241–250 of 302 posts
Earlier quoted context omitted.
Are they pretty sure it’s different than antimatter? Last I heard we weren’t sure if it has negative gravity or not.
If it's antimatter why isn't it coalesced into antigalaxies that we would be able to see visibly, and is instead diffused as a dark fluid?
Earlier quoted context omitted.
You're talking about relativistic mass, but that isn't a commonly used concept anymore. Today it is indeed the norm to think of "mass" as being the mass at rest. Massless particles are really massless and have momentum without having mass.
In which case, we seem back to having two things distorting spacetime. Could mass just be the momentum of massless particles going in really small circles such that you can pack them together and call them matter?
Earlier quoted context omitted.
You're talking about relativistic mass, but that isn't a commonly used concept anymore. Today it is indeed the norm to think of "mass" as being the mass at rest. Massless particles are really massless and have momentum without having mass.
In which case, we seem back to having two things distorting spacetime. Could mass just be the momentum of massless particles going in really small circles such that you can pack them together and call them matter?
Stupid non-physicist question here: we know the universe is expanding, but where does it expand at? Are there physical areas which we can observe growing? And if so, shouldn't that mean that we observe distant objects as lighter than their interactions with closer objects would imply?
Space expands everywhere, also right in front of you, it is just a very small effect over short distances. Two points one meter apart expand away from each other with a speed of 0.07 nanometers per year. For short distances electromagnetic and strong forces just pull everything immediately back together as things want do move apart. Over larger distances, think the scale of galaxies, gravitational forces keep the sta…
No, what you wrote is contradicted by evidence from within the inner solar system (MESSENGER's observations of solar mass loss were highly sensitive; we have excellent VLBI too), from other star systems, especially eclipsing binaries and systems with occluding planetary systems) and from galactic dynamics (LSR tests, peculiar motions, and lots of DM/dynamics evidence for galaxies at different redshifts). It is not how the standard model of cosmology works either. Grossly, 0.07 nanometers per metre per year expansion everywhere would be impossible to hide from solid state physics, and even several everyday polymers and ceramics; an expansion term would have to appear in accurate descriptions.
We cannot say there is no expansion at these scales, but the expansion is highly constrained and evidence requires that it effectively vanishes. A fifth-force mechanism like quintessence requires a shutdown mechanism inside galaxies that contain star systems like ours or TRAPPIST-1 or PSR~J0337+1715. Additionally it likely would need a shutdown in small structures (like asteroids) ejected from galaxy clusters by violent events, although we will not spot those in practice any time soon. Where there's a shutdown mechanism there's also a wake-up mechanism that has to be considered too, and high-redshift observations constrain the wakeup of quintessence action to relatively late times. What suppressed quintessence for a bit more than three billion years after the formation of the cosmic microwave background?
These type of modellers run into the hard problem that as they work out the parameters of their theory, they (so far) find they all are consistent with the standard cosmology. (That shouldn't be too surprising; LambdaCDM, the standard cosmology, was carefully built to concord with evidence "forward-compatibly".)
One way to put it is in the name: cosmic expansion, rather than universal expansion, distinguishing between effects apparent at the largest scales, and effects apparent everywhere in the universe, at all scales.
LambdaCDM is a model of an universe well-described by an expanding Robertson-Walker metric and matter in the large obeying the Friedmann equations. The expanding metric includes a term for the cosmological constant. The matter is essentially a space-filling fluid at rest, isotropic, homogeneous, and being diluted away by the expansion of the background Robertson-Walker spacetime. (It is in a special frame of reference in this model in which Dark Energy arises as a non-diluting homogeneous fluid imposing constant isotropic tension on the matter fluids. In general frames Dark Energy is just the cosmological constant.)
