The first thing to keep in mind is that the universe is expanding. And the second thing is that we receive information through light and the speed of light is finite. So actually, if we are talking about the universe as a whole, we do not know its diameter, we do not know if it is finite or infinite. What we do know is the diameter of what we call the observable universe, which is the part of the universe from which we can receive information.
Taking into account that the universe is expanding and that we know how fast it has expanded, not only the current one, but also in each previous stage of its history, we can calculate the distance that the last photon that we can receive has traveled, such as if we were to say, the last ray of light. That is the diameter of the observable universe. And basically, that order of magnitude is always related to the rate of expansion of the cosmos. We take that value that gives us the expansion rate, if we divide it by the speed of light, we obtain the distance of the observable universe. This diameter, assuming we are in the center, is about 28.5 billion parsecs (a parsec is the unit of length used in astronomy and is equivalent to 3.2616 light years). In light years it would be around 92,000 million light years (and a light year is the distance that light travels in a year: 9,460,730,472,580.8 km). But the important thing is that this is the diameter of the universe that we can observe because, as they are expanding, as it travels towards us, the light that comes from the past has to travel a greater distance. So there is a limit to what we can observe. Further, we do not know. There are different theoretical models, but we do not have direct information.
Another important matter to keep in mind is that when I say that we are at the center of the observable universe, it does not mean that we are at the center of the universe, but that as we are the observers we see what is around us, we are in the center of the universe. universe that we can observe.
Also, as I said before, the rate of expansion of the universe has been changing over time. It is related to the Hubble constant. In 1929, the American astronomer Edwin Hubble observed the expansion of the universe and saw a relationship between distance and speed and realized that this relationship was constant. With the cosmological model that explains how the cosmos expands depending on which is the dominant component of the content of matter and energy, if it is more dominated by radiation, by relativistic particles or, nowadays, that it is dominated by non-relativistic particles , by matter and by that rare component that makes the universe expand in an accelerated way and that is dark energy, because going back in time we can calculate what is the value of the parameter or Hubble constant in each epoch. And with that we can calculate what is the distance that the light has traveled.
Mar Bastero Gil She is a tenured professor and researcher at the High Energy Theoretical Physics Group (FTAE) of the University of Granada.
Question sent via email by Jairo Aldana G
Coordination and writing: Victoria Toro
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