We’ve all heard of the Big Bang – the beginning of our universe; starting as a tiny, extremely dense and hot point that rapidly expanded. While this theory explains everything around us in space with incredible accuracy, there are some aspects of the Big Bang and its implications that still have scientists confused. The horizon problem is one such problem, and it will be the topic of this piece.
We live in a universe that looks the same in all directions. In particular, the cosmic microwave background (CMB) is uniform in all directions. The CMB is the cooled remainder of the first light that ever travelled through the universe – it is considered a “shockwave” of the Big Bang.
The Horizon Problem boils down to this – the only way for two regions to have the same conditions, is that they are close enough together for information to be passed between them – so they can “balance out” to the same state. Our universe has a speed limit – the speed of light – and nothing can travel faster than this. In one second, light travels 300,000 kilometres. Here is the problem, if two regions are far enough apart that light has not had enough time to travel between them, then the regions cannot exchange information and hence are isolated from each other. There is a limit to how far their light can have travelled – their horizon. So how can two separated regions of space be in the same state, when they have not interacted? Imagine you are at a party, and only get to interact with people in your vicinity – you do not mingle with the people at the opposite side of the room; thus you haven’t exchanged information – it’s the same idea.
So how have scientists proposed we solve the horizon problem? A leading theory that speaks to scientists is inflation. Inflation means that the universe began to rapidly expand mere fractions of a second after the Big Bang, as quick as a flash before the more gradual expansion associated solely with the Big Bang. This suggests that the separated regions in question interacted before inflation began. Prior to inflation, the universe was a singularity or point, and the regions were much closer together than if there had only been expansion associated with the Big Bang. The theory of inflation to solve the horizon problem was first suggested in the 1980’s and since then, scientists have produced more than 200 inflationary models. Space missions to test the validity of some of these models have so far shown that the universe obeys the simplest inflationary models.
The Horizon Problem (2017), Bahia Si Lakhal and Ameur Guezmir, DOI:10.1088/1742-6596/1269/1/012017
From Eternity to Here, chapter 14, “Inflation and the Multiverse:” Sean Carroll
Image credit: Wikipedia – https://en.wikipedia.org/wiki/Horizon_problem