Cosmology question...

[ollypenrice]

I was discussing the expansion of the Universe with a guest and explaining that the expansion of the Universe exempts galaxy recession velocities from the speed limit of c and generates the idea of the observable and unobservable universe. But then we thought about what the difference is between proper motion and recession due to the Hubble flow.

I suggested that proper motion involves a galaxy going into an accelerated frame of reference (thereby invoking general relativity and the speed limit) whereas the expansion of the universe does not put galaxies into any kind of acceleration. The speed limit, therefore, does not apply.

Is this a reasonable explanation or is it incorrect?

Olly

[George Jones]

Short answer: in cosmology, the speed of light is strictly greater than the speed of light!

Suppose distant galaxy A has cosmological recessional speed v_rec with respect to us, and, at A, there is a particle launched in a direction away from us with speed v with respect to A. With respect to us, the speed of the particle is V = v_rec + v. This applies even to light (photons) at A. Then, the speed with respect to A is v = c, and with respect to us is V = v_rec + c. For photons at A, V for us (the first "speed of light" in the opening sentence) is greater than c (the second "speed of light" in the opening sentence).

However, v_rec < V, i.e., with respect to us, the recessional speed at A is less than the speed of light at A.

Also, it is a (persistent) myth that general relativity is required to handle acceleration.

Hopefully, I will get a chance to post a much more detailed answer.

[acey]

All velocities are relative to something and for the universe as a whole the natural rest frame is that of the Hubble flow, i.e. if everything is at rest with respect to the cosmological expansion then all measured redshifts will be exactly proportional to distance. Any deviation constitutes the relative motion of source and receiver with respect to this. For very large distances the deviation tends towards zero, i.e. the universe as a whole is indeed seen to be at rest relative to the comoving co-ordinates defined by the Hubble flow. The universe is getting bigger but the little bits riding on it are effectively standing still while space expands.

[Tiki]

I was discussing the expansion of the Universe with a guest and explaining that the expansion of the Universe exempts galaxy recession velocities from the speed limit of c ...

How is this? Surely only an observer unaware that he lives in a universe with an increasing Hubble Constant (ie, dH_o/dt>0 ) might erroneously record superluminal velocities of long travelled photons. The observer is ignorant of the proper distance.

Only the incautious have so far recorded velocities > c.

[ollypenrice]

How is this? Surely only an observer unaware that he lives in a universe with an increasing Hubble Constant (ie, dH_o/dt>0 ) might erroneously record superluminal velocities of long travelled photons. The observer is ignorant of the proper distance.

Only the incautious have so far recorded velocities > c.

I'm not suggesting that that anyone has recorded velocities greater tha c.

I suspect that I may be missing yout point/joke!!

Olly

[George Jones]

Surely only an observer unaware that he lives in a universe with an increasing Hubble Constant (ie, dH_o/dt>0 ) might erroneously record superluminal velocities of long travelled photon.

The Hubble constant has been decreasing for the entire age of the universe, accelerated expansion notwithsatnding.

[Tiki]

I suspect that I may be missing yout point/joke!!

I think it is me who's been missing the point.

http://arxiv.org/pdf/astro-ph/0310808v2.pdf is well worth a look.

[George Jones]

I think it is me who's been missing the point.

http://arxiv.org/pdf...h/0310808v2.pdf is well worth a look.

This is a very famous paper.

V = v_rec + v

This equation is equations (19) and (20) from the paper.

The Hubble constant has been decreasing for the entire age of the universe, accelerated expansion notwithsatnding.

1) Positive R double dot means accelerated expansion. 1) Negative H dot means decreasing Hubble constant. Differentiating the expression (given, e.g., between equations (25) and (26)) for the Hubble constant H = (R dot)/R shows that 1) and 2) are not mutually exclusive. In fact, 2) have been true for the entire observable lifetime of the universe (including now), and 1) is currently true.

Sorry for the technical turn of the thread.

[sam mayo]

so fish =pie * laterally, god this thread makes me feel like a teletubby at a physics convention i.e means im thick as pigs ****

[ollypenrice]

Thanks for the input and particularly for the link to the paper - which I will need to read eight times at least.

However, what I'm looking for above all is a short qualitiative description of the difference between movement in the normal sense and movement as in recession due to the Hubble Flow. Is it that a pair of superluminally receding galaxies are still more or less at rest with regard to the Hubble Flow?

