Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: EmmaHildyard on 28/06/2019 12:25:13

Title: How old is the observable universe?
Post by: EmmaHildyard on 28/06/2019 12:25:13
Leslie asks...

My question is related to the age of the observable universe...

The last time that I looked it was in the region of 46.5 billion years...

I've read many explanations on the internet but have just not been able to grasp why, given an age of 13 billion years for the universe, the observable universe is 46.5 billion years...how could light have travelled 46 billion years (to enable us to see that 'boundary' and I guess it is  46.5 billion in all directions? ...that's if I understand the concept of 'observable universe') but the age of the universe is only 13 billion?


Can you help?
Title: Re: How old is the observable universe?
Post by: Halc on 28/06/2019 13:17:57
Quote from: Leslie
My question is related to the age of the observable universe...

The last time that I looked it was in the region of 46.5 billion years...

I've read many explanations on the internet but have just not been able to grasp why, given an age of 13 billion years for the universe, the observable universe is 46.5 billion years...how could light have travelled 46 billion years (to enable us to see that 'boundary' and I guess it is  46.5 billion in all directions? ...that's if I understand the concept of 'observable universe') but the age of the universe is only 13 billion?
Light has not traveled 46 billion years.  The figure is computed using comoving coordinate system.  The furthest material which ever could have had a causal effect on here/now is currently about 46.5 billion light years away.

Comoving distance is not proper distance except at events where the universe appears to be the same age (13.8 billion LY) to a comoving observer as it does here.  Light did not actually travel from that material that is now so far away since the universe was opaque back then, but other causal influences such as gravity waves were capable of penetrating that soup.

The universe at the here/now event is 13.8 billion light years old (in our inertial frame).
In an inertial frame, that material that is the furthest observable thing was under 7 BLY away when it emitted what we observe now, but nobody seems to use an inertial frame when describing cosmological distances.  They tend to use a proper/comoving sort of coordinate system (I don't know the name of it) where things can move faster that light speed (due to expansion of space).  Distances are measured using meter-sticks that are all moving at different speeds, as opposed to the inertial system where the meter sticks are all stationary in some inertial frame. This alternate coordinate system, that distant material emited its influence when it was still very close by (like under a million light years away) and the light only getting here now from that.  Space expansion actually pushed the light away before it finally comes back to us after 13.8 billion years.
Title: Re: How old is the observable universe?
Post by: Bill S on 01/07/2019 18:27:22
Halc has done a great job with the explanation.  Iím not attempting to better it; just to grab the opportunity to have  someone cast a critical eye over my attempt, of a few years ago, to get my head round this.

Times relate to the present; 13.8 billion years (by) after the Big Bang.
 
At 1.6 by post Big Bang something sent some light towards where Earth is now.
 
At that time the emitter was 4.8 billion light years away.
 
The light took 12.2 billion years to reach Earth (because 13.8 - 1.6 = 12.2). 

In that time the distance between the emitter and Earth's location increased to 23.9 billion light years.  Obviously, the light has not actually travelled 23.9bly in 12.2by. 

Undoubtedly, there are those who can derive these  figures for themselves, but for us hitch-hikers, cosmic time/distance charts can be found at:
 
http://www.einsteins-theory-of-relativity-4engineers.com/TabCosmo7.html.   
Title: Re: How old is the observable universe?
Post by: yor_on on 01/07/2019 22:08:13
I don't f*ng know :)

The light we measure is about 13.8 ~ billion light years 'old'
If you count in the 'expansion' it's 'bigger'
If you want to define the acceleration?

Be my guest?

But I will tell you one thing I do believe.
The universe is infinite.
Title: Re: How old is the observable universe?
Post by: yor_on on 01/07/2019 22:11:27
The point is that we use 'light' as the measure stick.
Every 'fishbowl' will get the same measurement, no matter where you define yourself to be situated it will be the same.
Title: Re: How old is the observable universe?
Post by: Bill S on 02/07/2019 18:35:55
Quote
I don't f*ng know :)

I know that feeling,well; but,surely, someone must know something; or is that only Ol' man river?  :)
Title: Re: How old is the observable universe?
Post by: jeffreyH on 02/07/2019 20:23:33
Let's think about the expanding balloon analogy of the expanding universe. We draw our dots on the balloon to represent the galaxies. Then we start to inflate it. Now let's imagine that we can set marbles rolling around the surface. All moving at a set speed. The distance the marbles have to travel between galaxies is increasing but the speed of each marble stays the same. Now if we could attach a pencil to each marble as it moves to trace out a sine wave this would appear to stretch out over the path of the marble on the surface of the balloon. This is Doppler shift kitchen science and demonstrates how we know the universe is expanding. Also why the universe is larger than the light we detect would have us believe.
Title: Re: How old is the observable universe?
Post by: Bill S on 03/07/2019 19:07:06
 I like the analogy, Jeffrey, but it was more the maths than the mechanics for which I was looking for criticism.
Title: Re: How old is the observable universe?
Post by: jeffreyH on 03/07/2019 22:41:52
I like the analogy, Jeffrey, but it was more the maths than the mechanics for which I was looking for criticism.

Well at the moment I have no time for maths. I have too much other stuff to do. Keep asking the questions though.