Superluminal Speeds and All That Jazz

In summary: SR tells us that there is nothing beyond that. GR tells us that there might be something beyond that, but we can't know for certain. The theory of general relativity allows for the possibility of something called a 'wormhole.' A wormhole is a shortcut in space-time, and if you could find one and go through it, you would experience a very high-speed travel. But we don't know if they exist or not, and if they do, we don't know how to get to them. In summary, SR teaches that "nothing can be observed to travel faster than light." GR teaches that there might be something beyond that, but we can't know for certain
  • #1
oldman
633
5
Superluminal Speeds and All That Jazz

A popular prejudice is sometimes expressed as “nothing can travel faster than light”. But Special Relativity (SR), from which this prejudice is derived, in fact teaches only that “nothing can be observed to travel faster than light”. The meaning of ‘observed’ here is technical and can be unpacked by explaining how distances are measured and clocks are synchronized with light signals in SR. It may then come as a relief to realize that SR creates no mysterious speed barrier. SR does not prevent one traveling to a star, measured on Earth to be light-years away, in just a few hours; SR dictates only that travel-time be measured by the traveler on his/her clock, not by an Earth-bound stay-at-home.

It would be misleading, though, to call the traveler’s speed ‘superluminal’, say because at home on Earth the distance the that the traveler covered was measured as a few light-years. From an SR point of view, this would be like comparing apples and oranges. In nature as described by SR there are no superluminal speeds that can be measured as such.

What about superluminal speeds in General Relativity (GR)? ‘Superluminal speeds’ are certainly talked about when dealing with models of the universe based on GR, in many threads in this forum. Sufficiently distant parts of a model expanding universe are sometimes said to ‘separate at superluminal speeds’. To me, this seems in an inverse sense just as simplistic as saying that “nothing can travel faster than light”.

In GR speed is not a simple concept: First, in GR ‘to separate’ is generally taken to mean to
increase ‘the proper distance between’. Second, ‘proper distance’ means the sum of a chain of ‘local’ proper distances, measured as in SR, over a span in which spacetime is perceptibly non-Euclidean (‘distance’ could also measured by radar, as in SR, with possibly different results). Third, proper-distance measurements are to be made simultaneously, at the same instant of a defined ‘time’ (in consensus cosmology ‘simultaneous’ means when observers measure the same density of mass/energy, locally, in a model universe.) In GR the concept of ‘speed’ needs a lot of unpacking!

If, after all this unpacking, one gets an answer that exceeds c, there is in my view nothing to make it worthwhile to describe a speed which in practice you cannot measure or observe as ‘superluminal’. Using 'superluminal' suggests that something special or exceptional is being uncovered. In cosmology, getting an answer for a speed greater than c in an expanding universe means simply that one is extrapolating a model into unknowable regions beyond the (dynamic) red horizon of the observable universe. In observable nature as described by GR there are also]no superluminal speeds that can be measured as such.

Why then bother with the word ‘superluminal’ at all, I ask?
 
Space news on Phys.org
  • #2


oldman said:
...Why then bother with the word ‘superluminal’ at all, I ask?

Beats me :biggrin:
I don't particularly like the word. I use it rarely. I prefer to talk about standard model distances increasing at rates > c. Some other posters use it and when I occasionally do it's mainly just to make contact with other people's way of talking.

I'm not sure what you mean by "unknowable regions beyond the (dynamic) red horizon of the observable universe." Could you clarify? Maybe we mean different things by observable universe.

For me, when we map the CMB we are looking at matter which is now about 46 billion lightyears away from us. We can't see what that matter is doing at this very moment, but we can tell certain things about it such as variations in its density. To me it's clear that matter is part of our observable universe. The distance estimate is part of the standard LCDM model which I believe we have educational function to help people understand---as a home base or common point of departure. If we choose to deviate, it's OK but it's more efficient if as a minimum we all know the basics of the standard picture of cosmology we are deviating from.

You mention distance. Distance has always been a complicated issue in astronomy. Everybody should be alert to this. How do we define various distance measures? Which one is the prevailing or foremost, in terms of which others are conveniently expressed? Etc. You give a definition which (perhaps phrased slightly differently) I would basically agree with. It is convenient to use as one's primary idea of present-day distance and the Hubble Law is based on it.

In order to sketch out and visualize the universe we live in, we need some primary idea of distance. It's how you talk about geometry. And there is a prevailing definition. Given this measure of distance we are immediately confronted by the question of the rate at which various distances increase and for full many a distance that we deal with in cosmology, for instance the distance to pretty much any object we see with a redshift z > 1.4, that rate of increase is greater than c.

I'm not sure if you are having trouble with this, or objecting to it? Should we not use those words? Should we not say that suchandsuch distance is increasing at a rate of 2c or 3c or whatever? Should we use more words every time we need to refer to a rate of change of distance----some caveat or qualifying phrase that you have in mind and would like to recommend? I'm not entirely sure what you are saying in your post, Oldman.

How should we be talking about the standard picture of the layout of the universe, according to you? And my earlier question was what do you mean by "unknowable regions beyond the (dynamic) red horizon of the observable universe" ?

Do you mean regions beyond, say, 46 billion lightyears? Or do you mean regions beyond, say, 13.7 billion lightyears (the distance which is increasing at exactly c) because in the standard picture we see stuff currently way way out beyond that. Perhaps you'd clarify?
 
Last edited:
  • #3


marcus said:
...I don't particularly like the word. I use it rarely. I prefer to talk about standard model distances increasing at rates > c. Some other posters use it and when I occasionally do it's mainly just to make contact with other people's way of talking.
I agree. This is a neutral and sensible description.

