The Edge Of The Universe

[Davey-T]

1 minute ago, Astro Noodles said:

if objects in space/time can travel at  no more than the speed of light,

IIRC on a recent Stargazine Live episode the guy speaking ( can't remember his name ) said that there was no speed limit on the expansion of the space containing distant galaxies so it could be expanding faster than the speed of light.

Dave

[Robindonne]

I go partially with Knighty2112.  We just dont know.  And maybe thats the most exciting part🙂

[vlaiv]

16 minutes ago, Astro Noodles said:

I see it is actually called 'Inflation Theory'.

Don't confuse Inflation with expansion of universe.

Inflation is something that happened very early after big bang and then it stopped. Regular expansion has been going on ever since and is now picking up in pace (or rather it started picking up in pace after about 5-6By). Current expansion is feature of GR when applied on cosmological scales.

18 minutes ago, Astro Noodles said:

What I mean is that if the universe was created by the big bang 13.8 billion years ago, which is generally accepted, an if objects in space/time can travel at  no more than the speed of light, then the furthest observable object can be no more than 13.8 billion years ago. The furthest observed objects fall into the 13+ billion year light travel distance. However, the red-shift of these objects suggests that they are more than twice that distance away.

Couple of issues here.

First is that objects can't travel faster than speed of light - locally thru space/time with respect to another object. On scales involved and due to expansion of universe - they can move away from each other faster than speed of light - but they won't actually be "traveling" at all in local sense - it is space that is expanding between.

Second is that red shift suggests only 13.7By distances not twice that. Not sure where you've got that info.

Third is that particle horizon is actually at 46Bly away from us at the moment.

Do watch those videos that I've posted - they explain how different distances come together because of all expansion, light travel and time passing.

21 minutes ago, Astro Noodles said:

The way understand it as a layman is that the universe itself is expanding but the physical mechanism that causes this to happen is posited to be dark energy. But as dark energy has yet to be observed or measured directly in any way, in my mind it is not clearly defined yet. 

No. Dark energy is responsible for accelerated expansion of universe. Not for regular expansion of universe.

Let me make an analogy - you throw a ball upwards. It will move away from earth and slow down - at one point it will stop and fall back to earth. If you throw the same ball with enough speed - it will never slow down enough to fall back - but will instead continue to infinity. This is called escape velocity.

Similar thing happens with big bang - it has some initial speed of expansion - and how much "stuff" there is in universe - determines how much gravity there is to resist this expansion. If there was no dark energy - universe would still expand but it would be slowing down. Since there is dark energy  - that acts as rocket engine in above ball analogy - universe is speeding up - or rather rate of expansion is speeding up.

In any case - we don't need to measure dark energy in laboratory - fact that universe is expanding in accelerated manner is evidence of phenomena we call dark energy (regardless what it actually is).

There is reason why we call it energy and it is related to GR. In GR curvature of space time is related to energy content in that space time (mass or other forms of energy).

Different types of energy have different way of acting in expanding universe - matter for example "dilutes" when you add more space between - density of matter decreases with expansion. Light behaves differently - it gets stretched / red shifted so it also looses a bit of energy - at different rate.

Thing we call dark energy - remains constant as it is property of space - and you can think of it as "density of space" - although space is stretched - more space is "created" in between already existing space - so density of that remains the same.

This is important to understand because dark energy was not always acting in the way to accelerate universe. At earlier stages when things were closer together - density of matter and radiation was dominant thing and it slowed down expansion of universe but as time went by and universe expanded - matter and radiation diluted and became less dominant and at one point dark energy being constant started to be more important and universe started to expand in accelerated manner.

There is a reason why we often see image like this. It shows what happened across the time. At very beginning - we had inflation period - but that was very brief period and it was before time of last scattering. Then for some time universe expanded with slowing down - but only recently it started to expand in accelerated manner.

Left is shape of universe with acceleration slowing down - right is graph of universe with accelerated expansion.

First image is combination of these two shapes - it stars as slowing down universe but then after threshold is reached where matter and radiation are diluted enough and density falls down - dark energy starts being significant term and shape starts to change.

[andrew s]

The current best theory us that the Universe is spatially infinite and always has been!

Spacetime (not space) was finite at t=0 and has been expanding ever since. 

The greatestest distance we can see is called the observable Universe and is from the CMB we see to day. It is further (in light years) than the age of the Universe because of the metrical expansion in the Universe.

The most distant objects we can image are receeding faster than the speed of light due to the expansion but that is due to space expanding not the kinematics of the objects.

