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> the finding of accelerated expansion on which the dark energy model is based Dark energy arises because of the decomposition of 4-d spacetime into volumes of

> the finding of accelerated expansion on which the dark energy model is based Dark energy arises because of the decomposition of 4-d spacetime into volumes of 3-d space each distinguishable by a time coordinate, and choosing as the timelike axis the one in which matter is -- on average and at the largest scales -- at rest against the 3-d spatial coordinates. That fixes a preferred frame -- the cosmological frame -- for observers who are comoving with the spatial coordinates. Such observers will see no dipole anisotropy in the cosmic microwave background (or the cosmic neutrino background, or other relic fields), and will be comfortable with approximating the distribution of visible matter far from them as a homogeneous isotropic fluid, in spite of local inhomogeneities and local anisotropies (like, for example, the planet each of us has between her- or himself and a large percentage of the rest of the universe). All comoving observers in such a universe will generally agree that in one timelike direction the visible matter is denser and the CMB is hotter and that in the other timelike direction the visible matter is sparser and the CMB is colder. Let's pick the former direction as the past. All observers in such a universe -- even non-comoving ones -- will agree that it takes energy to separate two massive bodies that are gravitationally bound. So in the past, when comoving matter was denser, it would take more energy to separate the bits of matter from one another than in the present, when comoving matter is sparser. Something is keeping objects like galaxy clusters fixed against the comoving coordinates, instead of giving them peculiar velocities against these coordinates as they gravitationally attract one another.[1] That is dark energy, and we can experimentally determine its value in the cosmological frame. The rate at which visible matter is diluting away along the cosmological frame's past->future direction is what what we try to measure. Standard candles are useful for helping fix a distant galaxy to comoving coordinates. As our confidence in the location of distant matter improves, so does our confidence in the average value of dark energy. There are a sizable number of relationships involving measurable quantities like angular diameter, surface brightness, emission/absorption line "forests", and so forth that help us fix galaxies in the cosmological frame of reference, so throwing questions upon only one coordinate:observable relationship is far from fatal to our estimates of dark energy density. It sure is interesting though! So, back to "accelerated expansion": dark energy represents something working against the tendency of matter to gravitatonally collapse. Whether that something is constant or evolves is something we can determine by observation. Whether that something is uniform everywhere outside galaxy clusters or not is also something we can determine by observation. Current evidence is consistent with a constant uniform feature of the action of large scale structures. In particular, while normally gravitating matter is diluting away in the past->future direction we picked out in our cosmological frame of reference, dark energy is not diluting away. If, still staying in the cosmological frame, we switch our spatial coordinates from comoving coordinates on the cosmological frame to spherical Earth-centric coordinates, we have to adjust our understanding a little. A distant galaxy has a lower radial distance at earlier times than at later times. We can identify this with a recession velocity, and use that to explain the cosmological redshift and the presence of various cosmological horizons. But this is a coordinate-dependent explanation. It is perfectly fine to stick with comoving coordinates and resort to other explanations for the cosmological redshift. Indeed, it was that galaxies at extreme distances still look like galaxies that led to the metric expansion of space in the first place: small-angular-diameter galaxy clusters can't really be moving relativistically away from us or they would look VERY different structurally. Additionally, we haven't abandoned Lorentz covariance, so treating matter receding over the horizon as moving superluminally has to be considered a non-physical artifact of a choice of coordinates and frame of reference. And sure enough, just by changing the coordinates alone, these problems disappear. [1] individual galaxies within a cluster have peculiar velocities against the comoving coordinates that the cluster's centre of momentum stays fixed against. Likewise, massive elements within galaxies (e.g. Earth) have peculiar velocities against the comoving coordinates thanks to local gravitational interactions (e.g. Earth orbits the sun). Each of these introduces an anisotropy in the view of the overall distribution of matter in the universe and in the view of the CMB by an observer with a peculiar velocity. Such observers are not strictly comoving ones. But they're generally awfully close to comoving, because they're almost certainly embedded within a larger structure that is comoving.

