Expanding universe over the speed of light?

[Sporally]

I have a question on the laws of relativity i'm not entirely sure about...

Is the expansion of the universe dependent on relativity? If no, does that mean you can observe galaxies far away moving faster than c or shouldn't the speed of light be accounted for in Hubble's law?

[Ken G]

Your question is a common one, and it stems from confusing special relativity and general relativity. Special relativity applies in what is known as "flat" spacetime, which involves no gravity (or weak gravity for the purposes at hand), and cannot involve any "expansion of the universe." Expansion of the universe is a feature of certain solutions of general relativity, which involve a curved spacetime (so have an important gravitational effect). You can think of it using the picture that "space itself is expanding." The rule that says matter cannot be accelerated past c is a rule from special relativity, which really means that you can't have two objects that pass each other in the same place and time with a relative speed greater than c (unless the matter has some unusual history, like always going faster than c, but the existence of any matter like that is pure speculation and we might just ignore it).

However, "speed" in cosmology is not the speed of one object passing another, it is merely the rate of change of distance of two widely separated objects. That is actually a matter of the coordinates used to talk about distances and times, as there is not a single observer at the same place and time as the two objects being so compared. General relativity allows for many types of coordinates, and the standard ones that are used for cosmology allow distances between two widely separated objects to increase at a rate faster than c, without violating any of the rules of relativity. Indeed, if "space itself" can be viewed as expanding at a rate that does not change from place to place in the universe (that's part of the "cosmological principle"), then you can get arbitrarily large rates of increase of distance by considering objects arbitrarily far away.

What's more, we actually see light coming from objects whose distance from us, in the standard coordinates, has always been increasing at a rate faster than c. At first that sounds impossible, but it works out to be possible, and indeed you could get the same effect on a rubber sheet with ants crawling across it-- you could pull the ends of the sheet away from each other at a speed faster than the speed that ants can crawl, yet the ants could still manage to get from one side of the sheet to the other (this might be hard to try in practice, but the math is straightforward if you can do calculus).

[kevin1981]

I was about to right EXactly what Ken said :D

[Sporally]

Nothing worse than hitting the reply button and start writing when someone else has already started writing. Happened to me several times aswell ;)

Never studied general relativity and this is what's seems to be the problem. So observing a galaxy far far away (not talking Star Wars :-D ) would you see time going backwards? I guess not since light has to travel the same distance in the 'flat space' while the universe is expanding. What i'm saying is that if you see two galaxies at different distances, the furthers away is seen at a lot earlier time than the second one due to the speed of light (before the light reaches us). But when the universe is expanding i don't think we can say that the changing distances causes the galaxies life to be shown going backwards, right?

[Ken G]

What i'm saying is that if you see two galaxies at different distances, the furthers away is seen at a lot earlier time than the second one due to the speed of light (before the light reaches us). But when the universe is expanding i don't think we can say that the changing distances causes the galaxies life to be shown going backwards, right?Right. This is where distance gets tricky. Let's imagine for simplicity that all galaxies began at the same very early time (that's not true, they formed over a wide range of time), so that when you see a galaxy when it is young, it means you see light that was emitted a long time ago. Then the youngest galaxies you see emitted their light relatively early in the expansion of the universe, but that doesn't mean they were farther away when they emitted that light than a galaxy that you see when it is a bit older. That's because the youngest-looking galaxies we see actually had distances from us that were growing at a rate faster than c, so they were actually rather close when they emitted their light, but the light was not initially making any headway-- so might eventually pass by an older-looking galaxy that is farther away from us when it is emitting its light than the younger-looking galaxy was. From that point on, the light from the younger-looking galaxy, and the light from the older-looking galaxy, will travel together, and we see them both in our telescope. By this point, the younger-looking galaxy is really really far away, due to the continued expansion of the universe, but it was still not all that far when it emitted its light-- not as far as the older-looking galaxy was. So there is more than one kind of "distance" we can talk about in an expanding universe, and even that is all within the single standard coordinate system.

But in terms of your question, note that if we watch the same galaxy for days or years, we will be seeing later and later times at that galaxy, because the light from that galaxy can never pass by earlier light from the same galaxy. Expansion won't change that-- light does not pass light, if it trails at one point it will always trail. However, the amount of time that passes for the galaxy we are watching will be much shorter than the time we watch for-- that's due to the expansion.

[Sporally]

Thanks, i think i've gotten it all cleared out by now, or else i will get back to this thread if i have more questions in this area :)

[WayneFrancis]

I was about to right EXactly what Ken said :D

yea me too...but not as good...well then I then I guess it would have been exactly the same but its the thought that counts. :P

[Sporally]

Always :)

[kevin1981]

yea me too...but not as good...well then I then I guess it would have been exactly the same but its the thought that counts. :P

hehe, I was just being humorous as you probably know. It's just Ken G gives very insightful, clear answers when it comes to physics so i thought i would make a joke :)

[Sporally]

Indeed!

[forrest noble]

Sporally,


Expanding universe over the speed of light?

I have a question on the laws of relativity i'm not entirely sure about...
Is the expansion of the universe dependent on relativity?

In the expanding universe model, a part of the Big Bang model, accordingly space itself is expanding rather than any real motions of galaxies away from each other as a whole. Einsteins Cosmological Equations which model the universe according to the present standard interpretation, were not concerned with the expansion of the universe since at the time of his proposal such expansion was generally not considered since our Milky Way was the only known universe at the time. In his initial proposed equations, Einstein (which he later took out) had what has been called a cosmological constant, its purpose being a counteraction to gravity pulling things together so that there could be no big crunch. So the answer to your question is no, the expansion of the universe is not dependent on the equations of General Relativity or Special Relativity.


If no, does that mean you can observe galaxies far away moving faster than c or shouldn't the speed of light be accounted for in Hubble's law?


