As it stands, the Big Bang theory is accepted as the theory explaining how everything came to be. Cosmic microwave background radiation (CMB) is one of the main pieces of evidence supporting this theory. Despite the universe constantly expanding, astronomers have found that the CMB in the entire observable universe has a uniform temperature. How is this possible?
The Idea Behind The Horizon Problem
Whenever two things are the same temperature, it means that those two things had time to interact. A drop of blue and red food dye in a bowl of water would form purple after a while. The same should be applicable to our universe, right? Despite being true for a time, further observation found that CMB is at a uniform temperature all throughout the universe. It is even the same temperature in regions of space that have never even interacted with each other.
First, some background. Cosmic microwave background radiation can be considered to be the left-over radiation that resulted from the Big Bang. As previously mentioned, all the CMB in the observable universe is at a uniform temperature (2.72548 ± 0.00057 K). This implies that the horizon of the light that left 13.8 billion years ago from the edge of the universe interacted with the light that left the universe’s opposite edge.
What is The Horizon Problem?
This is where things start getting tricky because the universe has not existed long enough for this to happen. These two horizons could not have been able to travel 28 billion years to overlap and exchange temperatures. This is the Horizon Problem. The scientific community has since come together and formed theories in hopes of effectively solving the Horizon Problem. Unfortunately, there is currently no way to confirm any of these due to the limitations of our knowledge and technology.
The Cosmic Inflation Theory
One of the most accepted theories is the cosmic inflation theory. The cosmic inflation theory was first presented by American theoretical physicist and cosmologist Alan Guth in 1980. This theory states that the universe underwent an incredible increase in volume in 10-32 seconds after the Big Bang. Before the universe underwent inflation, everything now within the observable universe was close enough to interact. After the rapid expansion, areas that were once in contact have now been spread too far apart to interact.
The main questions in science we ask are “why” and “how”, and for the most part, we can answer these; until we get to the “why” and “how” of how the universe came to be. I believe the Horizon problem is another instance of this. We cannot confidently say that the universe underwent such an unimaginable expansion in such a short time, or even how. There isn’t even a way of explaining the uniformity of the CMB even now as it stands. Along with making our heads hurt, these questions create the drive and motivation to answer those two questions.