oldandtired 9y ago
I just enjoy the way you use all these words to say that the theoretical entities exist when experiment has not shown any evidence of their existence. Try to remember that a model is only a model and a theory is only a theory that is one possible explanation of the reality we observe. It is an attempt to get a handle on understanding the universe around us. When you and others go off and start proclaiming the actual existence of these theoretical entities without at least the experimental confirmation of such, then you are setting yourself to be considered in the same light as attendees at the local ufo conventions. It annoys me when theoretical entities are declared as existing when the evidence is only slightly in their direction or the only evidence is that they need to exist based on the current mathematical models being used. When said and done, dark energy and dark matter are mathematical entities required by the prevailing theory. The prevailing theory is only one of a number of possible models. Which one you subscribe to is up to you and your view of how the current evidence stacks up. At this point, none of the models seems to cover enough bases to make it the most likely scenario. So, for me, this is still an open question. The current prevalent model has at it basis, some, from my point of view, untenable inconsistencies. The first and foremost is that gravity (whatever model you might ascribe to) is the majority controlling force. The inconsistency is that model uses gravity to give rise to "Black Holes" and yet has not stopped the "Big Bang" from happening. When you start to play around with reconciling both entities, it becomes a bit of a problem. You then add time dilation effects due to gravity (as per the various models in use) and you get other strange quirks/inconsistencies appearing. The upshot of this is that people should be careful to not declare the actual existence of theoretical entities from incomplete models just based on belief. This just turns the supposed science into religion and religion is considered anathema amongst certain groups within the science communities. The theories and models are supposed to be helpful working explanations that can change or be thrown out based on new evidence. They have a particular usage profile and range of applicability. If you go outside of that usability or applicability range then the theory/model will start to fail. As we were taught in undergraduate engineering all those decades, you use the applicable model/theory for the specific circumstances being investigated.
- raattgift 9y ago
  > not shown any evidence of their existence What? Do you deny that highly similar spiral galaxes seen face-on are roughly similar in star-count? Do you deny the observation that there are highly similar spiral galaxies that occupy very different angular diameters on the sky? Do you deny the observation that spiral galaxies with lower angular diameters have higher redshifts? And that the redshifts squeeze the entire emissions spectrum from such galaxies? Do you deny the Lyman-alpha forest? These are all observations that someone with a decent back-yard observatory could make on his or her own. So what observations do you want to throw out? Parallax? The observation that stars within our galaxy and M31 differ on average only in the spectral redshifting from stars in the smaller-angular-diameter set that Vesto Slipher identified in 1914? Again, this is the sort of astronomy[1] you could do in a reasonably dark field near your home. > It is an attempt to get a handle on understanding the universe around us. Sure, that's what science is. And a viable theory is one that does not conflict with observations to date. Theories that are viable today can become inviable tomorrow; all it takes is a new observation. There are literally tens of thousands of theory-challenging astronomical observations made in any 24-hour period in 2017, and theorists get very excited by even the slightest inconsistency with the standard cosmology. Unfortunately such inconsistent observations are extremely rare, and tend to vanish under repeated observations (by the same and by other observatories). > At this point, none of the models seems to cover enough bases to make it the most likely scenario What exactly have you read about "the models" and the "bases"? Serious question. Is it anything more than mathless popsci articles or comments in technology-focused Internet discussion forums? > gravity to give rise to "Black Holes" and yet has not stopped the "Big Bang" from happening Go back and re-read the comment you replied to. The thrust of it is that galaxies exist even though in the past they were much closer together. Why haven't they collapsed gravitationally into something much denser? There are a variety of possible answers, but (paraphrasing myself), the most attractive one is that there is a component of the action of galaxies that acts against their mutual gravitational interactions. With suitable coordinate conditions, that can be seen as a fifth force, and with other coordinate conditions, that can be seen as a simple feature of the vacuum, and with other coordinate conditions, that can be seen as a fine balance between inertia and gravitation, and so forth. But unless you want to argue that the cosmological frame is an improper choice and defend that position, you're stuck with the reality that in that frame it is most natural to identify the action component with an energy density. Put more simply, yes indeed, the crucial question is why hasn't all the visible matter collapsed into a big black hole? Since you identified that question yourself, you're on the cusp of understanding, yet you reject the most obvious answer: there is something