Relative to us, the EM radiation from galaxies moving away from us at twice the speed of light at the time the radiation was emitted, accordingly cannot be seen by us because such radiation would have to be moving faster than the speed of light to reach us. We can observe galaxies that are presently, according to the present model, moving away from us faster than the speed of light because at the time this light was emitted these galaxies were moving away from us at relative speeds of less than the speed of light. The Hubble formula/ "law" allows our looking back to the beginnings of the universe according to the Big Bang model. Also it is generally accepted that there will always be vast quantities (or almost unlimited quantities) of galaxies that will seemingly always remain outside of our observable universe.

[melech]

Isn't this quite contrary to what Ken G is saying in post 2, in which he says that "... we actually see light coming to us from objects whose distance from us, in the standard coordinates, has always been increasing at a rate greater than c"?

[Sporally]

In his initial proposed equations, Einstein (which he later took out) ...
"My greatest error", or something along those lines. Yes, i remember that.


Relative to us, the EM radiation from galaxies moving away from us at the speed of light at the time the radiation was emitted, accordingly cannot be seen by us because such radiation would have to be moving faster than the speed of light to reach us.
Does such galaxies really exist? I mean, after BB those whole universe was smaller than anything imagineable, however, the inflation didn't begin until a few seconds after the birth of the universe, AFAIK, making it easy for light to travel the entire universe - shouldn't that acommodate for 'not being able to see the fartest galaxies', but instead have the light streched amazingly out?

[speedfreek]

We can observe galaxies that are presently, according to the present model, moving away from us faster than the speed of light because at the time this light was emitted these galaxies were moving away from us at relative speeds of less than the speed of light.

As melech pointed out, this is contrary to Ken's post. Using the standard coordinates, we can see galaxies that have always been receding faster than c.

And this doesn't just apply to a few isolated examples, it applies to all galaxies whose light was emitted more than 9.1 billion years ago, which represents most of the comoving volume of the observable universe!

[caveman1917]

I mean, after BB those whole universe was smaller than anything imagineable, however, the inflation didn't begin until a few seconds after the birth of the universe,

It was about 10-36 seconds i think, until 10-32 seconds.


AFAIK, making it easy for light to travel the entire universe - shouldn't that acommodate for 'not being able to see the fartest galaxies', but instead have the light streched amazingly out?

There was no light back then, inflation started at the symmetry breaking of the strong and electroweak forces. The only particles up until then would have been higgs bosons. It is only after inflation ended that photons would form.

[speedfreek]

Does such galaxies really exist? I mean, after BB those whole universe was smaller than anything imagineable, however, the inflation didn't begin until a few seconds after the birth of the universe, AFAIK, making it easy for light to travel the entire universe - shouldn't that acommodate for 'not being able to see the fartest galaxies', but instead have the light streched amazingly out?

The inflationary epoch lasted only the tiniest fraction of the first instant after the BB, and a whole lot more happened after inflation before even the first second was up.

But in this discussion we can ignore any of the universe that was inflated beyond our horizon during that first fleeting fraction of a second, for here we are dealing with what is known as the observable universe, which is defined by what we have seen.

One way to understand how we can see galaxies that have always been apparently receding faster than light is to turn the situation around and look at it from the point of view of one of those distant galaxies. So now we have the distant Milky-Way, receding faster than light! Can the light from our galaxy reach the Milky-Way if it is receding faster than light? One might think not, but at least the light should be able to reach closer galaxies, which are receding at less than c. Then consider the scenario from the point of view of those galaxies as that light passes them on its journey towards us - when the light passes a galaxy that is receding from here at less than c it is only a matter of time until it gets here!

[Ken G]

Yes, forrest noble was mistaken. At least it was an educational error-- let all our mistakes be such! Incidentally, this might be a good time to mention that cosmological redshifts don't depend on the rate that the distance was increasing when the light was emitted, they depend on the total amount of expansion that has happened since the light was emitted, which is something quite different. That's why in cosmology, one really needs general relativity, not special relativity (where the relative speed from an inertial observer when the light is emitted is all the inertial observer would need to know, and nothing that the source did subsequently would matter).

[Webbo]

The inflationary epoch lasted only the tiniest fraction of the first instant after the BB, and a whole lot more happened after inflation before even the first second was up.

But in this discussion we can ignore any of the universe that was inflated beyond our horizon during that first fleeting fraction of a second, for here we are dealing with what is known as the observable universe, which is defined by what we have seen.

One way to understand how we can see galaxies that have always been apparently receding faster than light is to turn the situation around and look at it from the point of view of one of those distant galaxies. So now we have the distant Milky-Way, receding faster than light! Can the light from our galaxy reach the Milky-Way if it is receding faster than light? One might think not, but at least the light should be able to reach closer galaxies, which are receding at less than c. Then consider the scenario from the point of view of those galaxies as that light passes them on its journey towards us - when the light passes a galaxy that is receding from here at less than c it is only a matter of time until it gets here!

Not necessarily because the universe wouldn't be stationary in the meantime. When, as you say, the light reaches the intermediary closer galaxies, the expansion has exponetially increased the distance during transit so the original galaxies you were approaching are even further away than they were when the light started its journey. They must be further than the distance the light has travelled because they were receding faster than light at the begining of the journey, so they cannot possibly be approached. So each step takes longer than the last with the ultimate goal receding to infinity. There will be an event horizon that cannot be reached.

[Webbo]

For example;

Say that you are a light particle and you wish to travel to an object 100,000 ly away and local expansion (accross 1 ly) is 0.1% of light speed. Locally you are therefore travelling at 1000 times the expansion rate. However, after 1 year you travel 1 ly but the remaining distance is now 99,999 x 1.001 = 100,099. After 2 years the remaining distance is 100,098 x 1.001 = 100,198.1. After 3 years its 100,197.1 x 1.001 = 100,297.3 and so on. Not only is the distance increasing, it's accelerating away.

