13.8 lightyears?

[mergatroid]

In a recent Youtube video, a working astronomer named Dr Becky claimed that we can see for 13.8 lightyears in all directions.

Is that a factual statement? In order words it's not 5 lightyears in one direction, 18 lightyears in another etc.

If so, what is the reason for this strange phenomenon?

[vlaiv]

That is true - except you made an error - it is 13.8 billion light years in every direction rather than just 13.8 light years.

Phenomenon is not strange at all if you understand that universe is 13.8 billion years old and that light that is further away than that did not yet have time to reach us.

There are some additional details to the whole story that might not be essential in understanding that the edge of observable universe is 13.8 billion light years away (or, just to make things unnecessarily complex - 46 billion light years away in co-moving coordinates ).

[Mr Spock]

I think you mean 13.8 billion light years...

It's as far as we can see in any direction.

[Paul M]

Just like if you are at sea, away from any land. Your horizon is the same distance in all directions, irrespectivebof where you are or which direction you are travelling.

The visible visible universe has just such an horizon for the reasons given about. Go where you want in any direction and you still seem to be at center!

[mergatroid]

33 minutes ago, vlaiv said:

Phenomenon is not strange at all if you understand that universe is 13.8 billion years old and that light that is further away than that did not yet have time to reach us.

That sounds like a thelogical argument. There's no reason to believe that it is onlyh 13.8 billion years old, other than faith in such a notion.

[mergatroid]

20 minutes ago, Paul M said:

Your horizon is the same distance in all directions,

The horizon exists because the Earth is curved. Are you saying the universe is curved?

20 minutes ago, Paul M said:

Go where you want in any direction and you still seem to be at center!

Ah, so you are making a theological argument that the Earth is at the center of the entire universe. Got it.

[Paul M]

1 hour ago, mergatroid said:

The horizon exists because the Earth is curved. Are you saying the universe is curved?

Ah, so you are making a theological argument that the Earth is at the center of the entire universe. Got it.

OK, I'll play.. 😇

There is much debate about the nature of the Universe. The last time i read up on the subject it, it seems that on the large scale the universe is close to flat or slightly curved. Of course not in any sense I can explain for you!

I'll not bite on theology. My point was quite clear that wherever the observer is in the universe, the edge for them and every other coincident observer is about 13.8b light years away. So absolutely no special case for any observer anywhere. 

[mergatroid]

1 hour ago, Paul M said:

wherever the observer is in the universe, the edge for them and every other coincident observer is about 13.8b light years away.

OK, so we're not in the exact center of the universe then. That's good. However why would our ability to view distant stars break off at 13.8 billion light years? That would suggest that photons have a lifespan and they just extinguish if they travel too far. But if they do that, then red shift would also be in doubt because a photon would die presumbly due to loss of energy, and red shift is also a loss of energy. If red shift isn't reliable, then we don't really know how far away celestial objects are.

[Mr Spock]

6 hours ago, mergatroid said:

That's good. However why would our ability to view distant stars break off at 13.8 billion light years? That would suggest that photons have a lifespan and they just extinguish if they travel too far.

No, they are just too faint to see. That is why we are building telescopes like Webb so we can see farther.

[Steve]

8 hours ago, Paul M said:

I'll not bite on theology. 

Well done, that man 🙂 

We don't have theological arguments at SGL. 

[michael.h.f.wilkinson]

6 hours ago, mergatroid said:

OK, so we're not in the exact center of the universe then. That's good. However why would our ability to view distant stars break off at 13.8 billion light years? That would suggest that photons have a lifespan and they just extinguish if they travel too far. But if they do that, then red shift would also be in doubt because a photon would die presumbly due to loss of energy, and red shift is also a loss of energy. If red shift isn't reliable, then we don't really know how far away celestial objects are.

Photons don't have a to have a finite lifespan (in fact, they are literally timeless according to general relativity) just a finite speed. If the current, most accurate estimate of the age of the universe of 13.8 billion years (i.e. obtained by scientific method, nothing to do with theology) is correct, we can only see photons arriving from 13.8 billion light years away at this point in time. As the universe ages, light from more distant sources will arrive. In fact, according to current scientific understanding, the early universe was too dense and hot to be transparent, and the heavily red-shifted glow of that early phase is the cosmic microwave background radiation. This must be from slightly less that 13.8 billion light years away.

[andrew s]

6 hours ago, mergatroid said:

OK, so we're not in the exact center of the universe then. That's good. However why would our ability to view distant stars break off at 13.8 billion light years? That would suggest that photons have a lifespan and they just extinguish if they travel too far. But if they do that, then red shift would also be in doubt because a photon would die presumbly due to loss of energy, and red shift is also a loss of energy. If red shift isn't reliable, then we don't really know how far away celestial objects are.

