The event horizon is the boundary beyond which an event cannot affect or be observed by an observer. Information about an event, or light from it, reaches an observer at the speed of light.
Therefore, if the source of the event is moving away from the observer faster than the speed of light, or if light cannot escape from the source of the event, the observer will have no information about the event. Typical examples are black hole event horizons and cosmological event horizons.
Black hole event horizon
A black hole is a very dense body of matter with a very large gravitational pull, so that any matter or light that gets close enough to the black hole cannot escape its gravitational pull. According to Einstein’s theory of general relativity, when light travels close to a massive object like a black hole, it travels through space distorted by the object’s gravity, and when it gets closer than a certain distance, determined by the mass of the black hole, the light’s path is eventually bent toward the black hole (i.e., it cannot travel away from it).
The imaginary plane around the black hole that corresponds to this particular distance is called the black hole event horizon and corresponds to the Schwarzschild radius. Light or information from events beyond the black hole’s event horizon (inside the event horizon) cannot reach an outside observer.
Event Horizon Telescopes
An event horizon telescope (EHT) is an array of telescopes or antennas made up of several radio telescopes linked together for the purpose of observing the event horizon (or more precisely, the outside of the event horizon) around a supermassive black hole.
The Event Horizon Telescope is an array of telescopes using very-long-baseline interferometry (VLBI), linking radio telescopes from around the world to create an effective diameter the size of the Earth, which has been used to image the supermassive black hole at the center of the Milky Way and the supermassive black hole at the center of M87 (the Virgo A galaxy).
Cosmological event horizon
The cosmic event horizon is an imaginary sphere whose radius is the maximum distance that light originating from us in the present can reach an observer in the future. The event horizon of a black hole is due to its massive mass, while the cosmic event horizon is due to the expansion of the universe.
The speed of light is finite, so the time it takes for light from an object far away from us to reach us increases with the distance to the object.
However, because the universe is expanding, light from an object that is far enough away from us will never reach us, no matter how much time passes. This object exists outside of our cosmological event horizon.
Since the cosmological event horizon is caused by the expansion of the universe, the distance to the cosmological event horizon depends on how the universe is expanding in the future, i.e., on the cosmological model. In the standard cosmological model, the distance to the cosmological event horizon is about 17 billion light-years.
This means that light originating from an object more distant than 17 billion light-years from us now will never reach us. Alternatively, we can say that we exist outside the cosmological event horizon of the object.
Particle horizon
The particle horizon is the maximum distance that light that originated in the past (from a particle) can reach us now after traveling for the age of the universe, and is a different concept than the cosmological event horizon. The finite size of the particle horizon is also due to the finite speed of light.
If the speed of light were infinite, no matter how far away a particle was, its light would be visible to us as soon as it was emitted. However, even with a finite speed of light, if the universe is not expanding and is infinite in age, there would be no particle horizon. In an unexpanding universe of infinite age, light from even the most distant particles would be able to reach us. In reality, the universe we live in has a finite age and is expanding, so it has a finite size particle horizon.
If the universe were neither expanding nor contracting, the size of the particle horizon would be the age of the universe at the time of observation multiplied by the speed of light. However, in reality, the universe has been expanding since its inception, so the actual size of the particle horizon, taking into account the expansion of the universe, will be larger than the speed of light multiplied by the time it took for the light to reach us.
Observable Universe
The observable size of the universe is the maximum distance that light from any celestial object in the past can reach us now, and is related to the particle horizon. The reason the observable size of the universe is finite is because the speed of light and the age of the universe are finite.
If the speed of light were infinite, then no matter how far away an object is, its light would be visible to us as soon as it is emitted. If the speed of light were finite, but the age of the universe were infinite, then light from any object, no matter how distant, would still reach us.
If the universe were not expanding or contracting, the radius of the observable universe would be about 13.8 billion light-years (i.e., the age of the universe multiplied by the speed of light). In reality, however, the universe has been expanding ever since it first began.
This means that the universe has continued to expand in the time it took for light from an object farther away from us to reach us, so the distance to that object is greater than the speed of light times the time it took for the light to reach us.
For example, light from an object at redshift 10 takes about 13 billion years to reach us, but the proper distance to that object is about 31 billion light-years, not 13 billion light-years.
The current size (radius) of the observable universe, as calculated by scientists to account for the expansion of the universe, is about 46.5 billion light-years, meaning that an object that is 46.5 billion light-years away from us is the farthest object we could theoretically observe.
The horizon problem
The horizon problem is one of the problems with the Standard Model and is related to the initial conditions of the universe. On a small scale, there are many different sizes of structures in the universe, including planets, stars, galaxies, galaxy clusters, supergalaxies, and megastructures, and their distribution is not uniform in space.
For example, some regions of the universe have galaxy clusters with high concentrations of galaxies, while others have voids with very few galaxies. However, if you average the distribution of these structures over a large enough scale, the universe is uniform and isotropic.
A good example of the uniformity and isotropy of the universe is the cosmic background radiation, which is uniformly isotropic to within one part in ten thousand in all directions.
The fact that the cosmic background radiation is uniform in a region far beyond the particle horizon from which it originates is an initial condition problem that is difficult to explain using standard big bang theory. The universe cannot exchange information with each other at distances greater than the particle horizon because observations show that the temperature and density are the same in regions far beyond the particle horizon.
One theory that has been proposed as a solution to this problem is the theory of inflation. The inflationary theory proposes that there was a period of explosive expansion of the universe at a very high rate early in its history.
The reason why the inflationary theory can solve the horizon problem is that, according to this theory, regions that are now outside the horizon were inside the horizon before the inflation, so they were able to exchange information about each other’s states. This naturally explains why regions that are now outside each other’s particle horizons have similar temperatures and densities.
However, the causes and mechanisms of this rapid expansion, as well as the reasons why it stops, are not yet clearly understood, because it is difficult to determine which of the many theories of rapid expansion is correct based on the observations made to date.