[Ken G]

No, calculus is the tool for doing that calculation, and it reveals that when the expansion has any two points separating at a fixed rate, then it makes no difference what that rate is, light will eventually traverse it. Of course, the actual expansion appears to be speeding up (perhaps due to a cosmological constant), so there is in fact a "horizon" beyond which we will never see. Even so, speedfreek's point holds about what galaxies we can see now, which have always been separating from us faster than c, because I expect his answer includes the acceleration of the expansion.

[forrest noble]

Melch, Speedfreek,


Isn't this quite contrary to what Ken G is saying in post 2, in which he says that "... we actually see light coming to us from objects whose distance from us, in the standard coordinates, has always been increasing at a rate greater than c"?

Sorally,


Does such galaxies really exist? I mean, after BB those whole universe was smaller than anything imagineable, however, the inflation didn't begin until a few seconds after the birth of the universe, AFAIK, making it easy for light to travel the entire universe - shouldn't that acommodate for 'not being able to see the fartest galaxies', but instead have the light streched amazingly out?

As most have heard and read, we can see, according to the present BB model, nearly 13 billion years back in time at a redshift of 7. This includes potentially all the galaxies between here and there. At that time galaxies were accordingly close together so that light coming from any existing galaxy on our half of the "divide" could still have the possibility of being observed . According to the Inflation model following the BB, energy/ matter moved away in all directions at super luminous speeds. That energy/ matter moving away from us at that time at super luminous speeds (on the other half of the divide of the BB) will seemingly forever remain unobservable.

Ken G,

correction for my quote above:
........Relative to us, the EM radiation from galaxies moving away from us at the speed of light....... should have been:
Relative to us, the EM radiation coming from galaxies moving away from us at twice the speed of light .........

[Ken G]

According to the Inflation model following the BB, energy/ matter moved away in all directions at super luminous speeds. That energy/ matter moving away from us at that time at super luminous speeds (on the other half of the divide of the BB) will accordingly forever remain unobservable.That's only true because of the acceleration of the expansion, which in some sense implies that "inflation is not completely over." Had that not been the case, as was believed only a few decades ago, your statement would still incorrect, even after amending for inflation-- we would eventually be able to see everything, even what "inflated" away from us. The "superluminous speed" is irrelevant to whether or not we get to see something (at least at some point in its history), what matters is how long we can wait, and whether or not the continued acceleration of the expansion prevents us.

[speedfreek]

There will be an event horizon that cannot be reached.

Yes, the cosmological event horizon, currently 16 billion light-years away in comoving coordinates, around 2 billion light-years beyond the Hubble Horizon. This means that due to the acceleration of the expansion we will never see an event that happens today (13.7 billion years after the Big Bang) if it happens more than 16 billion light-years away.

But the Hubble distance, where galaxies recede at c, is currently only around 14 billion years away. So, we will still, in the future, see events from galaxies that are, always have been, and always will be, receding faster than c.

[Webbo]

No, calculus is the tool for doing that calculation, and it reveals that when the expansion has any two points separating at a fixed rate, then it makes no difference what that rate is, light will eventually traverse it. Of course, the actual expansion appears to be speeding up (perhaps due to a cosmological constant), so there is in fact a "horizon" beyond which we will never see. Even so, speedfreek's point holds about what galaxies we can see now, which have always been separating from us faster than c, because I expect his answer includes the acceleration of the expansion.

And there's your problem right there. The expansion of two fixed points are not seperating at a fixed rate. You're forgetting that the newly created expanded space will also expand, hence, 1ly will expand to 2ly over a specific period of time, but after the same period of time, each of those ly's will expand to 2ly. Therefore after 1yr the 2 points are 2ly apart, after 2yr they are 4ly, after 3yr 8ly apart and so on.

So, even though the expansion is constant, the 2 fixed points accelerate apart.

[speedfreek]

As most have heard and read, we can see, according to the present BB model, nearly 13 billion years back in time at a redshift of 7.
Make that 13.66 billion years back in time at a redshift of z=1089.

[Ken G]

And there's your problem right there. The expansion of two fixed points are not seperating at a fixed rate. You're forgetting that the newly created expanded space will also expand, hence, 1ly will expand to 2ly over a specific period of time, but after the same period of time, each of those ly's will expand to 2ly. Therefore after 1yr the 2 points are 2ly apart, after 2yr they are 4ly, after 3yr 8ly apart and so on.No, certainly not.


So, even though the expansion is constant, the 2 fixed points accelerate apart.If that were true, it would not have taken detailed observations of type Ia supernova, primarily over the last decade or so, to detect the acceleration of the expansion. But it isn't true, you are describing the situation in the speculated inflationary epoch, or how a dark-energy-dominated future of our universe might be. But it is not the right way to analyze the observable history of our own universe, which has been much closer to a constant expansion rate-- meaning that two points have had their separation increasing at a nearly constant rate, and in many cases that rate is > c, and we still observe it (as per speedfreek's remarks).

[Webbo]

Yes, the cosmological event horizon, currently 16 billion light-years away in comoving coordinates, around 2 billion light-years beyond the Hubble Horizon. This means that due to the acceleration of the expansion we will never see an event that happens today (13.7 billion years after the Big Bang) if it happens more than 16 billion light-years away.

But the Hubble distance, where galaxies recede at c, is currently only around 14 billion years away. So, we will still, in the future, see events from galaxies that are, always have been, and always will be, receding faster than c.

Incorrect, once an object is receding faster than c, new light from that object cannot reach us because of the cumulaive effect of the expansion, as per my previous examples.

[Webbo]

No, certainly not.
If that were true, it would not have taken detailed observations of type Ia supernova, primarily over the last decade or so, to detect the acceleration of the expansion. But it isn't true, you are describing the situation in the speculated inflationary epoch, or how a dark-energy-dominated future of our universe might be. But it is not the right way to analyze the observable history of our own universe, which has been much closer to a constant expansion rate-- meaning that two points have had their separation increasing at a nearly constant rate, and in many cases that rate is > c, and we still observe it (as per speedfreek's remarks).

The light we see now was omitted long before the event horizon was reached.

