The expanding universe and the speed of light.

[Preserve]

I read somewhere that we can see galaxies that are farther away than what the age of the universe is and they say that this is because the universe is expanding. I'm still a bit unclear on how an expanding universe can do this. So if anyone can clarify is this is true or not. This then led to me to believe that the fact that the universe expands has an effect on the speed of light. The speed of light should be faster in the direction that the universe expands and it should be slower in the direction against the speed of light.

[EdEarl]

Studies of Type 1A supernova in the late 1990's showed that the Universe is expanding at an accelerating rate. Which leads to the conclusion that there is an event horizon beyond which we cannot see, because the Universe is expanding faster than the speed of light beyond that event horizon. This conclusion does not mean that matter is moving faster than the speed of light; rather, it means the fabric of space between concentrations of matter is expanding at an accelerating rate.

 

The Hubble deep field observations have observed galaxies whose light was emitted almost 13 Ga in the past. During that time, the Universe has continued to expand, taking galaxies Hubble photographed as 13 Ga old light past the event horizon. Thus, those galaxies are currently past our ability to see them. In the future the light that they have already emitted will shift more into the red, into the infra red, to microwaves, radio waves, and eventually disappear from our vision.

 

The speed of light is invariable, instead its wavelength becomes longer (redder) as the source emitting that light recedes from use faster; conversely, light becomes bluer (shorter wavelengths) as the source emitting light approaches us faster.

[swansont]

The expansion has added space between us and distant galaxies. The further away, the more space is added per interval of time. This gives rise to an apparent velocity that can exceed c. The restriction on c holds for inertial frames and objects moving through space, not on space itself.

[Spyman]

I read somewhere that we can see galaxies that are farther away than what the age of the universe is and they say that this is because the universe is expanding. I'm still a bit unclear on how an expanding universe can do this. So if anyone can clarify is this is true or not.

We can currently not see light emitted from a distance of 13.6 billion lightyears, but we can see light that was emitted close to 13 billion years ago and calculate that the object emitting this light has been brought much much further away by the expansion of space, during the time it took for that light to reach us.

 

There are three distances involved in this, the distance from which the light was emitted, the distance light had to travel to reach us and the distance the emitter has when we recieve the light. These distances are not the same and mixing them up with each other can give very confusing conclusions.

 

Think of a rubber band with a unstretched length of ten units and a wheel with a circumference of one of those units. If the wheel traverses the unstretched band it will take ten rotations and the ends of the band are also ten rotations of the wheel apart, both when the wheel start and stop.

 

But if the band expands during the traverse of the wheel then these distances will be very different. If the band is streched from the lenght of 10 rotations to 20 rotations while the wheel make the journey then the wheel starts 10 rotations away but the starting point will be 20 rotations away when the wheel reaches the other end and as measured by the wheel the travel distance was something between 10 and 20 rotations depending on when and how fast the expansion was.

 

According to standard models of cosmic expansion an object with a measured redshift of around 1.4 was emitting the light we receive today around 9 billion years ago from a distance of 5.7 billion lightyears and this object is now thought to be more than 13.7 billion lightyears distant, however it took this light 9 billion years to reach us because space expanded during its travel time.

 

Astronomers can today observe objects with much higher redshift up to and above 8.55 which corresponds to a emitting distance of 3.17 billion lightyears around 13 billion years ago and an estimated object distance now of around 30 billion lightyears.

Edited September 23, 2013 by Spyman

[Delbert]

I understand Doppler shift is how we measure or infer speed of retreat or expansion. And according to this thread there are stars moving away at the speed of light or greater than that of light, presumably there is a frequency to which light is shifted by Doppler indicating or specifying a speed at or greater than the speed of light. Now, my mathematics is rubbish, so perhaps others might know what this frequency shift is or how to calculate it.

[EdEarl]

A galaxy in that part of space receding from us faster than the speed of light cannot be seen, because the light from such a galaxy does not travel fast enough to reach us.

[Delbert]

A galaxy in that part of space receding from us faster than the speed of light cannot be seen, because the light from such a galaxy does not travel fast enough to reach us.

That was the reason for my query.

 

So, if we can't see or detect them, how do we come to the conclusion that such things are not only there but moving away from us at a speed at or greater than the speed of light?

[imatfaal]

That was the reason for my query.

 

So, if we can't see or detect them, how do we come to the conclusion that such things are not only there but moving away from us at a speed at or greater than the speed of light?

It important to bear in mind that the clusters are not moving through space at a speed that is greater than that of light - better to think of it as the distance between two clusters increasing at such a rate that not even light could bridge. It is very similar but a nicer way to conceptualise and avoids problems (relative velocity etc in SR). These are not really problems in GR - but that is complicated.

