds, not a single one will vanish from our sight completely. This is the fascinating science behind this phenomenon.

The Universe has been engaged in a colossal cosmic race since the initial moments of the hot Big Bang. On one side, there is the rapid expansion rate, which drives apart any two points in space. On the other side, the formidable force of gravity pulls all matter and energy towards one another, working against this initial expansion. Given this scenario, there are three potential outcomes:

1. The expansion is too strong for the available matter and energy, leading to a Universe that expands endlessly.
2. There is too much matter and energy for the initial expansion, resulting in a maximum size followed by a contraction, culminating in a Big Crunch.
3. The Universe exists at a delicate balance between the first two scenarios, where the expansion rate approaches zero but never fully recollapses.

For generations, scientists have attempted to ascertain which of these scenarios accurately describes our Universe. The observations that eventually emerged were nothing short of astonishing.

Instead of conforming to any of these three outcomes, the Universe behaves differently. During its initial few billion years, the expansion rate and matter-energy density seem to balance almost perfectly, with both dropping over time as they approach a state where the expansion rate nears zero.

Distant galaxies appear to recede from us at a slower pace, even as they reach greater distances. Eventually, ultra-distant galaxies—whose light has been traveling towards us for billions of years—start to catch up, revealing their presence to us.

Then, around 6 billion years ago, these ultra-distant galaxies began to move away from us at an accelerating rate, marking the revelation of dark energy's influence.

The explanation for this phenomenon is quite straightforward. As the Universe expands, its volume increases while the number of particles remains constant. Consequently, the matter density decreases in relation to the Universe's scale cubed, while radiation density decreases even more sharply due to dilution and the stretching of its wavelength.

However, if energy is inherently present in space, the energy density remains constant despite the Universe's expansion. As the densities of matter and radiation diminish significantly, dark energy becomes increasingly significant. Today, 13.8 billion years post-Big Bang, it has emerged as the primary form of energy in our Universe.

What implications does this have for the Universe's expansion?

Several important, albeit non-intuitive, truths emerge when applying the mathematics of the expanding Universe to our observations. Here are some key points:

• Currently, the Universe extends 46.1 billion light-years in all directions, meaning that light emitted at the moment of the Big Bang has traveled to us from a distance of 46.1 billion light-years.
• Objects beyond a certain distance recede so quickly that even if we departed today at light speed, we would never reach them.
• The distance at which this occurs implies that approximately 94% of all galaxies within the observable Universe are unreachable, regardless of our actions.

This notion might suggest that the Universe is gradually disappearing. Over time, galaxies that are part of clusters—such as our connection to Andromeda and Triangulum—will stay bound together, but other independent clusters will drift away from each other increasingly as the Universe evolves. In about 100 billion years, we may find it impossible to reach any galaxies beyond our Local Group.

However, the number of galaxies we can see today is the highest it has ever been and will only increase over time. This counterintuitive reality arises from a deep understanding of General Relativity and the expanding Universe. As light travels through an expanding cosmos, even as distances increase, light emitted from more distant sources eventually reaches us.

Today, light that has been traveling for 13.8 billion years possesses these characteristics:

1. When that light was first emitted, the Universe was considerably smaller and the emitting object was much closer to us than 13.8 billion light-years.
2. As the Universe has expanded, that light has traversed 13.8 billion light-years at the speed of light.
3. If we were to mark the position of the light's emission, it would now be 46.1 billion light-years away.

If we were to inquire about the number of galaxies currently visible to us with an exceptionally powerful telescope, we could finally answer that there are approximately 2 trillion galaxies within our observable Universe.

What will happen to the number of galaxies we can observe in the future? Will we see more, fewer, or the same number?

To answer this, we must grasp how light navigates through the expanding Universe. Despite the accelerating expansion and the increasing receding speeds of distant galaxies, the cosmic horizon continuously expands. Since the Big Bang, light from increasingly distant sources has reached every location in the Universe. Currently, we can detect light that has traveled for 13.8 billion years, leading us to a cosmic horizon of 46.1 billion light-years.

However, as time progresses, we will be able to perceive light that requires progressively longer travel times—13.9 billion, 15 billion, or even 100 billion years. In the future, galaxies currently beyond our visibility will eventually come into view.

Our understanding of dark energy's role in the Universe's expansion allows us to calculate the extent of what will eventually be observable. Currently, this corresponds to any object within 61.3 billion light-years of us, roughly 33% farther than what we can currently see. As the Universe continues to evolve, all galaxies beyond our visible horizon will eventually reveal themselves.

In terms of volume, this represents an additional 135% of the Universe beyond what we can currently observe. If we presently see 2 trillion galaxies, in the distant future, with advancements in our ability to collect light from these faint objects, we might observe a total of 4.7 trillion galaxies—more than double the current count.

At present, there are around 2 trillion galaxies within our observable Universe, with only about 6% being reachable. This means the remaining 94% will always appear as they were in the past; we will never witness them as they exist 13.8 billion years after the Big Bang, as that light will never reach us. Yet, as time continues, even more galaxies will become visible, though only in their early stages, ultimately bringing the number of observable galaxies to roughly 4.7 trillion—more than twice the current tally.

All these galaxies were once in close proximity to us, and their light will eventually reach our eyes, even as the Universe continues to expand indefinitely. Although there are limits to what we can observe in the future, we have not yet reached that threshold. Moreover, nothing will ever truly vanish; photons will simply arrive less frequently and with diminished energy. If we know what to look for, the Universe of the far future will not only remain observable, but we will be able to witness more of it than ever.

Starts With A Bang is now on Forbes and republished on Medium on a 7-day delay. Ethan has authored two books, Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.