Consider this;

Two objects are 1,000,000 ly apart and space expands between them ata constant rate over a certain period of time to seperate them by 2,000,000 ly.

After the same period of time passes again what distance apart would you place them?

[speedfreek]

To clarify the picture I will put in some figures using the standard coordinates.

To put this into some sort of context:

We have currently seen events that occurred at a cosmological proper distance of only around 5.8 billion light-years away, 9.1 billion years ago. This is the Hubble distance as we currently see it, rather than where we think it is now.

Any light reaching us that has been travelling for less than 9.1 billion years was originally emitted inside our Hubble sphere, at a distance closer than 5.8 Gly.

Any light reaching us that has been travelling for more than 9.1 billion years was originally emitted outside our Hubble sphere, but when the universe was smaller, so it was still emitted at a distance closer than 5.8 Gly.

The galaxies that were 5.8 Gly away, 9.1 Gyr ago, are now thought to be around 14 Gly away, but in the future we will see events from galaxies currently 16 Gly away.

So, depending on how you look at it, we have a whole lot more to see yet, and a lot it has always been apparently receding faster than c.

I would recommend this article as a good introduction to the subject.
http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf (direct link to pdf file)

And here is the paper this article was based on.
http://arxiv.org/abs/astro-ph/0310808

[speedfreek]

Incorrect, once an object is receding faster than c, new light from that object cannot reach us because of the cumulaive effect of the expansion, as per my previous examples.

Please read the article and paper I posted above.

[Webbo]

Please read the article and paper I posted above.

I will. In the meantime point out why my examples are wrong if that's your position. Or answer my question in #28

[Ken G]

Two objects are 1,000,000 ly apart and space expands between them ata constant rate over a certain period of time to seperate them by 2,000,000 ly.

After the same period of time passes again what distance apart would you place them?You seem to think the answer is 4,000,000 ly, but that would only be true for an actual model of a universe in an inflationary era, as I said. It would certainly not be true in the observable history of our own universe, where people take great pains to observe type Ia supernova to determine the actual expansion law. What this means is, your question is ill posed-- you do not supply the necessary information to answer it, because it is quite unclear what you mean by "expands at a constant rate", or whether or not you intend that phrase to have anything to do with our own universe.

The necessary information to be relevant to our universe involves general relativity, the cosmological principle (both of which we may assume are implicit in your question, but still don't suffice because we had to strike the "at a constant rate" part because the meaning you give that phrase makes it irrelevant to the recent history of our universe), and the density of matter and energy in the universe (including dark matter and dark energy). That last bit is what all the hubbub is about. If dark energy is a cosmological constant, than in the distant future when it completely dominates the expansion, only then will the answer to your question be "4,000,000 ly." For the previous observable history, an answer much closer to correct is "3,000,000 ly", but even that is not exact, it must be corrected by the actual equations.

[Webbo]

The article posted by speedfreak says enough (my bold);


The recent discovery that the rate of cosmic expansion is accelerating makes things even more interesting. Previously, cosmologists thought that we lived in a decelerating universe and that ever more galaxies would come into view. In an accelerating universe, however, we are surrounded by a boundary beyond which occur events we will never see—a cosmic event horizon. If light from galaxies receding faster than light is to reach us, the Hubble distance has to increase, but in an accelerating universe, it stops increasing. Distant events may send out light beams aimed in our direction, but this light is trapped beyond the Hubble distance by the acceleration of the expansion. An accelerating universe, then, resembles a black hole in that it has an event horizon, an edge beyond which we cannot see. The current distance to our cosmic event horizon is 16 billion light-years, well within our observable range. Light emitted from galaxies that are now beyond the event horizon will never be able to reach us; the distance that currently corresponds to 16 billion light-years will expand too quickly. We will still be able to see events that took place in those galaxies before they crossed the horizon, but subsequent events will be forever beyond our view.

Thanks speedfreak.

[speedfreek]

Consider this;

Two objects are 1,000,000 ly apart and space expands between them ata constant rate over a certain period of time to seperate them by 2,000,000 ly.

After the same period of time passes again what distance apart would you place them?

Assuming a constant rate of expansion, 4 Gly. If the rate remained constant, then the Hubble distance would remain constant and objects could only pass out of our Hubble sphere with the expansion. But even with a constant expansion light can get anywhere, given enough time. Only with an accelerating expansion do we have a cosmological event horizon.

Consider this - when an object is "just the other side" of our Hubble horizon, it's light can easily cross into it. It's light can nearly reach all the way here, if that object adopts a highly simplified Special Relativistic view of the situation (using SR it would take infinite time to reach us, but less than infinite time to reach a place only a short distance away!). But SR is only valid locally - you can't apply it to the expansion of the universe in quite the way you might think.

Then consider that the rate of expansion was decelerating for more than half the history of the universe, so our Hubble horizon was growing, allowing light from "the other side" to pass into our Hubble sphere and onwards towards us.

As the rate of expansion more recently started to accelerate, there are indeed distant regions where any light emitted now will never reach us, but up to a certain distance we will still be able to see light emitted from those regions at some point in the past.

[speedfreek]

Thanks speedfreak.

Yup, it reflects exactly what we have been saying here. Glad to help. :)

[loglo]

Webbo
Did you miss this bit?


In the current standard model of cosmology, galaxies with a redshift of about 1.5—that is, whose light has a wavelength 150 percent longer than the laboratory reference value—are receding at the speed of light. Astronomers have observed about 1,000 galaxies with redshifts larger than 1.5. That is, they have observed about 1,000 objects receding from us faster than the speed of light. Equivalently, we are receding from those galaxies faster than the speed of light. The radiation of the cosmic microwave background has traveled even farther and has a redshift of about 1,000. When the hot plasma of the early universe emitted the radiation we now see, it was receding from our location at about 50 times the speed of light.

Do you agree we can see the CMBR? If so, then how if it was receding at 50c?

