The size of the observable universe

[keepthebeercold (Sven)]

Hello y'all,

after taking a very long break from my active interest in astronomy (15-20yrs) I've recently come back and I'm trying to get up to speed with all the new shiny discoveries.

One thing I've stumbled across is that apparently now the "observable" universe is a about 90Gly in diameter

Wikipedia
Paul Halpern

This confuses me.

I get the basic idea. The light source was 13.x Gly away some 13.x billion years ago and according to our prediction that light source should now be 46.x Gly away because of the expansion of space.
But it's trivially obvious that we've never observed anything further than 13.x Gly away, the rest is prediction/theory. Even if the theory behind it is 100% correct than we can only say - the last time we observed an object that is now 46.x Gly away was when it was only 13.x Gly away.

So why speak of an "observable" universe 90Gly across?

Or am I missing something easy (or tricky) here?

Thank you very much

[Eratosthenes (Peter)]

Galaxies that are very far away can effectively be traveling faster than the speed of light (space-time expansion rate is faster than the speed of light). Big bang inflation theory also has the Universe expanding at faster than the speed of light.

Perhaps you need to explain why the age* of the Universe should be equal to its size?

(lucky the Theory of relativity places a speed limit of "c" only on Baryonic Objects (and mass-less photons) existing in regular space-time. Space-time itself is not limited to the speed of light. After all, what is the mass of space-time? I get confused at the various estimates of the TOTAL size of the Universe when compared to the Observable size of the Universe, Some estimates predict the Universe to be 12 billion times greater than the observable Universe. The way things are heading, eventually the observable Universe will be so small that the night sky will be absent of stars and galaxies. Not that any humans will be around to witness this collective decay into infinite madness)

[Atmos (Colin)]

When we observe light that has taken 13 billion years for it to reach us we're looking at it as it was 13 billion years ago. As the universe has been expanding and the speed at which the space between objects increases, what was 13 billion light years away 13 billion years ago is now 46 billion light years away. Hope that actually makes sense.

In short, we see things were they were when they were emitted, not where they are now billions of years later

[Eratosthenes (Peter)]

Originally Posted by Atmos

When we observe light that has taken 13 billion years for it to reach us we're looking at it as it was 13 billion years ago. As the universe has been expanding and the speed at which the space between objects increases, what was 13 billion light years away 13 billion years ago is now 46 billion light years away. Hope that actually makes sense.

In short, we see things were they were when they were emitted, not where they are now billions of years later

but can the estimate of 46 billion light years be achieved with expansion rates less than the speed of light?

[Atmos (Colin)]

Anything within the observable universe must be expanding at a lower rate other than the speed of light otherwise it will have red shifted outside of our detection. Given time the extremities of the cosmos will begin to go dark but that's not going to happen within our lifetimes

[keepthebeercold (Sven)]

Originally Posted by Atmos

When we observe light that has taken 13 billion years for it to reach us we're looking at it as it was 13 billion years ago. As the universe has been expanding and the speed at which the space between objects increases, what was 13 billion light years away 13 billion years ago is now 46 billion light years away. Hope that actually makes sense.

Cheers ... that's the way I understand it and that's exactly what confuses me/ makes no sense to me.

Originally Posted by Atmos

In short, we see things were they were when they were emitted, not where they are now billions of years later

Precisely. We see light from something that was 13.x Gly away when the light was emitted. It shouldn't matter where the light source is now.
We have absolutely never observed anything more than 14Gly away (at time of emission), so why say limit of observability is 46Gly?

I suppose I'm getting hung up on what "observability" means, but I feel it's being misused to sound more impressive.

[keepthebeercold (Sven)]

Originally Posted by Eratosthenes

Perhaps you need to explain why the size of the Universe should be equal to its size?

Sorry, can you rephrase this? This way it sounds very tautological.