 

Any cluster that is red-shifted by 1.4 or more (that is to say the light reaching us now is red-shifted by that amount after billions of years travel) is such a distance that space-expansion will now be increasing the gap between themselves and us at a rate greater than 2.98e8 m/s. From a different perspective any event happening 16 billion light years away will never reach us as the position of light emitted now will never be coincident with earth.

[Airbrush]

So, if we can't see or detect them, how do we come to the conclusion that such things are not only there but moving away from us at a speed at or greater than the speed of light?

I believe it is by type 1a supernovae which gave scientists distances to galaxies. Then they measured the red shift of the galaxies at that distance and noticed the red shift was the same as galaxies at the same distance but in different directions. The red shifts agreed. So they can interpolate at what red shift, and distance from us, light emitted from stars in a galaxy, in a distant supercluster, could not reach us.

 

Beyond that limit, we cannot actually see anything, but it seems reasonable that what lies beyond that visual limit would not be terribly different from our observable universe.

.....This then led to me to believe that the fact that the universe expands has an effect on the speed of light. The speed of light should be faster in the direction that the universe expands and it should be slower in the direction against the speed of light.

It may seem that way, however it seems to me that the expansion of space only has the effect of slowing light by placing more space between us and the distant supercluster. The light is traveling the same speed, but through more space. Conversely, light does not "catch a ride" and surf the expansion of space and thus travel faster than the "normal" speed of light.

Edited October 4, 2013 by Airbrush

[Delbert]

From a different perspective any event happening 16 billion light years away will never reach us as the position of light emitted now will never be coincident with earth.

Can't say I understand this: Light emitted now will never be coincident with Earth. Am I to presume that from an earthly viewpoint the light is standing still, and for that reason will never get here? Presumably the implication is this speed business is circumvented by an expansion of 'space'. Well, have we any evidence? Like as we view stars or galaxies close to the area of disappearance (couldn't find any other way of expressing it), is the perceived or measured speed of light from these objects any different from the speed of light that we love and know? From what I can gather the answer is no. The speed of light is the same for everyone, whether or not the object is 16 billion light years away or not.

 

I understand Maxwell's calculations are based on physics and not relative speed. Which means, does it not, it applies to anything we can experience, measure, see or even simply perceive.

 

Correct me if I'm wrong, but this faster that light business for very distant objects appears to be an extension of classical physics into the cosmos. And as we all know, classical physics doesn't apply to the speed of light, or possibly anything else for that matter.

Edited October 4, 2013 by Delbert

[imatfaal]

Can't say I understand this: Light emitted now will never be coincident with Earth. Am I to presume that from an earthly viewpoint the light is standing still, and for that reason will never get here?

No you shouldn't presume that. The distance between earth and and object that is currently 16 GLyrs away is increasing so quickly that even light will not cross it.

Presumably the implication is this speed business is circumvented by an expansion of 'space'. Well, have we any evidence? Like as we view stars or galaxies close to the area of disappearance (couldn't find any other way of expressing it), is the perceived or measured speed of light from these objects any different from the speed of light that we love and know? From what I can gather the answer is no. The speed of light is the same for everyone, whether or not the object is 16 billion light years away or not.

Which is exactly the point of my first paragraph above - light always travels the same speed in empty space - but space can undergo metric expansion at the same time

I understand Maxwell's calculations are based on physics and not relative speed. Which means, does it not, it applies to anything we can experience, measure, see or even simply perceive.

The solution to the differential wave equation implied by maxwells equations only works at the speed of light. It was more einstein who realised that this speed is invariant and independent of the observers velocity - this was special relativity.

Correct me if I'm wrong, but this faster that light business for very distant objects appears to be an extension of classical physics into the cosmos. And as we all know, classical physics doesn't apply to the speed of light, or possibly anything else for that matter.

Classical physics is all we have at these scales - the general dividing line between classical and non-classical is the use of quantum mechanics/plancks constant etc; and we really do not do that for macroscopic objects especially not for cosmological objects. Einsteins two great theories of relativity are classical physics - non-classical started with things such as photoelectric effect. We do not have a non-classical theory of gravity

 

The expansion of the universe, and the fact that this expansion is accelerating are classical physics - they use GR and observation and are remarkably well documented.

[EdEarl]

 

Like as we view stars or galaxies close to the area of disappearance (couldn't find any other way of expressing it), ....

It is the "event horizon" beyond which we cannot see, and it is in some ways similar to the event horizon of a black hole.

[Delbert]

No you shouldn't presume that. The distance between earth and and object that is currently 16 GLyrs away is increasing so quickly that even light will not cross it.