Here's a quick first reply to your post.

Your detailed reply is really helpful. Thanks, Marcus. You asked "How should we be talking about the standard picture of the layout of the universe? " In answer to your question --- in a word: "carefully".

All I can offer is the opinion of a 'struggle veteran', as we say in these parts. I've found cosmology not an easy subject to understand, but with help I'm getting there.

In a way it's like learning Russian, where the first hurdle you have to overcome is unlearning your English alphabet and replacing it with the Cyrillic alphabet. Worse than learning French!With cosmology one has first to unlearn the simple meaning of words like distance and speed. You must then learn to re-interpret them in a relativistic context. Once this is done you can go back to using these words freely, but with implicit caveats. The advantage of Russian is that when it is heard, it is easily recognised. Cosmology-speak is not so easily recognised because cosmologists use the same vocabulary as Joe the Plumber. But, finally, after overcoming these hurdles, one comes to recognise that Joe's vocabulary is all we have, and that cosmologists and plumbers mean similar things after all.

This is why I think cosmology needs to be introduced with a health warning, as it were, and why one should be wary of talking loosely about 'superluminal speeds' in particular.

I'll answer the other points you raised 'just now' -- in our local patois meaning 'a bit later'.
 
  • #4


marcus said:
I'm not sure what you mean by "unknowable regions beyond the (dynamic) red horizon of the observable universe." Could you clarify? Maybe we mean different things by observable universe.

Perhaps. Here I was thinking of the CMB as an horizon --- writing loosely, I'm afraid, doing just what I'm recommending others not do. The reason we can't see beyond the CMB is (I think) because of it's physical nature; it's an opaque fog of (to us) of reddened radiation, rather than a model-dependent particle or event horizon. I wrote 'dynamic' because the CMB must also be expanding, and its expansion rate could vary. But we must have been in causal communication with events beyond the CMB, because that's where baryogenisis happened. Here be matter rather than antimatter, so it's not an horizon.

For me, when we map the CMB we are looking at matter which is now about 46 billion lightyears away from us. We can't see what that matter is doing at this very moment, but we can tell certain things about it such as variations in its density. To me it's clear that matter is part of our observable universe. The distance estimate is part of the standard LCDM model which I believe we have educational function to help people understand---as a home base or common point of departure. If we choose to deviate, it's OK but it's more efficient if as a minimum we all know the basics of the standard picture of cosmology we are deviating from.

Yes. I agree. Especially about the LCDM model. But getting to grips with all that this implies can be quite a long process.

You mention distance. Distance has always been a complicated issue in astronomy. Everybody should be alert to this... It is convenient to use as one's primary idea of present-day distance and the Hubble Law is based on it.

We seem to agree here.

In order to sketch out and visualize the universe we live in, we need some primary idea of distance. It's how you talk about geometry. And there is a prevailing definition. Given this measure of distance we are immediately confronted by the question of the rate at which various distances increase and for full many a distance that we deal with in cosmology, for instance the distance to pretty much any object we see with a redshift z > 1.4, that rate of increase is greater than c.

I'm not sure if you are having trouble with this, or objecting to it? Neither. Should we not use those words? Should we not say that suchandsuch distance is increasing at a rate of 2c or 3c or whatever? Yes, if we gain any physical insight by thinking of how fast distances increase. But I can't think of any -- is not 'rate of change of distance' here just a scientific curiosity? Should we use more words every time we need to refer to a rate of change of distance----some caveat or qualifying phrase that you have in mind and would like to recommend? I'm not entirely sure what you are saying in your post, Oldman.] I'm recommending that one doesn't bother with aspects of the model whose only function seems to be to evoke the reaction 'Well I never!' or 'Wow!'
My comments are underlined here.

You may see me as making much ado about nothing, Marcus; if so, you may well be right. But judging from the many confusions that arise in this forum I do believe that the careful presentation of cosmology is worth fussing over.
 
  • #5


oldman said:
...You may see me as making much ado about nothing, Marcus; if so, you may well be right. But judging from the many confusions that arise in this forum I do believe that the careful presentation of cosmology is worth fussing over.

I sense that you are concerned here with pedagogy, Oldman---the most efficient way to present modern cosmology especially to newcomers to the forum. It's an important concern and you make some good points, but I think you may be overlooking some aspects and I'll offer my different perspective.

As you say, the edge of the observable is about 46 billion lightyears from us---that's where the matter is that we see when we map the microwave background. We are looking at stuff that is currently 46 from here. That's important for noobs to understand, and fortunately we are all on the same page regarding the estimated age of around 13.7 billion.

Since coming to PF in 2003, I've seem something on the order of 100 posters bring us their insight that, in standard cosmo, the radius of the observable must be 13.7 billion lightyears because expansion started 13.7 ago and the edge can't receed faster than c.

One of the main jobs for the regulars has been to relieve noobs of that misconception. In standard cosmo, the distance to the farthest matter we are looking at currently is 46, not 13.7.

I've gradually acquired the viewpoint that the least fuss and bother way to deal with this is to immediately confront it and make a clean break with this endemic misconception.
Essentially it is a cost-benefit analysis type of choice.

1. It is very CHEAP timewise to picture visually how distances between stationary points can increase at a c+ rate. You simply look at a balloon with glued pennies and with photons wriggling across the surface at a fixed rate of one inch per minute.
There will be distances between pennies which are increasing faster than one inch per minute. But no penny ever outraces a photon in its neighborhood. Ned Wright provides the two computer animations of the balloons with wrigglers. To visualize (in an unparadoxical nice consistent way) how distances can increase at c+ rates, that's all you need.

