- #1
jonmtkisco
- 532
- 1
M. Chodorowski argues in his http://arxiv.org/abs/astro-ph/0610590v3" that the concept of "expansion of space" is not necessary to explain the expansion of the universe, and therefore Occam's Razor tells us that we should consider discarding the concept entirely. He says that expansion is sufficiently described by a kinematic model of the cosmic dust particles moving apart because they've always moved apart; coupled with GR to define the effects of the dust field's self-gravity. Applying GR is also necessary to explain why the universe can't be described by a single global Minkowski inertial frame, and therefore why SR is valid only in local inertial frames.
Chodorowski cites J. Peacock who asserts that the expansion of space is fictitious locally but is valid at the global level. An interesting dichotomy that Chodorowski disagrees with. It is noteworthy that Tamara Davis (of Davis & Lineweaver fame) is the referee for Chodorowski's paper. That doesn't mean she agrees with his conclusions, but he does ascribe some semi-supportive statements to her in the form of "private conversations."
Francis & Barnes et al argue in their http://arxiv.org/abs/0707.0380v1" that the concept of the expansion of space should be preserved because it is a useful teaching and conceptual aid. They worry that "throwing the baby of an intuitive framework out with the bathwater of misconceptions leaves us only with bare mathematics, which in the case of general relativity is particularly daunting for the uninitiated, and useless as a conceptual device." But then they define their idea of the expansion of space to be "neither more nor less than the increase over time of the distance between observers at rest with respect to the cosmic fluid." They suggest that newly-created space "wells up" between particles as a result (and definitely not as the cause) of those particles moving apart in the Hubble flow. They also say that any dynamic effects attributed to the expansion of space actually are dependent on the choice of coordinates and therefore that it is fruitless to ask whether they are physically "real" phenomena.
The ongoing dialogue I briefly summarized above touches on some fundamental concepts of big bang cosmology. In that context, I thought it would be useful to start a discussion about the idea that space itself might not be expanding, and to explore what the implications might be of extending that idea to its ultimate logical conclusions. For lack of a name I'll call this the "pure kinematic-GR framework".
1. Space is nothing. In this framework, space should be viewed as nothing, simply the empty interval between quantum particles. As such, space has no structure, no geometry, no curvature, no movement, and no ether; it simply doesn't exist. What does exist are matter and radiation. Particles, pictured as a homogeneous dust field, are moving away from each other. Does their movement apart "create" new space, or are they moving through pre-existing space?
2. Expansion "of" or "through" space. Chodorowski says "Galaxies do not move through space or in space. In a Machian view, they move instead with space: they simply enable space to exist." This sounds like waffling to me, especially from an advocate of the idea that space is not expanding. If it's not expanding now, then wasn't it already there? Why not simply start with the presumption that matter is expanding through pre-existing space; that is, through pre-existing physical coordinate regions?
3. Origin and Edge of the universe. Perhaps the most fundamental ground rule of big bang cosmology is the Copernican Principle, which says that no point or direction in space is any more important than any other. At very large scales, the distribution of matter, the Hubble flow, and the anisotropies of the CMB are all observed to be highly isotropic and homogeneous. Hubble's law accurately predicts (at very large scales) that recession velocity in all directions is proportional to distance, and this is quite reasonably believed to be true for all observers regardless of where they may be located in the universe.
If particles are moving apart because they were previously moving apart, then they must have been much closer together at some earlier era. They may have been tightly packed together at t=0, or even formed a singularity. This is the generally accepted picture, with variations on the theme. However, the accepted description is that the expansion did not begin at a single point or origin; rather, it began "everywhere" in the universe, in the sense that the initial size of the universe was limited to the region where the particles were (which may have been finite or infinite), and the movement of the particles away from each other "created" new space between them, together resulting in an expanding universe. The accepted explanation may be true, but I know of no reason why it is could not also be possible that the universe had its origin at a single coordinate point in space, subsequently expanding through pre-existing empty space.
Specifically, consider the scenario in which all of the particles in the universe departed the origin coordinate simultaneously, at various speeds ranging evenly from 0 to c. As Chodorowski says, this is like the Milne model, except with gravity. As these particles travel isotropically and homogeneously in every direction away from the origin, observers on each particle (or eventually on each galaxy) would observe all of the other particles moving away from them, with speed proportional to distance. Exactly in accordance with Hubble's law.
Consider the analogy of the NY Marathon. Imagine 100,000 runners starting approximately simultaneously, but each running at a different speed. After a couple hours, the runners would be spread out across much of the course. A middling runner would observe the faster runners ahead moving away, at "comoving" speeds that vary with distance. The same runner would observe runners behind falling yet further behind, again at "comoving" speeds that vary with distance. Consider a scenario where half of the runners start the race at the same time but run in the opposite direction. One of them doesn't run at all and remains at the origin. If the speeds of the runners in both directions were perfectly distributed by speed, the middling runner in one direction looking backwards would observe that the "Hubble flow" of runners behind was continuous all the way through the origin and then through the runners going in the opposite direction of the observer. If the observer did not know that the Marathon started on the Verazzano Bridge, he'd have no way of distinguishing the origin from any other point in the "Hubble flow" of runners traveling in both directions.