As has already been said it's because light from further away has not reached us yet.

The light from furthest away we can detect is the CMB. Every day we see a little bit further as light from the CMB seeeps past us.

The light from the CMB is not the oldest/furthest signal we could possibly detect. In principle we could detect gravitational waves from before the time of last scattering that released the CMB radiation. 

Regards Andrew 

[andrew s]

By the way I think the age is right but the distances are wrong. From NASA:

https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/educators/guide/age_size.html

Age: 13.7 Billion Years
Size: 94 Billion Light Years

The most distant objects in the Universe are 47 billion light years away, making the size of the observable Universe 94 billion light years across. How can the observable universe be larger than the time it takes light to travel over the age of the Universe? This is because the universe has been expanding during this time. This causes very distant objects to be further away from us than their light travel time. For additional information, see Ned Wright's Cosmology FAQ. http://www.astro.ucla.edu/~wright/cosmology_faq.html#ct2

Regards Andrew 

[ollypenrice]

10 hours ago, mergatroid said:

That sounds like a thelogical argument. There's no reason to believe that it is onlyh 13.8 billion years old, other than faith in such a notion.

It doesn't sound anything at all like a theological argument and there is very good scientific reason to believe the universe is 13.8 billion years old.  Almost a hundred years ago, convincing evidence was found to suggest that the universe was expanding. Since then, this evidence has grown considerably. If you run that expansion in reverse you find that the universe reduces to a point source which began its expansion 13.8 billion years ago.

7 hours ago, mergatroid said:

OK, so we're not in the exact center of the universe then. That's good. However why would our ability to view distant stars break off at 13.8 billion light years? That would suggest that photons have a lifespan and they just extinguish if they travel too far. But if they do that, then red shift would also be in doubt because a photon would die presumbly due to loss of energy, and red shift is also a loss of energy. If red shift isn't reliable, then we don't really know how far away celestial objects are.

Unlike other objects, the universe does not have a centre. This may seem absurd but consider this: we can observe all other objects from the outside. When we do so we see they have a surface, even it it's a diffuse one like a cloud's. This surface lets us think of the object's centre, a point at some kind of average distance from all parts of the surface. Since the universe cannot be seen from the outside (because any outside would be part of it and therefore inside it) we cannot define any kind of centre.

Your second point has been discussed already by astronomers and cosmologists.  Fritz Zwicky, in the 1930s, proposed that the redhift might be explained by his 'tired light hypothesis' but the evidence was against him. Light from very distance objects can never reach us because its source is receding from us a more than the speed of light. (Note that this does not violate the 'speed limit' of the speed of light because the galaxies sending out the light have not been accelerated by the expansion of the universe, it is simply that the medium of space between us is expanding.)  That part of the universe which lies within the volume not receding faster than the speed of light is known as the observable universe because it is observable in principle.

Olly

Edited January 30, 2023 by ollypenrice
Clarification

[EarthLife]

10 hours ago, mergatroid said:

The horizon exists because the Earth is curved. Are you saying the universe is curved?

Ah, so you are making a theological argument that the Earth is at the center of the entire universe. Got it.

Our science isn't saying we're at the center of the entire universe, it's saying we're at the center of the observable universe - the only part of the universe we can ever see/detect (due to it's limited so far life time and expansion rate), hence 13.8 billion light years in all directions.

The light from further away objects (beyond 13.8 light years) has not yet reached us, which simply means we can't see them (hence outside of the observable universe).

Edited January 30, 2023 by EarthLife

[andrew s]

1 hour ago, ollypenrice said:

Light from very distance objects can never reach us because its source is receding from us a more than the speed of light

A minor correction @ollypenrice we can see objects receeding faster than the speed of light. If I recall correctly the limit is currently about 3c. 

It's complex issue which is difficult to put in a post. The best I can offer is this paper https://arxiv.org/abs/astro-ph/0310808 

The diagrams and maths are a challenge but the text is approachable if you accept the mathematics is correct -  which it is.

As per another threads real understanding takes hard work and definitely requires the wearing of underpants. 

Regards Andrew 

Edited January 30, 2023 by andrew s

[saac]

13 hours ago, mergatroid said:

The horizon exists because the Earth is curved. Are you saying the universe is curved?

Ah, so you are making a theological argument that the Earth is at the center of the entire universe. Got it.

