Why is the edge of the universe the place to look for the oldest stars?

[kilzok]

Forgive me if this has been covered already.
If the universe has no center why do astronomers find the oldest stars at its edge? It seems to me that they should be everywhere.

 

[turbo]

By "oldest" in this sense, the stars have longer look-back times in BB cosmology. The oldest stars in terms of stellar evolution can generally be found in globular clusters surrounding nearby galaxies, including our own. The oldest stars in terms of their composition are believed to be those that have only hydrogen, helium, and a little lithium and little or no heavier elements. They have been called "fossil stars" because they are believed to have formed before generations of supernovae enriched the interstellar medium with heavy elements.

Good question, but one with lots of wrinkles. The popular press does little to clarify, and a lot to sensationalize.

 

[kilzok]

Thanks for the answer. Why do I think that you do a lot of investing?

 

[turbo]

Investing?

 

[nickthrop101]

the further you look away, the further in back in time you see, soo that would be the oldest stars :D

 

[alexjbuck]

The speed of light is finite so the "Edge" of the universe is in reference to the edge of the "visible universe", an ever growing sphere around the earth. There is no physical edge that we know of (nor does the concept of a physical "edge" actually make much sense...).

The stars at "the edge of the visible universe" emitted the light we are seeing billions of years ago (thats how long it took those photons to travel from those stars to us at earth) so we are seeing stars as they were billions of years ago.

Thus these are considered the oldest stars in the universe

 

[Jorrie]

The stars at "the edge of the visible universe" emitted the light we are seeing billions of years ago (thats how long it took those photons to travel from those stars to us at earth) so we are seeing stars as they were billions of years ago.

Thus these are considered the oldest stars in the universe

Yes, but in a sense, "stars at the edge of the visible universe" are amongst the youngest! At least, the present images we have of them show them in their (relatively) young state and we do not know if they even exist today.

 

[Chronos]

The oldest known stars reside right here in the milky way.

 

[tichy]

The oldest known stars reside right here in the milky way.

Explain?

 

[Jorrie]

Explain?

http://www.eso.org/public/news/eso0723/" at 13.2 billion years. There may perhaps be older ones, but I do not think we know their ages...

 

[lastdays33]

I'm curious about the idea of a finite universe. What happens to the light emitted from a star lying at the very edge, or end, of the universe? If its light is emitted 360 degrees and travels at 186,000 miles a second, what happens to the light that hits the edge of the universe? Does it bounce of the edge like a flash light off a mirror? Does it get absorbed by the edge and become part of the "edge" itself? Does it get sucked into the edge like a black hole and disappear forever? Or does it penetrate through the edge itself? If the light penetrates through the edge then can the universe be finite?

If there are answers to these questions then maybe I could begin to except that the universe has an end or actual "edge".

 

[Avijeet]

Forgive me if this has been covered already.
If the universe has no center why do astronomers find the oldest stars at its edge?

I would like to address the idea of edge first. The present observations show that we live in an expanding universe, so as far as our idea of space is concerned, the universe has an infinite spatial extent. A boundary would have made sense in a closed universe. So there is no edge of the universe.

The relevant question to ask is why oldest stars are found at the farthest distances. The answer as explained in earlier posts is that light has a finite speed, so farther the object the more time it takes to light from from it surface to reach us. The farthest stars are thus the oldest.

It seems to me that they should be everywhere.

It is indeed correct that stars that were in the early stages of universe would be distributed everywhere. The point is if a star is very far away, the fact that we see means it is very old. The light from the young stars from far away distances is yet to reach us!

 

[Avijeet]

I'm curious about the idea of a finite universe. What happens to the light emitted from a star lying at the very edge, or end, of the universe?

If the universe was finite (closed), it would mean having a positive curvature, something like a sphere. Just as you can keep walking on the surface of a sphere and never face a boundary, the same will happen to light rays. They can travel round the universe and shine on your back!