Thus, even if there were a physical origin point in the universe for the expansion of the particle flow, a middling observer in the comoving Hubble flow would have no clue whatsoever to enable her to distinguish the origin from any other point in the universe. She could not even discern which direction was "towards" the origin, vs. "away" or "transverse".
If the the universe is infinite in extent, then once the Hubble flow begins, the concept of an origin becomes meaningless. Its unique identity is immediately lost forever. On the other hand, in a finite universe it would theoretically be possible to reconstruct the location of the origin if one could observe the expanding outer surface (edge) of the dust field. Clearly however, we are not in a position to make such an observation. The fact that the CMB is homogeneous in all directions from our location indicates that we are too far from the edge (if one existed) to ever detect its presence. The CMB radiation coming at us from the "edge" direction would have been traveling ever since the recombination era, and our observations currently are obscured beyond that distance. (In the event there is an "edge", please don't ask me what lies beyond it. I have nothing to say about that subject.)
By measuring the Hubble parameter we could calculate how fast the dust field (at various observable locations) is moving with respect to the origin, and how long ago it must have departed the origin. But we would still have no clue in which direction the origin lies.
4. Proper velocity measurements. The tethered galaxy exercise involves bringing a distant test particle to zero proper velocity with respect to an arbitrarily selected origin point. After the test particle is untethered, its comoving and proper motion with respect to the origin are observed. An important thing to keep in mind with respect to the pure kinematic-GR framework is that any so-called origin point defined for such a problem isn't actually a fixed location relative to the actual (but undetectable) origin coordinate of the dust field's expansion. The "exercise origin" is itself comoving with respect to the "universal" origin (if it exists). This explains why a sphere of massless test particles shot away from the exercise origin will "expand with the Hubble flow", as described in section 2.6.2 of the Francis & Barnes paper. They describe this effect (rather confusingly) as "cosmological tidal forces", and in my view incorrectly attribute it to expanding space. Tsk, tsk.
5. Superluminal recession. As both Chodorowski and Francis & Barnes explain, the observation that distant galaxies are receding from us faster than the speed of light is a phenomenon which is entirely dependent on choice of coordinates in a gravitating dust field, and it has no bearing on whether space is or is not expanding. The observed redshifts can equally well be explained as a combination of SR doppler shift and gravitational redshift.
I'll stop here and hope to generate a robust discussion.
Jon
Chodorowski cites J. Peacock who asserts that the expansion of space is fictitious locally but is valid at the global level. An interesting dichotomy that Chodorowski disagrees with. It is noteworthy that Tamara Davis (of Davis & Lineweaver fame) is the referee for Chodorowski's paper. That doesn't mean she agrees with his conclusions, but he does ascribe some semi-supportive statements to her in the form of "private conversations."
Francis & Barnes et al argue in their http://arxiv.org/abs/0707.0380v1" that the concept of the expansion of space should be preserved because it is a useful teaching and conceptual aid. They worry that "throwing the baby of an intuitive framework out with the bathwater of misconceptions leaves us only with bare mathematics, which in the case of general relativity is particularly daunting for the uninitiated, and useless as a conceptual device." But then they define their idea of the expansion of space to be "neither more nor less than the increase over time of the distance between observers at rest with respect to the cosmic fluid." They suggest that newly-created space "wells up" between particles as a result (and definitely not as the cause) of those particles moving apart in the Hubble flow. They also say that any dynamic effects attributed to the expansion of space actually are dependent on the choice of coordinates and therefore that it is fruitless to ask whether they are physically "real" phenomena.
The ongoing dialogue I briefly summarized above touches on some fundamental concepts of big bang cosmology. In that context, I thought it would be useful to start a discussion about the idea that space itself might not be expanding, and to explore what the implications might be of extending that idea to its ultimate logical conclusions. For lack of a name I'll call this the "pure kinematic-GR framework".
1. Space is nothing. In this framework, space should be viewed as nothing, simply the empty interval between quantum particles. As such, space has no structure, no geometry, no curvature, no movement, and no ether; it simply doesn't exist. What does exist are matter and radiation. Particles, pictured as a homogeneous dust field, are moving away from each other. Does their movement apart "create" new space, or are they moving through pre-existing space?
2. Expansion "of" or "through" space. Chodorowski says "Galaxies do not move through space or in space. In a Machian view, they move instead with space: they simply enable space to exist." This sounds like waffling to me, especially from an advocate of the idea that space is not expanding. If it's not expanding now, then wasn't it already there? Why not simply start with the presumption that matter is expanding through pre-existing space; that is, through pre-existing physical coordinate regions?
