Dark Gravity discussion

[PB666]

2 hours ago, Steel said:

I believe there are still some WIMP candidates still in play, like the right-handed neutrino.

Also the above study has not actually ruled out axions totally, they could still exist with a mass outside the range that was excluded by the observations.

Unlikely, I think WIMPs are now pretty much excluded.

 

[PB666]

https://www.unige.ch/communication/communiques/en/2017/cdp211117/

 

[Steel]

On 25/11/2017 at 1:51 PM, PB666 said:

https://www.unige.ch/communication/communiques/en/2017/cdp211117/

 

http://backreaction.blogspot.co.uk/2017/11/astrophysicist-discovers-yet-another.html?spref=tw&m=1

[PB666]

3 minutes ago, Steel said:

http://backreaction.blogspot.co.uk/2017/11/astrophysicist-discovers-yet-another.html?spref=tw&m=1

Bottom line is that he creates a set of consistencies but does not explain the require time variance in space-time.

He could be right, but does not know enough or have enough insights to gauge variance to do the math. Part of the problem with early theory of relativity is that Einstein did the same, and later found the mathematical solution, but now in a post-relativity physics should anyone be allowed to do the same. Remember he advocated a cosmological constant. I still think the idea of a cosmological constant is wrong and even if it was right its not consistent over time and not what Einstein thought it was . . . .It was a guess.

My point is that OP-author may or may not be right, but people need to stop looking for massive particles and start looking at gravity itself.

[Steel]

14 minutes ago, PB666 said:

My point is that OP-author may or may not be right, but people need to stop looking for massive particles and start looking at gravity itself.

I didn't think either approach should be ruled out or favoured over the other until the evidence say so. 

There are tens, if not hundreds, of alternative theories of gravity to vanilla GR, and almost universally they agree with some observations but not others, or are not self-consistent.

[YNM]

On 25/11/2017 at 8:51 PM, PB666 said:

https://www.unige.ch/communication/communiques/en/2017/cdp211117/

 

1 hour ago, Steel said:

http://backreaction.blogspot.co.uk/2017/11/astrophysicist-discovers-yet-another.html?spref=tw&m=1

I guess it all comes down to the "openness" of EFE.

I can't quite read EFE straight off with the tensors, but I can read Friedmann equation. Honestly, there's no limit in what unknowns you want to throw in. Curvature and gravitational masses is just one thing. You can add electromagnetical or some other sort of things, and you can have them follow different power relation with spatial distance.

The addition of multiple dark-energy aspects has been tried from some time when computational powers allows it. I've seen people proposing multiple dark matters. So it's not so much a big deal, for you have a really hard time knowing...

The "problem" with dark gravity that I can see is, how are they different from curvature ?

[PB666]

2 hours ago, YNM said:

 

I guess it all comes down to the "openness" of EFE.

I can't quite read EFE straight off with the tensors, but I can read Friedmann equation. Honestly, there's no limit in what unknowns you want to throw in. Curvature and gravitational masses is just one thing. You can add electromagnetical or some other sort of things, and you can have them follow different power relation with spatial distance.

The addition of multiple dark-energy aspects has been tried from some time when computational powers allows it. I've seen people proposing multiple dark matters. So it's not so much a big deal, for you have a really hard time knowing...

The "problem" with dark gravity that I can see is, how are they different from curvature ?

I don't disagree with you but I will make a couple of points with regard to gravity that make gravity as a force stand out.

1. Gravity is not a force, even though we treat is as if it was. When you stand on the ground its not gravity pulling you to the ground, its electrostatic forces in your shoes and ground that push against your inertia, if these did not exist you would easily form an elliptical orbit around the Earths point mass. Gravitation is the warping of space-time cause by the anisotrophic distribution of energy in space. The problem is that space has at least one form of non-zero energy rest state, and it may have other forms.

2. Gravity in its purest form (Quantum space-time) was once the strongest force in the universe and composed of a particle that was presumptively most of the energy of the Universe during early inflation, but now gravity (space-time), which we all agree, to be a field that propogates through space time is the weakest of all forces. . .So small in fact its not possible to detect its particle, just its peculiar effects.

