# Thread: The size of the Universe?

1. ## The size of the Universe?

I apologise if this has been asked before (I did a search but couldn't find this question on the forum).
I've also searched the Internet and only found very vague answers which don't really satiate my curiosity.
...admittedly though, the wider Internet isn't always the best source of information, so maybe my information is coming from bad or out of date sources...

I saw some numbers being put forward for what is reportedly thought to be the age and size of our Universe.... and they confused me....

Age: 13.72 billion years - unless I am mistaken, we're currently pretty certain this number is accurate and precise to 2 significant figures.
Size (diameter) according to Wikipedia: 92 billion light-years (and most other sources I could find were between 93 and 150 billion light-years).

Here's the assumptions I made for the sake of my calculation below:

- The fastest speed possible is the speed of light (which is one light-year per year).
- The Universe has been expanding at the speed of light since the Big Bang (I know that's not necessarily thought to be the case, but I'm aiming to generate the largest possible size estimate I can here).

Considering that, shouldn't the farthest the Universe could possibly have expanded from the Big Bang singularity in all directions be 13.72 billion light-years?
Therefore the maximum possible diameter of the Universe would be 27.44 billion light-years...

Obviously there is a substantial discrepancy between my rudimentary calculation of 27.44 billion light-years, and even the lowest 'proper' estimate I found of 92 billion light-years.
I know there must be a perfectly logical explanation, so what am I missing here? What adjustment or factor am I failing to include in my calculation that astrophysicists put in to produce their estimates?

Cheers,
Nathan

2. ## Re: The size of the Universe?

Others will be along in a bit to chime in, and yes, this question has been asked many, many times before, on this forum, and in other places...

Universe - Wikipedia, the free encyclopedia

It provides current best theory, history, as well as alternative theories, along with numerous references and jumping off points.

The two biggest things you appear to be overlooking:

- current best theory suggests the Universe has an unknown size, and may be infinite
- the 'observable' Universe is limited by the light horizon from our location, and is expanding at a speed greater than the speed of light. The current best theory suggests an age of 13.798 +/- 0.037 billion years, and a light horizon with a radius of about 46 bn years. The Universe in total is most certainly much larger...

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4. ## Re: The size of the Universe?

ANd the other thing is that expansion is not limited to the speed of light. The universe is now thought to have expanded much faster than the speed of light in the first fraction of a second.

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6. ## Re: The size of the Universe?

Originally Posted by AustinPSD
- current best theory suggests the Universe has an unknown size, and may be infinite
- the 'observable' Universe is limited by the light horizon from our location, and is expanding at a speed greater than the speed of light. The current best theory suggests an age of 13.798 +/- 0.037 billion years, and a light horizon with a radius of about 46 bn years. The Universe in total is most certainly much larger...
So if we could observe an area larger than our Universe, what would define the perimeter?

7. ## Re: The size of the Universe?

Originally Posted by Bottle
So if we could observe an area larger than our Universe, what would define the perimeter?
That "if" is such a big "if" that we are talking science fiction. In the realm of science fiction, you can define the universe any way you want.

If the universe is indeed infinite, then the word "perimeter" has no meaning. The concepts of "infinite" and "perimeter" are mutually exclusive. For that matter, the word "universe" usually means "everything that is", in which case "an area larger than our Universe" has no meaning either.

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9. ## Re: The size of the Universe?

As Keith suggests, this is a non sequitur. By definition, the Universe is "all inclusive". Logic dictates that upon this basis, there is no perimeter, or area outside our Universe from which to observe it.

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11. ## Re: The size of the Universe?

Yes, it's a topic that pops up time and time again. We live in a Cosmos, universe isn't in the equation as was thought, there are multi Cosmos and hubble extreme deep field has showed us quite a lot. Hopefully when the "Webb" telescope is launched in 2018, NASA will have more data on this topic. As of now Speculation and theories is all we have. Ever expanding , in all directions with no end, infinite expansion. It's like a tree, it grows and branches off in all directions. I believe the" Cosmos is a tree of life". For more explanation ask Brian Greene or Laurence Kraus, they put up a good discussion on You tube.

12. ## Re: The size of the Universe?

Originally Posted by KeithBC
That "if" is such a big "if" that we are talking science fiction. In the realm of science fiction, you can define the universe any way you want.

If the universe is indeed infinite, then the word "perimeter" has no meaning. The concepts of "infinite" and "perimeter" are mutually exclusive. For that matter, the word "universe" usually means "everything that is", in which case "an area larger than our Universe" has no meaning either.
Thanks Keith, I get it now. The term Observable Universe has never really clicked with me on its full meaning until now , probably due to how it's visually represented in documentaries - my visual learning is much higher than other methods I guess.
I guess it's like our ball of light in a room to which we have no idea of the size or our position in it.

13. ## Re: The size of the Universe?

Relating to this question; We have mapped the "Cosmic Microwave Background Radiation." What is confusing to me, maybe to others as well, is that if we have seen THAT, is there nothing more beyond that? This is such a complex issue, I'm having a difficult time correctly phrasing the question. I don't understand why there are galaxies, etc. that we aren't able to see (or will never be able to see), but yet we have "seen" the "CMBR". Would the CMBR not be "more distant" than those galaxies? I hope you understand my question. I apologize for my obvious inadequacies. Thanks you, everyone. Interesting topic.

Lon ...

14. ## Re: The size of the Universe?

Originally Posted by magoo01
Relating to this question; We have mapped the "Cosmic Microwave Background Radiation." What is confusing to me, maybe to others as well, is that if we have seen THAT, is there nothing more beyond that? This is such a complex issue, I'm having a difficult time correctly phrasing the question. I don't understand why there are galaxies, etc. that we aren't able to see (or will never be able to see), but yet we have "seen" the "CMBR". Would the CMBR not be "more distant" than those galaxies? I hope you understand my question. I apologize for my obvious inadequacies. Thanks you, everyone. Interesting topic.

Lon ...
The farthest "back" we can see is 13 billion years. But that doesnt mean that is the limit to how far away things can go. Those things that *were* 13 billion light years away at the big bang have moved away from us since that time. We can see them since 13 billion years has passed, but those objects are now 46 billion light years away from us now (and 13 billion years older than what they look like to us right now).

If an alien in one of those galaxies we see at the "edge" 46 billion light years away were to look in our direction it would see our galaxy as it was 13 billion years ago. If that alien were to then turn 180 degrees and look the other way it would see more galaxies that are also 46 billion light years away in the other direction. To us that galaxy would be 92 billion light years away and far outside our ability to observe. Another alien in that galaxy 96 billion light years from us would *never* know we existed as we are more than twice as far away as light has ever had the chance to travel. Yet that alien would likely be able to look again in the opposite direction from us and still see another bunch of galaxies in its "deep field".

So... we can see stuff thats 46 billion light years away, but think about if someone were in one of those galaxies looking back at us and what they would be able to see. You can then realize that the alien in that galaxy isnt seeing some sort of barrier right outside its galaxy even though to us all we can see is the CMBR at that point.

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