The observable Universe contains an estimated two trillion galaxies. Most are already unreachable, and the situation is worsening.
Because the Universe gravitates as it expands, the rate of expansion has slowed dramatically since the hot Big Bang 13.8 billion years ago. However, around six billion years ago, distant galaxies began to accelerate their recession from us, an effect produced by the constant presence of dark energy. Today, over 94% of the galaxies we can detect are already out of reach, and in the far future, only the Local Group will remain.
It’s been nearly a century since scientists first theorized that the Universe is expanding, and that the farther away a galaxy is from us, the faster it appears to recede. This is not because galaxies are physically moving away from us, but because the Universe is filled with gravitationally-bound objects, and the fabric of space in which those objects reside is expanding.
However, this depiction, which was popular from the 1920s onward, has recently been revised. It’s just been 20 years since we first noticed that this expansion was accelerating and that as time passed, individual galaxies appeared to recede away from us faster and faster. Even if we travel at the speed of light, they will become unreachable in time. The Universe is disappearing, and there’s nothing we can do about it.
Because the speed of light is finite, when you look out at a star whose light arrives after traveling toward you for 100 years, you’re viewing a star that’s 100 light years away. When you look out at a galaxy, its light arrives after traveling for 100 million years toward you, you’re not staring at a galaxy that’s 100 million light years away. Rather, you’re seeing a galaxy that’s significantly farther away than that!
The reason for this is that the Universe is expanding on the biggest sizes — items that aren’t gravitationally bound together into galaxies, groups, or clusters. The longer a photon takes to travel from a distant galaxy to your sight, the larger the influence of the Universe’s expansion, meaning that the most distant galaxies are even further away than the amount of time the light from them has been traveling.
This simplified animation depicts how light redshifts and unbound object distances 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 travelling with the expanding Universe will be even more light years away today than the number of years it took the light emitted by it to reach us (multiplied by the speed of light). (Credit: Rob Knop)
This shows as a cosmic redshift. We fully expect light to arrive at its destination with a specific wavelength because it is emitted with a specific energy and thus a specific wavelength. If the fabric of the Universe were neither expanding nor contracting, but rather were constant, that wavelength would be the same. But if the Universe is expanding, the fabric of that space is stretching as shown in the video above, and hence the wavelength of that light becomes longer. The large redshifts reported for the most distant galaxies have absolutely verified this picture.
But we can do far more than simply determine that the Universe has and continues to expand. We can use all of the information we have gathered to determine how the Universe has expanded over time, which informs us what the Universe is made of.
The expanding Universe stretches the wavelength of light once it leaves a distant cosmic source. This causes a redshift, in which the light of more distant objects redshifts for longer periods of time, when other components of the Universe (such as dark energy, matter, or radiation/neutrinos) were more important.
We can reconstruct the complete expansion history of the Universe by measuring sources at various distances, discovering their redshift, and then measuring their intrinsic vs. apparent size or intrinsic vs. apparent brightness.
Furthermore, because the way the Universe expands is determined by the many forms of matter and energy present, we can understand what our Universe is made 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.
Once we know the components of the Universe to this degree of precision, we can simply apply this to the laws of gravity (as provided by Einstein’s General Theory of Relativity) and predict the fate of our Universe. When we first applied this to the discovery of a dark energy-dominated Universe, we observed something shocking.
To begin with, it meant that any galaxies that were not already gravitationally bound to us would gradually disappear from view. They would accelerate away from us at an ever-increasing rate as the Universe continued to expand, unchecked by gravitation or any other force. A galaxy would get more distant with time, implying that there was an increasing amount of space between that galaxy and ourselves. Because of the expansion of space, the galaxy appears to be moving away at increasing rates.
However, this leads to an unavoidable and disturbing conclusion. It means that, at a certain critical distance from us, a photon either leaving our galaxy for a distant one or entering ours from a distant galaxy can never reach us due to the expansion of the fabric of space itself. The rate of expansion of the Universe is so vast that distant galaxies become inaccessible to us, even if we move at the speed of light!
At the moment, that distance is “only” around 18 billion light years away, as the density of matter and radiation continues to fall, as does the total expansion rate (measured in km/s/Mpc).
Given that our observable Universe has a radius of 46 billion light years and that all regions of space contain (on average and on the largest scales) the same number of galaxies, this means that only about 6% of the total number of galaxies in our Universe are currently reachable by us, even if we left today and traveled at the speed of light.
It also means that, on average, between twenty and sixty thousand stars change from reachable to unreachable every second. The light they emitted a second ago will eventually reach us, but the light they emit right now will never.
It’s a scary, sobering concept, but there’s another way to look at it: this is the Universe reminding us how valuable every second is. It’s the Universe reminding us that if we ever want to go beyond our own Local Group — the gravitationally bound set of objects comprised of Andromeda, the Milky Way, and around 60 small satellite galaxies — every instant we wait is another opportunity lost.
Only around 6% of the estimated two trillion galaxies in our Universe are still reachable from the point of view of the Milky Way, and that number is decreasing all the time. As a result of the fast expansion of the Universe produced by dark energy, 94 percent of the galaxies in our observable Universe are already out of reach for humans. As time passes, every galaxy beyond our Local Group will suffer the same fate.
Unless we develop the capacity for intergalactic travel and head out to other galaxy groups and clusters, humanity will forever be stuck in our Local Group. As time passes, our ability to send and receive signals to what is beyond in the vast cosmic ocean will fade from view. The Universe’s accelerating expansion is relentless, and our gravity is insufficient to prevent it. The Universe is disappearing, and we’re absolutely powerless to stop it.