The size of the universe - a question.

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The size of the universe - a question.

Post by Rum » Sat Apr 28, 2018 8:42 pm

Was listening to a podcast today (In Our Time - brilliant - get it - there are 700 of them or more). I listen to one most days. Today's was about the size and age of the universe.

As people who are interested in these things here know the current estimate of the age of the universe is about 13.7 billion years. This is based on several areas of evidence, but basically we can't see light that is older than that or stars/galaxies beyond that time/distance. It appears from our perspective that everything kicked off then.

The universe is sometimes described as expanding in the way a balloon does - i.e. every point on/in it expands away from every other point. The 'view' from any of these points is more or less as ours is - in other words they should be able to see about 13.7 billion years into the past by viewing light as seen from their position.

Which brings me to my question and puzzlement. If you were sitting on a planet on the edge of the universe that we can see, the observer there would be able to see 13.7 billion light years in all directions - including in a direction that points away from us. Thus expanding the size (and age) of the universe by another 13.7 billion light years. And of course you can keep on doing this until the size (and age?) becomes infinite.

Am I missing something? Perhaps the space time geometry or something?

Clever peeps please point out the error in my thinking!

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Re: The size of the universe - a question.

Post by Brian Peacock » Sat Apr 28, 2018 8:59 pm

The Universe is big. Bigger than the biggest thing ever, and then some. Much bigger than that in fact, really amazingly immense, a totally stunning size, real 'wow, that's big', time. Infinity is just so big that by comparison, bigness itself looks really titchy. Gigantic multiplied by colossal multiplied by staggeringly huge is the sort of concept we're trying to get across here.

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Re: The size of the universe - a question.

Post by Animavore » Sat Apr 28, 2018 9:52 pm

You can't see light older than 13.7 billion years old because it has taken that long for the light to reach us. Any light beyond that, ie. in the direction away from us at the edge of our field of view, hasn't reached us yet. Not because the distance = the age of the universe. But the other way around.

As the universe ages so will the field of view. After 15 billion years you'll see that distance and so on.

This will happen until the expansion starts moving faster than the speed of light. At which point the galaxies around hours will be moving so fast the light will never reach us, plunging the sky into darkness.
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Re: The size of the universe - a question.

Post by Scot Dutchy » Sun Apr 29, 2018 10:06 am

There is a theory that multiple universes exist. My mind just stops there. You need Brian Cox.
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Re: The size of the universe - a question.

Post by cronus » Sun Apr 29, 2018 10:12 am

The expansion is fastest in the space between galaxies - difficult but not impossible to visualise. Harder to describe what you've seen, and be sure about it, but then where's the fun in figuring out the obvious? I don't think the expansion of the universe is happening. It looks like vacuum decay pushing space with things in it apart.
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Re: The size of the universe - a question.

Post by pErvinalia » Sun Apr 29, 2018 10:17 am

Rum wrote:
Sat Apr 28, 2018 8:42 pm
Was listening to a podcast today (In Our Time - brilliant - get it - there are 700 of them or more). I listen to one most days. Today's was about the size and age of the universe.

As people who are interested in these things here know the current estimate of the age of the universe is about 13.7 billion years. This is based on several areas of evidence, but basically we can't see light that is older than that or stars/galaxies beyond that time/distance. It appears from our perspective that everything kicked off then.

The universe is sometimes described as expanding in the way a balloon does - i.e. every point on/in it expands away from every other point. The 'view' from any of these points is more or less as ours is - in other words they should be able to see about 13.7 billion years into the past by viewing light as seen from their position.

Which brings me to my question and puzzlement. If you were sitting on a planet on the edge of the universe that we can see, the observer there would be able to see 13.7 billion light years in all directions - including in a direction that points away from us. Thus expanding the size (and age) of the universe by another 13.7 billion light years. And of course you can keep on doing this until the size (and age?) becomes infinite.

Am I missing something? Perhaps the space time geometry or something?

Clever peeps please point out the error in my thinking!
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Re: The size of the universe - a question.

Post by Animavore » Sun Apr 29, 2018 10:19 am

I literally explained it. :|
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Re: The size of the universe - a question.

