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Questions on the size and age of the Universe......


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Hello

Having just finished reading Brian Cox's Universal book, I'm struggling to get my head around the science behind the age of the universe (not to mention the general theory of relativity, but that's another topic). Can someone please help me clear the fog?

The edge of the observable universe is c. 13.5 billion l.y. away, the light having taken 13.5 billion years to traverse the intervening space. OK so far. The Big Bang is estimated to have occurred around 13.5 billion years ago. When the light left the first galaxies the universe was obviously a lot smaller than it is now. Since the speed of light is a constant, with only the frequency changing as the universe stretches, how come the light has taken 13.5 billion years to cross what was a much smaller distance when the light was emitted?

I'm sure I'm missing something really obvious, and am probably over-thinking the problem, but would be grateful if someone can tell me where I'm going wrong. Thanks.

A related point in the book, which is not explained in detail (I don't know why, as it's quite thought provoking) is that; in the 13.5 b.l.y. that the light from the edge of the observable universe has been travelling, the universe has continued to expand, and the actual distance of this point in space, although we can't see it yet, is nearer 47 billion light years away. This is scrambling my brain. Am I understanding this bit right?

Thanks muchly :) 

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The rate of expansion of the distant, earliest, parts of the universe is faster than the speed of light. As a result it will remain dark to us, forever unobservable. 

Jim 

Edited by saac
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That is about right, but there are some small details that you need to understand.

First is idea that universe was a lot smaller. We have no idea how large universe is. Observable universe is 13.7 (or .5 - don't know precise value) billion light years in radius.

This means that a light source that we now observe, that emitted EM radiation early after big bang was at a such "distance to us" that in subsequent expansion of universe light emitted before 13.7 billion years, traveling at speed of light traveled 13.7 billion years to reach us. It just tells you those two things - light traveled 13.7by and in doing so crossed distance of 13.7bly. This does not tell you by itself important things:

1. What was the rate of expansion of universe during this time (it depended on density of universe and was not constant over this period)

2. How close were we to that object at the time light was first emitted (one reaching us now) - this we can know if we know 1. and do some complex calculations

3. How far away that object moved away from us while light was in flight and due to expansion of universe. This we can figure out by knowing 1. and 2.

It is also not telling us how big universe is - it could be infinite, and infinite thing is still infinite when squeezed, so although "distances" were shorter right after big bang - universe could have been infinite then as well. We just have no clue how big it is. Only thing that we know is age, and if object emitted light early in time of universe and light is hitting us now - we can know distance traveled by light and we can know distance to that object in our "now" slice.

46bly is radius of observed universe in so called "co-moving" coordinates. You can imagine it like this: I throw you a slow moving ball that takes some time to reach you, and I start walking away. By the time you catch that ball, I'm no longer in exact position that I was when I threw it - I'm further away because I started walking away just after I threw the ball.

Actual picture is more like:

Object emitted light and was at the distance to us (there were no us at that time, but let's say it like that for sake of argument). That light traveled at the speed of light, but universe also expanded during the flight of light. It is the same effect as if we were moving away and light had to catch up with us. So after 13.7by it finally caught up with us. But since space expanded - it expanded not only between us and light that was catching up with us, but also between object that first emitted it and light - so similarly object was moving away from the light and us also. If we account for space expansion that object is now at 46bly away in our "now" slice of time (time is local thing in relativity so there is no global now).

Key points - objects are not moving relative to each other - space is stretching in between. Distance traveled by light is always speed of light * time interval.

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I think you may be assuming that galaxies were forming the instant the big bang took place, galaxy formation occurred  about 200-400 million years after the "Big Bang", there was expansion prior to galaxies forming, and a period during expansion where temperatures were a billion billion billion degrees where light itself could not yet escape, matter was so dense and hot. Has expansion ever overtaken the speed of light?

Just my two cents because, i get to be smart too (my mom says i am) lol.

 

Edited by Sunshine
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22 minutes ago, saac said:

The rate of expansion of the distant, earliest, parts of the universe is faster than the speed of light. As a result it will remain dark to us, forever unobservable. 