 

[lastdays33]

If the universe was finite (closed), it would mean having a positive curvature, something like a sphere. Just as you can keep walking on the surface of a sphere and never face a boundary, the same will happen to light rays. They can travel round the universe and shine on your back!

Ahh...that makes some sense. But then my question is what is the very edge of the universe made of? For light to bounce off and reflect back at us it must have some kind of material substance; i.e plasma, thick gas of some nature, or hard elements like iron. But for the universe to be finite the "edge" must be made of a material that doesn't allow for the penetration of any light particles or waves at all. If any particles or waves of light escaped the "edge" of the universe than that wouldn't allow for that to be the actual edge of the universe as it would continue as long as that wave or particle of light continued.

 

[marcus]

If the universe was finite (closed), it would mean having a positive curvature, something like a sphere. Just as you can keep walking on the surface of a sphere and never face a boundary, the same will happen to light rays. They can travel round the universe and shine on your back!

Ahh...that makes some sense. But then my question is what is the very edge of the universe made of? ...

Lastdays, in the concept Avijeet presented to you, there is no "edge".

So it does not make sense for you to ask what the "edge" might be made of.

I assume you know that space can be curved and lightrays follow the curvature. This was actually observed to happen as early as 1919 (the British astronomer Eddington's team of observers).

The surface of an ordinary sphere (the analogy Avijeet spoke of) is 2D. A 2D animal living in such a 2D space could only point in directions in his world, not in or out of the sphere. A ray of light that he shone pointing north would come all the way around and hit him from the south. Light follows space, whatever curvature it has.

That ordinary sphere picture is merely an analogy to the 3D concept of a space without any "edge" or boundary of any kind. Avijeet was trying to communicate that concept to you.

A ring is a finite one-dimensional space with no endpoints (no boundary).
A spherical surface is a finite two-dimensional space with no edge (no boundary).
The hypersphere we are talking about is a three-dimensional analog of that. Finite and with no boundary either. A 3D creature like us living here cannot point his finger in any direction that is out of the hypersphere.

It is easy to handle mathematically, and one of the most usual concepts of 3D space that astronomers (specifically cosmologists) use. The other very common one is infinite 3D space, which again has no boundary.

The OBSERVED portion of the universe is something else. It is that region from which light has been able to reach us. It has no hard physical limit. The part we can observe grows as more light has had time to come in. So don't confuse the currently observable portion with the whole thing.

The whole thing could be infinite, or it could be a finite 3D volume like a hypersphere surface, with no boundary. Data is fitted to either case. We don't know yet which will turn out best to assume. The estimates of the minimum size the circumference could be, in the case it is finite, have tended to increase over the years as more data comes in. It is under investigation so have to be patient on that one.

 

[lastdays33]

Thanks Astro/Cosmo. That helps to give me a better picture in my mind. Outside of the desire to think on the deeper questions in life I have no formal education in these matters so if I sound slightly foolish, its because I just might be.

However, I do understand about light being able to follow curvature. The question I have is with the example you used concerning the British astronomers. Did they find that light curved because of gravity or was there another reason for it to curve. What I mean is this: From what I understand gravity is actually a big blanket stretched taught. As objects of mass are placed on that blanket they create impressions by their mass pressing down onto the blanket. If you rolled smaller object onto the blanket near a larger object, the larger object's "impression" will draw the smaller object with less mass toward it causing it to spin around the larger object until it's finally drawn into it, like our moon to our earth. The question is, did the astronomers find that light curves around objects in space, merely following that objects gravitational "impression" or did they find that light bends on its own without any force acting upon it?

If the density of the universe is more than the "critical density" then it would be a closed 3D, spherical, finite universe that will eventually close in on itself, the "Big Crush". Would it be the gravity, caused by the density of the universe, that creates a curve and thus trapping light into its gravitational pull, therefor not allowing the light to travel past the limits of universe's gravity and then bounce of its "edges" throughout the sphere? It seems that if that is the case then the universe actually acts as some sort of a black whole, trapping light inside itself and never allowing light to escape.

