### Author Topic: How can the observable universe be 93 bil ly across and not 28?  (Read 1583 times)

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#### Beamin

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##### How can the observable universe be 93 bil ly across and not 28?
« on: May 09, 2018, 09:23:03 am »
This I could never figure out. I understand that you are always at the cnter because math. So shouldn't that make it 13.7 bly in direction and 13.7bly in the other? And if its expanding there should be more space then time to see things like the CMB but we see it. How?
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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #1 on: May 09, 2018, 11:00:28 am »

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#### Electro Detective

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #2 on: May 09, 2018, 11:12:35 am »
It's another of those 'numbers' someone pulled out of their...hat, and everyone rolled with it

I'll believe the 'math' when Zeus or one of his mob rock up with a tape measure and give up the real stats,

accurate to the closest parsec

#### Rerouter

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #3 on: May 09, 2018, 11:28:26 am »
Currently space is expanding away from us

The longer answer is by the time billions of years old photons have travelled to us, the space inbetween there origin point and us has expanded, far far away from us it would carry the biggest effect, then as it became closer and closer, the delay from the expansion lessens, (Its all relative in a uniformly expanding sphere)

This means what was expanding rapidly away from us at that point in times, Its photons are just reaching us, and we can extrapolate forward how much furthur away it is now.

#### Distelzombie

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #4 on: May 10, 2018, 01:01:58 am »
https://futurism.com/how-can-the-diameter-of-the-universe-the-age/
Aah, that's one of those horrible pages! It's just: "First, let's ... First, ... First, first first first first." all over. By the time you end up at the point the headline mentioned you've forgotten what the first, let's think about how a human creates memories: In a study researchers used mice to observe their ability to make memories and found some interesting results first-hands are very specialized for grabbing and manipulating objects. ...

(The question has already been extensively answered. I'm just here for no reason. You should be glad I wasn't the first Amendment was not originally part of the Bill of Rights...)

#### Nominal Animal

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #5 on: May 10, 2018, 04:18:52 am »
Distances in an expanding universe become easier to understand, if you first consider a simple example. (There! I said it. First.  )

Consider a flat rubber sheet being stretched. You have marked two points on the sheet, A and B. You have a small toy car with a motor, so it moves at a fixed velocity with respect to the surface it is on. You set it down so it travels from point A to point B.

If the distance between A and B is L0 when the car starts at A, and L1 when the car arrives at B, the distance L the car actually traveled is between L0 and L1.

The exact distance it did travel, depends on the rate of the expansion of the rubber sheet, the speed of the car, and the duration of the travel. In the case of our expanding universe, while the speed of light is constant, distances grow exponentially, because every point in space is expanding at the same rate. So, it's not like stretching the rubber sheet by moving the endpoints away from each other at a constant rate; you'd need to move the endpoints ever faster and faster. (Or, if you consider a balloon, inflate it faster and faster.)

The end result is that for short trips, L is approximately L1. The longer the trip, the closer L is to L0.

#### Beamin

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #6 on: May 10, 2018, 03:51:46 pm »
So where do we get 93 billion as observable? further thing we saw was 12.9bly away recently. Math would help the formula where 13.7bly X hublconstan= 93bly?
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#### Distelzombie

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #7 on: May 10, 2018, 05:58:02 pm »
We don't know. You came up with that. The observable universe IS 13.7 billion lightyears. Look at the first link again:
Quote
So, how can the universe be 93 billion light-years wide if it is only 13.8 billion years old and nothing can travel faster than light?
Edit: It doesn't have to be the same question that is answered, if it also answers yours.
« Last Edit: May 10, 2018, 06:00:42 pm by Distelzombie »

#### Nominal Animal

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #8 on: May 10, 2018, 06:32:33 pm »
Beamin: The light that we have seen, that started its travel 13.7 billion years ago, started from a point that is over 40 billion light years distant from us right now.

If you go back to my rubber sheet example, and let A be that faraway starting point, B is us, the car travels at c or 1 light year per year, and the rubber sheet is stretched at a scale factor a(t) (which tells how much the rubber sheet is stretched), you'll find that even though the cars odometer says 13.7 billion light years, point A is over 40 billion light years distant from point B when the car arrives at B.

The 93 billion light years figure refers to the size of the universe, if our models are right. It does not mean we can see objects that distance away. It means that the things that we can observe old light having originated from, must be right now over 40 billion light years away from us.

#### Beamin

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #9 on: May 13, 2018, 05:16:12 am »
That's what I don't get if the sheets expanding that would mean we would see less not more since the photons have to take a longer pather to get here and since there is only so much time that would create the illusion of seeing less then whats out there. If the sheet was contracting we would see more stars because further away star would be pushed toward us because they have a shorter path.
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#### Nerull

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #10 on: May 13, 2018, 05:42:18 am »
But that's precisely right. Our observable universe is not the entire universe.

We will never know anything about what is beyond our particle horizon, without FTL:
https://en.wikipedia.org/wiki/Particle_horizon

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#### IanMacdonald

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #11 on: May 13, 2018, 08:47:49 am »
I wonder if it's like a triangle of mirrors, and only seems to go on forever.

#### Nominal Animal

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #12 on: May 13, 2018, 09:11:08 am »
I wonder if it's like a triangle of mirrors, and only seems to go on forever.
You wonder right, excepting the mirrors. There is no edge, no boundary, no center. Our universe seems to be very much like the surface of a balloon being inflated, except that ours has three spatial dimensions instead of two.

The balloon analog fails in that it is being inflated in a three-dimensional universe itself; it only works as an analog if you consider the surface only, and do not try to infer other correlations with our universe from it.

#### BrianHG

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #13 on: May 22, 2018, 11:38:58 pm »

__________
BrianHG.

#### james_s

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #14 on: May 23, 2018, 01:11:23 am »
The concept of something going on forever has always kind of broken my brain, it's like dividing by zero. If it doesn't go on forever though what's at the end, and what's past the end? Aaah!

#### kj7e

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##### Re: How can the observable universe be 93 bil ly across and not 28?
« Reply #15 on: May 23, 2018, 02:07:04 am »
I have seen every PBS SpaceTime episode, most of the time the math is way over me (later episodes with Matthew O’Dowd).  Here is an early episode that covers this question;

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