Jim 

I thought nothing could travel faster than the speed of light?

Edited by Astrokev
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12 minutes ago, vlaiv said:

That is about right, but there are some small details that you need to understand.

First is idea that universe was a lot smaller. We have no idea how large universe is. Observable universe is 13.7 (or .5 - don't know precise value) billion light years in radius.

This means that a light source that we now observe, that emitted EM radiation early after big bang was at a such "distance to us" that in subsequent expansion of universe light emitted before 13.7 billion years, traveling at speed of light traveled 13.7 billion years to reach us. It just tells you those two things - light traveled 13.7by and in doing so crossed distance of 13.7bly. This does not tell you by itself important things:

1. What was the rate of expansion of universe during this time (it depended on density of universe and was not constant over this period)

2. How close were we to that object at the time light was first emitted (one reaching us now) - this we can know if we know 1. and do some complex calculations

3. How far away that object moved away from us while light was in flight and due to expansion of universe. This we can figure out by knowing 1. and 2.

It is also not telling us how big universe is - it could be infinite, and infinite thing is still infinite when squeezed, so although "distances" were shorter right after big bang - universe could have been infinite then as well. We just have no clue how big it is. Only thing that we know is age, and if object emitted light early in time of universe and light is hitting us now - we can know distance traveled by light and we can know distance to that object in our "now" slice.

46bly is radius of observed universe in so called "co-moving" coordinates. You can imagine it like this: I throw you a slow moving ball that takes some time to reach you, and I start walking away. By the time you catch that ball, I'm no longer in exact position that I was when I threw it - I'm further away because I started walking away just after I threw the ball.

Actual picture is more like:

Object emitted light and was at the distance to us (there were no us at that time, but let's say it like that for sake of argument). That light traveled at the speed of light, but universe also expanded during the flight of light. It is the same effect as if we were moving away and light had to catch up with us. So after 13.7by it finally caught up with us. But since space expanded - it expanded not only between us and light that was catching up with us, but also between object that first emitted it and light - so similarly object was moving away from the light and us also. If we account for space expansion that object is now at 46bly away in our "now" slice of time (time is local thing in relativity so there is no global now).

Key points - objects are not moving relative to each other - space is stretching in between. Distance traveled by light is always speed of light * time interval.

Thanks. I think I need to read your reply a few times - there's quite a lot in there to think about :) 

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7 minutes ago, Sunshine said:

I think you may be assuming that galaxies were forming the instant the big bang took place, galaxy formation occurred  about 200-400 million years after the "Big Bang", there was expansion prior to galaxies forming, and a period during expansion where temperatures were a billion billion billion degrees where light itself could not yet escape, matter was so dense and hot. Has expansion ever overtaken the speed of light?

Just my two cents because, i get to be smart too lol.

 

Thanks Sunshine. Yes, I realise that light was not "free" until after recombination, and that the universe had already expanded somewhat before light from the first objects was emitted, but I don't think that changes my question....or does it ?

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4 minutes ago, Klitwo said:

There's only "one" who knows the exact age and size of the universe....and he ain't talking!

All in time, there was a time when we couldn't know what caused that lightning strike, or why the earth shook, all in time.

Edited by Sunshine
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2 minutes ago, Astrokev said:

I thought nothing could travel than the speed of light?

Particles and information can't travel faster then the speed of light - but space is stretching, so although nothing is moving - distance between things get larger - and further something is - it looks like it is moving away faster.

imagine you have 4 objects. Each 1 unit away from others in a straight line.

o1 - o2 - o3 - o4

And in unit time, distance between each two objects increases by one unit of length.

At first instance distance between o1 and o4 is 3 length units.

At second instance distance between o1 and o4 is 6 length units - it looks like o1 and o4 are moving away with "speed" of 3 length units in 1 time unit. This is not the same as distance between o2 and o3 - which was 1 length in first instance and 2 in second - these two appear as if they are moving away from each other at 1 length unit / 1 time unit - or with "speed" of 1.

Further object is - it looks like it is moving faster from us, but in effect space is expanding between us and everything that is between us and that object. For some objects this cumulative "speed" of expansion will be larger than speed of light - but nothing moves - space is expanding.