Regards Andrew 

@Vladi listed while I was posting this 

Edited June 19, 2021 by andrew s

[ollypenrice]

58 minutes ago, Astro Noodles said:

I see it is actually called 'Inflation Theory'.

What I mean is that if the universe was created by the big bang 13.8 billion years ago, which is generally accepted, an if objects in space/time can travel at  no more than the speed of light, then the furthest observable object can be no more than 13.8 billion years ago. The furthest observed objects fall into the 13+ billion year light travel distance. However, the red-shift of these objects suggests that they are more than twice that distance away.

Either there is something wrong with the observational techniques (which seems unlikely), or the universe itself is expanding .

The way understand it as a layman is that the universe itself is expanding but the physical mechanism that causes this to happen is posited to be dark energy. But as dark energy has yet to be observed or measured directly in any way, in my mind it is not clearly defined yet. 

No, the theory of the expansion of the universe allows for the universe to be far larger than the light travel distance of 13.8 billion years. If the universe were not expanding then then no two points could be further than 13.8 billion lightyears apart.  But it is expanding, so the two points are moving apart during those 13.8 billion years. 

Conisder an alternative to lightyears. Let's go for pigeon days and say that they fly 500 miles a day. My pigeon reaches you with its onboard watch, which I started when I released it, recording one day. You therefore conclude that I must now be 500 miles away - but you'd be wrong, because I didn't release it from the station platform 500 miles from you. I released it from the train as it passed by the platform and, during the pigeon's flight, I have continued to travel away from you on that train.  In that time the train covered 1000 miles so, despite the fact that the pigeon flight time was 24 hours, our separation is acutually 1,500 miles.

So in this analogy the movement of the train driving us apart represents the expansion of the universe and pigeon days represent lightyears.

The flaw in the analogy (which is otherwise fit for purpose) is itself interesting. The train is moving. In the case of our two cosmological points getting further apart, they are not getting further apart because they are moving, in the normal sense, but because the space between them is growing. For us to move, in the normal sense, we must be accelerated. However, at cosmological distances, two galaxies can move apart without either of them being accelerated simply because each cubic metre of space has grown slightly since the last measurement. If we have enough of these cubic metres (and in cosmology we do!) all these little bits of expansion can add up to a prodigious expansion and cause galaxies to move apart at more than lightspeed. (Remember, they are not moving in the normal sense. They are not being accelerated apart. They can both be at rest while racing apart.)

Olly

Edited June 19, 2021 by ollypenrice

[Icosahedron]

The light reaching us from galaxies moving away from us is red-shifted. The "edge" of the observable universe is where light is red-shifted into the infrared range. Due to infrared interference from the sun and earth current telescopes can't be made sensitive enough to detect what lies further out. This is where the James Webb Space Telescope comes into play, an infrared telescope that will be positioned away from earth and shielded from the sun. It will allow us to observe what lies beyond the current "edge".

 

[Astro Noodles]

2 minutes ago, ollypenrice said:

No, the theory of the expansion of the universe allows for the universe to be far larger than the light travel distance of 13.8 billion years. If the universe were not expanding then then no two points could be further than 13.8 billion lightyears apart.  But it is expanding, so the two points are moving apart during those 13.8 billion years. 

Yes, I understand that. It is the mechanism of dark energy powering the expansion which I think needs more understanding.

[ollypenrice]

1 minute ago, Astro Noodles said:

Yes, I understand that. It is the mechanism of dark energy powering the expansion which I think needs more understanding.

Yes, I'm sure it does. A recent head of Fermilab didn't believe in dark enrgy at all, I don't think.

Olly

[andrew s]

18 minutes ago, ollypenrice said:

No, the theory of the expansion of the universe allows for the universe to be far larger than the light travel distance of 13.8 billion years. If the universe were not expanding then then no two points could be further than 13.8 billion lightyears apart.  But it is expanding, so the two points are moving apart during those 13.8 billion years. 

Conisder an alternative to lightyears. Let's go for pigeon days and say that they fly 500 miles a day. My pigeon reaches you with its onboard watch, which I started when I released it, recording one day. You therefore conclude that I must now be 500 miles away - but you'd be wrong, because I didn't release it from the station platform 500 miles from you. I released it from the train as it passed by the platform and, during the pigeon's flight, I have continued to travel away from you on that train.  In that time the train covered 1000 miles so, despite the fact that the pigeon flight time was 24 hours, our separation is acutually 1,500 miles.

