In the real world, no one knows exactly what the “true model” of gravity is but physicists are aware that in most situations it [gravity] is reminiscent of Newtonian gravity, and, under every situation that has been measured, gravity resembles general relativity. Let’s say, you don’t agree with that observation–do know that I don’t care about convincing you of what the real world observes; it’s not you that I’m attempting to convince. Rather, it’s the people–the bystanders–standing aside observing the argument/conversation/debate or what have you that I’m trying to convince [of what the evidence shows]. That’s a form of constructive criticism as it relates to science or scientific matters. Someone may have the knack for mathematical abilities to do interesting things in physics. Mind you, I personally think that it’s a shame that someone with mathematical aptitudes choose to waste them on physics, but to each his own. I mean, I surely hope someone like that can quickly realize how the path they may have chosen eventually leads to doing philosophy–but that’s just me talking.
In further reference to gravity, let’s say I have point A. One can argue that all forces are balanced at point A in which case point A does not decelerate. I have point B. One can also argue that all forces are balanced at point B in which case point B does not decelerate. Even with this observation you cannot say that the universe is decelerating. Starting with point A as your origin, take a look at point B and you’ll see that there is a force at point B pulling it toward point A. From point A’s point of view, point B would not be an inertial coordinate system since it’s accelerating and since it is accelerating, you switch between point A and point B–and the forces change. From point B’s point of view, it is an inertial coordinate system and point A is the one that’s accelerating. Whenever you’re switching between point A and point B, you have to change the forces to account for the fact that the coordinate systems are non-inertial. From point B’s perspective, you are a non-inertial reference frame from the point of view from point A. Subtract the forces acting upon it in order to make things inertial. This will cause the forces at point B to go to zero. Eventually, point A will go from zero force to the opposite of what was the force acting upon point B.
It’s imperative that you grasp exactly what you’re observing in relation to acceleration. Even if the sum of the forces is zero you’ll still see acceleration if you’re measuring the sum with respect to a non-inertial frame of reference. So, with a point that has been chosen, all other points fall into place and they become non-inertial and if the sum of all forces is zero in those frame references, you’ll still have acceleration. That’s the point of gravity–and it happens a lot. You could be in a free-fall in an elevator and you’ll measure zero force and in spite of the fact that all forces are zero in your local reference frame, you’ll still be accelerating since the forces are defined in a non-inertial reference frame.
Employ the use of a Newtonian model of cosmology which is the zeroth order approximation. You should be aware that the Newtonian model includes assumptions that are known to be wrong (especially with the “speed of light is infinite” claim) and therefore, results in drawn-out conclusions that are known to be wrong as well (Newtonian models do not take into account of geometric curvature). Newtonian cosmology comes into play for the simple reason that a good number of physicists just do not harbor enough of an understanding of general relativity to have reasonable discussions based on Friedmann-Walker models. Scientists can argue in a Newtonian world and then figure out the implications of once general relativity has been added in since the differences between Newtonian cosmology and Friedmann-Walker models are well-known. People have pretty good intuition about “things Newtonian“. If you’re in a falling elevator, the forces that are measured will be considerably different than from that of a stationary observer. In a Newtonian model, we are assuming that time is observer-dependent, but in general relativity, forces are used at all. You’ll end up with a complete mess if you were to apply the concept of “force” with general/special relativity.
Does anything sound nutty to you? It should, especially if you’re an aspiring physicists, because guess what? All productive physicists have at least one nutty idea. There’s a Nobel prize winning physicists that had convinced himself that black holes do not exist. Every other physicist he came in contact with would not mention the two words [“black holes”] around him, however, this particular physicist kept himself occupied with only spending the least amount of time contemplating on black holes and the majority of his time preoccupied with his own ideas and that’s what landed him the Nobel prize. Understand that it’s the ability to not be totally and completely consumed by one idea that distinguishes one scientist from another. If you have enough pokers in the fire eventually something is going to hit paydirt.
If you try to do cosmology, whether it’s Newtonian or not, with special relativity, you’ll end up with a mess. So the choice is either do things with Newtonian gravitation or with full-on general relativity. Ever wondered why Einstein is considered a genius? It’s because that once you’ve figured out that c (speed of light) is finite it’s no longer easy to come up with some theory of gravity that is, or would be, consistent. Current observations reveal that the universe is homogenous and isotropic, therefore, scientists take these observations and create models that are consistent with these observations and that leads us to FLRW (Friedmann-Lemaitre-Robertson-Walker metric). The problem with some scientists is that they tend to take these observations and begin inferring philosophy rather than physics and start treating isotropy and homogenity as if they were mathematical axioms–and they’re not. If you assume X, Y, and Z, you must conclude A. If A is not concluded then you must assume that something went wrong somewhere with X, Y, or Z.
Is dark matter a form of modified gravity? It’s as good as an idea as anything else at this point, but is it legitimate or a product of popular science? Popular science, to me, is a problem because popular science only references the final result; it does not mention the hundreds of alternatives that were tried prior to arriving at the point of imparting “dark matter”. Let me not forget to mention that popular accounts of science aren’t exclusively good at explaining the process of science and with that in mind, you can probably see why the majority of folks would rather sit in front of their TVs and watch Neil deGrasse Tyson ramble on about “sun-like” stars, black holes, quasars or take them on an cartoonish, inaccurate historical journey into the life of some nondescript, pedestrian 16th Century astronomer or what have you instead of picking up a book on chemistry, atmospheric pressure or acquire the knowledge to understand Boyle’s Law. It’s no wonder that the American “scientific community” gets laughed at by the likes of probable child molesters from Germany and by the natural-born retards who live in the dual-state of Australia where they fuck babies in the ass and high-five kangaroos.
But, I digress. There is, however, cosmological evidence for dark matter and it can only be refuted if there are alternative explanations for dark matter that can be presented. I’m no astrophysicist but I would suggest that if dark matter is some form of modified gravity and it appears that dark matter attracts in only one direction then an alternative explanation for the cosmological evidence of dark matter would be to look to see if dark matter appears on galactic scales and see if it produce baryon oscillations. There are lots of inconsistencies with LCDM (Lambda-Cold Dark Matter) and observations at the galactic scales and that is not particularly alarming yet it is an area of active research due to the fact that LCDM was not designed for modeling galaxy formation at cluster scales. If anyone in the scientific community (domestic or global) can come up with a gravitational model that acts like Newtonian gravity at galaxy cluster scales–and it turns out to be correct–I bet you it’ll include dark matter. To this very day, no group of researchers have come up with an alternative gravitational model without the inclusion of cosmological dark matter.
A researcher would ask: “How much of the gravity theory would has to change from Newtonian before you can eliminate dark matter?”
Answer: A lot.