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3 minutes ago, vlaiv said:

Particles and information can't travel faster then the speed of light - but space is stretching, so although nothing is moving - distance between things get larger - and further something is - it looks like it is moving away faster.

imagine you have 4 objects. Each 1 unit away from others in a straight line.

o1 - o2 - o3 - o4

And in unit time, distance between each two objects increases by one unit of length.

At first instance distance between o1 and o4 is 3 length units.

At second instance distance between o1 and o4 is 6 length units - it looks like o1 and o4 are moving away with "speed" of 3 length units in 1 time unit. This is not the same as distance between o2 and o3 - which was 1 length in first instance and 2 in second - these two appear as if they are moving away from each other at 1 length unit / 1 time unit - or with "speed" of 1.

Further object is - it looks like it is moving faster from us, but in effect space is expanding between us and everything that is between us and that object. For some objects this cumulative "speed" of expansion will be larger than speed of light - but nothing moves - space is expanding.

That's a neat explanation. Thanks. Now, back to my question....I need to think about this some more.....

 

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27 minutes ago, Astrokev said:

Ah ?

I know, the universe is just full of weirdness. But isn't it surprising that we can uncover the secrets; "the unreasonable effectiveness of mathematics in describing the universe".  :) 

Jim 

Edited by saac
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  • 2 months later...

Don't get your head in a tizz.

This is just a theory, developed by humans with 1 point of reference in a vast universe.

Like most other theories it will crash and burn, in time.

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On 03/02/2019 at 00:44, Astrokev said:

I thought nothing could travel faster than the speed of light?

It seems there are two kinds of 'travel,' if I have this clear. If this is an incorrect description I'd be most happy to be corrected.

Objects can travel away from each other because one or both of them have been accelerated by a force to drive them apart. This is the kind of movement with which we are routinely familiar. If I get in a jet at Heathrow to fly home I am accelerated away from Heathrow but towards Nice.

The remote galaxies are not doing this. They are all moving away from each other because the space between them is expanding. They are not feeling a force applied to them. They feel as if they are at rest in the centre of an expansion - but they all feel this.  The laws of relativity apply to accelerated objects but not to space itself - whatever that might be! It might well be an illusion so making our confusion perfect!

Olly

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On 03/02/2019 at 00:44, Astrokev said:

I thought nothing could travel faster than the speed of light?

There is a misconception here...

The speed at which light travels is with respect to the Space it travels through (technically with respect to objects that are “sitting still” in Space). The Universe does not expand at a particular speed, it expands at a speed per distance (68km/sec/megaparsec). 

The law that refers to the speed of light as being the ultimate speed in the Universe is Special Theory of Relativity. But STR is a localized law of physics and when we are speaking about a distant galaxy we are applying another law that is called General Relativity. And in the sense of the GR there is no concept like limited velocity ( velocity makes sense only on local regions of space). If it's not close, it doesn't count as a “velocity” in the way that STR cares about.

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As mentioned by Vlalv, it helps to think of it as space itself getting bigger, rather than things moving through space (although things do move through space, but for reasons other than the universe expanding).

re. Relativity - it depends where you are standing; sums things up nicely. 

Paul

Edited by Paul73
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This area is a mind boggling!  For instance, the CMB we observe today was initially moving away from us a speed well in excess of the speed of light. This was because the Hibble constant was very large initially. The CMB radiation eventually slowed down to zero and started to approach us and as it get to us to day is doing c exactly! 

This is because the Hubble constant is a ratio. It is a bit like the speed you travel  in your car divided by the distances traveled. However fast you go the distance travelled goes up so the ratio goes down! As an example if you do  a constant 60 mph in one hr the ratio is 60, in 2hrs 30, in 3hr 20 etc. 

This means the speed of distant galaxies we observe today are and were traveling away from us faster tha the speed of light up to 3.4c if I remember correctly. 

Regards Andrew 

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52 minutes ago, RolandKol said:

The answer is 42!

End of Story! :)

P.S. and yes, - Don't panic! :)

It’s been a long thread. What was the original question?

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