If this sounds foolish, again, thanks for bearing with me. If it is gravity that causes light to bend, outside of a legitimate black hole, light does not get sucked into a planet or comet or galaxy's gravity. It merely follows the curve that the object's mass creates on the "blanket" of gravity and continues on past it allowing us to see the light of stars that are either behind or partially hidden by other objects in space. If it's the gravity of the universe causing it to bend in on itself and create a sphere, light shouldn't be trapped by it but rather just go around it and continue on in 3D linear paths of waves and particles.

 

[lastdays33]

Thanks Marcus...not astro/cosmo :)

 

[marcus]

If the density of the universe is more than the "critical density" then it would be a closed 3D, spherical, finite universe that will eventually close in on itself, the "Big Crush".

Slight correction here. If density more than critical then presentday space is finite volume BUT it does not necessarily lead to Big Crunch . That was what people said before 1998, but after 1998 they realized that expansion of finite volume could continue forever. So "closed" does not mean "going to eventually crunch". It used to mean that.

People do gradually learn stuff. Correct mistakes etc.

Would it be the gravity, caused by the density of the universe, that creates a curve and thus trapping light into its gravitational pull, therefor not allowing the light to travel past the limits of universe's gravity and then bounce of its "edges" throughout the sphere? It seems that if that is the case then the universe actually acts as some sort of a black hole, trapping light inside itself and never allowing light to escape.

This is has elements of rightness. But don't make the mistake of picturing "edges".

In standard cosmology, the universe is ALL THE SPACE THERE IS. There is no outside.

So it is different from a black hole----a black hole has an outside. Light can pass by a black hole, get bent some, and continue on, remaining forever outside. Only if it gets within a certain distance will it be "sucked in".

The picture is very different with the universe because THERE IS NO SPACE OUTSIDE THE U and no "edge" or boundary, and NOWHERE FOR LIGHT TO GO. So there doesn't have to be anything to "reflect it back".

(There are some fancy theories involving multiple expansion processes and "domains" and "domain walls" but they are speculative, not based on observational evidence, so tend to be ignored by working cosmologists. We keep it simple and focus on standard cosmology here. So far the observational data is nicely consistent with there being just one universe, one set of physical laws, one expansion process, one start of expansion.)

If it is gravity that causes light to bend, outside of a legitimate black hole, light does not get sucked into a planet or comet or galaxy's gravity. It merely follows the curve that the object's mass creates on the "blanket" of gravity and continues on past it allowing us to see the light of stars that are either behind or partially hidden by other objects in space. If it's the gravity of the universe causing it to bend in on itself and create a sphere, light shouldn't be trapped by it but rather just go around it and continue on in 3D linear paths of waves and particles.

That all sounds right, except that a black hole (as long as the light doesn't get too close) affects light the same way as other massive objects. And remember there is always SOME curvature to space so that light NEVER goes in exactly linear paths. The thing about a closed (finite volume) U is that there is just enough curvature so that (if you could stop expansion and give it time) the light would circle around.

You may find this disappointing, if you like the idea of light circumnavigating the 3D hyperspbhere of space: because of expansion light can never make it all the way around. As far as we know.

To our best knowledge expansion will continue and is rapid enough that if you shone a flashlight off in one direction it would never come back from the opposite direction even given all future eternity. This is what the model that fits the data best so far says.

We live in a space that is either infinite or finite volume. If it is finite, like eg. a 3D hypersphere, then by the best observations we have it is already so big and expanding so fast that you could never circumnavigate even if you were a flash of light going at the speed of light. So in that sense (perhaps this is a little sad) it might as well be infinite.