2. It is very EXPENSIVE to leave the endemic misconception in place, because c+ expansion rates are physically built into Hubble law, and the FLWR metric. Rates of expansion of distance are what Hubble law is about: v = Hd. The v there is a rate of change of distance, the instantaneous distance between two stationary points. Since it's what the basic law is about, if you don't get that up front, you don't ever get to square one.

So I find it is actually hazardous to leave that misconception lying around for other posters to stumble on. One person spouting off can get several others confused, if they are vulnerable to thinking there is something funny or suspect about distances increasing at c+ rates.

3. There is a considerable bonus BENEFIT of coming to grips early on with some aspects of cosmology that are integral to the basic framework, like the business of stationary wrt background. You might say a basic cosmological injunction is "Dont move too fast relative CMB or the doppler hotspot will fry you!" And don't let the people in your thought experiments do that either, lest it fry them and fry your mind as well. :biggrin:
Stationary basically means rest wrt the matter which emitted the CMB that we are now looking at. And happily we find the other galaxies are not moving much relative to it either. There may or may not be some largescale drift but either way that is a small footnote that you can't even talk about without first getting the idea of approximate rest wrt the expansion process and bulk primordial matter. Also cosmic rest gives you an idea of cosmic simultaneity that is essential to standard cosmology. The Hubble law v = Hd relates v and d defined as if measured instantaneously at some given moment. The d is between stationary points. In other words, the most basic quantities we work with depend on an idea of rest and simultaneity which is special to the field.

4. Dealing up front with the standard cosmo pattern of increasing distances, including rates of increase, gives a handy way to define useful quantities. For example the Hubble radius of 13.7 billion lightyears is defined simply as that distance which is increasing at rate c.
The Hubble law is the proportionality between a distance and its rate of increase. So a distance of 27.4 (twice Hubble radius) would be increasing at rate 2c, and so on. These are physical relationships which I think it's good for beginners to assimilate early on. Part of understanding the standard model, which we should all get under our belts for starters. (Then if you want to cast doubt on the standard model and propose some alternative picture of the cosmos at least we all share a common starting point from which to explore variants. I don't object to skepticism and exploratory deviation, they are healthy! But first we all need to understand the basics of what we are being skeptical about and deviating from.)

Oldman, I'm hoping very much that you and I can agree on some of these points!
 
Last edited:
  • #6


marcus said:
I sense that you are concerned here with pedagogy...the most efficient way to present modern cosmology especially to newcomers to the forum. It's an important concern ...

Yes it is. In this thread I wanted to focus on the misconception that the FLRW model, in promoting talk of 'superluminal speeds' -- verboten and unobservable in SR, somehow generates an internal conflict in relativity. You tend to focus on misconceptions about distances and ages of edges rooted in similar prejudices, and I agree pretty much with all you say. We also have different approaches towards dispelling such misconceptions--- I like a clear introducion to basic concepts; you recommend the simple balloon analogy and practicing with online calculators. Thanks for setting out your views so clearly here.

There is plenty of room for both approaches. One can't have too much of a good thing! I hope that the final version of your sticky thread is a convincing mixture that helps newcomers to a better understanding of what modern cosmology is all about.

With this aim in mind, I'll transfer some residual misgivings I have about comments you made here on the balloon analogy, to that thread.
 
  • #7


I missed the part where FLRW suggests superluminal velocities. Spacetime is not constrained by special or general relativity.
 
  • #8


Chronos said:
I missed the part where FLRW suggests superluminal velocities. Spacetime is not constrained by special or general relativity.
I agree that spacetime is not constrained by SR or GR, Chronos. Spacetime is an invented coordinate space, rather than something tangible or perceptible --- it's as free as one's imagination. But in the FLRW model of an expanding universe there is often talk of how sufficiently far-apart elements of the cosmic fluid separate more rapidly than c, at what is often referred to as a 'superluminal speed'. Could you amplify your comment, please?
 
  • #9


oldman said:
...But in the FLRW model of an expanding universe there is often talk of how sufficiently far-apart ...
I think you would agree it is a simple one-line mathematical consequence of the usual form of the metric, would you not? I expect there is often talk, but it's concretely obvious too.

E.g. write down the FLWR metric, take the flat case for simplicity, normalize a(present)= 1 which is often done. Look at a spatial interval of length 1/a' where a' is the time derivative of the scalefactor. Then consider the rate at which the length of that interval is increasing---it is c by simple algebra. And any longer interval will be increasing faster than c. Just as a consequence of the way the FLWR metric is constructed.

Double that length and the metric will tell you it is increasing at 2c. Am I missing something? Seems like a direct math consequence of FLWR, by inspection so to speak.

I like switching the letters FLRW around to FLWR so that it spells "flower" :biggrin: Excuse the playful levity. :wink:
 
  • #10


marcus said:
...Seems like a direct math consequence of FLWR, by inspection so to speak.

Indeed it is. Especially transparent if one considers the metric along any line in the everywhere isotropic model universe the metric describes. There is even no need to use angular coordinates theta and phi.
 
  • #11


marcus said:
I think you would agree it is a simple one-line mathematical consequence of the usual form of the metric, would you not? I expect there is often talk, but it's concretely obvious too.

