How is it possible to see the universe that is vanishing due to dark energy?

In the 1920s, researchers unveiled the expanding nature of the universe through measurements of galaxy distances and the redshift of their emitted light. By the 1990s, it became evident that this expansion is accelerating, with distant galaxies moving even further away. This acceleration has been attributed to dark energy, contributing to the universe's gradual disappearance over time.

Currently, there are approximately 2 trillion galaxies within the observable universe, with 97% already unreachable, even at light speed. Yet, despite this limitation, we can still observe them. Surprisingly, new galaxies continue to emerge into our view as time progresses. While we may not be able to reach these fading galaxies, their light still reaches us. Here's how it works.

According to General Relativity, our gravitational theory, a static universe is impossible. Hence, it must either be expanding or contracting. This conclusion stems from the observations of uniformly distributed matter and energy throughout the universe, allowing us to calculate its spacetime evolution based on three key factors:

• The initial rate of expansion or contraction, including zero,
• The total amount of matter and energy present,
• The ratios of different energy types (such as dark energy, dark matter, etc.).

These parameters enable us to ascertain both the historical and future trajectories of the universe.

In recent decades, astronomers have mapped the current structure of the universe on extragalactic scales, observing how galaxies cluster and form large-scale structures. The Cosmic Microwave Background further enriches our understanding by revealing the seeds of the structures that developed into today's galaxies.

Tracing back to the Big Bang, we find a cohesive narrative: the universe has existed for 13.8 billion years, consisting of 68% dark energy, 27% dark matter, 4.9% normal matter, and 0.1% neutrinos and other forms of energy, and it is unlikely to collapse back in on itself.

If we observe a nearby galaxy, we see its evolution over time as it interacts with smaller galaxies, forming new stars through various processes. However, as it moves farther away, its redshift increases significantly, especially once it crosses a distance of about 15 billion light-years, marking a boundary between what can and cannot be reached at light speed.

Conversely, observing an ultra-distant galaxy reveals a different story. Today, we see it as it existed in the distant past, with its light redshifted and stretched from its original wavelength due to its long journey through the expanding universe.

As billions of years pass, the light we receive will become redder and fainter, indicating that it is from an increasingly distant point in time. Even after hundreds of billions of years, it will never evolve to match the age of our local galaxies.

Remarkably, we can also consider galaxies whose light has yet to reach us. The most distant observable object, 13.8 billion years after the Big Bang, is currently 46 billion light-years away. However, any object within 61 billion light-years will eventually allow its light to reach us.

This light is already en route, and will eventually arrive, allowing us to see even more galaxies in the future, despite the universe's expansion.

While we can currently observe 2 trillion galaxies, this number could rise to 4.7 trillion in the distant future.

So how can we see the disappearing universe if it is indeed vanishing?

This question prompts a deeper examination of the impact of dark energy on distant galaxies. In a hypothetical universe comprised entirely of matter (without dark energy), distant galaxies would decelerate over time rather than accelerating away.

In such a scenario, as time progresses, any visible object would gradually have its redshift decrease. As light continues to travel and reach us, distant galaxies would age, and there would be no limit to the number of galaxies we could observe.

In a decelerating universe, no cosmic horizon would restrict our view. Any distant galaxy could eventually send its light to us given sufficient time.

However, our universe is not decelerating; it is influenced by dark energy, which determines the scale and timeline of its acceleration. This reveals crucial details about galaxies positioned at various distances:

• Closer than 15 billion light-years: We will see them as they are today, 13.8 billion years post-Big Bang.
• Between 15 and 46 billion light-years: We will always see them, but they will appear younger than 13.8 billion years and are unreachable.
• Between 46 and 61 billion light-years: They will eventually come into view, but will never be as old as the earliest observable galaxies.
• Greater than 61 billion light-years: These galaxies will remain beyond our reach and visibility.

Our understanding of cosmic history is robust, built on the gravitational theory and the universe's current expansion rate and energy composition. Light will persist in traversing this expanding universe, allowing us to receive it far into the future, albeit with limitations on what we can observe.

The key to seeing these distant galaxies lies in their past proximity to us and the light they emitted when the universe was younger. Despite ongoing expansion and acceleration, these ancient photons will eventually reach us, illuminating new galaxies as time goes on. With advancements in technology, we can build larger telescopes to detect fainter and redder light, enabling us to explore up to 4.7 trillion galaxies, even in a universe dominated by dark energy.

Starts With A Bang is now on Forbes and republished on Medium thanks to our Patreon supporters. The author has written two books, Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.