[Webbo]

Assuming a constant rate of expansion, 4 Gly. If the rate remained constant, then the Hubble distance would remain constant and objects could only pass out of our Hubble sphere with the expansion. But even with a constant expansion light can get anywhere, given enough time. Only with an accelerating expansion do we have a cosmological event horizon.

Consider this - when an object is "just the other side" of our Hubble horizon, it's light can easily cross into it. It's light can nearly reach all the way here, if that object adopts a highly simplified Special Relativistic view of the situation (using SR it would take infinite time to reach us, but less than infinite time to reach a place only a short distance away!). But SR is only valid locally - you can't apply it to the expansion of the universe in quite the way you might think.

Then consider that the rate of expansion was decelerating for more than half the history of the universe, so our Hubble horizon was growing, allowing light from "the other side" to pass into our Hubble sphere and onwards towards us.

As the rate of expansion more recently started to accelerate, there are indeed distant regions where any light emitted now will never reach us, but up to a certain distance we will still be able to see light emitted from those regions at some point in the past.

Then you are contradicting this post you made earlier?


One way to understand how we can see galaxies that have always been apparently receding faster than light is to turn the situation around and look at it from the point of view of one of those distant galaxies. So now we have the distant Milky-Way, receding faster than light! Can the light from our galaxy reach the Milky-Way if it is receding faster than light? One might think not, but at least the light should be able to reach closer galaxies, which are receding at less than c. Then consider the scenario from the point of view of those galaxies as that light passes them on its journey towards us - when the light passes a galaxy that is receding from here at less than c it is only a matter of time until it gets here!

[Webbo]

Webbo
Did you miss this bit?



Do you agree we can see the CMBR? If so, then how if it was receding at 50c?

Because you are mixing what we see now with what we assume happened in the early universe. What if the assumption is wrong.

[Webbo]

Yup, it reflects exactly what we have been saying here. Glad to help. :)

You will need to clarify what you have been saying here as in one post you say it is possible to see light from objects receding at faster than c, and in others you state that it's not.

[speedfreek]

Ok, I see what has happened here. When I gave that earlier example, I thought I explained that I had turned the picture around, to aid understanding, but it seems it didn't work! :)

I will try it a slightly different way, instead.

If an object 100,000 light-years beyond our Hubble distance (which is a concept, rather than a physical barrier) emits light towards us, we do not assume the light just travels 100,000 light-years and then gets frozen on the spot, somewhere in space. The light will propagate towards that conceptual place at c and continue past it, just as the light emitted by our own Milky-Way doesn't get stopped in space by the observational limitations of some arbitrary distant galaxy.

Here's a picture for you. Except in a static universe, space is always flowing in one direction or another in relation to the Hubble distance, and the space carries the photons along for the ride, whilst those photons always propagate through that space at c!

If the rate of expansion is decelerating, as it was for around 8 billion years or so, then our Hubble distance was also moving out, to meet that light. As the rate slowed towards a constant rate, the Hubble distance slowed towards a constant distance, but light on the other side still moved towards it at c. As the rate of expansion then started to accelerate, the Hubble distance started to move away from the light, so light within a certain distance will still be able to reach and cross it, but light beyond that certain distance will not. That distance is the cosmological event horizon.

But none of this precludes us from observing light emitted from places that have always been apparently receding faster than c, or continuing to be able to do so for the forseeable future. Even when we eventually see the events that are happening now, 16 billion light-years away, we will be seeing events happening in places that have always been apparently receding faster than c.

[speedfreek]

Because you are mixing what we see now with what we assume happened in the early universe. What if the assumption is wrong.

Well, if light takes time to travel, then what we see now is what happened in the early universe.

[loglo]

Because you are mixing what we see now with what we assume happened in the early universe. What if the assumption is wrong.

I am not mixing anything, the quote was from the article. And why do you call it an assumption and not an observation? Hubble's Law is empirical after all.

[Geo Kaplan]

You will need to clarify what you have been saying here as in one post you say it is possible to see light from objects receding at faster than c, and in others you state that it's not.

Since you seem not to understand the mainstream view very well (and we're supposed to be providing mainstream answers here), perhaps it would be helpful for you to study, e.g., Prof. Wright's pages: http://www.astro.ucla.edu/~wright/cosmology_faq.html

[Webbo]

...light within a certain distance will still be able to reach and cross it, but light beyond that certain distance will not. That distance is the cosmological event horizon.

That what I have been saying all along, but it was certainly not clear that was the view at the beginning of this thread and it's certainly not the impression you gave earlier.

[Webbo]

Well, if light takes time to travel, then what we see now is what happened in the early universe.

So where's the observation measuring radiation receding at 50c?

[Webbo]

I am not mixing anything, the quote was from the article. And why do you call it an assumption and not an observation? Hubble's Law is empirical after all.

As before, where's the observation measuring radiation receding at 50c?

[Webbo]

Since you seem not to understand the mainstream view very well (and we're supposed to be providing mainstream answers here), perhaps it would be helpful for you to study, e.g., Prof. Wright's pages: http://www.astro.ucla.edu/~wright/cosmology_faq.html

Cosmological event horizons are mainstream. That why I am posting because the impression given earlier is not correct.

[speedfreek]

That what I have been saying all along, but it was certainly not clear that was the view at the beginning of this thread and it's certainly not the impression you gave earlier.

Before, I was refuting the assertion that we can only see galaxies that are receding faster than c because they were receding slower than c when the light was emitted, which is incorrect - they were never receding slower than light. The acceleration of the rate of expansion and the resulting cosmological event horizon are not really relevant to that particular discussion.

[forrest noble]

Webbo, Sporally

Re: Is it possible to see galaxies presently receding from us faster than the speed of light?

