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We are helpless to prevent the gradual disappearance of the Universe.

There are an estimated two trillion galaxies within the observable Universe. Most are already unreachable, and the situation only gets worse.
As the Universe expands, it also gravitates, so the expansion rate has slowed down tremendously since the hot Big Bang occurred some 13.8 billion years ago. However, about six billion years ago, distant galaxies started speeding up in their recession from us: an effect caused by the relentless presence of dark energy. Today, some ~94% of the galaxies we can observe are already unreachable by us, and in the far future, only the Local Group will remain.
Nearly a century ago, scientists proposed the idea that the Universe was expanding and that the speed at which a galaxy appears to move away from us is directly related to its distance. However, it is now known that this is due to the fabric of space expanding, rather than galaxies physically moving away. Recently, this understanding has been revised, as it has been discovered that the expansion of the Universe is actually accelerating. This means that galaxies will continue to move away from us at an increasingly rapid pace, making them ultimately unreachable, even if we could travel at the speed of light. Ultimately, the Universe is slowly disappearing and we are powerless to stop this phenomenon.
The Milky Way, as seen at La Silla observatory, is a stunning, awe-inspiring sight to anyone, and offers a spectacular view of a great many stars in our galaxy. Beyond our galaxy, however, are trillions of others, nearly all of which are expanding away from us. (Credit: ESO/Håkon Dahle)

When observing a star, its distance can be determined based on the amount of time its light takes to travel to Earth, as the speed of light is finite. However, when observing a galaxy whose light has traveled for 100 million years, it does not necessarily mean that the galaxy is 100 million light years away. In fact, due to the expansion of the Universe on large scales, the distance is much farther than that.

This is because as photons travel over greater distances, the expansion of the Universe has a greater effect on their journey. Therefore, the most distant galaxies that we observe are even farther away than the amount of time it took their light to reach us, as the Universe’s expansion has pushed them even farther away.

This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. The farther a galaxy is, the faster it expands away from us, and the more its light appears redshifted. A galaxy moving with the expanding Universe will be even a greater number of light years away, today, than the number of years (multiplied by the speed of light) that it took the light emitted from it to reach us. (Credit: Rob Knop)

When light travels through space, it has a specific wavelength and energy. If the fabric of the Universe remained constant, the wavelength of the light would also remain constant upon arriving at its destination. However, due to the expansion of the Universe, the fabric of space is stretching, causing the wavelength of the light to become longer. This is known as a cosmic redshift, and it has been observed in the light emitted by the most distant galaxies.

The significant redshifts detected in the light from these galaxies have provided strong evidence for the theory of an expanding Universe. If the Universe were not expanding, the wavelength of the light from distant galaxies would not have been stretched to the degree observed. Therefore, the redshift phenomenon provides confirmation of the theory of cosmic expansion.

Distant galaxies, like those found in the Hercules galaxy cluster, are not only redshifted and receding away from us, but their apparent recession speed is accelerating. Eventually, we will cease to receive light from beyond a certain point from them. (Credit: ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute)

It is not just the fact that the Universe is expanding that we can determine through observations; we can also use the information gathered to understand how the expansion has occurred throughout its history. This allows us to gain insights into the composition of the Universe.

As light travels from a distant object in the cosmos, the expanding Universe causes the wavelength of the light to stretch, resulting in a redshift. The amount of redshift increases with the distance of the object from us, reflecting the evolution of the Universe’s components over time. This allows us to study the impact of different components of the Universe, such as dark energy, matter, radiation, and neutrinos, on the expansion of the Universe at different times.

Two of the most successful methods for measuring great cosmic distances are based on either their apparent brightness (L) or their apparent angular size (R), both of which are directly observable. If we can understand the intrinsic physical properties of these objects, we can use them as either standard candles (L) or standard rulers (R) to determine how the Universe has expanded, and therefore what it’s made of, over its cosmic history. (Credit: NASA/JPL-Caltech)

By measuring sources at a whole slew of distances, discovering their redshift and then either measuring their intrinsic vs. apparent size or their intrinsic vs. apparent brightness, we can reconstruct the entire expansion history of the Universe.