 

[Chronos]

We reside at the temporal 'edge' of the universe - the most ancient location in all of space-time. The galaxies we observe billions of light years distant are as they appeared billions of years in our past. The individual stars that comprise these galaxies, of course, are undetectable at such vast distances - save for the occasional supernova. This, in part, is the reason astrophysicists are so keenly interested in distant supernovae. Observations of these events are critical to our understanding of the chemical evolution of the universe since the BB.

 

[lastdays33]

In standard cosmology, the universe is ALL THE SPACE THERE IS. There is no outside.

Is this an o.k. view to have: The amount of mass inside our universe is finite, limited. But that finite mass is expanding infinitely?

 

[lastdays33]

This was what I wrote before I thought of my previous post. Since I spent 3 hours thinking and rewriting it I thought I'd post it on principal alone :)

Marcus, all of this has been extremely helpful in my attempt to try to understand. I sincerely appreciate you taking your time to explain further for me!

This is my simple thinking: If the universe is finite than it has an end. If it has an end it has a limit (hypothetically). I understand what you mean that “the universe is ALL THE SPACE THERE IS. There is no outside” but you did say in “standard cosmology”. I am certainly not debating you since I am well out of my league to debate you on this matter. I am merely one who seeks answers by asking questions. But by suggesting that a finite universe exists and that all the space in a limited, finite universe is all the space that there is (there is nothing outside of it) you force me to put aside Newton’s law of motion. If there is an expanding, finite universe pushing out, there must be something on the other side of its finite limits that it’s pushing against or expanding into.

When the universe expands it creates more of itself, space. That's the picture in my mind...like a flooding river overflowing its banks. As more water is added the river gets wider and wider, consuming more land, making it part of the river. But using that analogy doesn't seem fair in that there is already existing land for the flooded river to consume. But what would the expanding, finite universe consume so that it creates more space? Is it like the land that was dry but is now flooded and consumed by the river? Or does it create space out of itself? If you blow into a balloon it gets bigger and its surface area increases but you are putting more mass into the balloon to make it bigger. Is the universe creating more mass in itself to expand or is the already existing mass simply expanding outwardly? If it’s creating more mass what is creating it and where is that coming from? And in the balloon example, as the surface area increases it is taking up air space and decreasing the limited, finite air space under our atmosphere; Newton’s law of motion: the opposite reaction to the increasing surface space is the decrease in air space. Please, these are not points that I am trying to make but rather legitimate questions.

Also, if the universe is finite then it must have a hypothetical end, or death. I was thinking of finite things, what happens when they reach their limit? For example, grass. Grass is finite; it lives than it dies. Grass has no afterlife (if a person believes in an afterlife) so does that mean that the grass no longer exists after it dies (I understand that I’m being philosophical here rather than empirical)? It lived within all of its existence yet when it died it still existed, and served another purpose; fertilizer. The grass didn't just disappear from existence, though its life did.

Obviously we will most likely not see the end of the universe. But if everything that is known to man ceases to exist as it is known, it actually still exists elementally in another form; like the grass no longer being just grass but dead grass or fertilizer. The same can be said for anything really; humans, cars, volcanoes, stars, planets. Once they stop existing as they were known, their elements remain, only their form changes. The universe seems to be the only thing that man knows that when it ceases to exist (hypothetically), it just disappears from existence all together, that is if it is finite.

Much like the question, “What mass or energy existed to create the big bang from?” (I believe there was some sort of big bang, in essence) the same question lies in its end or its death. What does the universe become if it is finite and it ends? Something, when it ends, always becomes something else. So, if the universe is finite and it ends, what does it then become? Using the grass analogy again, life, when it dies, becomes seed or feed for something else, becoming life again. Does the universe violate its own laws upon its hypothetical end/death and not become anything?

