The Misconceptions About the True Size of the Universe
In the 90s it was often said that the universe is huge, or more precisely 14 billion light years in hugeness. At the time, non-scientific magazines, documentaries and popular science didn’t necessarily distinguish between age and size. It was easy to assume that the size of the universe was the same as the age, i.e. 14 billion light years.
In addition to this there was rarely nothing called “the observable universe” in popular science. Today it is widely accepted that there is more universe beyond what you can actually see, and there is possibly an infinite number of universes like our own. Let’s sort out the old misconceptions.
The age of the universe according to our most precise measurements and observations is approximately 13,8 billion years. During that time the part of the universe that we can see (the observable universe) has expanded to 46–47 billion light years in radius.
This corresponds to a big bubble with a diameter that is 93 billion light years. As time goes by the age and size of the universe will increase. Since the most distant objects will move away from us at an ever increasing rate there is a limit to how much of the universe that we will be able to observe.
This limit is called the future visibility limit (or “future observable universe” if you like) and is estimated to be around 61 billion light years. This corresponds to a sphere with a diameter of 122 billion light years.
Since scientists expect the whole universe to consist of “more of the same stuff” it is easy to calculate the number of galaxies that is contained in the larger sphere. It turns out that there are more galaxies to discover than we can actually see today, but also that approximately 95% of all galaxies will be permanently beyond our reach (because they move away from us faster than the speed of light).
Looking beyond the observable universe we have the unobservable universe. This is the part of the universe that we know exists but will never be able to observe. It may be infinite (flat) or finite (curved), but at the moment we do not know.
If the universe has a limit to its size it will be curved and hence curve back on itself. Our best estimations (from a team of scientists at the University of Oxford) states that the universe is at least 250 times larger than the visible part. If you translate this into a sphere the diameter will be at least 23 trillion light years in diameter. But it may well be much larger than that!
It should be noted that in this case we cannot place ourselves in the middle of the universe (like in the observable universe) since there are no references — we simply cannot know where we are in this huge ocean.
The unobservable universe together with the observable universe is the actual “universe”. And this universe (the entire cosmos) may be just one of billions of universes (multiverse).
The misconception between age and size and the different types of universes is similar to the misconception that two distant objects cannot move faster away from each other than the speed of light.
Due to the expansion of space itself two galaxies at the opposite side of the observable universe (i.e. located 93 billion light years away from each other) is actually moving away from each other faster than the speed of light. How fast? Let’s apply the very latest measurements and estimates of the expansion rate.
This states that the expansion rate of the universe is approximately 73 km/s /Mpc (megaparsec). This translates to 22 km/s per million lightyears or roughly 3,4 times the speed of light at the edge of the observables universe seen from Earth. Two arbitrary galaxies at each end of the sphere would experience a red shift corresponding to almost 7 times the speed of light.
A similar misconception is the discussion about the “most distant objects” in the universe. These objects are typically distant galaxies, galaxy-clusters, supermassive black holes, quasars and similar, with a very high spectroscopically-confirmed redshift.
The distance of the most distant objects are given as “light travel time” (converted to light travel distance) but does not account for the fact that space expands while light is travelling through it. The actual distance (or proper distance) of the most distant confirmed objects are currently around 33 billion light years.
This means a few things. First of all we know that these are not the most distant objects in the observable universe that exists (becasue the radius is 13 billion light years greater). We also know that these objects are permanently beyond our reach because they travel away from us faster than the speed of light. And we have no idea whether these objects still exists, because the information we have is billions of years old.