at work preventing the gravitational collapse of galaxies. The only additional step is that in certain frames of reference, preventing two massive objects from eventually moving closer together under their mutual gravitational interaction requires work to be done. (Maybe there is an intermediate step: why don't galaxies individually collapse into giant black holes? Or why haven't the planets crashed into the sun? In both cases there is a lot of angular momentum that would have to be radiated away from the orbiting objects in order that they can get close enough for the gravitational interaction to produce a collision. That takes a long time. Gödel proposed that the entire universe is rotating around a central axis[2] so therefore there is a lot of angular momentum keeping distant galaxies in an orbit rather than colliding with nearby (to us) galaxies. Rotating metrics have been studied (thanks in large part to Gödel) and they produce very different observables of distant galaxies than the ones we have.) > the supposed science into religion and religion is considered anathema amongst certain groups within the science communities Georges Lemaître -- referenced in [1] below -- sure was religious! Nobody doubted his astrophysics chops. There are plenty of working cosmologists today who hold to some faith or other (they're certainly not all Christians! or even monotheists!). They go to where observations, experiments and other types of material evidence take them. As long as one does that, nobody will care what she or he believes about spirituality or even the presently extremely hard to observe early universe. But you, who does not seriously challenge the observations or evidence, refuse to entertain their implications, and do not offer up anything to explain why other than that you argue (wrongly) that others' collaborative efforts to rigorously and formally follow those implications are "religious" because inter alia (a) incompleteness of models means the whole model is entirely wrong (b) General Relativity is self-inconsistent (it sure isn't!) (c) General Relativity is not a successful tool at the relevant length scales (d) strong gravity is in play at length scales in which one can observe galaxies at all. Your argument is plainly based on ignorance and personal incredulity. It is easy to fix the ignorance part: there is a lot you can read; you might start with something like Carroll's textbook[3] working your way to Chapter 8. Wanting to reject the standard cosmology just because it doesn't sit right with your intuition is a character trait you'll just have to wrestle with, like any good scientist has to from time to time. But when your intuition isn't even informed and you insist that the standard cosmology (which you do not understand) is wrong, well, people will form adverse opinions. - -- [1] https://arxiv.org/abs/1108.4864 https://arxiv.org/abs/1108.4864 [2] https://www.wikiwand.com/en/G%C3%B6del_metric#/Cosmological_interpretation https://www.wikiwand.com/en/G%C3%B6del_metric#/Cosmological_... [3] https://www.goodreads.com/book/show/259680.Spacetime_and_Geometry https://www.goodreads.com/book/show/259680.Spacetime_and_Geo...

oldandtired 9y ago

I just enjoy the way you use all these words to say that the theoretical entities exist when experiment has not shown any evidence of their existence. Try to remember that a model is only a model and a theory is only a theory that is one possible explanation of the reality we observe. It is an attempt to get a handle on understanding the universe around us. When you and others go off and start proclaiming the actual existence of these theoretical entities without at least the experimental confirmation of such, then you are setting yourself to be considered in the same light as attendees at the local ufo conventions. It annoys me when theoretical entities are declared as existing when the evidence is only slightly in their direction or the only evidence is that they need to exist based on the current mathematical models being used. When said and done, dark energy and dark matter are mathematical entities required by the prevailing theory. The prevailing theory is only one of a number of possible models. Which one you subscribe to is up to you and your view of how the current evidence stacks up. At this point, none of the models seems to cover enough bases to make it the most likely scenario. So, for me, this is still an open question. The current prevalent model has at it basis, some, from my point of view, untenable inconsistencies. The first and foremost is that gravity (whatever model you might ascribe to) is the majority controlling force. The inconsistency is that model uses gravity to give rise to "Black Holes" and yet has not stopped the "Big Bang" from happening. When you start to play around with reconciling both entities, it becomes a bit of a problem. You then add time dilation effects due to gravity (as per the various models in use) and you get other strange quirks/inconsistencies appearing. The upshot of this is that people should be careful to not declare the actual existence of theoretical entities from incomplete models just based on belief. This just turns the supposed science into religion and religion is considered anathema amongst certain groups within the science communities. The theories and models are supposed to be helpful working explanations that can change or be thrown out based on new evidence. They have a particular usage profile and range of applicability. If you go outside of that usability or applicability range then the theory/model will start to fail. As we were taught in undergraduate engineering all those decades, you use the applicable model/theory for the specific circumstances being investigated.