E.g. write down the FLWR metric, take the flat case for simplicity, normalize a(present)= 1 which is often done. Look at a spatial interval of length 1/a' where a' is the time derivative of the scalefactor. Then consider the rate at which the length of that interval is increasing---it is c by simple algebra. And any longer interval will be increasing faster than c. Just as a consequence of the way the FLWR metric is constructed.

Double that length and the metric will tell you it is increasing at 2c. Am I missing something? Seems like a direct math consequence of FLWR, by inspection so to speak.

I like switching the letters FLRW around to FLWR so that it spells "flower" :biggrin: Excuse the playful levity. :wink:
The catch is that the FRW metric does not describe a vacuum between things. In fact in a FRW spacetime there is no vacuum, only matter.

A bit of a leap to map that to our known universe which appears mostly relative vacuum with just a few spots of dense matter would't you say? :smile:
 
  • #12


MeJennifer said:
A bit of a leap to map that to our known universe which appears mostly relative vacuum with just a few spots of dense matter would't you say? :smile:

Short answer to your question: no, I wouldn't say :smile:

Long answer: When you assume homegeneous and isotropic, it practically amounts to an approximation with matter uniformly spread out instead of concentrated in lumps. Cosmologists use the "dust" metaphor.
The validity of the FLWR model is not at issue here---it fits the data. The point is if you assume FLWR as a good approx, then you automatically get distances increasing at c+ rate.

Approximating a clumpy spangled world by a world with uniformly spread turns out to give a splendid approximation to reality, and of course with matter spread with ideal perfection there is no vacuum, but so what? All one needs to get over that hurdle is a "dustgrain" of motherwit :smile:, wouldn't you say?

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

Oldman, if you are reading, I still recall your description of early universe structure formation as curdling. You enjoy words and it occurs to me that "flower" sounds like "flour".
The Friedmann et al solution is sometimes called a dust solution, or dust universe. Matter being uniformly spread and pressureless, like a cloud of dust. FLWR
 
Last edited:
  • #13


marcus said:
The validity of the FLWR model is not at issue here---it fits the data.
It does not, that is why, in their "brilliance", some postulate dark matter and dark energy to make it fit.
That is like using astrology to predict stock prices and explain the discrepancies by dark and hidden forces.

I would say pretty obvious to the critical mind, not so obvious to those who have built an industry around it and have a history of aversion to be caught wrong.
 
  • #14


marcus said:
The validity of the FLWR model is not at issue here---it fits the data...

MeJennifer said:
It does not, that is why, in their "brilliance", some postulate dark matter and dark energy to make it fit.
That is like using astrology to predict stock prices and explain the discrepancies by dark and hidden forces.

I would say pretty obvious to the critical mind, not so obvious to those who have built an industry around it and have a history of aversion to be caught wrong.

a conspiracy? :biggrin:

Come on Jenny, FLWR fits the data excellently well. And it is doing exactly what one hopes a model will do---it provides us with two important numbers to explain: the dark energy density and the dark matter density.

It is now up to physicists theoretical, and physicists experimental, to find physicist explanations for these two numbers (which may even lead to an improvement in FLWR!). This is an exciting challenge to them and it will be interesting to see what they come up with.

So far we have a sweet simple model with two interesting parameters which we adjust (say to 0.73 and 0.27) and it fits masses of data stunningly. It is doing just what a model in a mathematical science is supposed to do, and it might seem foolish to disparage it for doing its job.

When FLWR is precisely adjusted to fit (e.g. by plugging 0.73 and 0.27) it is called LCDM or LambdaCDM----Lamda refers to the 0.73 parameter and CDM (cold dark matter) refers to the 0.27.

So FLWR alias LCDM is making us a present of these two fascinating numbers. And from everything I hear, legions of physicists are positively beside themselves with excitement and curiosity, and (finances permitting) the funding funds are flowing as well. Explaining 0.73 and 0.27 is becoming a major major focus of research worldwide.

So that's cool. :cool:
 
Last edited:
  • #15


marcus said:
a conspiracy? :biggrin:
No, job security.
 
  • #16


marcus said:
The Friedmann et al solution is sometimes called a dust solution, or dust universe. Matter being uniformly spread and pressureless, like a cloud of FLWR
Yes. I like this.
 
  • #17


MeJennifer said:
A bit of a leap to map that to our known universe which appears mostly relative vacuum with just a few spots of dense matter would't you say? :smile:

Judging from your posts, Jennifer, you seem to be sceptical of the model based on the FLWR metric. And you seem to understand matters relativistic pretty thoroughly -- I'm thinking of your recent posts in the relativity forum e.g. in the thread "Light velocity measurements".

So here's a thought about a 'leap' of a different kind for you:

The FRLW metric, which so neatly accounts for the modern-cosmology-founding interpretation of redshift, decribes change in the universe by allowing the the scale factor (which is the space-coordinate metric coefficient, presently unity) to vary with time.

Have you noticed that one could equally well account for the redshift by describing change with a time-coordinate metric coefficient that varies in time? (I don't mean c, which in this context is a unit-conversion factor) The accepted choice, made long ago, was the 'leap' I'm referring to.

In fact the only thing that the redshift is telling us is that the ratio of metric coefficients changes with time. It'sthehuge amount of other circumstantial evidence accumulated by astronomers that supports the FRLW metric with it's choice of varying scale factor.

But then you're sceptical about the effectiveness of this evidence, aren't you?
 
  • #18


oldman said:
So here's a thought about a 'leap' of a different kind for you:

The FRLW metric, which so neatly accounts for the modern-cosmology-founding interpretation of redshift, decribes change in the universe by allowing the the scale factor (which is the space-coordinate metric coefficient, presently unity) to vary with time.

