Universe - Size and Age

[Mowgli]

I was reading http://www.space.com/scienceastronomy/distant_galaxy_040216.html stating that they found a galaxy at about 13 billion light years away. I am trying to figure out what that statement means. To me, it means that 13 billion years ago, this galaxy was where we see it now. Isn't that what 13b LY away means? If so, wouldn't that mean that the universe has to be at least 26 billion years old? I mean, the whole universe started from one singular point; how could this galaxy be where it was 13 billion years ago unless it had at least 13 billion years to get there? (Ignoring the inflationary phase for the moment...)

 

I have heard people explain that the space itself is expanding. What the heck does that mean? Isn't it just a fancier way of saying that the speed of light was smaller some time ago?

[Klaynos]

From what I remember from hearing similar conversations with people and astrophycists.

 

No, because space is expanding then there is no requirement for the distance to be directly related to the time.

 

The speed of light changing is not a requirement of space expanding, and whether the speed has changed is under alot of debate at the moment.

 

Space expanding means that not only is the universe getting bigger, but the space between everything in the universe it getting bigger too... It's a concept that often causes people problems the same as "no there is no centre of the universe, because everywhere was the singularity where it all began"

[swansont]

I mean, the whole universe started from one singular point; how could this galaxy be where it was 13 billion years ago unless it had at least 13 billion years to get there? (Ignoring the inflationary phase for the moment...)

 

Ignoring all the rest, how would this mean the universe is 26 billion years old?

 

I have heard people explain that the space itself is expanding. What the heck does that mean? Isn't it just a fancier way of saying that the speed of light was smaller some time ago?

 

The speed of light is an inherent part of atomic structure, in the fine structure constant (alpha). If c was changing, then the patterns of atomic spectra would have to change. There hasn't been any confirmed data that shows that alpha has changed (there has been the occasional paper claiming it, but you need someone to repeat the measurements), and the rest is all consistent with no change.

[Martin]

to confirm or reinforce what swansont said

 

there are speculation and some fringe or nonstandard cosmologies that involve c changing over time (or alpha changing over time)

but the changing constants thing just gets more and more ruled out.

 

I've been watching for over 5 years and the more people look and study evidence the LESS likely it seems that there is any change. they rule it out more and more accurately with their data.

 

so it is probably best to ignore the "varying speed of light" cosmologies until one is thoroughly familiar with standard mainstream cosmology

================

 

you have misconceptions Mowgli

 

General Relativity (the 1915 theory) trumps Special Rel (1905)

 

they dont actually contradict if you understand them correctly, because SR has only a very limited local applicability, like to the spaceship passing by:-)

 

wherever GR and SR SEEM to contradict, believe GR. It is the more comprehensive theory.

 

GR does not have a speed limit on the rate that very great distances can increase. the only speed limit is on LOCAL stuff (you can't catch up with and pass a photon)

 

So we can and DO observe stuff that is receding from us faster than c. (It's far away, SR does not apply.)

 

this was explained in a Sci Am article I think last year

google the author's name Charles Lineweaver and Tamara Davis.

 

we know about plenty of stuff that is presently more than 14 billion LY away.

 

You need to learn some cosmology so you wont be confused by these things.

 

Also a "singularity" does not mean a single point. that is a popular mistake because the words SOUND the same.

 

a singularity can occur over an entire region, even an infinite region.

 

also the "big bang" model doesnt look like an explosion of matter whizzing away from some point. It shouldn't be imagined like that. The best article explaining common mistakes people have is this Lineweaver and Davis thing in Sci Am. I think it was Jan or Feb 2005 but I could be a year off. Google it. Get it from your local library or find it online. Best advice i can give

[Mowgli]

To swansont on why I thought 13 b LY implied an age of 26 b years:

When you say that there is a galaxy at 13 b LY away, I understand it to mean that 13 billion years ago my time, the galaxy was at the point where I see it now (which is 13 b LY away from me). Knowing that everything started from the same point, it must have taken the galaxy at least 13 b years to get where it was 13 b years ago. So 13+13. I'm sure I must be wrong.

 

To Martin: You are right, I need to learn quite a bit more about cosmology. But a couple of things you mentioned surprise me -- how do we observe stuff that is receding from as FTL? I mean, wouldn't the relativistic Doppler shift formula give imaginary 1+z? And the stuff beyond 14 b LY away - are they "outside" the universe?