On the LambdaCDM model we can overlay the 'swiss cheese' model; this is standard too, but we are departing from cosmology and heading toward astrophysics. The motivation of 'swiss cheese' is simple: at the largest scales, the Friedmann Robertson-Walker model sketched above describes all observations between well and extremely well. However, it does not describe gravitationally-bound systems like galaxy clusters, galaxies, or star systems. Those are much much better described with conventional clumping matter (and dark matter) inside a collapsing Tolman spacetime. Notably, none of these systems are homogeneous (they contain lumpy bits like planets and stars, and sparse bits like the interstellar medium) and all of them contain radiating orbiting material; that they are radiating implies gravitational collapse (there's also plenty of other evidence for that). The Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological model does not describe these systems.
In 'swiss-cheese', the FLRW cosmological model is the cheese. If we treat the cosmological-scale matter fluid as a dust that is only homogeneous at the largest scales, the dust can be lumpier in some places and sparser in others. The lumpiest bits represent galaxy clusters that remain gravitationally bound over cosmological times. We then "swiss" this lumpy cheese procedurally: we cut out a region of the Friedmann matter on the Robertson-Walker background and replace it with a region of galaxy-cluster matter on a Tolman background, and stitch the two together using junction conditions on the boundary. (Typically we do this to the densest lumps, the galaxy clusters, but we could do the same procedure in deep intergalactic space instead. A spherical region of that will not be exactly void, since it will contain at least photons and neutrinos produced in the early universe, likely a bit of baryonic matter from big bang and supernova nuclear synthesis, and possibly a small amount of dark matter too: however all this stuff is so sparse that it would not generate a collapsing spacetime metric -- calculated out, it would resemble a slightly perturbed expanding Robertson-Walker metric. So we don't put "holes" there; our "holes" contain galaxies.).
In a swiss-cheese model there is simply no expansion in the "holes", because the Tolman backgrounds that work do not contain a cosmological constant term. (The typical problem is that something that does contain expansion, like a Kottler vacuole, evolves away from a mass-compensating comoving void, e.g., the hole grows much too fast or in strange ways that do not match any of the tens of millions of galaxy clusters in our sky).
Since Tolman/FRW swiss cheese models [a] are tractable in practice and [b] match observations extremely well, it is perfectly reasonable to take the hint that nature doesn't expand at all inside the holes. That is, the non-expansion of the hole background carries down hierarchically; you don't need to introduce an expansion term into a metric describing an individual star or planet.
It would be very exciting to find any metric expansion within the solar system, or in another solar system under close scrutiny, or at any scale smaller than that of the swiss-cheese "holes".
Since real matter systems like stars and planets can be very well described as a "hole" generating a metric like Schwarzschild out to some boundary, there is no motivation to add in a metric expansion term (it would just vanish within the boundary).
Finally, inhomogeneous cosmologies generally import these results, just like they import the local and global measurements of H_0. The viable scale of fluctuations in the Hubble parameter in such approaches is large compared to solar systems. So there's no relief in a "well, your model is wrong" (incidentally, I'd agree, there are some real problems with swiss-cheese).
In summary, the evidence is against your claim, and other strands of evidence support theoretical frameworks in which there is no metric expansion around Earth/Earth-Moon. That includes frameworks in which one has explicit terms for such expansion: those terms must vanish around here.
- --
See also: the much less wordy way of taking the same position at http://curious.astro.cornell.edu/about-us/97-the-universe/ga...
Compare: https://medium.com/starts-with-a-bang/ask-ethan-if-the-unive...
Earlier quoted context omitted.
If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model. GR models gravity as fluctuations in the metric tensor of spacetime, which is a symmetric rank-2 tensor. This is all you need to know to conclude that a graviton (a quantize fluctation of that tensor) must be spin-2. https://en.wikipedia.org/wiki/Graviton mentions "[I]t can be shown…
Well, in classical GR, which is still our best and most accurate theory of gravity, gravity is only an apparent force and thus isn't (or can't be) mediated by anything. Stating as fact that a graviton must exist or otherwise GR "would be wrong" is a bit of stretch IMHO. The graviton is plausible speculation, backed by some theoretical justifications, but nothing more yet.
as far as I understand, the space-time curvature is just an interpretation of the math. Another perfectly acceptable interpretation is as a relativistic classical field that couples with the stress-energy tensor field (thus, indirectly, with any other field).