Have you noticed that one could equally well account for the redshift by describing change with a time-coordinate metric coefficient that varies in time? (I don't mean c, which in this context is a unit-conversion factor) The accepted choice, made long ago, was the 'leap' I'm referring to.

In fact the only thing that the redshift is telling us is that the ratio of metric coefficients changes with time. It'sthehuge amount of other circumstantial evidence accumulated by astronomers that supports the FRLW metric with it's choice of varying scale factor.

But then you're sceptical about the effectiveness of this evidence, aren't you?
It is absolutely true that in the expansion phase of an FRLW spacetime one can say with equal validity that all clocks speed up instead of all distances increase. Those statements are equivalent in GR.

However that does not help the support of the idea that a FRLW metric is a good approximation for the existing universe. Think about it, does our universe even remotely look like a pressureless fluid? To me it looks like mostly (relatively) empty space and some concentrated blobs of matter.

When one develops a model and it turns out that that model does not approximate reality one can do two things, either find a better model or stick with it and introduce things like dark forces and dark matter which, perhaps conveniently, cannot be directly detected.
One might as well try to falsify fairies. :smile:
 
  • #19


MeJennifer said:
It is absolutely true that in the expansion phase of an FRLW spacetime one can say with equal validity that all clocks speed up instead of all distances increase. Those statements are equivalent in GR...

I certainly don't mean to insist on using the FRLW metric, although it does seem to me to look like a good approximation to try first. Neither do I like labelling what is going on in our evolving universe "an expansion phase". A pox on the word expansion! But perhaps like you, I'm still somewhat sceptical of the kludges that have been introduced to fix various problems.

Although I may well end up believing in fairies, as you put it, a small niggle keeps scratching at the back of my mind --- is it possible that something else is going on, something that changes the ratio of metric coefficients, something that looks very like expansion, or clocks speeding up --- something really wild like the relative unfolding of the dimensions we are so familiar with, something no one has yet explored or tried to model, what with everybody being so obsessed with how well 'expansion' seems to work; up to the point where kludges have to be brought in?

Mostly I keep such wild thoughts to myself, but they break out now and then. Nuff said.
 
Last edited:
  • #20


oldman said:
...Neither do I like labelling what is going on in our evolving universe "an expansion phase"...

:confused:

We are in a phase of widespread increasing distances between objects at CMB rest, according to pattern called Hubble law. So what better do you propose to call it? Would you feel comfortable saying we are in a:

1. Hubble phase, or Hubble law phase. (accurate, but too technical to communicate to many listeners)

2. increasing-distances phase (accurate, but too many syllables).

It is easier to say expansion (3 syllables) than increasing-distances (6 syllables). So what's wrong? As long as people don't misunderstand and over-extend the analogy with material substance. Aren't you being a bit fussy about words here?

The basic point is you either buy GR or you offer a better mathematical model of how geometry evolves. If you buy GR then you have no right to expect distances to stay the same. In fact you expect us quite possibly to be in an increasing-distances phase. And the kicker is the idea of being collectively at rest with respect to the matter in the early universe---at rest relative CMB.
Modulo small proper motions and possibly some drift, but on the whole a remarkable collective stationarity.

Well these are the most obvious features of the universe. What words would you like better? I'm flexible about words as long as the mathematical model clearly underlies them. Interpretation is just verbal trimmings. So as long as the words are easy to say, and fairly descriptive, I'm cool.

So what words do you like?
 
  • #21


marcus said:
So what words do you like?

I was replying specifically to MeJennifer and her use of the word "expansion phase" here. You are talking about the consensus model, which she doesn't seem to accept. When talking of this model "expansion" is of course quite acceptable as a description of consensus. Anything else would be clumsy, as you say.

However I'm not yet entirely convinced that the consensus interpretation is correct. I do "buy GR" and I don't "expect distances to stay the same". But, on the other hand, you don't have the right to expect "clocks to always run at the same rate" . All we do know for certain is that physics is invariant in time and space (consequently that GR is a most elegant and satisfying description of both gravity and of this invariance), and that there is a redshift. The rest is interpretation --- which may well have been correctly made --- circumstantial evidence indeed suggest that this is so. But people are fallible, and the consensus interpretation does involve a clutch of ad hoc kludges.

I therefore cavil at certainties, as when you write "We are in a phase of widespread increasing distances between objects at CMB rest, according to pattern called Hubble law." Yes, it looks as if we are, but you write as if this were dogma.

I do like "the idea of being collectively at rest with respect to the matter in the early universe---at rest relative (to the) CMB." Sorts the ether nonsense out nicely.
 
  • #22


oldman said:
...at rest relative (to the) CMB.
I understand what people mean by it but being at rest relative to radiation is just nonsense. Radiation travels at light speed and nothing can be at rest relative to light speed.
 
  • #23


The concept of the CMB rest frame seems to raise some interesting issues. As I understand it, the CMB rest frame is not actually making any comparison to a relative velocity to radiation, but rather two events both associated with CMB decoupling.

On the basis that decoupling took place throughout the spatial universe at the same time, give or take a few years, then the CMB ‘photons’ received on Earth today from all directions should have the same temperature-wavelength. If the Earth had a relative velocity in a given direction, this would cause a Doppler shift of the wavelength this direction with respect to the opposite direction.

If this description is essentially correct, would a model of an expanding homogeneous and isotropic universe infer any meaning to the magnitude and distribution of the relative velocity of observable galaxies with respect to the CMB frame?
 