I hope the following explanation will help give you a picture of what is involved:

If a galaxy is moving away from us at the speed of light or greater at the present moment, the redshift of its EM spectrum would be 1 or greater. This would be at a total observed wave length of 2 or greater. When the observed wavelength is 2 the redshift is 1. The redshift is designated as "Z," and the wavelength is designated as "Z+1." Also realize that, according to the present BB model, space is expanding instead of galaxies moving away from each other.

http://hyperphysics.phy-astr.gsu.edu/hbase/astro/hubble.html

Consider a particular galaxy with a redshift of one (meaning it is receding from us at the speed of light). Consider a point on a line half-way between us and the other galaxy. At that point light from both galaxies is traveling at the speed of light in opposite directions toward the other galaxy. Relative to that point each galaxy is receding at only 1/2 the speed but the light from the other galaxy at that point is moving at the speed of light. This is the reason the light will catch up to us and we can observe it at a redshift of 1. Relative to that point each galaxy is only moving at half the speed of light but the light at that point is moving twice as fast as each galaxy's recession velocity at that point.

On the other hand at a redshift of greater than 2, the light at the divide between the two galaxies will never quite catch up to us since our galaxy is receding faster than the speed of light relative to the dividing point. So the light from a galaxy (redshift greater than 2) being emitted now, including light emitted after the light at the half-way point, will not be observable to us in the future, about 8 1/4 billion years from now. But we would still be able to see the galaxy due to its light coming toward us that was past this present half-way point. It would accordingly take approximately another 3 billion years, about 11 billion years total, for the galaxy to entirely disappear from our view. This model does not consider the dark energy idea, the accelerated expansion of the universe, which could change these estimated time periods.

Bottom line is that according to the present BB model, there will come a time when seemingly all galaxies presently receding from us, will disappear from our view if "we" are still here in those times. This horizon is called The Observable Universe (sometimes called our event horizon).

Hope this explanation helps.

[speedfreek]

So where's the observation measuring radiation receding at 50c?

The observation is the CMBR and the measurement is the wavelength, relative to the predicted wavelength at the time of release, based on the current mainstream theory. It depends on a combination of how much the universe has scaled up whilst that radiation has been travelling towards us, how long it took to reach us, and how far away we think it was originally emitted from!

The end result is transformed into a picture where the particle horizon, the "emission distance" of the CMBR, the edge of our observable universe, has always been receding from us superluminally, was receding at over 50c when the CMBR was released and still recedes at over 3 times the speed of light today, if we choose to view the picture using the standard comoving coordinates where the universe is 13.7 billion years old and has a radius of ~46 billion light-years.

The event horizon currently lies 16 billion light-years away, whereas the particle horizon, the place where the CMBR we currently detect was released from if that place moves with the expansion of the universe, lies around 30 billion light-years further away and represents the edge of our observable universe (cosmological definition) at a distance of over 46 billion light-years in all directions. It represents the most distant place we have received a photon from.

[Ken G]

Cosmological event horizons are mainstream. That why I am posting because the impression given earlier is not correct.You are sowing misconceptions right and left, is what you are doing. There are two kinds of horizons that can easily be confused, and neither has any direct connection with what was separating from us at a rate faster than c when the light was emitted. The first is due to the finite age of the universe, which says that at the present time, there is a limit to how far we can see, because there just wasn't time for light to get to us from any farther. That is often called the "Hubble sphere" or some such thing, although further details are required to specify what coordinate distances are being used. Then there is a wholly different kind of horizon, which has to do with hypothetical future observations, which asks is there a limit on how far we will ever see. That only happens at all if the expansion is accelerating, which it does appear to be.

As I said above, for a constant expansion, which is generally taken to mean an expansion where the rate of separation of any two points is constant (i.e., not exponential like you describe), then there is no such horizon-- we would eventually see gas that is arbitrarily far beyond our current Hubble sphere. But since the expansion does appear to be accelerating, then there is a limit to how far we will ever see, which is pretty much what we have already seen. I repeat that neither of those horizons has any direct connection with what is separating from us at c, or 2c, or at what point in time it was separating at that rate, until you know the nature of the acceleration of the expansion and do a careful integration using calculus. That calculation is what speedfreek is referring to.

[Ken G]

If a galaxy is moving away from us at the speed of light or greater at the present moment, the redshift of its EM spectrum would be 1 or greater. No, no, and more no. That is not how cosmological redshifts work at all. Again, the redshift we would eventually receive (many billions of years from now) from a galaxy whose distance from us currently increasing at a rate equal to c cannot be calculated in any easy way. It requires determining the total amount that the universe expands between now and such a time when the signal is finally received. That will depend on what dark energy is.

[forrest noble]

Retraction of my posting #49


........When z is larger than 1 then cz is faster than the speed of light and, while recession velocities faster than light are allowed, this approximation using cz as the recession velocity of an object is no longer valid. (bold added)
This quote from the link below that such interpretations/ approximations are no longer considered valid.

http://www.astro.ucla.edu/~wright/doppler.htm
http://iopscience.iop.org/0004-637X/565/1/1/fulltext

I guess the point would then be that regardless of the presently estimated recession velocity for a particular redshift, according to the present LCDM interpretation, there will come a time in the future when each of the galaxies that are presently observable, would fall beyond our visible universe.


We calculate the extent of the observable universe and the portion of it that can be seen at different epochs.

quote from the link below

Dark Energy and the Observable Universe

cdsweb.cern.ch/record/502882/files/0105547.ps.gz

[Geo Kaplan]

I guess the main point would then be that regardless of the estimated recession velocity, accordingly there would come a time in the future when all galaxies having a continued recession velocity away from us, will fall beyond our visible universe.

http://www.astro.ucla.edu/~wright/doppler.htm

I'm glad to see your citation of Wright's article. Studying the others on his site would help educate you on a great many things, and hopefully would reduce the volume and frequency of erroneous posts here. Remember: The Q&A section is for mainstream answers.

[speedfreek]

Then you are contradicting this post you made earlier?
I can see how you might think that, but the point you were making relates to a slightly different question to the one I was answering.

I was answering

"How can we see objects that have always been receding faster than c?",

whilst your point addresses questions like

"Will we be able to see all objects that recede faster than c?"

or

"How much of the history of those superluminal objects will we be able to see play out?"