So in this analogy the movement of the train driving us apart represents the expansion of the universe and pigeon days represent lightyears.

The flaw in the analogy (which is otherwise fit for purpose) is itself interesting. The train is moving. In the case of our two cosmological points getting further apart, they are not getting further apart because they are moving, in the normal sense, but because the space between them is growing. For us to move, in the normal sense, we must be accelerated. However, at cosmological distances, two galaxies can move apart without either of them being accelerated simply because each cubic metre of space has grown slightly since the last measurement. If we have enough of these cubic metres (and in cosmology we do!) all these little bits of expansion can add up to a prodigious expansion and cause galaxies to move apart at more than lightspeed. (Remember, they are not moving in the normal sense. They are not being accelerated apart. They can both be at rest while racing apart.)

Olly

Sorry @ollypenrice the Universe is and always have been spatially infinite according to the best current theory. This has nothing to do with the speed of light or the age of the Universe.

Regards Andrew 

Edited June 19, 2021 by andrew s

[andrew s]

11 minutes ago, Astro Noodles said:

Yes, I understand that. It is the mechanism of dark energy powering the expansion which I think needs more understanding.

The simplest theory of dark energy is a very small non zero  curvature to spacetime I.e. a small cosmological constant. Regards Andrew 

[Astro Noodles]

Look guys, I'm not some kind of flat earth conspiracy nut or something. I just think that a lot more work needs to be done by cosmologists and physicists on the mechanisms of expansion and dark energy. 😁

I'm not sure that I'm satisfied with the approach that the universe is expanding, we can measure that, but we need a mechanism to explain it, so let's invent dark energy to fit into the hole in our theories! and then do the science later. I do accept that something needs to fill in the hole.

 

Edited June 19, 2021 by Astro Noodles

[Ouroboros]

33 minutes ago, ollypenrice said:

No, the theory of the expansion of the universe allows for the universe to be far larger than the light travel distance of 13.8 billion years. If the universe were not expanding then then no two points could be further than 13.8 billion lightyears apart.  But it is expanding, so the two points are moving apart during those 13.8 billion years. Olly

This I understand and is what I couldn’t remember. So thanks, Olly.  

No one is allowed to post any more now otherwise it’ll just confuse me!  

Edited June 19, 2021 by Ouroboros

[Astro Noodles]

45 minutes ago, vlaiv said:

n any case - we don't need to measure dark energy in laboratory - fact that universe is expanding in accelerated manner is evidence of phenomena we call dark energy (regardless what it actually is).

Thanks for the detailed explanation Vlaiv. 

I am not trying to deny science or re-invent the universe. It just seems to me that the statement of yours I have quoted just doesn't seem like a scientific definition to me, although it does concur with everything I have seen and heard.

My point is that the phenomena we understand as dark energy is not fully understood.

[andrew s]

24 minutes ago, Astro Noodles said:

My point is that the phenomena we understand as dark energy is not fully understood.

Nothing in Science is fully understood.  We have models that work well or less well at predicting observable effects.  We judge a theory on its usefulness  and to a degree its elegance. No one know what energy or entropy "really "are. We are more familiar with energy so feel comfortable with it as opposed to say entropy. 

Regards Andrew 

[vlaiv]

26 minutes ago, Astro Noodles said:

Thanks for the detailed explanation Vlaiv. 

I am not trying to deny science or re-invent the universe. It just seems to me that the statement of yours I have quoted just doesn't seem like a scientific definition to me, although it does concur with everything I have seen and heard.

My point is that the phenomena we understand as dark energy is not fully understood.

I understand what you are saying, but at some point we simply need to accept that nature behaves in some fundamental way that maybe can't be explained further with more basic principles / constituents.

Why does energy bend space / time? Why is it that space can be bent?

Part of that question comes from our preconception of what space is supposed to be like. We did to much Euclidean math and we are used to thinking of space in terms of some sort of coordinate system that is "rigid". That in turn creates sensation of wondering when we encounter idea that both space and time can be warped.

We can ask - why is there dark energy and what it is - in the same sense that we can ask  -what is time and why does it exist, but we also must be prepared to accept that it is fundamental phenomena and that we might never find better answer than just - because "it does".

[Marvin Jenkins]

WOW! And I thought The Edge, was the guitarist from U2.

Marv

[bingevader]

But surely the universe is flat, just like the earth?! 😉

Then there must be an edge!? 🙃😄

[michael8554]

It's the "Fabric of Space" that is expanding, carrying Matter with it - Galaxies, stars, planets etc - which aren't getting "bigger".