How does quantum gravity go from the strongest force in the universe to the weakest force in the Universe with gauge invariance? if Quantum gravity can vary then why cannot space-time similarly vary.  Oddy if you try to detect  gravity in different ways you get a slightly different result, and although we know what Standard gravitational parameter for the earth and sun to about 10 digits, the gravitational constant is only seven. All the other basic constants of the universe are know to at least 10 digits. Thus gravity as a force stands out for several reasons. It should be subject to ongoing critique as to the core of its nature. 

 

[YNM]

@PB666 Ah, I see. A quantum version of modified gravity...

[K^2]

10 hours ago, PB666 said:

Thus gravity as a force stands out for several reasons. It should be subject to ongoing critique as to the core of its nature. 

Anything in science is subject to critique, but I should point out that theory that yields EFEs is the most robust, most scrutinized, and most well tested, both numerically and quantitatively, theory that we have for anything. We have tests of General Relativity and Quantum Electrodynamics to 12 digits, and both of these arise from the same underlying principles.

I also would like to point out that while gravity is not really a force in Classical sense, neither is electrostatic force. Both of these are gauge forces of symmetry groups. The difference is that gravity is a gauge force of an extrinsic degree of freedom, and all other forces are due to intrinsic DoF symmetries. This means that gravity can be viewed as a fictitious, or inertial force of the frame of reference, with no such analogue existing for intrinsic DoFs.

Strength of gravity in early history of the universe, and we're talking tiniest fractions of a second here, is in the fact that gravity is highly non-linear, and density of the early universe was rather high. And while we have no closed form expression for Quantum Gravity, beyond being able to write down the Lagrangian, which causes the worst kind of divergences, we do have effective field theories that describe it adequately on all length scales significantly above Plank's length, and all of these are consistent with our expectations of early universe expansion.

Dark energy and dark matter... *sigh* Let me just say that it's much easier to believe that there are yet-undiscovered matter fields responsible for that, be they WIMPs or something else entirely, than to think that the theory itself is fundamentally broken. The later is, of course, a possibility, and a number of teams are probing various ways in which it can be wrong, but nobody seriously expects a discovery more shattering than, "Oh, looks like there is another degree of freedom that we haven't realized existed."

A more interesting question is why the fundamental symmetries exist. But once we agree that they do, Lagrangian we have in Standard Model is the only possible one, and all of physics neatly follows as a matter of rigorous mathematical theorem. So the only possibilities are that the fundamental symmetries are broken in ways we haven't detected yet, that there are more symmetries we haven't discovered yet, or that we got all of the physics right, and the rest is just math. We have really good scientists looking at all 3 possibilities.

[Diche Bach]

Where was the big bang? Where is the point in the observable universe from which everything inflated?

[PB666]

10 hours ago, Diche Bach said:

Where was the big bang? Where is the point in the observable universe from which everything inflated?

 

On 11/27/2017 at 1:40 AM, K^2 said:

Anything in science is subject to critique, but I should point out that theory that yields EFEs is the most robust, most scrutinized, and most well tested, both numerically and quantitatively, theory that we have for anything. We have tests of General Relativity and Quantum Electrodynamics to 12 digits, and both of these arise from the same underlying principles.

I also would like to point out that while gravity is not really a force in Classical sense, neither is electrostatic force. Both of these are gauge forces of symmetry groups. The difference is that gravity is a gauge force of an extrinsic degree of freedom, and all other forces are due to intrinsic DoF symmetries. This means that gravity can be viewed as a fictitious, or inertial force of the frame of reference, with no such analogue existing for intrinsic DoFs.

Strength of gravity in early history of the universe, and we're talking tiniest fractions of a second here, is in the fact that gravity is highly non-linear, and density of the early universe was rather high. And while we have no closed form expression for Quantum Gravity, beyond being able to write down the Lagrangian, which causes the worst kind of divergences, we do have effective field theories that describe it adequately on all length scales significantly above Plank's length, and all of these are consistent with our expectations of early universe expansion.

Dark energy and dark matter... *sigh* Let me just say that it's much easier to believe that there are yet-undiscovered matter fields responsible for that, be they WIMPs or something else entirely, than to think that the theory itself is fundamentally broken. The later is, of course, a possibility, and a number of teams are probing various ways in which it can be wrong, but nobody seriously expects a discovery more shattering than, "Oh, looks like there is another degree of freedom that we haven't realized existed."