3. Origin and Edge of the universe. Perhaps the most fundamental ground rule of big bang cosmology is the Copernican Principle, which says that no point or direction in space is any more important than any other. At very large scales, the distribution of matter, the Hubble flow, and the anisotropies of the CMB are all observed to be highly isotropic and homogeneous. Hubble's law accurately predicts (at very large scales) that recession velocity in all directions is proportional to distance, and this is quite reasonably believed to be true for all observers regardless of where they may be located in the universe.
If particles are moving apart because they were previously moving apart, then they must have been much closer together at some earlier era. They may have been tightly packed together at t=0, or even formed a singularity. This is the generally accepted picture, with variations on the theme. However, the accepted description is that the expansion did not begin at a single point or origin; rather, it began "everywhere" in the universe, in the sense that the initial size of the universe was limited to the region where the particles were (which may have been finite or infinite), and the movement of the particles away from each other "created" new space between them, together resulting in an expanding universe. The accepted explanation may be true, but I know of no reason why it is could not also be possible that the universe had its origin at a single coordinate point in space, subsequently expanding through pre-existing empty space.
Specifically, consider the scenario in which all of the particles in the universe departed the origin coordinate simultaneously, at various speeds ranging evenly from 0 to c. As Chodorowski says, this is like the Milne model, except with gravity. As these particles travel isotropically and homogeneously in every direction away from the origin, observers on each particle (or eventually on each galaxy) would observe all of the other particles moving away from them, with speed proportional to distance. Exactly in accordance with Hubble's law.
Consider the analogy of the NY Marathon. Imagine 100,000 runners starting approximately simultaneously, but each running at a different speed. After a couple hours, the runners would be spread out across much of the course. A middling runner would observe the faster runners ahead moving away, at "comoving" speeds that vary with distance. The same runner would observe runners behind falling yet further behind, again at "comoving" speeds that vary with distance. Consider a scenario where half of the runners start the race at the same time but run in the opposite direction. One of them doesn't run at all and remains at the origin. If the speeds of the runners in both directions were perfectly distributed by speed, the middling runner in one direction looking backwards would observe that the "Hubble flow" of runners behind was continuous all the way through the origin and then through the runners going in the opposite direction of the observer. If the observer did not know that the Marathon started on the Verazzano Bridge, he'd have no way of distinguishing the origin from any other point in the "Hubble flow" of runners traveling in both directions.
Thus, even if there were a physical origin point in the universe for the expansion of the particle flow, a middling observer in the comoving Hubble flow would have no clue whatsoever to enable her to distinguish the origin from any other point in the universe. She could not even discern which direction was "towards" the origin, vs. "away" or "transverse".
If the the universe is infinite in extent, then once the Hubble flow begins, the concept of an origin becomes meaningless. Its unique identity is immediately lost forever. On the other hand, in a finite universe it would theoretically be possible to reconstruct the location of the origin if one could observe the expanding outer surface (edge) of the dust field. Clearly however, we are not in a position to make such an observation. The fact that the CMB is homogeneous in all directions from our location indicates that we are too far from the edge (if one existed) to ever detect its presence. The CMB radiation coming at us from the "edge" direction would have been traveling ever since the recombination era, and our observations currently are obscured beyond that distance. (In the event there is an "edge", please don't ask me what lies beyond it. I have nothing to say about that subject.)
By measuring the Hubble parameter we could calculate how fast the dust field (at various observable locations) is moving with respect to the origin, and how long ago it must have departed the origin. But we would still have no clue in which direction the origin lies.
4. Proper velocity measurements. The tethered galaxy exercise involves bringing a distant test particle to zero proper velocity with respect to an arbitrarily selected origin point. After the test particle is untethered, its comoving and proper motion with respect to the origin are observed. An important thing to keep in mind with respect to the pure kinematic-GR framework is that any so-called origin point defined for such a problem isn't actually a fixed location relative to the actual (but undetectable) origin coordinate of the dust field's expansion. The "exercise origin" is itself comoving with respect to the "universal" origin (if it exists). This explains why a sphere of massless test particles shot away from the exercise origin will "expand with the Hubble flow", as described in section 2.6.2 of the Francis & Barnes paper. They describe this effect (rather confusingly) as "cosmological tidal forces", and in my view incorrectly attribute it to expanding space. Tsk, tsk.
5. Superluminal recession. As both Chodorowski and Francis & Barnes explain, the observation that distant galaxies are receding from us faster than the speed of light is a phenomenon which is entirely dependent on choice of coordinates in a gravitating dust field, and it has no bearing on whether space is or is not expanding. The observed redshifts can equally well be explained as a combination of SR doppler shift and gravitational redshift.
I'll stop here and hope to generate a robust discussion.
Jon
Last edited by a moderator: