The vast, immeasurable universe, seemingly infinite, could be home to extraterrestrial residents.
According to Vice, for centuries, people from various cultures have been captivated by the allure of distant horizons. This fascination drives us to embark on perilous journeys across continents, across oceans, and even into space.
Nevertheless, the mysteries of the cosmic horizon, or the edge of the observable universe, remain an enigma yet to be unraveled.
As the name suggests, the observable universe refers to the portion of space that humans can observe from modern observatories. As astronomical sight has improved over centuries, scientists have discovered that traveling into space is also a means of journeying back in time.
For example, when looking at a star that is 8 light-years away, like Sirius, the image you see is actually Sirius as it was 8 years ago.
The observable universe is merely a tiny fraction of the vast, infinite expanse of the entire universe. Unveiling the mysteries of space is a mission humanity has undertaken.
What lies beyond the cosmic horizon? (Image: Science Alert).
This mission is not easy, as there are certain limits to the distance we can observe directly due to the light from the cosmic microwave background (CMB). Moreover, the cosmic horizon – the boundary separating the observable and unobservable universe – poses an additional barrier that complicates observation.
What exists in the vastness beyond our reach? Unexplainable physical phenomena, new worlds, or even extraterrestrial life forms? Can we explore them, or will they forever remain in the realm of human hypothesis?
What is the Cosmic Horizon?
If we assume the universe is 13.8 billion years old and light travels at a constant speed, the radius of the universe could be around 13.8 billion light-years. However, there is something that moves faster than light: the expansion of the universe.
“Photons could traverse from A to B in the early universe more easily than in later space because initially, the distances between them were much shorter,” explains Malcolm Fairbairn, an astrophysicist at King’s College London, UK.
For instance, the cosmic microwave background radiation. The faint light we see in every direction is the light emitted when the universe was just a few hundred thousand years old.
Photons from the cosmic microwave background may be about 13 billion years old, but they come from a region currently about 46 billion light-years away from us.
“As soon as humanity became aware of the expansion of the universe, we began to consider the distance from where signals are received based on how the universe expanded in the past,” says Fairbairn.
This distance is referred to as the cosmic horizon and will recede further as the universe ages. If civilizations still exist in the Solar System after a billion years, it is likely that future observers will discover new galaxies and stars when their light finally reaches Earth.
In 1964, researchers Penzias and Wilson stood at the 15 m Holmdel Horn Antenna during the first detection of cosmic microwave background radiation, electromagnetic radiation generated during the early universe. (Image: NASA).
However, the cosmic horizon is not the only boundary scientists identify on a cosmic scale. As the universe expands, objects that are farther away recede from Earth more quickly. At a defined boundary, known as the Hubble horizon or sphere, objects begin to recede faster than the speed of light.
The current cosmic horizon is significantly broader than the Hubble horizon by several billion light-years. This allows us to observe objects moving away from Earth at speeds classified as “superluminal motion,” much faster than the speed of light.
Based on known physical principles, these objects do not move through space faster than light. Instead, spacetime itself is expanding faster than light. This means that if extraterrestrial beings exist in a galaxy we can see beyond the Hubble horizon, we still cannot communicate with them within the cosmic horizon.
Any message we send, even at the speed of light, will never catch up with the spatial expansion beyond the Hubble horizon.
Are there extraterrestrials beyond the cosmic horizon?
Of course, no one can know what exists beyond the cosmic horizon. However, theoretically, beyond the cosmic horizon is a world that is completely similar to the one inside.
This cosmic homogeneity may disappoint dreamers of fantastical worlds. Nonetheless, it also gives rise to a new hypothesis: the possibility that life has developed elsewhere in the universe.
For decades, scientists have tried to prove the hypothesis of extraterrestrial life. However, this remains a hypothesis without any solid evidence.
Tomonori Totani, an astrophysicist at the University of Tokyo, Japan, proposes a new way to validate this by expanding the scope of “natural emergence” – a term referring to the origin of life. Instead of solely searching for life within the observable universe, he broadens the scope of his research to “the entire expanding universe”, encompassing everything beyond the cosmic horizon.
Totani states that searching for natural emergence beyond the observable universe is entirely feasible, as it is widely known that the universe has no limits.
Quantum fluctuations during the expanding universe period created energy at a point in space, leading to the formation of stars, galaxies, and other large-scale structures in today’s universe. (Image: Forbes).
“However, the search for natural emergence is not easy, even if the scope is limited to the observable universe,” Totani notes, “Nonetheless, if life arises in the true size of the expanding universe, explaining life on Earth becomes much simpler.”
Totani is not the first scientist to propose the idea of life within the unobservable universe. He has utilized the argument for an infinite universe, which can contain a multiverse of many universes, and it is very likely that life exists as suggested by Eugene Koonin, a senior researcher at the US Biotechnology Center.
While this hypothesis may provide a new direction for researchers, the existence of civilizations beyond the cosmic horizon remains a challenging hypothesis to accept.
Is it possible to observe things beyond the cosmic horizon?
The answer is no. Although we may still receive some new light beyond the current horizon in the coming billion years, due to the current rate of expansion, the universe will stretch to the point where only the nearest galaxy systems will be visible.
Fairbairn states that if the universe is filled with materials, radiation, and photons, we will eventually be able to observe further as the cosmic horizon gradually recedes. However, due to the effects of dark matter, that will never happen.
Nonetheless, nothing can halt the progress of scientists studying the unobservable universe. According to recent research, they have found a way to detect gravitational waves, which were generated 1/1,000 of a second after the Big Bang.
A theoretical representation showing how each universe (small particles) is interconnected. (Image: Vice).
According to scientists, gravitational waves are created by disruptions in spacetime, such as collisions of black holes and neutron stars, which are events that occur in relatively recent cosmological history. Additionally, these strange waves could also be generated by physical processes that occurred right after the universe was born.
Fairbairn notes that these gravitational waves appear earlier than any previously known signals. And because they are unaffected by cosmic microwave background radiation, we can observe further, even back in time. Gravitational waves will provide reliable data about the changes in the cosmic horizon in the past.
The truth about a world beyond the cosmic horizon may seem difficult to accept, but nonetheless, we already have a rather marvelous observable universe, even if it is just a small part of an infinitely vast entity.