A more interesting question is why the fundamental symmetries exist. But once we agree that they do, Lagrangian we have in Standard Model is the only possible one, and all of physics neatly follows as a matter of rigorous mathematical theorem. So the only possibilities are that the fundamental symmetries are broken in ways we haven't detected yet, that there are more symmetries we haven't discovered yet, or that we got all of the physics right, and the rest is just math. We have really good scientists looking at all 3 possibilities.

And yet every time they set out to find such 'dark' matter they end up proving that dark matter 'flavor of the day' is not the cause of dark matter. Who said gravity was broken?

Quote

Fictitious forces do not appear in the equations of motion in an inertial frame of reference: in an inertial frame, the motion of an object is explained by the real impressed forces. In a non-inertial frame such as a rotating frame, however, Newton's first and second laws still can be used to make accurate physical predictions provided fictitious forces are included along with the real forces. For solving problems of mechanics in non-inertial reference frames, the advice given in textbooks is to treat the fictitious forces like real forces and to pretend you are in an inertial frame.^[9][10]

Treat the fictitious forces like real forces, and pretend you are in an inertial frame.

— Louis N. Hand, Janet D. Finch Analytical Mechanics, p. 267

It should be mentioned that "treating the fictitious forces like real forces" means, in particular, that fictitious forces as seen in a particular non-inertial frame transform as vectors under coordinate transformations made within that frame, that is, like real forces. -Wikipedia

Hand and Finch are handing us some humorous physical critique. But if you ever try to calculate an orbit using a Cartesian system you can see why its no laughing matter, the first statement is that the position vector points in the direction of an equally fictitious point mass. But even as we consider energy and gravity, there are apparently as set of n-body nodes that allow passage of objects to areas of our solar system, that if you were in a hurry to go to would cost a considerable amount of dV. They are not violations of space time, but apparently space-time magic. Thus although gravity has been known for quite some time, the connotations of what we know are still being investigated, and we have yet to investigate to any great degree what is going on outside our solar system.

But here in lies the problem, we cannot be in every reference frame to test gravity, a force that we admit is fictitious and judge its flavor.

One statement of the nature of the Universe in 'that tiniest  fraction of a second' was that there was no space and no time, there was no space-time. We could call it quantum space-time, but that might be as fictitious as gravity, because when space and time merge, neither of them make any sense. So then its a quantum state that via something. . . . inflation . . . . . gives us space-time. And more illusive is that in its current state it goes unobserved.

Quote

Quantum gravity theory for the highest energy scales

The general approach to deriving a quantum gravity theory that is valid at even the highest energy scales is to assume that such a theory will be simple and elegant and, accordingly, to study symmetries and other clues offered by current theories that might suggest ways to combine them into a comprehensive, unified theory. One problem with this approach is that it is unknown whether quantum gravity will actually conform to a simple and elegant theory, as it should resolve the dual conundrums of special relativity with regard to the uniformity of acceleration and gravity, and general relativity with regard to spacetime curvature.

Such a theory is required in order to understand problems involving the combination of very high energy and very small dimensions of space, such as the behavior of black holes, and the origin of the universe. -wkipedia .  quantum gravity.

The question concerning space is that it might lack observable symmetries at high scales and during transitions (dare it be said because there is the implication of passage of time possibly bidirectional) from higher scales to other scales. The connection of space-time with the early universe may be an inferred symmetry that forces background independence of space-time. As such, although the universe inflated from a point in space, that position become irrelevant in any test that involves space-time (even if it could be inferred). But if we accept the background independence of space-time, then inflation did not occur in one place, but in any place, and a quantum space-time did not exist in a place but in all places. "This and other similar results lead physicists to believe that any consistent quantum theory of gravity should include topology change as a dynamical process." -wikipedia. This is fine up unto the point that the quantum space-time dynamic is undefined and is not easily connected to space-time. As the evaluation of the most important transition in space-time cannot be resolved there is a hanging assumption that we can see enough of space-time behavior at the smallest scale that we can know its behavior is consistent with the larger scale. However some experiments indicate there is much more fluctuation in the gravitational constant than other universal constants one of them was conducted with near zero hv energy. But how does space time behave in the emptiest of empty space, we cannot test because we are highly energetic, do we have a proxy that reports what is going on between the stars and galaxies? We grant Einsteins definition of gravity as a warpage of space-time that exist as a field and propagates at the same rate as other mass-less fields, but this does not in and of itself grant it status as a universal constant.