  • #24


mysearch said:
...As I understand it, the CMB rest frame is not actually making any comparison to a relative velocity to radiation ...
Yes, you're correct. MeJennifer had the wrong end of my stick here. I wasn't being careful enough. In the consensus model the CMB radiation provides a personal reference frame of rest for any observer --- simply a frame in which she/he observes this radiation to have the same-temperature black-body spectrum no matter in what direction it is being observed. In the consensus model observers at rest in such frames separate from each other --- their (carefully defined) 'proper distance' apart increases with time --- they are said to partake of the 'Hubble flow' as the model universe expands in a GR way.

So there is no single absolute frame of rest, no universal ether, just different frames for different folks.
If the Earth had a relative velocity in a given direction, this would cause a Doppler shift of the wavelength this direction with respect to the opposite direction.
It actually does.
... would a model of an expanding homogeneous and isotropic universe infer any meaning to the magnitude and distribution of the relative velocity of observable galaxies with respect to the CMB frame
Here you touch on very recent observations of systematic motions whose origin has been tentatively attributed to happenings outside our observable universe. But this is altogether off-topic for this thread -- I don't have a reference, either.

I'm off to the warm sandy shores of the Indian Ocean for the weekend.
 
Last edited:
  • #25


mysearch said:
...
If this description is essentially correct, would a model of an expanding homogeneous and isotropic universe infer any meaning to the magnitude and distribution of the relative velocity of observable galaxies with respect to the CMB frame?

Probably safest not to talk about CMB frame. The word frame suggests a rigid reference frame with a fixed distance scale---technical connotations engrained in our minds from special relativity. The CMB does provide a universal criterion of rest, being at rest with respect to CMB (no doppler hotspot) or with respect to the matter of the early universe. But two things both at rest relative CMB can nevertheless have the distance between them increasing. So in the orthodox sense of frame, they don't belong to the same frame. I think you understand this and were just talking about motion of galaxies with respect to CMB.

so your question makes better sense if you just leave off the word frame at the end.If I understand your question right, I think the answer is yes. Note that inferences in cosmology are rarely absolute or final---astronomers are constantly refining their ideas.

When people were talking some 10-15 years ago about the Great Attractor located in the direction of constellations Hydra and Centaurus in the southern hemisphere, but too far away to see or too obscured by intervening stuff, what they were really observing was a collective DRIFT of various clusters of galaxies relative to CMB rest.

There was another result like that recently, but not yet confirmed, purporting to have identifed a statistical drift in a sample of over a thousand galaxies, again relative to CMB rest.

Each galaxy has its own individual motion relative CMB, though in general it's difficult or impossible to determine (except for the radial component of motion relative to us---some small deviation from the expected redshift, receding just a bit too fast, or not fast enough, to fit the overall pattern.)

With nearby clusters, such as Virgo cluster, it's easier to gauge than for more distant. And of course for our galaxy, and our local group of 10-20 galaxies it is even easier. So there are published figures on the various individual motions----speed and direction----for some mostly nearby things. These are comparatively SMALL (a few hundred km/s) and not part of the expansion process.

For example the solar system is going 380 km/s relative to the CMB---in the direction of Leo. We know that because there is a large doppler hotspot in that direction. A few microkelvin hotter CMB in that direction and a few microkelvin colder spot in the opposite.

This 380 km/s is the composite of our orbital speed within the galaxy and the galaxy's own collective motion, which is about 500 km/s in a different direction closer to Centaurus (actually a small constellation near Centaurus called Crater which means wine-cup.)

Virgo cluster (the nearest really big cluster) is also going some 500 km/s in a Centaurusish direction, as I recall----as well as they can determine. All these motions are different from Hubble law recession and they are calculated relative to CMB rest.

If all galaxies were perfectly stationary relative to CMB then they would still have their recession redshift, but they wouldn't have these small individual deviations. But they seem to have some mostly random deviations. And when people study these individual deviations from perfect Hubble law recession, they can come up with inconclusive suggestions of some sort of large scale coincidence or DRIFT. I mentioned that earlier. It is the sort of thing which is a teaser. If it is confirmed then it suggests there is a departure from uniformity and a concentration of mass in some direction, hitherto beyond our ken----like the fabled Great Attractor. It could be just a percentagewise minor blip in the otherwise uniform distribution but a small percent increase density over a very large volume could make a big gravitational effect on us, and explain a drift (relative to CMB rest) of ours and surrounding galaxies.

So your question is, does cataloguing individual motions relative CMB let us infer any meaning? And the answer is YES, if it is confirmed to be not perfectly random but has some overall drift direction then it it would force us to modify our assumption of uniformity, and let us infer an estimated size and direction of non-uniformity. This then if confirmed would represent early universe structure and would have to be included in the models of early universe structure formation. What's a plausible explanation for why this deviation? Is it compatible with the prevailing inflation scenarios? etc etc.
 
Last edited:
  • #26


oldman said:
Here you touch on very recent observations of systematic motions whose origin has been tentatively attributed to happenings outside our observable universe. But this is altogether off-topic for this thread -- I don't have a reference, either.

I'm off to the warm sandy shores of the Indian Ocean for the weekend.

Damn! That is so enviable! Enjoy the warm sandy shores. Where do live the rest of the time?

I'll get a reference for that recent study that purports to detect a drift, in a sample of over 1000 galaxies.
 
  • #27


First thanks for all the background information:
But two things both at rest relative CMB can nevertheless have the distance between them increasing.
I agree. However, I wanted to outline a conceptual model to clarify a few ideas and what I meant by a CMB frame. Imagine a very large, but arbitrary, spherical volume of space defined by a radius [r]. On the surface of this conceptual spherical volume, there are a number of distant galaxies, which have no velocity with respect to CMB. As such, I believe they define a volume of space, which will expand in-line with the scale factor, i.e. a(t).

