Although the Universe may be infinite in extent, in the generally accepted Big Bang cosmology we can only see a small fraction of it. This is known as the observable universe.
Field of distant galaxies – Image Credit NASA
There are three widely used definitions of cosmological horizons, which are limits imposed by cosmology to how far away we can see. The purpose of this post is to try and explain in a non-technical way these different definitions. I hope I succeed!
The Particle Horizon (boundary of the observable universe)
As discussed in a previous post , when we look at a distant galaxy its light will have taken millions (or even billions) of years to have reached us. if we could construct a cosmological-sized ruler between the galaxy and the Earth then we would measure a quantity known as the proper distance to the galaxy. This is the separation between the Earth and the galaxy at a given time. Because the Universe is expanding, the proper distance between two distant objects (which aren’t held together by gravity) increases with time. In the example below, light is emitted from a galaxy which 300 million years ago was at a proper distance of 294 million light years from Earth. However, the Earth is moving away from the emitted light photons all the time they are travelling towards us. So these photons actually travel 300 million light years to reach Earth. When they reach us the proper distance of the galaxy will be 306 million light years.
The particle horizon is the theoretical maximum proper distance we can see to at the current time. It is a spherical shell approximately 46.5 billion light years in radius around the Earth. When we look at distant objects we are looking back in time and light from an object at the particle horizon will have been emitted at the beginning of the Universe and will have been travelling towards us for the entire age of the Universe.
All the objects we observe today lie inside the particle horizon, which forms the boundary of the observable universe. If an object lies beyond the particle horizon, then the Universe is not old enough for its light to have had enough time to reach us.
At the exact instant of the Big Bang the particle horizon would have been zero and as the Universe ages the particle horizon increases. This is for two reasons.
(1) As the age of the Universe increases, light can travel a greater distance before it reaches us.
(2) Because the particle horizon is the proper distance of the furthest object we can see, due to the expansion of the Universe, as the Universe ages the proper distance between two distant objects increases.
The time axis shows the time since the Big Bang, the purple dashed line marks the current particle horizon.
In reality, we cannot see all the way to the particle horizon. As readers of my earlier post will know, the early Universe was far too hot for atoms to exist. It contained a plasma of positively charged hydrogen and helium ions and negatively charged electrons. Electromagnetic radiation, of which light is an example, cannot pass through plasma. The oldest radiation we can detect is the cosmic microwave background (CMB) which was emitted when the Universe was only 400 000 years old, at which time it had cooled sufficiently for individual atoms to exist. The CMB radiation we observe today has been travelling towards us since this time and was emitted from a spherical shell of points, which lie at a proper distance of approximately 46 billion light years from Earth.
The Event Horizon and Hubble Sphere
As described previously the Universe is expanding. The further away an object is the faster it is receding from us.
There is a clear relationship between the recessional velocity and the distance of a galaxy. This relationship is known as Hubble’s Law and is written as
v = HoD
- v is the velocity an object is moving away from us
- D is the object’s distance
- Ho is a constant known as the Hubble constant. If v is measured in kilometres per second and D is in megaparsecs (Mpc) (1 Mpc =3.26 million light years) then Ho is approximately 70 km/s per Mpc. The Hubble constant measures how fast the Universe is expanding. In reality, the Hubble constant changes over time (it is generally believed to be decreasing) and so is more correctly called the Hubble parameter H(t). The Hubble constant is the value of the Hubble parameter today. However, the current rate of change of the Hubble constant is very small. It will take hundreds of millions of years to fall by 1% from its current value.
Assuming that Hubble’s law is valid at all distances, at a separation from us of more than 4,300 Mpc (or 14 billion light years) a galaxy will be receding at a velocity greater than 300 000 km/s which is the speed of light. In which case any light it emitted today could never reach us. The Hubble sphere is an imaginary sphere centred on the Earth of radius 4,300 Mpc. If the Hubble parameter didn’t change over time, we could only see objects which emitted light today located inside the Hubble sphere.
The event horizon is the largest proper distance from us from which light emitted now will reach us at some point in the future.
- If an object lies closer than the event horizon then its light will reach us.
- If an object lies further away than the event horizon then it so far away that light emitted now will never reach us.
If the Hubble parameter didn’t vary over time, then the event horizon would simply be the radius of the Hubble sphere (14 billion light years). In most cosmological models, even though the Universe is expanding, the value of the Hubble constant is falling over time. The net effect of this is that the event horizon is larger than the radius of the Hubble sphere and gradually changes over time.
The graph below shows how the event horizon changes over time. In the current model of the Universe the event horizon will gradually increase with time but at a slower and slower rate reaching a maximum value of around 18 billion light years.
Since Hubble’s law v= H(t)D predicts superluminal recession at large distances (i.e. distances greater than c/H (t) ) it is sometimes wrongly stated that it needs some kind of “special relativistic correction” to prevent a galaxy moving away from us a velocity greater than the speed of light.
In fact, there is no contradiction with special relativity when faster than light motion occurs outside the observer’s inertial frame and in any case general relatively not special relativity is needed to describe the Universe as whole.
Distant galaxies are receding from us superluminally. This means we will never be able to see their light emitted at the current time. However, they are at rest locally and motion in their own local inertial frames remains is well described by special relativity. For more details see Davis and Lineweaver (2003).
Davis, T A and Lineweaver, C H (2003) Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe, Available at: https://arxiv.org/abs/astro-ph/0310808 (Accessed: 30 April 2021).