[Eratosthenes (Peter)]

Originally Posted by Atmos

When we observe light that has taken 13 billion years for it to reach us we're looking at it as it was 13 billion years ago. As the universe has been expanding and the speed at which the space between objects increases, what was 13 billion light years away 13 billion years ago is now 46 billion light years away. Hope that actually makes sense.

In short, we see things were they were when they were emitted, not where they are now billions of years later

....during the inflation period post-big bang, the expansion rate of space-time exceeded the speed of light. The region of space where Very distant outer galaxies exist is also expanding at a rate greater than the speed of light. This is why the age of the Universe (14 billion ly) is about 1/7th of the diameter of the Universe

[Eratosthenes (Peter)]

Originally Posted by keepthebeercold

Sorry, can you rephrase this? This way it sounds very tautological.

yes, meant to say 'Why the "age" of the Universe should be equal to its size'.

[keepthebeercold (Sven)]

Originally Posted by Eratosthenes

yes, meant to say 'Why the "age" of the Universe should be equal to its size'.

Cheers. No, the size need not have anything to do with its age.

As far as "observability" goes we are bound by the speed of light, though.

Maybe it's easier if I just ask the other way around - in what sense is an object that is 46Gly away observable? From what I gather not at all, except we have certain observation data from when it was only 13.x Gly away.

[Eratosthenes (Peter)]

Originally Posted by keepthebeercold

Cheers. No, the size need not have anything to do with its age.

As far as "observability" goes we are bound by the speed of light, though.

Maybe it's easier if I just ask the other way around - in what sense is an object that is 46Gly away observable? From what I gather not at all, except we have certain observation data from when it was only 13.x Gly away.

and yet space (time) itself can expand at rates that exceed the speed of light.

So two adjacent galaxies may "not be moving" relative to one another but the distance between them could be increasing at a rate greater than the speed of light. A quasi-pseudo-paradox

[keepthebeercold (Sven)]

Originally Posted by Eratosthenes

and yet space (time) itself can expand at rates that exceed the speed of light.

So two adjacent galaxies may "not be moving" relative to one another but the distance between them could be increasing at a rate greater than the speed of light. A quasi-pseudo-paradox

Whether this is in jest or serious ... I still don't get what possible meaning of "observable" people are using when they say something 46Gly away is observable.

Isn't that like me sitting on my deck chair in Sydney and saying I can observe cars in Melbourne, because auntie Lizzy visited two weeks ago and parked hers in my garage and I have a theory that predicts this car now to be in Melbourne?

[sjastro]

Originally Posted by keepthebeercold

Whether this is in jest or serious ... I still don't get what possible meaning of "observable" people are using when they say something 46Gly away is observable.

Isn't that like me sitting on my deck chair in Sydney and saying I can observe cars in Melbourne, because auntie Lizzy visited two weeks ago and parked hers in my garage and I have a theory that predicts this car now to be in Melbourne?

The definition of an "Observable Universe" is when distant photons emitted from an object (which might be travelling at superluminal velocities) become subluminal as they are within the Hubble sphere of the local observer.
The Hubble sphere is an expanding sphere where maximum radius is based on the distance where the recession velocity of an object is travelling at speed c.
The velocity of expansion of the sphere is based on the current value of the Hubble constant.

An object beyond this sphere exceeds c but could still be observed for a very simple fact, it is emitting photons back towards the observer.
Since the Universe is in an accelerated fame of reference rather than an inertial frame, the rules of special relativity do not apply and the speed of light is not the same for all observers.

If the object is receding at a speed V, the speed of photons moving towards the observer is V-c. For photons initially just outside the Hubble sphere, this non constant speed can become less than the expansion velocity of the Hubble sphere with time in which case the photons become subliminal and observable.

To get back to your original question the Universe has expanded to 46 billion light years in the period of a 13.x billion year time of flight of a photon to the observer.
The photons are still observable as they remain subluminal and are within the Hubble sphere of the observer.

The calculation of the 46 billion light year radius is based on factors such as the value of the Hubble constant and the curvature of the Universe.

