# Clarification about the size of the Universe

• I
• amorphos_b
In summary, the size of the universe is constantly changing due to expansion and the observable universe is estimated to have a radius of about 45 billion light years. The number of galaxies can be estimated by counting them or by estimating the average density of the universe and dividing it by the average mass of a galaxy. The concept of a curved space only applies to a universe with more than five stars and there is no need for anything outside the universe to explain its content. However, understanding the complexities of the universe requires a strong understanding of mathematics and theories, and speculation without proper knowledge can lead to inaccurate assumptions.

#### amorphos_b

Clarification upon size of universe

when we look at distant galaxies we are looking back in time to an ever shrinking universe. Which I would visualise as conical. If then, we could see all the galaxies as they really are right now, then surely those cones would all disappear, and the universe would be vastly bigger? e.g. by the same degree as the cones reduce over time, the universe would be at least that much greater in size. The cones in every direction would be straight instead.

Secondly some galaxies crash into others, and maybe as a result some perhaps split off – if e.g. a galaxy was traveling in a different direction, some of the velocity would push or pull some stars away from others. Even if we don’t ever get more galaxies, we would get less from collisions.

So how can we know how big the universe is now, and how many galaxies it is composed of?

What research have you done on this? the age/size of the universe is a common topic so should not be hard to find good info.

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amorphos_b said:
Which I would visualise as conical.
Why? Where are you getting this from?

amorphos_b said:
If then, we could see all the galaxies as they really are right now, then surely those cones would all disappear, and the universe would be vastly bigger? e.g. by the same degree as the cones reduce over time, the universe would be at least that much greater in size. The cones in every direction would be straight instead.

Secondly some galaxies crash into others, and maybe as a result some perhaps split off – if e.g. a galaxy was traveling in a different direction, some of the velocity would push or pull some stars away from others. Even if we don’t ever get more galaxies, we would get less from collisions.
This all looks like personal speculation, which is off limits here.

As noted above, there's quite a lot of speculation in what you wrote. Answering the answerable questions that I see:

Distance in a curved spacetime is a complicated concept. You are correct that things that look like they are (e.g.) 10 billion light years away are now further away due to expansion. Our models predict that the observable universe currently has a radius of about 45 billion light years.

Secondly, galaxy dynamics is complicated. I don't think any naive model of galaxies colliding with each other will ever tell you anything plausible. To get an estimate of the number of galaxies we simply count them. If the number density changes with distance (which it probably will, since we're looking back in time) then we can derive an estimate of the current density and multiply by the volume of the observable universe. I doubt it's an exact science, which is why the hundred billion number you quoted in your last thread has only one significant figure.

You could also estimate galaxy numbers from the average density of the universe, which you can estimate from the recession rates. Divide that density by the average mass of a galaxy to get the number of galaxies per unit volume. Again, the error bars will be sizeable.

amorphos_b said:
if we imagine that there are only 5 stars in the universe, it is rather hard to imagine that there is nothing outside of universe.
It doesn't matter how you vary the content of the universe, you never need anything outside it to explain anything. We can make accurate predictions with only the universe and the stuff in it.

A universe consisting of only five stars would not have a curved space (at least not in the sense that the term is used in cosmology - it would not even have a uniquely identifiable notion of "space"). It would be difficult to see where one could start to end up with such a thing - but you can describe it without reference to anything outside the universe.

I'm afraid you are running into a fairly common problem, which is that this kind of thing is interesting and you want to speculate about it, but you haven't studied enough maths to properly understand the models and consequently your speculation instantly wanders into the weeds. It's like the difference between listening to music and playing an instrument. Any fool can hit a button on a media player, but you have to put in some hours if you want to be able to play even in the back room of a pub.