In fact I think it is possible to come up with a space-time curvature interpretation of the EM field, for a toy universe where everything has a EM charge.
Now, given that the gravitational field couples with other fundamental fields and those are quantized, the gravitational field must necessarily also be quantized. The math straight forward quantization works fine for low energies, thus the spin-2 boson described elsethread. The issue is that at low energy, the gravitational effects are so small that it is impossible to come up with an experiment that would detect the difference between a classical field and a quantized field.
At high energy the straightforward derivation breaks down because of infinities (other quantum fields had similar issues, but the math tricks used to resolve them do not work with the gravitational field). There are multiple theories (string theory, loop gravity, etc) that try to resolve this problems, but the experimental apparatus required to distinguish between them are colossal (as in particle accelerators with radii measured in AU).
Earlier quoted context omitted.
Well GR is potentially very wrong both at very large scales (seemingly requiring dark matter and dark energy) and very small scales (due to known present incompatibilities with quantum mechanics). It's only been shown to be a highly accurate model everywhere in between.
I think both dark matter and dark energy are perfectly consistent with GR. Hitherto unobserved matter would contribute to the stress energy tensor and dark energy is (I believe - has this changed?) the cosmological constant.
One model for dark energy is indeed the cosmological constant, but there are other approaches, most notably quintessence (https://en.wikipedia.org/wiki/Quintessence_(physics)).
Earlier quoted context omitted.
Space expands everywhere, also right in front of you, it is just a very small effect over short distances. Two points one meter apart expand away from each other with a speed of 0.07 nanometers per year. For short distances electromagnetic and strong forces just pull everything immediately back together as things want do move apart. Over larger distances, think the scale of galaxies, gravitational forces keep the sta…
> Space expands everywhere, also right in front of you, it is just a very small effect over short distances ... .07 nanometers per year but two points one kilometer apart move away with 70 nanometers per year because there are a thousand meters in between them No, what you wrote is contradicted by evidence from within the inner solar system (MESSENGER's observations of solar mass loss were highly sensitive; we have e…
"Grossly, 0.07 nanometers per metre per year expansion everywhere would be impossible to hide from solid state physics, and even several everyday polymers and ceramics; an expansion term would have to appear in accurate descriptions."
On a small scale, the effects of the expansion are overcome by other forces -- this doesn't mean that the expansion doesn't occur - merely that its effect is locally overcome and so it doesn't pull apart arrangements of atoms and molecules which are bound together by other forces.
This is even described in your second link.
Earlier quoted context omitted.
> "Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact." From what I have seen they can get precise post-dictions but not precise pre-dictions because there are so many free parameters (basically you can put dark matter wherever you need it). > "I've given you the link to the complete articles and also highlighted the major points. Try to read and u…
> The claimed predictions from the 1970s and 1980s are not mentioned on that page. They are mentioned, they just aren't elaborated: you have to do your own homework if you apparently "understand" the subject enough to claim your own opinion. A begin of that homework, however, as I've already written, you even have on the initial page that you chose to post, specifically: "2013: ΛCDM provides an excellent fit to the i…
No, they were not mentioned. If they were, you would be quoting it.
>'"2013: ΛCDM provides an excellent fit to the improved CMB data"
As I've said, that's the "item 3" from the 5 item list, and it's ultimately confirmed by ESA's Planck satellite results in 2013. That is a strong confirmation.'
You have to read the entire quote. This "fit" was a post-diction, not a pre-diction: "in order to obtain a fit, we have had to nearly double the baryon density over what it was so confidently known to be before it wasn't."
Post-dictions can be interesting as part of model development but are unable to confirm a model. The new, (possibly adjusted) model then need to be checked against new data.
Rubbish, it's turtles all the way down!
Earlier quoted context omitted.
Wouldn't that allow for a "perpetual motion" setup where a blob of normal mass is in front of a blob of negative mass? The negative mass would push the positive mass forward, and then be attracted to it and follow. What am I misunderstanding?
It would be perpetual motion, but momentum and energy would remain the same. Both depend on mass (so negative mass accelerating creates negative momentum and energy).