Olly

[Tiki]

Thanks for the input and particularly for the link to the paper - which I will need to read eight times at least.

However, what I'm looking for above all is a short qualitiative description of the difference between movement in the normal sense and movement as in recession due to the Hubble Flow.

Not sure what you mean by 'normal' but understanding the 'proper distance' is key. It could be that you are trying to compare events describable in terms of local reference frames rather with events that aren't.

Is it that a pair of superluminally receding galaxies are still more or less at rest with regard to the Hubble Flow?

I am yet to get my head around the superluminal stuff but I don't see why not.

[Tiki]

Not sure what you mean by 'normal' but understanding the 'proper distance' is key. It could be that you are trying to compare events describable in terms of local reference frames rather than with events that aren't.

I am yet to get my head around the superluminal stuff but I don't see why not.

Edited the only way i know.

[George Jones]

Thanks for the input and particularly for the link to the paper - which I will need to read eight times at least.

However, what I'm looking for above all is a short qualitiative description of the difference between movement in the normal sense and movement as in recession due to the Hubble Flow. Is it that a pair of superluminally receding galaxies are still more or less at rest with regard to the Hubble Flow?

Olly

It is true that very distant galaxies are (nearly) at rest with respect to the Hubble flow, but I think that the real "problem" is that there is a mismatch between "speed" as defined in special relativity and "speed" as defined in cosmology. I would like to write a better version of the following:

the simple arithmetic for calculating speed and distances doesn't apply when the space you are trying to measure distances in is warped, not just by gravity but by time.

Exactly. Because of spacetime curvature between "over there" and "over here", it is difficult to define the speed of an object "over there" with respect to us "over here" in a way that respects all of our everyday experiences with speed. This leads to a first explanation for the possibility of recessional speeds greater than the speed of light.

Special relativity prohibits speeds greater than the speed of light. Cosmology, however, is governed by the curved spacetime of general relativity, to which special relativity is a good *local* approximation. Consequently, we will never see anything moving faster than the speed of light in our local neighbourhood. Stuff at the edge of the universe is not in our local neighbourhood, and thus is not governed by the laws of special relativity.

Alternate (more technical) explanation for recessional speeds greater than the speed of light.

speed = distance/time, so if different definitions of distance and time are available, we can have have differing definitions of speed. The definitons of distance and time used in cosmology lead to cosmological recessional velocities that correspond not to velocity in special relativity, but to something different called rapidity (sometimes called the "velocity parameter"). In special relativity, there is a relationship between velocity and rapidity, which, for some reason is not used in cosmology. If this relationship were used in cosmology, then a recession rapidity of 3.4 corresponds to a recessional speed of 0.998 times the speed of light.

[ollypenrice]

Thanks, I think I'm getting there! 'Normal movement' wasn't a very helpful phrase, I know. What I meant was the kind of movement we create when we apply a force to a body to accelerate it, as in a rocket, for instance. GR limits the maximum speed attainable by the rocket. However, once the rocket reaches cosmological distances its recession from us will also be driven by the expansion of the universe (by comparison with which its rocket motor will be insignificant) and at a sufficient distance the Hubble Flow will take its recession beyond c. Is that correct? (I realize that such a rocket is hypothetical.)

Olly

[Treeden]

... and at a sufficient distance the Hubble Flow will take its recession beyond c. Is that correct?

Would that be "... beyond c relative to us"?

Please excuse my interjection Olly, but I've been following this with some interest, and although it is beyond my understanding at present, I am keen to try and get my head round it.

Rob

[ollypenrice]

Would that be "... beyond c relative to us"?

Please excuse my interjection Olly, but I've been following this with some interest, and although it is beyond my understanding at present, I am keen to try and get my head round it.

Rob

Hi Rob, welcome aboard! Yes, beyond c relative to us. This part of the universe will never communicate with us because the Hubble Flow is driving it away from us beyond c so its light will never reach us.

Olly

[Treeden]

Hi Rob, welcome aboard! Yes, beyond c relative to us. This part of the universe will never communicate with us because the Hubble Flow is driving it away from us beyond c so its light will never reach us.