 

I will certainly look up and read the authors you mentioned. Thanks.

[swansont]

To swansont on why I thought 13 b LY implied an age of 26 b years:

When you say that there is a galaxy at 13 b LY away, I understand it to mean that 13 billion years ago my time, the galaxy was at the point where I see it now (which is 13 b LY away from me). Knowing that everything started from the same point, it must have taken the galaxy at least 13 b years to get where it was 13 b years ago. So 13+13. I'm sure I must be wrong.

 

That would depend on how you do your calibration. Looking only at a Doppler shift and ignoring all the other factors, if you know that speed correlates with distance, you get a certain redshift and you would probably calibrate that to mean 13b LY if that was the actual distance. That light would be 13b years old.

 

But as Martin has pointed out, space is expanding; the cosmological redshift is different from the Doppler shift. Because the intervening space has expanded, AFAIK the light that gets to us from a galaxy 13b LY away is not as old, because it was closer when the light was emitted. I would think that all of this is taken into account in the measurements, so that when a distance is given to the galaxy, it's the actual distance.

[jackson33]

Mow; in short i am in the steady state camp and feel the universe, much as we see nearby is the same where ever it may end.

 

however from the big bang camp; they explain this expansion we see, that is known in matter to that 14.2 billion year distance. that light rays received over days of lens openings into one spot and computer enhanced in the final image is from that time. whats there now, no one can guess. however according to the theory those objects moved on out and are at least near 27 billion light years out now. since the theory is a little confusing on matter or its formation after a cooling process, whether matter is increasing or not is questionable. if, yes, then new matter would be forming, as it did after the BB. if not then what matter has formed is spreading.

 

expansion of space is purely in the Big Bang Theory. in SSU, the expansion of the universe itself into what should be infinity space (no limit or end), would not occur and with out cause seems unlikely. that is, no little factories out there producing new matter.

[Martin]

 

I will certainly look up and read the authors you mentioned.

 

this post has 5 or 6 links to that Sci Am article by Lineweaver and Davis

http://scienceforums.net/forum/showthread.php?p=142965#post142965

 

it is post #65 on the Astronomy links sticky thread

 

it turns out the article was in the March 2005 issue.

 

I think it's comparatively easy to read---well written. So it should help.

when you've read the Sci Am article, ask more questions---your questions might be fun to try and answer:-)

[abskebabs]

I have a query related to the subject of this thread.

Considering the fact that that the Universe is expanding at an accelerating rate(also is the the acceleration increasing too?), when working out the age of the Universe do relativistic corrections have to be made, because we have been in the process of being separated from other parts of the Universe at different rates throughout the Universe's "history"? Or is it ok because from our perspective we are in a "proper time"? Finally do the rules of GR still apply even if there is a rate of change of acceleration or a change in that etc etc etc?

[Martin]

I have a query related to the subject of this thread.

Considering the fact that that the Universe is expanding at an accelerating rate(also is the the acceleration increasing too?), when working out the age of the Universe do relativistic corrections have to be made, because we have been in the process of being separated from other parts of the Universe at different rates throughout the Universe's "history"? Or is it ok because from our perspective we are in a "proper time"? Finally do the rules of GR still apply even if there is a rate of change of acceleration or a change in that etc etc etc?

 

The rules of GR apply.

I assume you realize that some ordinary SR stuff is not applicable (SR applies in a consistent way only to local affairs---you can't catch up with and pass a photon, but distant stuff not governed by SR can be receding at 3 or 4 times the speed of light).

 

Anyway yes, the rules of GR apply.

 

You can consider the cosmogist estimate of age of universe to be PROPER TIME associated with the material that now forms our galaxy. But as a practical business I havent ever heard of cosmologists making any "relativistic corrections" to the age.

 

there is a basic model---Friedmann equations (look it up on Wiki)---and they just solve that model.

 

Cosmology today is largely data-fitting. They have huge amounts of data on 4 or 5 things (CMB, galaxy surveys, supernovae, and other stuff as well) and they ADJUST THE PARAMETERS OF THE FRIEDMANN model to get the best fit to all the data---that is how they measure the parameters like the current value of the Hubble and the density of dark energy and the overall curvature etc.