Here is a more scientific (and mathematical) treatment of the subject.
If assumes a knowledge Minkowski space diagrams, light cones, Robertson-Walker metric etc.

https://arxiv.org/pdf/astro-ph/0310808v2.pdf

[keepthebeercold (Sven)]

sjastro, thank you very much for your explanation and the provided link. I had a quick look at it and I think I should be able to digest it with a bit of effort.

Now I only have to find a few quiet hours of reading time.

Small edit: as you've given me such a great answer, may I ask whether you'd be able to point me towards some good reading material, be it blogs, articles or books, concerning the current understanding of the expansion of space? I'm wondering about questions like: if space expands, do our measuring sticks not expand with them and if so how do we measure the expansion? Or - does the expansion of space assume or imply that space at Planck level is continuous and not discrete?
Thanks again.

[sjastro]

Sven,

Here are some useful links on Cosmology and expansion.

http://members.iinet.net.au/~sjastro...andingUniv.pdf
http://members.iinet.net.au/~sjastro...ance_scale.pdf
http://members.iinet.net.au/~sjastro...y/acc_univ.pdf

Then there is Ned Wrights tutorial, somewhat more technical but it goes some way to explain the diagrams in ariv.org link which is central to understanding the paper.

http://www.astro.ucla.edu/~wright/cosmo_01.htm etc.

This blog which Shiraz pointed out has some valuable information on cosmology.
http://www.preposterousuniverse.com/blog/

Our measuring sticks on a global (cosmological) scale do expand, but on a local scale the sticks are rigid for two reasons.
(1) The effects of expansion are minimal on a local scale.
(2) Local scales are generally associated with gravitationally bound systems, (Earth, Solar system, galaxy, local cluster) where gravity prevents expansion from occurring.
This can of course can change if dark energy increases with time in which case in the very distant future expansion might occur even at the smallest scales.

The Planck level is the quantum mechanical version for spacetime of the Compton wavelength for an object.

If you shrink an object down to its Compton wavelength it no longer behaves as an object described by classical physics but is quantum mechanical in nature.
Similarly spacetime at very small scales is quantum mechanical in nature.
A "point" in spacetime is undefinable due to uncertainty in the "position" of the point.

It is one of the problems with General Relativity being a scale dependant theory.
Point sources such as Black Holes should in theory not exist as spacetime at such small scales should "smear out" the point source.
Similarly our knowledge of the Big Bang only goes back to 10^-43 second after the bang as before this the Universe was quantum mechanical.

Steven

[Eratosthenes (Peter)]

Originally Posted by keepthebeercold

.....Isn't that like me sitting on my deck chair in Sydney and saying I can observe cars in Melbourne, because auntie Lizzy visited two weeks ago and parked hers in my garage and I have a theory that predicts this car now to be in Melbourne?

yes



What's important is that of all the Scientific disciplines in the Religion of Science the least complicated and intellectually rudimentary and simplistic is Physics.

Chemistry becomes more complicated, followed by Biology which is even more complicated again. On top of these we have psychology and economics which are so complicated that as predictive religious tools, they are unable to generate collective results.

Then we have the field of mathematics which is the simplest of all Art Philosophies - more of a mystical religious circus full of priests and freaks. Mathematicians admit by definition that their profession is the purest of religions and their proofs are baseless in reality.

No wonder the dynamite kid Nobel refused to include Mathematics in the Prize ceremony. Economics gets a nod though. Mathematics was then forced to hold farcical circus events every 4 years to present their priests with the laughable Fields medal. It gets worst still - the recipients must be under the age of 40. Apparently the human brain and its mathematical abstractness ceases to be when a person reaches the age of 480 months.

The dogmatic Mathematical circus scam has gone on long enough imo.

[markbakovic]

-(i^2)

[keepthebeercold (Sven)]

@Steven - thanks again, your effort is much appreciated.

Sven