In addition, since the way the Universe expands is determined by the various types of matter and energy present within it, we can learn what our Universe is made out of:

  • 68% dark energy, equivalent to a cosmological constant,
  • 27% dark matter,
  • 4.9% normal (protons, neutrons and electrons) matter,
  • 0.1% neutrinos and antineutrinos,
  • about 0.008% photons, and
  • absolutely nothing else, including no curvature, no cosmic strings, no domain walls, no cosmic textures, etc.
The relative importance of different energy components in the Universe at various times in the past. Note that when dark energy reaches a number near 100% in the future, the energy density of the Universe (and, therefore, the expansion rate) will remain constant arbitrarily far ahead in time. Owing to dark energy, distant galaxies are already speeding up in their apparent recession speed from us. (Credit: E. Siegel)

With a precise understanding of the components of the Universe, we can apply Einstein’s General Relativity to determine the future of our Universe. However, the implications of applying this knowledge to the discovery of a Universe dominated by dark energy were startling.

It revealed that all galaxies that are not gravitationally bound to us will eventually vanish from view. As the Universe expands unchecked by any force, these galaxies will speed away from us at an ever-increasing rate. Over time, the distance between these galaxies and ourselves will increase, leading to an accelerated motion away from us due to the expansion of space.

The GOODS-North survey, shown here, contains some of the most distant galaxies ever observed, some of which have had their distances independently confirmed. A great many the galaxies imaged in this image are already unreachable by us, even if we left today at the speed of light. (Credit: NASA, ESA, and Z. Levay)

The implications of this expansion are distressing and inevitable. At a certain distance from us, the fabric of space expands so rapidly that photons leaving our galaxy towards a distant one or approaching ours from a remote galaxy will never reach us. Despite moving at the speed of light, these distant galaxies become unreachable due to the Universe’s expansion rate.

Currently, the distance beyond which galaxies become unreachable is about 18 billion light years away, as the density of matter and radiation continues to decrease, causing a decrease in the overall expansion rate when measured in km/s/Mpc.

Given that the observable Universe has a radius of approximately 46 billion light years and that every region of space contains the same number of galaxies on the largest scales, it means that only about 6% of the total number of galaxies in the Universe are within our reach, even if we depart now and travel at the speed of light.

The size of our visible Universe (yellow), along with the amount we can reach (magenta). The limit of the visible Universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years. However, beyond about 18 billion light-years, we can never access a galaxy even if we traveled toward it at the speed of light. (Credit: Andrew Z. Colvin and Frederic Michel, Wikimedia Commons; Annotations: E. Siegel)

On average, between twenty and sixty thousand stars become unreachable every second, meaning the light they emit at present will never reach us. However, the light they emitted a second ago will someday reach us. While this thought may be unsettling, it also serves as a reminder of the value of every moment. The Universe is encouraging us to take advantage of the time we have and to not waste any opportunity to explore beyond our Local Group. This group of gravitationally bound objects includes Andromeda, the Milky Way, and approximately 60 small, satellite galaxies.

The different possible fates of the Universe, with our actual, accelerating fate shown at the right. After enough time goes by, the acceleration will leave every bound galactic or supergalactic structure completely isolated in the Universe, as all the other structures accelerate irrevocably away. After another few tens of billions of years, only the Local Group will be reachable any longer. We can only look to the past to infer dark energy’s presence and properties, which require at least one constant, but its implications are larger for the future. (Credit: NASA & ESA)

Out of the estimated two trillion galaxies that exist in our Universe today, only a meager 6% of them can be accessed from the Milky Way’s perspective. This number is continually dwindling due to the Universe’s accelerated expansion caused by dark energy. As time progresses, an increasing percentage of galaxies will become out of humanity’s reach, even beyond our Local Group.

Unless we advance our capability to travel between galaxies and venture out to other clusters and groups, we will remain confined within the Local Group. The ability to send and receive signals beyond the cosmic ocean will gradually disappear as time passes. Despite our attempts to defy the Universe’s relentless expansion, the strength of gravity is insufficient to overpower it. The Universe is evaporating before our eyes, and we are incapable of halting it.

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