Thus we are left using _our_ co-moving space time, our measures of gravity,  as a reference frame and we make the following assumption for right or wrong that:

1. The space we look through has massive or energetic particles in it, some that we cannot observe, that explain the motion of objects by the laws of space-time that we observe here on Earth.
2. That further out the space as we look through it has a force acting on it that is somehow defying space-time as we know it, pressing energetic objects apart at faster speeds. 

or

A mistake was made . . . . (Other than the creation of the Universe, which is generally agreed to have been a bad move).

Occam's razor may slice this one quickly and soon.

Edit: I should add that we create a "faith" when we believe that the fluctuations in the universal gravitational constant will eventually work themselves out as some sort of experimental or procedural flaws, either these fluctuations are real and have some sort of meaning or something about the experiments that were performed lack some type of fine understanding of space-time.

Edited November 29, 2017 by PB666

[Steel]

3 hours ago, Diche Bach said:

Where was the big bang? Where is the point in the observable universe from which everything inflated?

It was everywhere. Because, as far as we can tell, all space is expanding, which means if you rewind to the beginning of time there was no space in-between everything and the entire universe was in one infinitesimal point. Thus the big bang occurred everywhere at once, from this infinitesimal point which was the entire universe

Edited November 29, 2017 by Steel

[Diche Bach]

12 hours ago, Steel said:

It was everywhere. Because, as far as we can tell, all space is expanding, which means if you rewind to the beginning of time there was no space in-between everything and the entire universe was in one infinitesimal point. Thus the big bang occurred everywhere at once, from this infinitesimal point which was the entire universe

So everything in the universe is equidistant from the center of the universe? I find that very difficult to reconcile with common sense.

When you say "all space is expanding" I suspect you are making a bit of a simplification there. My understanding is that things which are farther away _from us_ appear to be moving away _from us_ faster than objects that are closer _to us_. Moreover, I don't have the impression that the Sol system and everything within its immediate proximity (including the pixels on this screen, Trump's hair piece, and zooplankton in the Marianas Trench) are expanding at the same rate as the distant universe, eh?

There are clusters and superclusters of galaxies whose relative attractive interactions are the subjects of study and whose "filament" like forms across the universe are the subject of various synthetic graphics. I also find that rather difficult to reconcile with the idea that everything "is" the center of the universe.

Not to be confrontational, but yeah; 

[Steel]

8 minutes ago, Diche Bach said:

So everything in the universe is equidistant from the center of the universe? I find that very difficult to reconcile with common sense.

When you say "all space is expanding" I suspect you are making a bit of a simplification there. My understanding is that things which are farther away _from us_ appear to be moving away _from us_ faster than objects that are closer _to us_. Moreover, I don't have the impression that the Sol system and everything within its immediate proximity (including the pixels on this screen, Trump's hair piece, and zooplankton in the Marianas Trench) are expanding at the same rate as the distant universe, eh?

There are clusters and superclusters of galaxies whose relative attractive interactions are the subjects of study and whose "filament" like forms across the universe are the subject of various synthetic graphics. I also find that rather difficult to reconcile with the idea that everything "is" the center of the universe.

Not to be confrontational, but yeah; 

Sort of. It's a difficult concept to try and get across via text. Simply (or as simply as I can) the big bag happened simultaneously everywhere in the universe, because at the point of the big bang the entire universe was in the same place because it was an infinitesimal singularity.

 

You are right, it is a bit of a simplification. Over large scales things are moving away from each other due to the expansion of space. Of course, over small distances gravity dominates and things stick together - that's why we have stars and a solar system and Donald Trump.

[Diche Bach]

Gravity causes pubic grabbing(?) . . . 

[Green Baron]

Simply let go of the idea that the universe exploded into something. It began very small, but we can not define the initial moment, physics start when it already had a certain size, somewhere at the start of what is called the "inflation". And yes, the expansion doesn't make halt in our proximity or in us, we just don't realize as we have no other reference.