It might sound that I'm a proponent of an infinite universe (which I am) but in this case I am not making any attempt to argue the point of an infinite universe. I'm merely trying to understand what a finite universe might be. We don't know one way or the other whether it is finite or infinite so to say one is right or wrong is inappropriate. And as you said early, it's expanding so fast and far it might as well be infinite. But if the universe is finite and has a hypothetical end, according to the laws of motion and physics, it must continue in some other form. Or, it utterly annihilates those laws all together in its death and becomes nothing. This seems unreasonable since we exist with the universe itself and follows its laws and rules. To supersede its own rules in its end, or its death seems to allow for even greater questions about whether it is finite or infinite. It will lead to ask whether it is alive, cognitive and able to make a choice to overrule its very own rules. Or, was it programmed to do so by another force.

I asked a lot of questions here and completely don't expect you to answer them all, if any. My side of the conversation turned more philosophical than cosmological. However, I think the philosophy of the cosmos is just as important as the empirical study of it. It is so much easier for me to think of a universe that is infinite. If it is it has no end. If it has no end it has no limits. To me that would explain so much more in life. Just for example, an atom. We continue to break an atom down to smaller and smaller pieces like quarks. If we could take an up quark and split it I’m sure we’d find new particles inside of that. And if we took that new particle and split it, we’d find new particles inside ad infinitum. There was a time in man’s history when we didn’t know atoms even existed. So like the ability to split an atom infinitely (my theory), that explains the, to me, the infinity of the universe. If the universe is infinite, atoms came from the creation of the universe, than atoms are infinite.

However, it excites me and I enjoy thinking about the questions posed by a finite universe, especially if “standard” cosmology thinks along these lines.

 

[marcus]

Lastdays,
it sounds like your latest post is this one #20

In standard cosmology, the universe is ALL THE SPACE THERE IS. There is no outside.

Is this an o.k. view to have: The amount of mass inside our universe is finite, limited. But that finite mass is expanding infinitely?

and this post #21 is one you wrote earlier and decided to post on principle, (or one might say for the record).

This was what I wrote before I thought of my previous post. Since I spent 3 hours thinking and rewriting it I thought I'd post it on principal alone :)

... If the universe is finite than it has an end. If it has an end it has a limit (hypothetically). I understand what you mean that “the universe is ALL THE SPACE THERE IS. There is no outside” but you did say in “standard cosmology”...
...
Also, if the universe is finite then it must have a hypothetical end, or death...
...
Much like the question, “What mass or energy existed to create the big bang from?” (I believe there was some sort of big bang, in essence) the same question lies in its end or its death...
It might sound that I'm a proponent of an infinite universe (which I am) but in this case I am not making any attempt to argue the point of an infinite universe. I'm merely trying to understand what a finite universe might be. We don't know one way or the other whether it is finite or infinite so to say one is right or wrong is inappropriate. ...

Both posts are useful, I think.
Your #20 is an advance over #21 because it seems to include the idea that the universe COULD be spatially finite and temporally infinite.

The amount of mass inside our universe is finite, limited. But that finite mass is expanding infinitely

Our basic premise (the Cosmological Principle) is that the U looks approximately the same from every point in space. I.e. roughly uniform, space and matter coextensive, so big empty space "outside" of where the matter is, to put it crudely. That's a working postulate for Cosmo as a professional field---fits the data and the math models of gravity we have so far.

So from your blue statement we can conclude also that if you measure space by volume, which seems reasonable enough
The amount of space, the spatial volume of our universe is finite, limited. But that finite volume is expanding infinitely.

That is certainly an OK statement. As you suggest, we don't have enough data to favor spatial infinite over finite or to prefer finite over infinite. We can't say. We have to keep the options open by modeling both ways and fitting the data to either type model.

That red statement, which (because of the basic working postulate of Cosmo) follows from the blue, is certainly an OK way to imagine the world and it is the kind of thing the balloon model can be used to illustrate.

I hope you have watched, or will watch, the short animation at Ned Wright's website. A 2D toy model of one possible way the U could be.

(The main alternative, infinite amount of matter spread out evenly in infinite geometry, expanding forever, is harder to depict, even though it could be the case.)