I'm sorry to be a bore here, but my understanding is that 16 billion light years (assuming that's 16 GLyrs) is older than the understood age of the universe at 13+ billion years.

 

There's also the business if we can't see the things how do we know they are there? It seems to be that what all we're doing is extending the universe beyond what we can see (extending it in what appears to be a classical way), and then assuming this unseen area must also have stars and things in it like the bit we can see.

 

But as far as I'm aware classical science breaks down with things like the universe and the speed of light. Like I think in classical science we can easily make light travel faster than the speed of light by simply walking forward shining a torch. The light from the torch will be travelling faster than with the torch standing still. But we know that is not the case, according to Maxwell and later Einstein. So why do we assume these things exist 16 billion light years away, 3 billion years older than the understood age of the universe?

The expansion of the universe, and the fact that this expansion is accelerating are classical physics - they use GR and observation and are remarkably well documented.

If we're using classical science as you say, then it seems to me our conclusions will almost certainly shown to be wrong. If not now then in the future.

 

There's also the business of does classical science take into consideration that what we're looking at all those billions of light years away almost certainly don't exist anymore, or if by some weird consequence they do, they're doubtless no where near where we see them!?

 

In other words, would it be a fair assumption to say: what we see and what we're making all these theories and conclusions about, aren't actually there!!!

[imatfaal]

I'm sorry to be a bore here, but my understanding is that 16 billion light years (assuming that's 16 GLyrs) is older than the understood age of the universe at 13+ billion years.

That's a clip round the back of the head by the physics teacher sort of statement! Lightyears is a measure of distance not time. Aside from that - this discrepancy is the very point; there is lots of stuff further away from us than a simple lightspeed expansion can account for. The radius of the observable universe is around 44Glyrs

There's also the business if we can't see the things how do we know they are there? It seems to be that what all we're doing is extending the universe beyond what we can see (extending it in what appears to be a classical way), and then assuming this unseen area must also have stars and things in it like the bit we can see.

Isn't that extension the nub of all science? But in this case we can "see" the stuff we are talking about - but we can also make predictions about our ability to "see" it in the future

But as far as I'm aware classical science breaks down with things like the universe and the speed of light. Like I think in classical science we can easily make light travel faster than the speed of light by simply walking forward shining a torch.

Careful - you are using a definition of classicial that is completely different to that used by almost everybody else. SR and GR are classical theories - non-classical starts with quantum mechanics. If you insist on using personal definitions of terms of art then discussion is futile.

The light from the torch will be travelling faster than with the torch standing still. But we know that is not the case, according to Maxwell and later Einstein. So why do we assume these things exist 16 billion light years away, 3 billion years older than the understood age of the universe?

If we're using classical science as you say, then it seems to me our conclusions will almost certainly shown to be wrong. If not now then in the future.

This goes back to my first response - heuristically you can consider it as the insertion of more space in between distant objects this will solve the provision against travelling faster than light (it's not travelling just more space is being inserted) or more formally you can look at in terms of GR rather and SR and get involved in comoving coordinates and local velocities which also allays fears about ftl

 

There's also the business of does classical science take into consideration that what we're looking at all those billions of light years away almost certainly don't exist anymore, or if by some weird consequence they do, they're doubtless no where near where we see them!?

It most certainly does not make sweeping assumptions about existence or lack of same

In other words, would it be a fair assumption to say: what we see and what we're making all these theories and conclusions about, aren't actually there!!!

Questions of simultaneity and the lack of it are dealt with mathematically and rigorously - information outside our lightcone is unavailable without exception. What is happening "now" somewhere else is always a matter of speculation - some of that is well-informed and based on sturdy models other stuff is make believe. What we can say is that today 5th October 2013 we observe light impinging on our instruments from a distant galaxy that is red-shifted to such an extent (if our assumptions about the universe are correct ) that the galaxy will be increasing its separation with earth at a rate that is higher than could be bridged by light.

[EdEarl]

 

From: Wikipedia

In 1998, observations of type Ia supernovae also suggested that the expansion of the universe has been accelerating

Prior to 1998, everyone thought the expansion of the universe was slowing, which would mean all the galaxies in the Universe could be seen indefinitely (if there was a telescope big enough to observe them). Previously, the idea did not exist that there was an event horizon that defined an observable universe from things outside our field of observation.

 

Since 1998, cosmologist have accepted that the expansion of the universe is accelerating, which means space time is expanding since mass cannot move faster than the speed of light. Thus, there exists an event horizon at the edge of the observable universe, beyond which space time is accelerating faster than the speed of light, carrying galaxies with it. Beyond the event horizon, we cannot see anything, because light from beyond it is moving too slow to ever get to us. The even horizon is moving away from us at the speed of light, and everything beyond it is moving faster.