 

So when they have measured the basic parameters as well as they currently can. then they plug those parameters into the Friedmann model and crank it back to zero scalefactor---and that is the age of the universe.

 

the age is just the elapsed time in the Friedmann model (with good parameters)

[abskebabs]

The rules of GR apply.

but distant stuff not governed by SR can be receding at 3 or 4 times the speed of light

You've lost me with this... You're saying the main postulate of special relativity does not necessarily apply to things that are not "local"? Why is that? I didn't think GR refuted SR, I thought it was an extension to cover the things SR did not(like accelerating frames).

[swansont]

You've lost me with this... You're saying the main postulate of special relativity does not necessarily apply to things that are not "local"? Why is that? I didn't think GR refuted SR, I thought it was an extension to cover the things SR did not(like accelerating frames).

 

SR assumes flat spacetime. The expansion of space isn't covered.

[wilgory]

I got this from the nasa site. It seems to settle the "age" question. What do you guys think?

 

http://www.gsfc.nasa.gov/news-release/releases/2002/h02-73.htm

[timo]

You're saying the main postulate of special relativity does not necessarily apply to things that are not "local"? Why is that? I didn't think GR refuted SR, I thought it was an extension to cover the things SR did not(like accelerating frames).

Let me also add a reply here since it neatly continues what I said earlier today (http://www.scienceforums.net/forum/showpost.php?p=330894&postcount=12).

The field equations for gravity, i.e. the dependency of the spacetime geometry of the sources and the dynamics of spacetime geometry (e.g. gravitational waves), are not part of SR. The calculations about the shape/structure of the universe are nothing but calculations of spacetime geometry as a function of the particle content using some rather broad (symmetry) assumptions - the Friedman equations are already the simplified results that incorporate the symmetry assumptions and boundary conditions and only take some scalar inputs describing the sources. The calculations are GR calculations, hence there´s no need for GR corrections to the calculations.

 

As Swansont said, SR assumes a special static form of spacetime and a special form of coordinates to describe this spacetime. The term "local" in physics refers to a sufficiently small area around a point of interest - the size of that area generally depends on situation and tolerable error (similar to an "epsilon-delta enviroment", in case you ever heard that term in maths). In the GR case, for a sufficiently small area of spacetime (generally, "area" also refers to a sufficiently small time interval, not only spacial distances) you can often assume a flat gemometry and use the common SR coordinates. This not only rules out any curvature effects, but also any effects due to dynamics of spacetime (due to the limited time interval). A classical example of how a geometry can locally be approximated with a seemingly completely different geometry is earth. Assuming it was a sphere, you still can (and usually do) approximate it as being flat locally, like on a city map.

 

Distant galaxies moving away with speeds (note that I explicitely did not use the term "velocity") so large that a light-ray could never reach them are an effect of spacetime dynamics (the expansion) and therefore not covered by SR - they are also very far away so you wouldn´t necessarily expect the locality assumption to be valid anyways.

[abskebabs]

Let me also add a reply here since it neatly continues what I said earlier today (http://www.scienceforums.net/forum/showpost.php?p=330894&postcount=12).

The field equations for gravity, i.e. the dependency of the spacetime geometry of the sources and the dynamics of spacetime geometry (e.g. gravitational waves), are not part of SR. The calculations about the shape/structure of the universe are nothing but calculations of spacetime geometry as a function of the particle content using some rather broad (symmetry) assumptions - the Friedman equations are already the simplified results that incorporate the symmetry assumptions and boundary conditions and only take some scalar inputs describing the sources. The calculations are GR calculations, hence there´s no need for GR corrections to the calculations.

 

As Swansont said, SR assumes a special static form of spacetime and a special form of coordinates to describe this spacetime. The term "local" in physics refers to a sufficiently small area around a point of interest - the size of that area generally depends on situation and tolerable error (similar to an "epsilon-delta enviroment", in case you ever heard that term in maths). In the GR case, for a sufficiently small area of spacetime (generally, "area" also refers to a sufficiently small time interval, not only spacial distances) you can often assume a flat gemometry and use the common SR coordinates. This not only rules out any curvature effects, but also any effects due to dynamics of spacetime (due to the limited time interval). A classical example of how a geometry can locally be approximated with a seemingly completely different geometry is earth. Assuming it was a sphere, you still can (and usually do) approximate it as being flat locally, like on a city map.