This volume of space contains baryon matter, radiation, cold dark matter and dark energy. As the volume expands, the energy density of the first three dilutes, but dark energy remains constant. The increasing relative % of dark energy causes the rate of expansion to accelerate. However, our reference galaxies maintain their relative position while remaining at rest with respect to the CMB. As such, they seem to provide a frame of reference within an expanding universe. So attempting to answer my own original question:
If this description is essentially correct, would a model of an expanding homogeneous and isotropic universe infer any meaning to the magnitude and distribution of the relative velocity of observable galaxies with respect to the CMB frame?
Within the context of the conceptual homogeneous model outlined, it would seem that any velocity with respect to CMB is an anomaly, which corresponds to the observation that the universe is not uniformly homogeneous, at least, on the local level. As such, most galaxies, solar systems and planets will have been shifted from CMB rest due to localised distributions of gravitational matter, while still basically conforming to the recessional velocity of an expanding universe?
 
  • #28


mysearch said:
...
Within the context of the conceptual homogeneous model outlined, it would seem that any velocity with respect to CMB is an anomaly, which corresponds to the observation that the universe is not uniformly homogeneous, at least, on the local level. As such, most galaxies, solar systems and planets will have been shifted from CMB rest due to localised distributions of gravitational matter, while still basically conforming...

That puts it very clearly. This agrees with what I think is the mainstream cosmic picture and I accept it pretty much verbatim without reservation*.

What you are saying is most motion of macroscopic objects relative CMB is due to stuff falling. Localized unevenness, differences in primordial density, would have started stuff falling from low-density regions to higher.

To the best of my knowledge (and I hope some more knowledgeable person can correct me if I am wrong) most of that kind of individual object motion would have arisen that way and must continue to arise that way.

There are violent events which can impart motion by other means, but they can't compete with simple falling to explain most motion. Examples are supernovae, and gravitational slingshot events that somehow manage to hurl compact objects and may even give them enough speed to escape from their home galaxies. But I think overall by far the most common source of anomalous motion must be simple falling.

One of the most interesting questions is: How did the primordial unevenness in matter distribution originate in the first place? One conjectured explanation traces non-uniformity to "quantum fluctuations"----I put that in quotes because I'm not sure by what mechanism the quantum fluctuations would have become actualized as real largescale non-uniformity. Who collapsed the wave function? Why isn't the cat still in two inconsistent conditions? Like everybody else I have read various discussions of this but I remain uncertain about the origin of primordial structure i.e. density variations.

If you ever read the Roman author Lucretius he traces it all back to an accidental Swerve, an accidental deviation from uniformity that triggered the whole blarsted coagulation. Like us he dimly perceived that there was a problem and like us he hopefully offered his suggestion :biggrin: In the subsequent 2000 years we apes have made enormous strides and yet certain things remain unresolved.

*apart from a few quibbles about wording.
 
  • #29


marcus said:
When people were talking some 10-15 years ago about the Great Attractor located in the direction of constellations Hydra and Centaurus in the southern hemisphere, but too far away to see or too obscured by intervening stuff, what they were really observing was a collective DRIFT of various clusters of galaxies relative to CMB rest.

There was another result like that recently, but not yet confirmed, purporting to have identifed a statistical drift in a sample of over a thousand galaxies, again relative to CMB rest.
I hope they are still talking about the Great Attractor and put these new observations in context with the GA.
I’ve seen but a few comments on this new “collective DRIFT” but they have not been clear on direction or scale of the new observations.
I can only assume it must be confirming the “Great Attractor” observations in effect and direction or they would have announced any conflict with it.

As I understand the Great Attractor observation is based on our Local Group (Less than 50 Galaxies). Do you know if the 1000 galaxies used in the “Collective DRIFT” observations included our local group and nearby.
Or might they have focused on an area away from us and Hydra-Centaurus area in an attempt to find a triangulation to the Great Attractor.
That strikes me as a good approach, picking a large area away from us and the GA should give a different angle pointed towards the GA if it is there. But if it gives a vector to a new Great Attractor Great for that distant area parallel to the vector we have to our GA, it would iindicate some form of universally Drift at an even larger scale of an asymmetric universe. That would be a new form of a non-homogenous indication.

Hope you find that reference,
some detail on how it compares to the LG GA observations should be interesting.
 
  • #30


RandallB said:
...
Hope you find that reference,
some detail on how it compares to the LG GA observations should be interesting.

Hi Randall, thanks for reminding me! I did say I would get links to the two papers by Kashlinsky et al that claimed to see indications of a "dark flow".

It wasn't confirmed (so far anyway) and flaws were found in their analysis, which Ned Wright lists here:
http://www.astro.ucla.edu/~wright/dark-flow-errors.html

He also gives links to the two papers, so I don't need to. His conclusion warns that their conclusions cannot be trusted.
 
  • #31
More Jazz

When trying in a dim sort of way to comprehend the consensus model of the universe --- the one Marcus is trying to build a 'same page to get on' about --- I've found it useful to imagine toy models in which extreme circumstances prevail. Sometimes this leads on to questions that I don't have answers for. Hence this further Jazz.

I find the vastness of the model observed universe quite unimaginable, with its remote boundary now at a proper distance of about 46 million light years. This distance is imposed by the tiny Hubble constant, presently about 2.4 x 10 ^ -19 per second.