In the very early Universe, that Fabric briefly expanded at many multiples of the speed of light, which Einstein's Theories allow for.

Beyond the "edge" of the Observable Universe at 13.8 Billion LYs, the galaxies and stars are being carried away at faster than the speed of light, so we will never be able to observe those galaxies.

So the real "edge" of "our" Universe is currently about 40B+ LYs away from us.

However, consider an astronomer looking at earth from a Planet 13.8 BLY away from us.

If he turns his scope 180 degrees, he will see galaxies a further 13.8 BLY away.

And an astronomer in one of those galaxies will see........ etc etc

So how big is the Universe ?

An infinite number of 13.8 BLY "bubbles" ?

Michael

 

 

 

 

 

[vlaiv]

40 minutes ago, michael8554 said:

So how big is the Universe ?

An infinite number of 13.8 BLY "bubbles" ?

That really depends on curvature of universe.

If curvature is zero or negative - we have infinite universe. If curvature is positive we can have closed universe with finite spatial extent.

In some special cases, zero curvature can also mean closed universe - but that requires non isotropic universe and cosmological principle states that universe is both isotropic and homogeneous on large scales.

Currently measured value of curvature is very close to 0. It is actually 0 with some margin of error (0.1% or so) - so we can't really tell if it's zero or just very small within measurement error. In fact - this is a problem as we can never measure it to be exactly 0 - there will always be some measurement uncertainty and we will always wander if is perhaps just small positive or small negative value.

In case it is small positive value - then universe can be finite and size of universe in this case is at least x18 size of observable universe - so it is at least 18 times larger than what we can see.

[Robindonne]

Interesting read. But dont we have to admit that this isnt really debatable, or very debatable but open ended.  So many theories where we just have to stick to the most accepted one.  Dont mean to hurt every well explaining individual, on the contrary, I love any detailed explanation, but I can't stop myself from realizing after every explanation I read that I've read a possibility and not a fact.  I sincerely hope we never find out and that we are all wrong for the reason the universe will be explored forever. 
Dont let it give away its biggest secret please🙏🏼.

Edited June 20, 2021 by Robindonne

[vlaiv]

3 hours ago, Robindonne said:

Dont mean to hurt every well explaining individual, on the contrary, I love any detailed explanation, but I can't stop myself from realizing after every explanation I read that I've read a possibility and not a fact.

Anything we conclude is only a possibility not a fact.

You could say that things falling in gravity is a fact, but I'll tell you that that has been our experience so far - but we can't really state that as a fact - as it can change at any moment. Only thing that you can say is that: Within your experience it has been so. Was it so 10000 years ago? We can easily say that it is very very very likely that it was so - but we don't know that for a fact.

This should not bother you though as any scientific theory can fall at any moment - and when it does, we will work to create better one - that fits more data.

There is no clear point at which we can say - you know, expansion of universe only fits this much data and now we gathered some more data and we now consider it to be a fact.

Our confidence in theory grows with each new confirmation and any deviation is cause to stop and examine what is going on - no matter how well established theory is. Lambda CDM is rather good cosmological model that explains many things. It self is not perfect - as we can now attest - we have different measured values for Hubble constant from two sources - so either one of them is making serious measurement error - or our model is somehow flawed. Further investigation will tell.

[saac]

16 hours ago, bingevader said:

But surely the universe is flat, just like the earth?! 😉

Then there must be an edge!? 🙃😄

and there is not ergo it is not  

[andrew s]

Actually we can say gravity worked  up to and beyond 10000yrs ago by looking at suitably distant galaxies.  Regards Andrew 

[Barry Fitz-Gerald]

I have never quite understood the problem with 'Dark Energy' as empty space is not empty as we all know - it is filled with the electromagnetic field which can have any value it likes due to quantum uncertainty - and even a negative value is still a value, its just that to us humans a negative value means less than nothing. Try telling that to empty space.  I understand that attempts to reconcile the predicted value of this 'vacuum energy' with the currently accepted value of 'dark energy' produces a mis-match of several orders of magnitude, but the fact remains that phenomena such as the Casimir effect shows that there is a 'vacuum energy' which can act on the physical universe and to me seems a logical candidate for the force behind the expansion of space. I suspect those cosmologist have got their decimal point in the wrong place - should have paid more attention in maths.

[vlaiv]

50 minutes ago, andrew s said:

Actually we can say gravity worked  up to and beyond 10000yrs ago by looking at suitably distant galaxies.  Regards Andrew 

Yes, at those distances, but not under the apple tree