 

Galaxies that we see near the even horizon, will one day move through the event horizon and become invisible. The reason they move through the event horizon is because space time between us and them is expanding at an accelerating rate, and as their apparent velocity exceeds the speed of light, they will disappear.

 

Note: each massive object creates a gravity well in which space time is curved by the mass. This curve is a contraction of space time, which is opposite to the expansion of space time over large distances.

Edited October 5, 2013 by EdEarl

[between3and26characterslon]

Studies of Type 1A supernova in the late 1990's showed that the Universe is expanding at an accelerating rate....

 

I'm no expert but I disagree with this statement. Studies of type 1a supernovea show that the farther distant an object the faster it is moving away from us but this only implies objects are accelerating away from from us, it does not prove it.

 

Unless there is evidence that a known object is moving away from us measurably faster now than at some point in the past I don't think we can conclude an accelerating Universe.

 

Maybe it's my lack of understanding, I am open to evidence or explanation.

[EdEarl]

 

I'm no expert but I disagree with this statement. Studies of type 1a supernovea show that the farther distant an object the faster it is moving away from us but this only implies objects are accelerating away from from us, it does not prove it.

 

Unless there is evidence that a known object is moving away from us measurably faster now than at some point in the past I don't think we can conclude an accelerating Universe.

 

Maybe it's my lack of understanding, I am open to evidence or explanation.

The Wikipedia article referenced below is an overview of the science, and gives further references.

 

From: Wikipedia

In 1998, observations of type Ia supernovae also suggested that the expansion of the universe has been accelerating

[Toffo]

Let's calculate how much Doppler shift there is when a clock + light source combo is receding at velocity c:

 

It takes one minute for a second hand of a clock to make one round around a clock face.

 

In one minutes time the distance to the clock increases so that it takes one minute more time for light to travel from the clock to a still standing observer.

 

So it takes one minute + one minute until a still standing observer has seen that a full circle has been completed by the second hand.

 

So the Doppler shift is: 100 % increase of wave length, or 50% decrease of frequency.

Edited October 5, 2013 by Toffo

[EdEarl]

Let's calculate how much Doppler shift there is when a clock + light source combo is receding at velocity c:

 

It takes one minute for a second hand of a clock to make one round around a clock face.

 

In one minutes time the distance to the clock increases so that it takes one minute more time for light to travel from the clock to a still standing observer.

 

So it takes one minute + one minute until a still standing observer has seen that a full circle has been completed by the second hand.

 

So the Doppler shift is: 100 % increase of wave length, or 50% decrease of frequency.

That is for non-relativistic velocities, for example sound.

See: https://en.wikipedia.org/wiki/Relativistic_Doppler_effect

[Toffo]

That is for non-relativistic velocities, for example sound.

See: https://en.wikipedia.org/wiki/Relativistic_Doppler_effect

 

Yes I left time dilation out.

 

A galaxy receding at c is not time dilated, because it's not moving.

 

To calculate the Doppler shift of the galaxy we need to know the increase of time delay in one minute for example.

 

In univere whose expansion is not accelerating the Doppler shift will be less than z = 2 at velocity c, I guess.

[Delbert]

That's a clip round the back of the head by the physics teacher sort of statement! Lightyears is a measure of distance not time.

Well, I'm not sure that's correct, since what we see is what it was like X billion years ago. Which is the past, and not what it is like now - if it actually exists in the here and now!

Isn't that extension the nub of all science? But in this case we can "see" the stuff we are talking about - but we can also make predictions about our ability to "see" it in the future

I'm sorry, but such predictions over such a vast period of time I think is stretching things a bit - perhaps more than a bit. It seems to me the universe might not necessarily conform to our 'common sense' understanding. Even common sense extending into relativity and quantum mechanics. As I think I've mentioned previously (maybe not here) about QM, relativity and the like, if the universe was really something tangible, solid, made of something and all the other attributes our common sense powers of reasoning might attribute, it seems to me we really would have a problem explaining it!

 

Perhaps it's the case that the universe can only live in the past, we can never know about anything as to what it is 'now'.

 

In an attempt to explain this concept to someone once, I used the analogy of an exchange of cheques. Like, I write you a cheque for (say) £1000, which is credited to your account the next day, but not debited from my account for two days. On the second day you write me a cheque for a £1000... ...and so on. In other words we've created £1000, so long as we keep writing cheques to each other, which only exists because of a time delay in balancing the accounts. Although I think the banks have since tightened up their procedures, so we might not be able to do it these days!

 

As I think a Nobel prize winning physicist once said: Any amount of energy can be borrowed, so long as it's paid back within in a short enough period of time. And since energy is exchangeable with what we can matter...!!