Post by pErvinalia » Sun Apr 29, 2018 10:19 am

Animavore wrote:
Sat Apr 28, 2018 9:52 pm
This will happen until the expansion starts moving faster than the speed of light. At which point the galaxies around hours will be moving so fast the light will never reach us, plunging the sky into darkness.
Expansion of what? In relation to what? I suspect you are one of the millions of lay people who think they understand how this stuff works, but really don't... ;)

Although, I'm happy to be proved wrong. :food:
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Re: The size of the universe - a question.

Post by pErvinalia » Sun Apr 29, 2018 10:20 am

Animavore wrote:
Sun Apr 29, 2018 10:19 am
I literally explained it. :|
Haha, you just beat my in pointing out how your explanation makes no sense. :)
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Re: The size of the universe - a question.

Post by Animavore » Sun Apr 29, 2018 10:21 am

pErvinalia wrote:
Sun Apr 29, 2018 10:19 am
Animavore wrote:
Sat Apr 28, 2018 9:52 pm
This will happen until the expansion starts moving faster than the speed of light. At which point the galaxies around hours will be moving so fast the light will never reach us, plunging the sky into darkness.
Expansion of what? In relation to what? I suspect you are one of the millions of lay people who think they understand how this stuff works, but really don't... ;)

Although, I'm happy to be proved wrong. :food:
The expansion of the universe in relation to itself.
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Re: The size of the universe - a question.

Post by pErvinalia » Sun Apr 29, 2018 10:21 am

Yeah nah. Circular argument is circular.
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Re: The size of the universe - a question.

Post by Animavore » Sun Apr 29, 2018 10:24 am

pErvinalia wrote:
Sun Apr 29, 2018 10:20 am
Animavore wrote:
Sun Apr 29, 2018 10:19 am
I literally explained it. :|
Haha, you just beat my in pointing out how your explanation makes no sense. :)
It makes plenty of sense. How could we see further than the age of the universe if the light from parts beyond the light-year distance corresponding with the age of the universe haven't reached us yet?
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Re: The size of the universe - a question.

Post by Animavore » Sun Apr 29, 2018 10:29 am

pErvinalia wrote:
Sun Apr 29, 2018 10:21 am
Yeah nah. Circular argument is circular.
It's not circular at all. The universe is observed expanding. And it expands the same in all directions. Further galaxies appear to move away faster than nearer ones. Exactly what you would observe if the universe was expanding the same in all directions from all points.

QED.
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Re: The size of the universe - a question.

Post by pErvinalia » Sun Apr 29, 2018 10:32 am

Animavore wrote:
Sun Apr 29, 2018 10:24 am
pErvinalia wrote:
Sun Apr 29, 2018 10:20 am
Animavore wrote:
Sun Apr 29, 2018 10:19 am
I literally explained it. :|
Haha, you just beat my in pointing out how your explanation makes no sense. :)
It makes plenty of sense. How could we see further than the age of the universe if the light from parts beyond the light-year distance corresponding with the age of the universe haven't reached us yet?
I'm not saying you can see further than the age of the universe. I'm questioning the explanations that claim we can see as far as the age of the universe. It makes no sense to me if you consider the universe like an expanding balloon. The light from the big bang would have past everything at time 0+1. We'd never see it.

The other point to consider is that every point is expanding away from every other point. But what does "expanding" mean? This is the bit I was really referring to in your post. Is it spacetime expanding? If it is, then the speed of light relative to points in space wouldn't change as spacetime expanded. in which case we would never see the occurrence of 'going dark' as you described.
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Re: The size of the universe - a question.

Post by pErvinalia » Sun Apr 29, 2018 10:33 am

Animavore wrote:
Sun Apr 29, 2018 10:29 am
pErvinalia wrote:
Sun Apr 29, 2018 10:21 am
Yeah nah. Circular argument is circular.
It's not circular at all. The universe is observed expanding. And it expands the same in all directions. Further galaxies appear to move away faster than nearer ones. Exactly what you would observe if the universe was expanding the same in all directions from all points.

QED.
That's a reasonable conclusion. But it's not an explanation. What is expanding, and what is it in relation to, given the universe is everything in existence?
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