# Particle and event horizons

• BzzBzzz
In summary, in a flat universe with dark energy and normal matter, the sum of the density parameters for matter and the cosmological constant equals 1 at the present time. If the density parameter for the cosmological constant is larger than a certain value, the particle horizon will be larger than the cosmological event horizon. The particle horizon represents the past and the event horizon represents the future. The particle horizon always grows, while the event horizon tends to the Hubble sphere in a cosmological model dominated by the cosmological constant. Objects beyond the event horizon but within the particle horizon, such as far quasars, will never be able to send their light to us in the future.

#### BzzBzzz

In a flat universe with dark energy (cosmological constant) and normal matter like ours, the sum of the density parameter for matter (Omega m) with the density parameter for the cosmological constant (Omega lambda) at the present time equals 1.

If Omega lambda is larger then a certain value the size of the particle horizon is larger then the size of the cosmological event horizon. What does this mean? What happens in a Universe where the particle horizon is bigger then the event horizon?

I would say there is no relation between them. The particle horizon is related to the past and the event horizon is related to the future. To define our particle horizon one shall consider which would be the current position of a particle of zero mass sent at the beginning of time from our comoving position. On the other hand, our event horizon is the current position of objects whose light will never reach us (in future). The particle horizon grows always and the event horizon tends to the Hubble sphere (c / H) in case of a cosmological model dominated by the cosmological constant (like the current one). Objets beyond the event horizon but inside the particle horizon may be for example very far quasars; their light, emitted in past, is reaching us now, but the light they are supposedly emitting now, will never reach us in future.

## 1. What is a particle horizon?

A particle horizon is a theoretical boundary in space beyond which light and other particles have not had enough time to reach us since the beginning of the universe. It represents the farthest distance we are able to observe in the universe.

## 2. How is a particle horizon different from an event horizon?

A particle horizon is based on the time light has had to travel since the beginning of the universe, while an event horizon is the boundary around a black hole from which nothing, including light, can escape due to the strong gravitational pull. In other words, a particle horizon is a distance limit, while an event horizon is a point of no return.

## 3. What is the relationship between particle and event horizons?

The particle horizon is used to define the observable universe, while the event horizon is a concept used in the study of black holes. However, the two concepts are related because the particle horizon limits the distance at which we can observe events, including those that may occur near an event horizon.

## 4. Can the particle horizon change over time?

Yes, the particle horizon is not a fixed boundary. As the universe expands, the particle horizon also expands, allowing us to observe more distant objects. This means that the observable universe is constantly growing.

## 5. How is the particle horizon related to the Big Bang theory?

The particle horizon plays a crucial role in the Big Bang theory. It is used to determine the size and age of the observable universe, which helps to support the theory of the universe beginning as a singularity and expanding rapidly in the early stages.