Not really a theological argument more a mathematical argument and no Earth not at the centre.    Take a look at the Hubble Lemaitre graph. Measure the gradient and then take the reciprocal of the gradient. That provides the age of the universe measured in seconds. Easy enough task from there on in.  The Hubble Lemaitre law shows a directly proportional relationship of recessional velocity (distant galaxies)  against distance allowing derivation of the simple relationship V = Hd, (H being Hubble constant).  With this, v = d/t,  and the reciprocal of the gradient of the line it is now a simple matter to see that the age of the universe is 13.8 billion years old. 

Hubble Lemaitre Law

Jim

 

[ollypenrice]

3 hours ago, andrew s said:

A minor correction @ollypenrice we can see objects receeding faster than the speed of light. If I recall correctly the limit is currently about 3c. 

It's complex issue which is difficult to put in a post. The best I can offer is this paper https://arxiv.org/abs/astro-ph/0310808 

The diagrams and maths are a challenge but the text is approachable if you accept the mathematics is correct -  which it is.

As per another threads real understanding takes hard work and definitely requires the wearing of underpants. 

Regards Andrew 

Thanks. I hadn't come across this refinement but I last took a Cosmology course over 20 years ago so either I've forgotten, or the thinking is new - or they were just keeping it simple for people like me!

Olly

[Malpi12]

Would it confuse things even more if this simple soul said :-

Although the "observable" universe is getting bigger as time passes, there is going to be less  & less to observe, as more "things" will be expanded beyond c (or 3c ?)
Only the Local Group and the Milky Way will remain, gravitation-bound, for awhile. Until the increasing rate of expansion trumps even that.
So make the most of what Webb can still see.

But I could be wrong ? I have a feeling that I am missing something ! Cos that bit about Webb dont feel right

Edited January 30, 2023 by Malpi12

[AstroKeith]

11 minutes ago, Malpi12 said:

Would it confuse things even more if this simple soul said :-

Although the "observable" universe is getting bigger as time passes, there is going to be less  & less to observe, as more "things" will be expanded beyond c (or 3c ?)
Only the Local Group and the Milky Way will remain, gravitation-bound, for awhile. Until the increasing rate of expansion trumps even that.
So make the most of what Webb can still see.

But I could be wrong ? I have a feeling that I am missing something ! Cos that bit about Webb dont feel right

You're right.

Except we're unlikely to be around then!

[Malpi12]

10 minutes ago, AstroKeith said:

You're right.

Except we're unlikely to be around then!

Thanks !  And yes I used "awhile" as homework for my chums

 

[Malpi12]

Sorry, pressed send too soon

The bit about Webb that I am thinking now is :-

How close to the edge do things need to be to get lost in say 1month, 1 year, 1 million years.

I mean if Webb can see back to the first light approx 100,000 y after last scattering (300000ish y after BB) do we need to wait 100000y till we start missing things over the c (3c) cliff edge  ?
 

Edited January 30, 2023 by Malpi12

[AstroKeith]

2 minutes ago, Malpi12 said:

Sorry, pressed send too soon

The bit about Webb that I am thinking now is :-

How close to the edge do things need to be to get lost in say 1month, 1 year, 1 million years.

I mean if Webb can see back to the first light approx 100,000 y after last scattering (300000ish y after BB) do we need to wait 100000y till we start missing things over the c (3c) cliff ngs ?
 

Actually the c/3c cliff does not exist. The paper referenced by Andrew above, although dated now, does 'prove' that it's not a barrier. Its all about frames of reference.

The problem I believe with seeing yet further back, or continuing to see what we can just see now, is distance and red shift. The radiation is weaker and even more 'red shifted'. Actually 'red shifted' is now not quite right as we have shifted beyond red. What we will need is a super sensitive and high resolution version of COBE.

A useful way to think of the expansion, is not that things are moving through space to be further away, its that the space itself between us that is expanding.

[Malpi12]

Yes, thanks, got the bit about the space between, I prob. should not have said "cliff edge",
Similar - the CMB is not far away- the glow is still all round us but now very cold and weak.

I am thinking The Cold Dark End, or The Big Rip was part of this -observable- scenario ?

I am off out to a party in a min. so a few bottles may help to clarify
 

13 minutes ago, AstroKeith said:

Actually the c/3c cliff does not exist. The paper referenced by Andrew above, although dated now, does 'prove' that it's not a barrier. Its all about frames of reference.

The problem I believe with seeing yet further back, or continuing to see what we can just see now, is distance and red shift. The radiation is weaker and even more 'red shifted'. Actually 'red shifted' is now not quite right as we have shifted beyond red. What we will need is a super sensitive and high resolution version of COBE.

A useful way to think of the expansion, is not that things are moving through space to be further away, its that the space itself between us that is expanding.

 

[AstroKeith]

I recommend Katy Mack's book  - 'The End of Everything (Astrophysically Speaking)'

She writes really well.