As an example of how your point relates to mine, let us consider a galaxy we can see with a redshift of z=3. This galaxy has always been outside of our Hubble sphere, so it has always been receding superluminally. We see that galaxy as it was 11.5 billion years ago, when it was a little over 5 billion light-years away.

If it has since moved with the expansion of the universe it (or whatever has happened in that region of the universe since!) will now be somewhere around 21 billion light-years away, beyond our cosmological event horizon, but we will continue to be able to see that comoving region of the universe until we are seeing it as it was around 4 or 5 billion years ago, when it was something around 14 or 15 billion light-years away. Any light emitted by that galaxy "now" is on the other side of our event horizon but it will be many tens of billions of years before we see that galaxy approaching our event horizon.

We can see a galaxy that always has a superluminal recession speed, and we will continue to be able to see at least another 7 billion years of that galaxy's history play out, as it recedes by around 10 billion light-years, but we will observe the history of that recession play out over a much longer time span from our point of view.

(The above description uses the current "best fit" parameters for the Lambda-CDM cosmology)

[Sporally]

Just have to say my brain is more or less lost in this discussion, but i will get back to this thread once in a while and maybe some day i will understand it all :) Guess i'll be needing a sketch of it before i will understand it correctly (if even possible to sketch on a paper). But thanks Forrest Noble, your post surely made it a bit clearer to me!


..., so there is in fact a "horizon" beyond which we will never see.
I know we will never be able to reach the galaxies furthest away (unless talking about wormholes-stuff), but will we be able to see the furthest galaxies travelling a little faster than c if we catch up with a galaxy in the middel moving less than c away from us. I guess not...


So, depending on how you look at it, we have a whole lot more to see yet, and a lot it has always been apparently receding faster than c.
So i guess that means that one day or another, we will stop seeing new galaxies even though the sphere following the age of the universe (so to say) will move slower outwards compared to the expansion of the universe. I know this is scientifically the best way to explain it :)

[Ken G]

I know we will never be able to reach the galaxies furthest away (unless talking about wormholes-stuff), but will we be able to see the furthest galaxies travelling a little faster than c if we catch up with a galaxy in the middel moving less than c away from us. I guess not...I agree with your guess-- I doubt we'll ever travel far enough to help get a more distant view, but we might travel somewhere to get a closer view.



So i guess that means that one day or another, we will stop seeing new galaxies even though the sphere following the age of the universe (so to say) will move slower outwards compared to the expansion of the universe. I know this is scientifically the best way to explain it.Yes, I think acceleration of expansion makes that true (although I'd take your "even though" and replace it with "because").

[Sporally]

I agree with your guess-- I doubt we'll ever travel far enough to help get a more distant view, but we might travel somewhere to get a closer view.
DOUBT!? Doubt as in theoretical possible or doubt as in 'unless we've gotten the entire set of laws on physics wrong'?


Yes, I think acceleration of expansion makes that true (although I'd take your "even though" and replace it with "because").
Then i will add an 'Of course' :)

[Ken G]

DOUBT!? Doubt as in theoretical possible or doubt as in 'unless we've gotten the entire set of laws on physics wrong'?"Doubt" as in, I can't tell without doing a calculation that I feel unmotivated to do, because there's no reasonable expectation we'll ever achieve speeds fast enough to change our perspective on the distant universe. The only reason for speed is to get to relatively nearby places we'd like to take a closer look at.

[Sporally]

Thought some bright scientists had already done this :) But fair enough, i can live without this information.

[Webbo]

The observation is the CMBR and the measurement is the wavelength, relative to the predicted wavelength at the time of release, based on the current mainstream theory. It depends on a combination of how much the universe has scaled up whilst that radiation has been travelling towards us, how long it took to reach us, and how far away we think it was originally emitted from!

The end result is transformed into a picture where the particle horizon, the "emission distance" of the CMBR, the edge of our observable universe, has always been receding from us superluminally, was receding at over 50c when the CMBR was released and still recedes at over 3 times the speed of light today, if we choose to view the picture using the standard comoving coordinates where the universe is 13.7 billion years old and has a radius of ~46 billion light-years.

The event horizon currently lies 16 billion light-years away, whereas the particle horizon, the place where the CMBR we currently detect was released from if that place moves with the expansion of the universe, lies around 30 billion light-years further away and represents the edge of our observable universe (cosmological definition) at a distance of over 46 billion light-years in all directions. It represents the most distant place we have received a photon from.

This is full of so many assumptions that it just proves my point.

[Ken G]

What point was that? There are indeed assumptions that must be made about the model parameters to say anything about the observability of the light being emitted, which is also true of your remark that "once an object is receding faster than c, new light from that object cannot reach us because of the cumulaive effect of the expansion" , which is true for light emitted now (though not quite for the reasons in your examples), but that's because the universe has entered a phase of accelerated expansion. It wasn't true for the light emitted at times that we can now observe. I believe this is the reconciliation of what has been said so far.

[Webbo]

As I said above, for a constant expansion, which is generally taken to mean an expansion where the rate of separation of any two points is constant (i.e., not exponential like you describe), then there is no such horizon-- we would eventually see gas that is arbitrarily far beyond our current Hubble sphere.

This is totally incorrect. If this were true then an object receding greater than c due to expansion would have always been receding at greater than c. This is a contradiction to the accepted expansion rate of circa 75km/sec/Mpc, and it's a contradiction of the explanation that the reason it is greater than c, is because of the cumulative effect of each Mpc of expansion between those 2 points which is continually increasing. What you should be stating is rate of seperation of any fixed distance is constant. You need to clarify your understanding of expansion if you have based it on this misconception.

You also seem unable to grasp the fact that expanded space is also subject to the same rate of expansion, hence 1 to 2, 2 to 4, 4 to 8 etc. There is no extra property of expanded space that makes it reject further expansion.

