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Wolfgang Korsus Dipl.-Ing.NT, Astrophysiker
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During conversations with highly interested friends, by email or by other means (University of Cambridge, Oxford) people and students ask me questions again and again, which belong one hundred percent to astronomy. It fits very well, because answering questions is also part of my job. But some of these questions are much more popular than others and I thought it would be a good idea to answer these so-called „standard questions“ in my articles. So I will do that, in certainly irregular intervals and I start with the absolute favorite among the questions.
They start with: „How big is the universe?“
This question can be answered quickly and you will not believe it! So again: How big is the universe?
We do not know!
But now I hear most of you saying, „That’s a rather unsatisfactory answer.
You were lucky!
Science can also give a better answer, of course, first of all we have to clarify what is meant by this „universe“.
We will make the question and the facts connected with it a little easier for ourselves. We define the „universe“ much simpler. It is everything that we can see, measure and register in space.
To put it even more simply, it is everything that we can observe. Now the question must be asked a little bit differently: „How big is the observable universe?“
In detail, we see, we observe, we measure, we register and we perceive something. ………….and this is connected with us in some way. For example, we can see a star because it emits light and it hits the detectors of telescopes on our planet, among others. We can also see gas clouds whose radio waves hit radio telescopes, also X-ray radiation, it hits X-ray telescopes. It follows from this:
If you want to see something, you have to register electromagnetic radiation on our planet.
But radiation, as we know, basically needs time to spread.
…….and how does it (the radiation) do that? at the speed of light. So, the light had a so-called maximum time to reach us, namely the time that corresponds to the previous life span of the universe.
….and how old is our universe ? ……namely 13.819 billion years The light that takes longer to reach the earth has logically not yet reached us, because the universe is not yet old enough for that? ……..from this it follows that the oldest objects in the universe that we can observe are therefore not older than 13.819 billion years.
Unfortunately, the situation is a bit more complicated. Because it was not until almost 400,000 years after the Big Bang (were the conditions such that light could propagate).
With this knowledge one could now come to the following conviction:
Yes, then the observed universe is only 13.819 billion light-years large. Because one can look from the earth also only 13.819 billion years in all directions.
But what have we not thought of now ! That means in plain language, this universe has always been expanding constantly. In the time in which the light from a distant galaxy has been on its way to us, the universe becomes bigger and bigger. You can also express it in more extreme terms: It really did take 13.819 billion years to reach the Earth. But in this time this universe has grown and the distance is therefore even greater!
I will now dispense with the necessary calculations and simply say, because it follows that we can look almost 46.6 billion light years in any direction. The answer to the question „How big is the (observable) universe?“ follows: It has a diameter of about 93 billion light years!