Yes, it is simple. The expansion simply adds up with distance. Imagine the distance between 2 points 1m apart gets larger by 1cm in 1 minute. A third point another meter away "flees" with 2cm/minute, and so on. And there is no speed limit to this expansion. The speed limit is "only" for things with mass inside this expanding space, it is a "local" speed limit. Pedestrian area :-)

As to the center: the universe is thought of to have probably no "edge", or the other way round, to be infinite. Should become clear if we adopt the idea that it did not explode into something. No edge, no center. Or, with a little philosophy, the "center" is everywhere and so why not right here.

Hope that wasn't totally wrong, ready for being corrected.

Edited November 29, 2017 by Green Baron

[monophonic]

Is the balloon analogy still applicable? It has been so long since I first heard it half of the "understandable explanations" have since been pointed and laughed at.

[wumpus]

On 11/29/2017 at 1:45 PM, Diche Bach said:

So everything in the universe is equidistant from the center of the universe? I find that very difficult to reconcile with common sense.

In terms of raw space, relativity assumes no preferred origin so no.  In terms of a "center of mass of all mass" that feels weird but appears true (you wouldn't expect the "Hubble effect" without it).  Sounds like there are two obvious ways to achieve that:

The universe never had a center of mass that wasn't everywhere (meaning that if all matter was gathered together, there was no empty space around it)
The universe did have empty space around it, but the energy bounced around during early expansion to become homogenous.  The "center of mass is everywhere" has been true ever since.

I expect the truth to be far weirder.

[YNM]

On 29/11/2017 at 9:24 AM, Diche Bach said:

Where was the big bang? Where is the point in the observable universe from which everything inflated?

Everywhere ?

The most common explanation is with the surface (2D) of a blown balloon. It doesn't expand anywhere, it doesn't expand from somewhere.

 

Alas, things rarely goes so easy. May computing power and insight fuel the next big cosmological discoveries.

Edited December 1, 2017 by YNM

[Diche Bach]

This thread is driving me to consider conversion to pastafarianism and becoming a missionary . . .

The idea the everything observable originated from a single "point" seems to me to breach so many terms of internal validity right from the outset that I suspect the average Joe and Sally just sort of stare blankly but are satisfied that it sounds sufficiently mysterious that it seems a fitting alternative to a benevolent elder  beardy guy pointing and say "Let their be light."

I have complete faith . . . no no, I mean uhm . . . BELIEF that the empirical observations which have led the hundreds or perhaps even thousands of brilliant minds who make up the modern day cosmologists to develop the models they have developed are as accurate as they can be for a "large-brained" bipedal ape with fairly limited perceptual and cognitive abilities. The fact that there are apparently enormously important and powerful forces that we cannot observe but whose effects are unmistakable in creating discrepancies in what we CAN observe and what we have modeled with our symbolic logical languages for characterizing reality (physics math and the rest) seems fitting somehow. Okay, right, gotcha, your tellin' me the universe seems to have "inflated" from a single point about 14 billion years ago and that matter as we know it didn't even really exist for quite some time and the laws of physics and chemistry also didn't really exist for quite a while . . . and oh yeah, there also seems to be this mysterious force (well maybe two of them that are opposing or maybe one of them that functions differently in different contexts) "dark matter/gravity" and "dark energy" which are pretty much completely inexplicable but pretty obviously real and important to how the universe hangs together / flies apart, etc.

I certainly don't have answers and I'm also no supernaturalist or "anti-science" type. The fact that we've been led to these paradoxes by empirical observation is precisely why I find the whole thing believable if still a bit 'unfinished' or 'unclear;' but that to me is the exciting part of it! Mystery! The Unknown! Perhaps we are on the cusp of discoveries about the nature of the universe which are so mindbogglingly wondrous and will seem so obvious and edifying once they are made clear in the descriptions of a discoverery that we will undergo a watershed that could only be compared to that of the Copernican transformation. I relish such notions and I find it a shame that so few "scientists" seem to.

[sevenperforce]

You know when you're cooking pasta on the stove and the pot starts to fill up and overflow, because more bubbles are being formed each second at the surface (aided by starch) than can pop?

