First we had Newton's laws of gravity. It worked quite well in the solar system scale and everyone was happy. Then Einstein formulated the special theory of relativity and Newtonian gravity was found to be incompatible with that. So Einstein formulated the General Theory of Relativity (Einsteinian Gravity) which solves that problem. As a bonus, his theory could also explain the eccentric behavior of Mercury's orbits, which Newtonian Gravity couldn't. It also
predicted that galaxies should be able to bend light (lensing) and this was confirmed by Arthur Eddington. However, for many galaxies, the amount of lensing is way
off from what the Einsteinian theory predicts. Similarly,
rotational velocities of stars in a galaxy are also way off
from what is predicted by both Einsteinian and Newtonian
theory. In addition, when you apply the Einsteinian theory
to the entire universe, you can't explain the observable
geometry of the universe. That is also way off. Dark matter
and Dark Energy are just a very clever way of saying "we have no idea what's going on with gravity in the large scale
(beyond solar system scales)". No one has ever observed
any dark matter or dark energy so far. It is entirely possible that Einsteinian theory of Gravity is just an
approximation that works well for small scales and that
we need a new theory of gravity at the galactic scale
or the scale of the universe. It is also entirely
possible that Dark Matter and Energy don't actually exist.
"The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is an exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic expanding or contracting universe that is path connected, but not necessarily simply connected."
This is probably the only part of your post that actually came close to the truth. Dark energy is far more of a placeholder than dark matter at this point.
1. I was referring to the fact that the observable matter of the universe is not enough to generate the nearly flat and accelerating universe.
2. I am quoting from the article.
"In addition to the Chandra observation, the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes were used to determine the location of the mass in the clusters. This was done by measuring the effect of gravitational lensing, where gravity from the clusters distorts light from background galaxies as predicted by Einstein's theory of general relativity."
Since they are calculating the mass by using General Theory of Relativity (indirect measurement, you have to trust the
theory for this), I don't see how this can be called direct detection.
'The most serious problem facing Milgrom's law is that it cannot completely eliminate the need for dark matter in all astrophysical systems: galaxy clusters show a residual mass discrepancy even when analysed using MOND.'
'Besides these observational issues, MOND and its generalisations are plagued by theoretical difficulties. Several ad-hoc and inelegant additions to general relativity are required to create a theory with a non-Newtonian non-relativistic limit, the plethora of different versions of the theory offer diverging predictions in simple physical situations and thus make it difficult to test the framework conclusively, and some formulations (most prominently those based on modified inertia) have long suffered from poor compatibility with cherished physical principles such as conservation laws.'
Basically, people have tried to fiddle with the laws as you suggested, but none of actually managed to fit the data better than GR + dark matter, plus the fiddling often just looked like curve-fitting without any good theoretical justification. Of course, the jury's still out.
It's the precession of Mercury's perihelion, not the eccentricity of its orbit: apart from the perihelion precession issue for Mercury and, to a lesser extent, Venus, the solar system is quite Newtonian. Also, the first lensing experiment measured the amount of light deflection by the sun, not galaxies.
> Dark matter and Dark Energy are just a very clever way of saying "we have no idea what's going on with gravity in the large scale (beyond solar system scales)".
While I agree with definition of dark matter that you provided, keep in mind that there are hypothesis that try and explain it as something that exists. For example: WIMPs.
People seem think think this allows our current theories to be wrong.....
Perhaps there might be small issues with Einstein theories, personally I doubt it, but, to me, I really don't think small fixes to the theory will explain this stuff away.
If you look at the equations, relativity looks like a refinement to Newtonian physics: at small masses and low velocities, relativity makes the same predictions as Newtonian physics.
However, it is not correct to call this a "small improvement." Relativity was a radical change to our understanding of how the universe behaves. In a Newtonian universe, information (including gravitation, electrical attraction, and light) travelled instantaneously. This does not happen in relativity, and nor does it happen in the universe we live in. Einstein did not merely improve upon Newton's work; Einstein proposed a theory which explained the ways in which we do not live in a Newtonian universe. Thomas Kuhn goes into this concept in detail in "The Structure of Scientific Revolutions."
There's nothing in Newtonian theory that requires light to travel instantaneously, and Ole Romer first demonstrated that light had a finite speed in the 1670s, roughly a decade before Newton published the "Principia".
It's not actually clear that there is any problem with current theory - it may just be that our understanding and application of it is incorrect. You can have a perfect mathematical model, but if just one of your premises is incorrect, the output can be nonsensical.
I am a physicist (well, I studied as one, before going technology entrepreneur), and have always sat in the "dark matter is bullshit" camp - so my opinion is likely tainted.
I think a far more likely explanation, as this paper could suggest, is that something far more mundane is happening.
For instance, we already understand tidal forces - that bodies can become tidally locked, that there is an inertial frame shear that drags bodies along around a rotating centre of mass, and exchanges energy between an orbiting body and the centre.... So why is it infeasible that galaxies are tidally locked, and the lack of fall-off in radial velocity is down to the gravitational shear from the galactic core being far more intense than we anticipate, causing the behaviour we see. It doesn't require a rewrite of relativity, "just" of galactic evolution.
Either that, or galaxies are sprites, and lazily rendered ones, at that.
> For instance, we already understand tidal forces - that bodies can become tidally locked, that there is an inertial frame shear that drags bodies along around a rotating centre of mass, and exchanges energy between an orbiting body and the centre.... So why is it infeasible that galaxies are tidally locked, and the lack of fall-off in radial velocity is down to the gravitational shear from the galactic core being far more intense than we anticipate, causing the behaviour we see. It doesn't require a rewrite of relativity, "just" of galactic evolution.
Because someone has indubitably already ran the numbers. The tidal force only has a non central, orbit changing influence on spatially extended bodies, and its effects even on objects as large as our solar system are completely negligible, as it doesn't seem to have a measurable effect on the orbits of the planets. The rotation curves are way off, which is a huge chunk of excess angular momentum that no tidal force can conjure. Where would it even come from, the rotational angular momentum of component stars?
Acknowledge and embracing the assumption that the speed of light is a universal constant -- disregard of reference frame? And as a consequence, allowing space and time to curve? That is not a small fix. That is a fundamental faith jump.
>It is also entirely possible that Dark Matter and Energy don't actually exist.
Exactly. The whole basis for belief in "dark matter" and "dark energy" is basically: "our theories on how the universe works don't compute, so there must be some invisible things (dark matter & energy) we cannot detect since our theories can't be wrong."
Except for the slight problem that observations of the bullet cluster show gravitational lensing inconsistent with the observed mass of physical matter. MOND has no explanation for this.
No our theories were wrong, that was obvious, dark matter and dark energy are (part of) the new theory
We're going with them because all the other new theories end up requiring fractional dimensions or some similarly bizarre space time and still don't end up explaining things as well as "I dunno guys maybe there's just some stuff we can't see". We already know there's a bunch of stuff we can't see, dark matter is just some other kind of stuff we can't see. Dark matter is on the correct side of the razor.
The shift here is that dark matter is detected only in aggregate. And massive aggregates at that. What are the properties of dark matter? Where does it come from? How does it interact with other matter, light or dark? No idea, because it's only detected as a correction term.