Olly

Thanks Olly. My understanding of this is like a 1000 piece jigsaw that I have emptied onto the floor in front of me. I'm currently working my way through the edge pieces so I can build the main structure. Then, hopefully, I can start filling in the detail.

It might take a while but I know it will be a beautiful picture when it's finished.

[Tiki]

..... This part of the universe will never communicate with us because the Hubble Flow is driving it away from us beyond c so its light will never reach us.

Olly

I believe that this light can actually reach us. A superluminal photon heading in the direction of away could quite easily scattered back towards us. The chief point being (see George above) that it will only have a velocity of 'c' should we measure its velocity in our local reference frame.

[grey]

While many main stream "scientists" claim that the Big Bang and inflation are correct, i put forward to you a more plausible theory that some cosmologists and plasma physicists are slowly but surely bringing to light..

(noteables: I. Velikovsky, K. Birkeland, I. Langmuir, H. Alfven, H. Arp, A. Perrat)

Alfven contended that the energetic activity ascribed to "black holes" could be better explained by electromagnetic forces and that these forces shape galaxies..

"Today's scientists have substituted mathematics for experiments, and they wander off through equation after equation, and eventually build a structure which has no relation to reality:" - Nikola Tesla

hec, mathematicians proved that heavier than air flight was impossible! Don't get me wrong, Math is critical and a vital tool for science, but let's not put the cart before the horse.

gravity is an infinitesimally week force and does not allow for credible explanations of what is really going on out there.. the big bang, black holes and galactic formation has been made more esoteric than ever by mathematicians who now dominate the realm of cosmology and spread all kinds of assumptions and fudge factors to validate absurd theories. dark matter, dark energy, these are what we in the academic world call "major" fudge factors.. how is it that 70% of the universe is comprised of dark energy (credited for the expansion of space, another questionable theory) and dark matter (invisible, undetectable and not made of "normal" stuff) accounts for 28% of the universe and only 2% left for "normal" matter??

expansion (inflation theory) - based on Hubble's law that the red-shift measured in galaxies is directly proportional to their distances, hence the further a galaxy is the faster it's moving away, not so, there have been many peculiar objects that directly contradict this assumption, most notably NGC-7603 that has a red-shift of 8700Km/sec while a quasar connected to the galaxy via a filament has a red-shift of 16800km/sec, and recent discoveries of 2 more quasars within the connecting filament have red-shifts of 117000km/sec and 72000km/sec... this is completely contrary to currently accepted theories... there are too many examples to cite but i leave you all with this.. would Hubble have come to the same conclusions if he had measured quasi-stellar red-shifts? i don;t think so, if not for Hubble's law, there is no need to run the clock back into a singularity, therefore, no big bang.. let alone the CMB (cosmic background radiation) temperature measurements which do not coincide with big bang predictions of 50 Kelvin, CMB is 3K, more akin to a stable non-expanding universe (predicted almost to the decimal place in a stable, non-expanding universe model)

a plasma universe.. 99.99% of all we see in the universe can be reproduced in labs and scaled to cosmological magnitudes when using plasma physics to test and explain it all

electromagnetism can explain just about everything we observe in the universe today without the need for dark stuff.. and EM is 39 magnitudes, that's 1 with 39 zeros, stronger than gravity, and is more plausible than the gravitational model. I personally think gravity is a weak component of the electromagnetic force and not a separate force at all as is currently theorized.. that is more likely the case than a dark theory, and not anywhere as far out there. Recently scientists at CERN have discovered that the weak nuclear force and strong nuclear forces actually "look and feel" very much like the electromagnetic force at higher energy states.. now there's a unified theory if i ever heard one.. all forces are fundamentally EM.

Einstein himself was quoted to say that not even he understood relativity after it was invaded by mathematicians, nor did he believe that black holes were possible.... he was also quoted to say that relativity left him feeling like something was missing.. let's face it quantum physics and relativity are completely disjointed. Plasma physics can account for the very large and the very small... a "unified" theory.

do yourselves a favor, research it yourselves and draw your own conclusions. Cosmology today has been made to be a realm of funky math...

big bang - i don't think so, electric universe - more likely..

lookup plasma cosmology or electric universe

just my "hubble" opinion

clear skies

[grey]

excuse the double vid... pasted twice by mistake

[grey]

this may interest you as well

http://www.youtube.com/watch?v=P4zixnWeE8A