The expansion of the universe is like that. Let's say you're a microbe inside one of those bubbles somewhere in the middle of the layer of foam rising larger and larger in the pot. You don't know where in the pot you are; you don't know how large or small it is, or where the edge is, or if there even is an edge. All you know is that suddenly, all the bubbles around you are swelling and getting bigger, and everything is moving away from everything else.

That's how metric expansion works. Everything is expanding in every direction; there is no particular "center", at least not one we're aware of. 

[Diche Bach]

1 minute ago, sevenperforce said:

You know when you're cooking pasta on the stove and the pot starts to fill up and overflow, because more bubbles are being formed each second at the surface (aided by starch) than can pop?

The expansion of the universe is like that. Let's say you're a microbe inside one of those bubbles somewhere in the middle of the layer of foam rising larger and larger in the pot. You don't know where in the pot you are; you don't know how large or small it is, or where the edge is, or if there even is an edge. All you know is that suddenly, all the bubbles around you are swelling and getting bigger, and everything is moving away from everything else.

That's how metric expansion works. Everything is expanding in every direction; there is no particular "center", at least not one we're aware of. 

That actually sounds fairly digestible! 

[sevenperforce]

To say that at the Big Bang the "whole universe" was "compressed to an infinitesimal point" isn't entirely inaccurate but it's not as accurate as it could be. The fact is, we don't know the shape of the exouniverse. The endouniverse we CAN observe is relatively flat (in terms of gravitational curvature), so even if the exouniverse isn't not infinite, we're definitely only seeing a very small portion of it. The exouniverse could be infinite, or it could be an amorphous sphere, or it could be a long thin squiggly line, or it could be a four-dimensional hypertoroid, or any other conceivable bounded or unbounded shape. Whatever that shape was, originally, was compressed down to its smallest possible definition at the Big Bang. Everything we see was just a very tiny "infinitesimal point" within the greater exouniverse. 

Edited December 1, 2017 by sevenperforce
fixed typo

[Diche Bach]

2 minutes ago, sevenperforce said:

To say that at the Big Bang the "whole universe" was "compressed to an infinitesimal point" isn't entirely inaccurate but it's not as accurate as it could be. The fact is, we don't know the shape of the exouniverse. The endouniverse we CAN observe is relatively flat (in terms of gravitational curvature)

I see people say this and while I of course do not disbelieve the claim it certainly doesn't make any sense to me. Gravity is what holds everything "down" on Earth and to escape that gravity one has to exert force in a limited array of directions. The same seems to be true for the relationship between virtually all objects between which gravitational interactions can be observed: they are not "flat" they are curvilinear. So yeah, if we look at some massive signal like the cosmic microwave background maybe that suggests "flat?" But given that all the baryons seem to be subject to curved gravitational dynamics, how is it actually edifying to refer to "the universe" as being gravitationally flat? Even the filiament structures of superclusters are thought of as long term emergent patterns of the very same sorts of gravitational dynamics that cause rocks and stars and balls of gas and galaxies to swirl around one another in curves, eh?

 

8 minutes ago, sevenperforce said:

so even the exouniverse isn't not infinite, we're definitely only seeing a very small portion of it.

Oh dear, that double negative I imagine was a mere typo, but what now the mystery is killing me!  Are you saying the exouniverse (great term by the way, did you come up with those distinction!?  )  IS infinite or IS NOT infinite.

Much like Clarke's quote on exobiologicals, whether they exist or do not . . .  “Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.”

[YNM]

41 minutes ago, sevenperforce said:

The expansion of the universe is like that. Let's say you're a microbe inside one of those bubbles somewhere in the middle of the layer of foam rising larger and larger in the pot. You don't know where in the pot you are; you don't know how large or small it is, or where the edge is, or if there even is an edge. All you know is that suddenly, all the bubbles around you are swelling and getting bigger, and everything is moving away from everything else.

You can tell you're rising up though. Not in the universe.

Also, that would makes it that your "reachable" universe gets bigger and bigger. It also happens in ours without the need for expansion - the extent of the observable universe gets bigger and bigger by nature. The universe's expansion as is called by cosmologist isn't that expansion.