raattgift 9y ago
> not shown any evidence of their existence What? Do you deny that highly similar spiral galaxes seen face-on are roughly similar in star-count? Do you deny the observation that there are highly similar spiral galaxies that occupy very different angular diameters on the sky? Do you deny the observation that spiral galaxies with lower angular diameters have higher redshifts? And that the redshifts squeeze the entire emissions spectrum from such galaxies? Do you deny the Lyman-alpha forest? These are all observations that someone with a decent back-yard observatory could make on his or her own. So what observations do you want to throw out? Parallax? The observation that stars within our galaxy and M31 differ on average only in the spectral redshifting from stars in the smaller-angular-diameter set that Vesto Slipher identified in 1914? Again, this is the sort of astronomy[1] you could do in a reasonably dark field near your home. > It is an attempt to get a handle on understanding the universe around us. Sure, that's what science is. And a viable theory is one that does not conflict with observations to date. Theories that are viable today can become inviable tomorrow; all it takes is a new observation. There are literally tens of thousands of theory-challenging astronomical observations made in any 24-hour period in 2017, and theorists get very excited by even the slightest inconsistency with the standard cosmology. Unfortunately such inconsistent observations are extremely rare, and tend to vanish under repeated observations (by the same and by other observatories). > At this point, none of the models seems to cover enough bases to make it the most likely scenario What exactly have you read about "the models" and the "bases"? Serious question. Is it anything more than mathless popsci articles or comments in technology-focused Internet discussion forums? > gravity to give rise to "Black Holes" and yet has not stopped the "Big Bang" from happening Go back and re-read the comment you replied to. The thrust of it is that galaxies exist even though in the past they were much closer together. Why haven't they collapsed gravitationally into something much denser? There are a variety of possible answers, but (paraphrasing myself), the most attractive one is that there is a component of the action of galaxies that acts against their mutual gravitational interactions. With suitable coordinate conditions, that can be seen as a fifth force, and with other coordinate conditions, that can be seen as a simple feature of the vacuum, and with other coordinate conditions, that can be seen as a fine balance between inertia and gravitation, and so forth. But unless you want to argue that the cosmological frame is an improper choice and defend that position, you're stuck with the reality that in that frame it is most natural to identify the action component with an energy density. Put more simply, yes indeed, the crucial question is why hasn't all the visible matter collapsed into a big black hole? Since you identified that question yourself, you're on the cusp of understanding, yet you reject the most obvious answer: there is something at work preventing the gravitational collapse of galaxies. The only additional step is that in certain frames of reference, preventing two massive objects from eventually moving closer together under their mutual gravitational interaction requires work to be done. (Maybe there is an intermediate step: why don't galaxies individually collapse into giant black holes? Or why haven't the planets crashed into the sun? In both cases there is a lot of angular momentum that would have to be radiated away from the orbiting objects in order that they can get close enough for the gravitational interaction to produce a collision. That takes a long time. Gödel proposed that the entire universe is rotating around a central axis[2] so therefore there is a lot of angular momentum keeping distant galaxies in an orbit rather than colliding with nearby (to us) galaxies. Rotating metrics have been studied (thanks in large part to Gödel) and they produce very different observables of distant galaxies than the ones we have.) > the supposed science into religion and religion is considered anathema amongst certain groups within the science communities Georges Lemaître -- referenced in [1] below -- sure was religious! Nobody doubted his astrophysics chops. There are plenty of working cosmologists today who hold to some faith or other (they're certainly not all Christians! or even monotheists!). They go to where observations, experiments and other types of material evidence take them. As long as one does that, nobody will care what she or he believes about spirituality or even the presently extremely hard to observe early universe. But you, who does not seriously challenge the observations or evidence, refuse to entertain their implications, and do not offer up anything to explain why other than that you argue (wrongly) that others' collaborative efforts to rigorously and formally follow those implications are "religious" because inter alia (a) incompleteness of models means the whole model is entirely wrong (b) General Relativity is self-inconsistent (it sure isn't!) (c) General Relativity is not a successful tool at the relevant length scales (d) strong gravity is in play at length scales in which one can observe galaxies at all. Your argument is plainly based on ignorance and personal incredulity. It is easy to fix the ignorance part: there is a lot you can read; you might start with something like Carroll's textbook[3] working your way to Chapter 8. Wanting to reject the standard cosmology just because it doesn't sit right with your intuition is a character trait you'll just have to wrestle with, like any good scientist has to from time to time. But when your intuition isn't even informed and you insist that the standard cosmology (which you do not understand) is wrong, well, people will form adverse opinions. - -- [1] https://arxiv.org/abs/1108.4864 https://arxiv.org/abs/1108.4864 [2] https://www.wikiwand.com/en/G%C3%B6del_metric#/Cosmological_interpretation https://www.wikiwand.com/en/G%C3%B6del_metric#/Cosmological_... [3] https://www.goodreads.com/book/show/259680.Spacetime_and_Geometry https://www.goodreads.com/book/show/259680.Spacetime_and_Geo...