In a climate of constant technological progress and where the mysteries of deep space have found answers before only unimaginable, it will probably happen to you to ask how big is the universe. If the race to space exploration has made us understand how our planet is just a speck in a cosmic desert boundless, in the same way not everyone knows precisely what is the actual extent of the universe that was created, at least to hear the most accepted cosmological model, thanks to the gigantic and thunderous explosion of the Big Bang.
An explosion apparently occurred more than 13 billion years ago, and that gave way to everything we know, and also to all that has remained unknown for the moment. With the technological evolution, astronomers are able to look back in time to the moments just after this event so crucial, but are still limited in quantifying the size of the cosmos, necessarily determined by a number of different factors, which include not coincidentally the shape and expansion.
What is the universe
Before discovering, in case it is actually known, how big the universe is, a fundamental step to take is to understand what is really meant by this term. In scientific literature, and in particular in that related to astronomical phenomena, the name Universe is identified with the complex that includes all space and everything it contains. Specifically, it is matter, energy, planets and stars, galaxies and the contents of intergalactic space more generally. In essence, we can define it as "the set of all things that exist", and to the naked eye is visible only a very small part of it.
Making a calculation from our local time frame, the universe originated about 13.8 billion years ago through the aforementioned Big Bang, at present the best accepted hypothesis by scientists, although with its obvious limitations. According to this model, were four fundamental forces to play a key role in the "space genesis": strong nuclear, weak nuclear, gravitational and electromagnetic. All associated with the formation of protons, electrons and neutrons, which with the lowering of temperatures gave rise to helium nuclei, deuterium nuclei and atoms.
In this phase, the universe continued to cool and expand, creating the right conditions for the formation of galaxies and stars. A real path of expansion following the Big Bang, which has found its peculiar balances thanks to the presence of gravitational attraction, which tends to reduce the distancing between galaxies and prevent infinite expansion. Nowadays, scientists hypothesize two different theories about the future evolution of the universe: the theory of the closed Universe and the theory of the open Universe.
According to the first theory, the universe would have the tendency to expand first, and then go successively to contract until reducing itself in a very small mass of high density; for the second one, on the contrary, the universe should expand indefinitely considering that the amount of matter present is not sufficient to stop the expansion itself. At the dawn of the '80s, then, came into play a new model, known as the inflationary universe, in which the universe would have quickly increased by 1050 times its diameter than predicted by the most popular traditional theory.
How big is the universe
The observable part of the universe from Earth with the technical equipment at our disposal amounts to a total diameter of 93 billion light years. Just the scientific and meticulous observation of the universe and its motions, has allowed us to understand that in the past it has been governed by the same physical laws and constants for most of its history and throughout its observable extent, with obvious inferences in its initial, and most delicate phase. Entering more specifically, we can say that the observable universe is precisely the region of the universe visible from Earth, and looks approximately like an infinitely large sphere, with a radius equal to 43 billion light years.
To make a comparison understandable even to the less experienced, we must think that the diameter of a typical galaxy is 30,000 light years, and the typical distance between two neighboring galaxies is 3 million light years. For example, our Milky Way has a diameter of about 100,000 light years, and the closest galaxy to our planet, Andromeda, is located approximately 2.5 million light years from us.
It goes without saying that, in cosmology, the truly observable universe is "brushed" as a vast area of space enclosed by a sphere centered on an observer. Sphere that in fact encloses everything he can observe. Generally speaking, we then mean the entire portion of the universe that can be investigated by mankind, and therefore the sphere centered on our Earth, but at the same time it is easy to understand that each position in space inevitably possesses its own specific observable universe.
If for absurdity the universe was not, as previously stated, in continuous expansion, we could easily answer the question "how big is the universe?" with a trivial calculation: in practice, the radius of the observable universe would be equal to the distance traveled by light in the time elapsed from the beginning of the universe, corresponding to 13.8 billion light years, and the Big Bang. However, considering that the expansion is actually continuing, to date the actual distance of this horizon is significantly larger, and cannot be defined with absolute precision.
Today, according to the estimates considered most reliable, that same point is 46 billion light years away, thus making that of the observable universe a sphere with a diameter of about 93 billion light years, as claimed in the opening of this paragraph. These dimensions could go to contain about 7×1022 stars, organized into about 2×1012 galaxies, equal to two thousand billion, in turn agglomerated in groups and clusters of galaxies and superclusters.
However, more recent and precise observations, have put on the plate new estimates, according to which the hypothetical number of galaxies in the universe extrapolated from the structure of the so-called Cosmical Optical Background currently known would result only a fraction of the real one. With a consequently higher number of at least one or two orders of magnitude, not counting another 90% of galaxies in the observable universe not detectable with the telescopes we have, unfortunately not yet powerful enough.
The age of the universe
All the previous considerations, allow us to understand that it is not unanimously possible to define how big is the universe. Universe that, now in acceleration phase in its unstoppable expansion, puts by itself a limit to its observable portion. The "boundaries" are marked by the so-called cosmological horizon, which is defined as the region of the universe beyond which every object moves away from the observer at speeds greater than light. It goes without saying that this horizon represents and delimits the maximum distance with which one can no longer have causal contact: according to this astronomical principle, there will never exist the possibility to observe or exchange any signal or information generated from now on with regions beyond the horizon, with the elements "going out of reality" of the observer himself.
In fact, as many of you already know, the horizon is a physical limit imposed by the finite speed of light or any other radiation emitted by celestial objects, so that this radiation takes a certain amount of time to reach the observer, and that is infinitely much longer than the average human lifespan.
Real difficulties related to the size of the universe, and that also arise in wanting to determine its age, although we have arrived at a convention rather "accurate", within the limits of the possible. With this terminology, generally it is indicating the time passed between the Big Bang and the present day, so precisely 13.82 billion years following the model of Big Bang. However there are various measurements that allow at least to estimate the age of the universe. Among the most popular, we have first of all the measurements of cosmic background radiation and expansion of the universe. The first ones, to explain it in the simplest possible way, consider the cooling time of the universe from the Big Bang. The studies of the expansion of the universe, instead, allow to calculate a probable date of the beginning of the expansion itself.
You will understand by yourself that, not knowing precisely what came before the Big Bang, the actual age of the universe could be much higher than the estimates made nowadays by the scientific community. On the other hand, because of the inevitability of the universe, both in terms of time and size, very often the term universe is improperly used in common jargon to refer to the observable universe. This is because, as we explained earlier, unobservable physical phenomena are paradigmatically indescribable to human scientific knowledge.