Using your description 2 objects A&B that were say 10ly apart expand to 20ly apart after a given period of time (say 1 billion yrs). After the same period of time they expand to 30ly apart (to maintain a constant rate of seperation). However, if true, an object placed between them, C, after the first 1by (at 10ly from each to be centered), would after the second billion yrs only be 15ly from each. Therefore you have decreased the rate of expansion for A-C & C-B. This is just wrong and describes a local deceleration of the expansion over each Mpc.

[Webbo]

What point was that? There are indeed assumptions that must be made about the model parameters to say anything about the observability of the light being emitted, which is also true of your remark that "once an object is receding faster than c, new light from that object cannot reach us because of the cumulaive effect of the expansion" , which is true for light emitted now (though not quite for the reasons in your examples), but that's because the universe has entered a phase of accelerated expansion. It wasn't true for the light emitted at times that we can now observe. I believe this is the reconciliation of what has been said so far.

I think my whole point was that it was new light emiited now cannot cross the event horizon. The only reason I posted was because there were two examples mentioned that did not give that impression. Namely the "ants crossing the rubber sheet" and the "travel to a closer star and then bridge the gap" examples. Both of these gave the impression that it is just a matter of time and they will reach their destination, when in fact, they would only reach their destination if they started the journey before their destination reached a recessional speed greater than the speed they can travel.

[Ken G]

This is totally incorrect. If this were true then an object receding greater than c due to expansion would have always been receding at greater than c. Yup. That's the usual meaning of "constant expansion." It is what would happen if there were no important gravitational sources and no cosmological constant, pretty much as would be true if our universe had neither dark matter nor dark energy.


This is a contradiction to the accepted expansion rate of circa 75km/sec/Mpc, and it's a contradiction of the explanation that the reason it is greater than c, is because of the cumulative effect of each Mpc of expansion between those 2 points which is continually increasing.Incorrect on both counts. Your claims would require that the "Hubble constant" be constant in time, but it isn't.

What you should be stating is rate of seperation of any fixed distance is constant.That's closer, but it would also be incorrect. Pick a blob of gas, and if its rate of separation from us remains constant with time, then that is what I (and most people) mean by constant expansion, and furthermore, due to the way dark matter and dark energy have offset each other over the last 13.7 billion years, it is a rough but reasonable approximation for what the expansion has actually done. In stark contrast to the way you imagine the expansion happening, which was only true in the inflationary epoch, or in a distant future where dark energy dominates.


You need to clarify your understanding of expansion if you have based it on this misconception.Well, I can agree that one of us needs to recognize their misconceptions. I believe several others on the thread have given you resources to do that. I guess it comes down to if you want to know the truth, or believe you are right.
You also seem unable to grasp the fact that expanded space is also subject to the same rate of expansion, hence 1 to 2, 2 to 4, 4 to 8 etc. There is no extra property of expanded space that makes it reject further expansion.As I have told you several times, that statement is only true in a universe completely dominated by dark energy. That hasn't been our universe, though may be in the distant future when the mass density is much lower than it is now.

Using your description 2 objects A&B that were say 10ly apart expand to 20ly apart after a given period of time (say 1 billion yrs). After the same period of time they expand to 30ly apart (to maintain a constant rate of seperation).Yes, and as I also said, that is not some kind of general property of how the expansion has to work, that's how it would basically work with no dark matter and no dark energy, and is also an approximation to the average way it has actually worked (with different numbers) over the last 13.7 billion years. Any modern cosmology text will explain that to you.


However, if true, an object placed between them, C, after the first 1by (at 10ly from each to be centered), would after the second billion yrs only be 15ly from each. Correct. No problem with that.

Therefore you have decreased the rate of expansion for A-C & C-B. This is just wrong and describes a local deceleration of the expansion over each Mpc.Uh, no.

[speedfreek]

I think my whole point was that it was new light emiited now cannot cross the event horizon. The only reason I posted was because there were two examples mentioned that did not give that impression. Namely the "ants crossing the rubber sheet" and the "travel to a closer star and then bridge the gap" examples.

As Ken said, the reconciliation here is that my description was related to the light we currently observe, our past light cone, and was good enough for that purpose.

To put that into context, all the light we see that was emitted from galaxies with apparent recession speeds that were, and still are, greater than c, was emitted more than 9 billion years ago. All that light has been within our Hubble sphere, in regions receding from here at less than c, for the past 9 billion years, but we think the acceleration only started to have any measurable effect around 5 - 6 billion years ago.

In your quote you say new light emitted now cannot cross the event horizon, but that could be a little misleading - it might be better to say that light from beyond that horizon will never reach us, rather than implying the possibility that any light emitted just beyond stops dead, and cannot cross the horizon. It is the "event" horizon, after all.


Both of these gave the impression that it is just a matter of time and they will reach their destination, when in fact, they would only reach their destination if they started the journey before their destination reached a recessional speed greater than the speed they can travel.

I do know what you are getting at of course, but I have to say that light travels at c, and I expect there is a lot of light being emitted "today" in regions of the universe beyond our Hubble sphere, which is starting its journey after its destination reached a recessional speed greater than c, and yet it will reach its destination!

I am referring of course to all the light emitted "today" between the edge of our Hubble sphere and our cosmological event horizon. All that light emitted between 14 and 16 billion light-years away, from objects apparently receding faster than light, due to the expansion of the universe.

To illustrate how the event horizon is not directly linked to the Hubble sphere via recession speed in the way you might think, I would recommend the spacetime diagrams on pages 3 and 11 of the paper in the second link (http://arxiv.org/abs/astro-ph/0310808) at the end of my post #29.

One last thing. Consider a galaxy like Andromeda, a mere 2.5 million light-years away. To an observer somewhere in Andromeda, their cosmological event horizon is 2.5 million light-years more distant in their direction than ours is. Light from beyond our cosmological event horizon almost reaches here! What actually stops it from reaching here?

[Sporally]

Thanks for all your replies, it did indeed enlighten me even though you couldn't agree on it all - but at least i got smarter after this discussion :)