Since I have no idea at all what determines the numerical value of the Hubble constant, I feel free to imagine a toy model that expands as absurdly fast as I like. Why not? --- Alan Guth did just this!

Sometimes I find it more comfortable to imagine a table-top model of the observed universe, choosing a Hubble constant of the order of 10 ^ +7 per second. The observed-universe boundary is then only a few tens of meters away. I also like to imagine the Hubble constant to be eternally constant, so that there are absolutely no gravitational tidal forces that can distort the shapes of everyday objects like myself, my steel ruler and mechanical tick-tock clock with which I set up coordinates and explore simple physics, as ruled by SR with the ordinary value for c.

Just as I begin to think that in this tabletop universe I could ignore such an absurd rate of expansion of the universe around me, and expect ordinary physics to prevail, I remember that in this toy universe extreme redshifts would occur for light signals transmitted between points of spacetime. I imagine that large redshifts would in this case affect the workings of atomic and particle physics, e.g. interparticle interactions.

Which brings me to the general question: GR assumes that local physics obeys the same covariant laws with the same c throughout spacetime. Should one imagine an upper limit for the Hubble constant in an expanding universe, to ensure that the expansion leaves enough local 'room' in spacetime for the workings of say, QED or QCD as we understand these theories, to remain perceptibly unaffected? Or is local physics likely to be somehow changed by very rapid expansion, as occurs in, say, the inflationary scenario?
 
  • #32


oldman said:
.
...I find the vastness of the model observed universe quite unimaginable, with its remote boundary now at a proper distance of about 46 million light years.

three orders of magnitude


This distance is imposed by the tiny Hubble constant, presently about 2.4 x 10 ^ -19 per second.

one order of magnitude

... no idea at all what determines the numerical value of the Hubble constant,

General Relativity. It's determined dynamically by GR. The Friedmann eqns derive from GR, and the first Friedmann eqn (which dates from around 1922) specifies the changing numerical value of the Hubble parameter. Indeed, the square of that parameter constitutes the righthand side of that equation. Calling it a constant was an unfortunate misnomer. You can see from the equation that the square is proportional to density, so it has to decline as the universe thins out. It has always been known not to be constant.

... I also like to imagine the Hubble constant to be eternally constant, ...

Then you are choosing to take leave of General Relativity. You'll be needing a new theory of gravity. Hope you find one and it goes all right. :biggrin:
 
  • #33


marcus said:
General Relativity. It's determined dynamically by GR. The Friedmann eqns derive from GR, and the first Friedmann eqn (which dates from around 1922) specifies the changing numerical value of the Hubble parameter. Indeed, the square of that parameter constitutes the righthand side of that equation. Calling it a constant was an unfortunate misnomer. You can see from the equation that the square is proportional to density, so it has to decline as the universe thins out. It has always been known not to be constant...


I'm not clear what your comments about orders of magnitude mean. Have I got numbers wrong, perhaps? Probably.

But let me clarify why I said that I have no idea what determines the numerical value of H.

H is, via Friedmann I, and for a spatially flat geometry, expressed in terms of two variables: namely mean density and Lambda. We have some idea of what the density of our universe is, but no idea at all of what determines Lambda. To match the presently observed H and flat geometry we accept an appropriate value for Lambda, perhaps calling it dark energy. We have no idea why Lambda, and hence our H, have the values they now do. So I still do have no idea why H is what it is!

If I choose to imagine an absurdly large value for H in my toy model, I am in effect choosing a Lambda to suit my fancy. Since there is a 10 ^ 120 discrepancy between the postulated value of Lambda and its rationale as vacuum energy, what's wrong with this liberty?

I take your point about H varying. I was trying to exclude from consideration the tidal forces that are caused by the rate of expansion varying. These might be confused with non-existent forces that are often erroneously attributed to expansion itself.

Thanks muchly for your good wishes about my getting hold a new theory of gravity. I'll remember them kindly. But for the moment I'll stick with GR!
 

1. What is superluminal speed?

Superluminal speed refers to any speed that is faster than the speed of light. In physics, the speed of light is considered the fastest possible speed, so any speed that exceeds it is considered superluminal.

2. Is superluminal speed possible?

Currently, there is no scientific evidence or theory that supports the possibility of superluminal speed. According to Einstein's theory of relativity, the speed of light is a fundamental limit and cannot be surpassed.

3. How is superluminal speed measured?

Superluminal speeds are measured in multiples of the speed of light, which is approximately 299,792,458 meters per second. For example, if an object is traveling at 2 times the speed of light, it would be considered to have a superluminal speed of 2c.

4. What are some examples of superluminal speeds?

Some examples of superluminal speeds in science fiction include warp speed in Star Trek and hyperspace in Star Wars. However, these are purely fictional concepts and have no basis in real-world physics.

5. What are the potential consequences of achieving superluminal speeds?

If superluminal speeds were possible, it would have significant implications for our understanding of physics and the universe. It could potentially violate the principles of causality and lead to time travel, as well as challenge the concept of a universal speed limit. However, as of now, these consequences are purely speculative as superluminal speeds have not been proven to be achievable.

Similar threads

Replies
28
Views
922
Replies
6
Views
372
Replies
23
Views
1K
  • Cosmology
Replies
11
Views
1K
Replies
11
Views
8K
Replies
19
Views
2K
Replies
3
Views
1K
Replies
4
Views
981
  • Cosmology
Replies
8
Views
1K
  • High Energy, Nuclear, Particle Physics
Replies
4
Views
968
Back
Top