 

Distant galaxies moving away with speeds (note that I explicitely did not use the term "velocity") so large that a light-ray could never reach them are an effect of spacetime dynamics (the expansion) and therefore not covered by SR - they are also very far away so you wouldn´t necessarily expect the locality assumption to be valid anyways.

 

Thanks for your reply. Once again I think I am starting to understand this, just like I did with your previous explanation. I am guessing now that when you allow for a dynamical spacetime then things like a light ray not catching up with a galaxy could occur, am I right in such an ssumption? Also when we speak of "dynamic spacetime" does that basically mean not "flat" everywhere, as you said is the case everywhere, or do you also mean changing with respect to time(man, this gets puzzling sometimes!)? It may take me a lttle time t grapple with the ideas, but the learning process is simmering away regardless...

 

I guess that is what this forum fulfills its task for me at the moment is that I can come with conceptual questions from my field of study and yield useful answers to them. I thank you for partaking in this continuing endeavour.(sorry if that was a bit melodramatic:embarass: )

[timo]

I am guessing now that when you allow for a dynamical spacetime then things like a light ray not catching up with a galaxy could occur, am I right in such an ssumption?

Yes. In fact in the simplemost models (not sure about more sophisticated models), galaxies are not assumed to move at all. They stay in their positions but distances between them increase due to the changing geometry. The common example is a balloon on which you mark two points. When you blow up the balloon, the two points stay in place but the distance between them increases because the geometry of the balloon changes (the radius increases). In SR you´d have the points on a piece of paper.

 

 

Also when we speak of "dynamic spacetime" does that basically mean not "flat" everywhere, as you said is the case everywhere, or do you also mean changing with respect to time(man, this gets puzzling sometimes!)?

I meant changing over time hoping that no one examines the statement too closely. The statement "dynamic spacetime" in that sense does of course have a catch if taken too seriously: If time is already part of spacetime in whatever sense, who can it change with time? I cannot answer the question right now, I assume that saying spacetime was dynamic in the sense of "it changes with time" is simply not correct. You might be able use "dynamic" in the sense of "changes with respect to particle content".

 

In the case of cosmology, however, there is something similar to saying "spacetime changes over time". In the coordiante system used in cosmology you can tell space from time. There, you can tell how space changes with time. In above analogon: You can tell how the radius of the balloon changes with time.

[foodchain]

Yes. In fact in the simplemost models (not sure about more sophisticated models), galaxies are not assumed to move at all. They stay in their positions but distances between them increase due to the changing geometry. The common example is a balloon on which you mark two points. When you blow up the balloon, the two points stay in place but the distance between them increases because the geometry of the balloon changes (the radius increases). In SR you´d have the points on a piece of paper.

 

 

 

I meant changing over time hoping that no one examines the statement too closely. The statement "dynamic spacetime" in that sense does of course have a catch if taken too seriously: If time is already part of spacetime in whatever sense, who can it change with time? I cannot answer the question right now, I assume that saying spacetime was dynamic in the sense of "it changes with time" is simply not correct. You might be able use "dynamic" in the sense of "changes with respect to particle content".

 

In the case of cosmology, however, there is something similar to saying "spacetime changes over time". In the coordiante system used in cosmology you can tell space from time. There, you can tell how space changes with time. In above analogon: You can tell how the radius of the balloon changes with time.

 

 

Not to cut into something ongoing, just a quick question. So the physical phenomena of a body, say a galaxy in reference to say how light behaves, back the physical phenomena again, is what is used as a gauge or a reference to size of that body or its influence?

 

I am still a bit puzzled on it though. I get this from the idea that in math, many things can come to exist that more often then not through research and experimentation come to exist, is this surely so with what you are talking about? I mean down to the smallest levels of particles and so on, I just don’t know if such has been factored into the equations or even if that is necessary, yes its a bit of a particle bias but I do tend to hold one, even while not really knowing what I am talking about half the time.

[timo]

Sry Foodchain but I don´t understand anything of what you wrote, not even the grammatical structure of your sentences. Can you rephrase your question?