How far can we travel? Is there a line we will never cross? Are there places we’ll never get to, no matter how hard we try?
The hardest part is imagining the distances that spacecraft have to travel to reach other planets, such as Mars or asteroids. We can talk about tens of millions of kilometers. Still, the numbers are not enough to feel the feeling of isolation and loneliness that astronauts will endure during an interplanetary journey. How long will the Soyuz capsule take to reach the International Space Station?
Distance is perhaps the most relative concept in our connected world. The time it takes to get somewhere now depends only on the availability of transportation and money.
We are on a quiet arm of the Milky Way, a medium-sized spiral galaxy 100,000 light-years in diameter containing billions of stars, gas clouds, dark matter, black holes, neutron stars, and planets. Seen from afar, our galaxy appears dense, but in reality, it consists mostly of space. On average, there is a distance of 5 light-years between two stars.
With current technology, it would take us thousands of years to reach the nearest star system. The Milky Way is pretty big, and it’s not the only one. The Milky Way, along with the Andromeda galaxy and about 30 other galaxies, including dwarf galaxies, form the “Local Group,”; a region of space where galaxies are spread over a diameter of 100 million light-years.
Our local cluster is one of 100 clusters in the “Laniakea” supercluster, which is just one of the millions of superclusters that make up the observable universe.
Now, let’s assume for a moment that we have a bright future; humanity becomes a type 3 civilization, not exterminated by aliens, and develops the technology necessary for interstellar travel based on the current understanding of physics. In this best-case scenario, how far could we go? Well, we could just explore the Local Group. It is the largest structure that humanity will be a part of. Although the Local Group is huge, it represents only 0.00000000001% of the observable universe.
The simple fact that there is a limit to us and that there are so many other galaxies that we will never be able to reach is a little scary. Why can’t we travel further? Well, it all has to do with the nature of nothingness. “Nothing,” or space, is not empty but has intrinsic energy, the so-called “quantum fluctuations.” On a small scale, there is constant action, particles, and antiparticles arising and annihilating. You can imagine this quantum vacuum as bubbles: some with denser areas and others with less dense areas.
Now, let’s go back to 13.8 billion years ago when the structure of space consisted of nothing. Just after the Big Bang, in an event known as cosmic inflation, the observable universe expanded from the size of a ball to a diameter of trillions of kilometers in just a few fractions of a second. This sudden universe expansion was so rapid and extreme that all quantum fluctuations were stretched out, and subatomic distances became galactic distances with uneven density. After inflation, gravity began to attract matter. On a large scale, the expansion was too fast and strong to overcome gravity, but on a small scale, gravity won out, forming dense regions of matter, leading to galaxies like the one we live in today. Only galaxies in the Local Group are gravitationally attracted.
But can we leave the Local Group? Here, dark energy makes everything more complicated. Six billion years ago, dark energy took over. It is responsible for the expansion of the universe. We don’t know why or what this energy is, but we can see its effect.
Dark energy is responsible for the large-scale acceleration of the universe, i.e., it causes the second derivative of the cosmic scale factor a(t) to be positive. But on a smaller scale, clumps of matter where the effect of the four forces (strong nuclear energy, weak nuclear energy, electromagnetism, and gravity) are much stronger than the repulsive effect of dark energy.
In the early universe, there were larger, cooler regions around the local group from which clusters of thousands of galaxies emerged. A lot of galaxies surround us. These structures and galaxies outside the local group are not gravitationally bound to us. The further the universe expands, the greater the distance. Over time, dark energy will push everything further away from us, making all other galaxy clusters eventually unreachable.
The nearest group of galaxies is already millions of light-years away and is moving away from us at a speed we only dream of ever reaching. We might leave the Local Group to travel into intergalactic space in the dark, but we’ll never get anywhere.
While the Universe will expand further, the Local Group will contract under the force of gravitational attraction. In a few billion years, the Andromeda galaxy will collide with the Milky Way and form a single giant galaxy called Milkdromeda.
At some point, the galaxies outside the local group will be so far away that the few photons that reach us will have such a long wavelength that they will no longer be detected. Once this happens, no information from outside the local group will be able to reach us. The universe will disappear, in all directions, forever.
A person born in the distant future in our galaxy will believe that there is nothing else in the universe outside of his galaxy. When they look far into the universe, they will see only emptiness and darkness. They won’t be able to see cosmic radiation, and they won’t be able to learn about the Big Bang. They will believe that the universe is static and eternal. Milkdromeda will be an island in the dark, getting darker and darker.
However, there are trillions of stars in the Local Group, and it is big enough for humanity. After all, we didn’t manage to leave our solar system, and we had enough time to explore the galaxy. We are incredibly lucky to exist at the right time to see the future and our most distant past.