I Can the MOND theory be experimentally tested?

[KurtLudwig]

I have read about LISA Pathfinder. Can a satellite with gravitational test instruments, such as a torsional pendulum, test the MOND theory at L1 and L2?
Maybe both MOND and dark matter are needed to explain velocities of some remote stars and the formation of large galactic structures.

 

[Vanadium 50]

. Can a satellite with gravitational test instruments, such as a torsional pendulum, test the MOND theory at L1 and L2?

No. The acceleration is way too high.

 

[alantheastronomer]

The recent observations of gravitational waves from colliding black holes and neutron stars has shown that modified theories of gravity can be ruled out.

 

[Vanadium 50]

Alan, I really, really wish you wouldn't make such authoritative statements. Especially with your "the astronomer" name (when, as I understand it, you are not). And double especially when you are wrong.

The observations of gravitational radiation certainly constrain MONDy models, but they do not eliminate them. Furthermore, there is more to modified theories of gravity than just MOND (and indeed, there are families of alternatives to GR with a tuneable parameter which when made sufficiently small recovers all the predictions of GR.)

 

[alantheastronomer]

I really, really wish you wouldn't make such authoritative statements.

Then take it up with Kip Thorne, since the statement's his!

 

[PAllen]

Then take it up with Kip Thorne, since the statement's his!

Please provide a reference. If you do, I am guessing we will see qualifiers in his statement that are missing in yours.

 

[KurtLudwig]

After having read up more on LCDM vs MOND, I would like to share two papers: "A Tale of Two Paradigms: the Mutual Incommensurability of LCDM and MOND" by Stacy McGaugh, a professor of physics at Case Western Reserve University and "Modified Newtonian Dynamics, an Introductory Review" by Riccardo Scarpa, a Spanish research astronomer and professor. Also, I have read all articles pertaining to dark matter and gravity on Wikipedia.
Dark matter was needed to start the condensation of matter during the very early formation of structures in our universe. However, LCDM is very weakly constrained, since dark matter can be added and positioned where ever it is needed to prove one's hypothesis. It is stated in literature, that dark matter is distributed in a halo (sphere), yet it is only needed in the peripheries of galaxies. This would require it to be distributed in a donut shape around a galaxy. One flaw of LCDM model is that it cannot be disproved.
MOND explains the rotation curves of stars in galaxies by simple mathematical formulas. It is tightly constrained by a0 = 1.2 x 10^-10 m/s^-2. We must keep in mind, that Newton's Law of Universal Gravitation was never tested in regimes of very low gravity. Currently, there is no relativistic theory of MOND and there is no universally accepted theory of quantum gravity.
Quoting Stacy McGaugh:"Where one makes clear predictions, the other one tends to be mute. This makes comparisons of the two fraught. The conclusion one comes to depends on how one chooses to weigh the various lines of evidence."
Maybe both LCDM and MOND are needed to explain the various observations in cosmology and astronomy. This is not elegant, but then quantum physics is not elegant. Somehow, in the weak gravity regime of MOND, quantum gravity comes into play and Newtonian gravity needs to be mathematically modified. Please delete this paragraph if these opinions are not allowed on this website.

 

[ohwilleke]

The recent observations of gravitational waves from colliding black holes and neutron stars has shown that modified theories of gravity can be ruled out.

What the observation actually says is that any boson carrying a gravitational wave in a quantum gravity generalization of general relativity must have a zero or very nearly zero rest mass.

This is because the gravitational waves from the fairly distant collision and the electromagnetic waves from that collision arrived very close in time to each other (not identically, but we don't know for sure how far apart they should be in theory because the details of what goes on in the complex process of a black hole colliding with a neutron star isn't fully understood with the requisite precision).

There are some modified gravity theories that have multiple bosons, at least one of which is massive. The observational constraints on those theories are quite strict as a result of this observation.

But, among other things, it can't rule out toy model MOND because the original 1983 version of toy model model is expressly limited in its domain of applicability to circumstances where relativistic effects are inconsequential, and because it is a phenomenological relationship without a stated mechanism. Also, MOND proposes a very subtle effect that is visible only in weak fields and is swamped in strong gravitational fields like the ones involved in a black hole-neutron star merger, so it isn't a good place to observe such things.

The observation is not relevant to quantum gravity theories in which all of the force carrying bosons have zero rest mass, or to classical modified gravity theories in which gravity waves are carried over space time at the speed of light, or to formulas like MOND, the Tully-Fischer relationship or the Radical Acceleration Relationship (RAR) that merely relate observed quantities without providing a mechanism.

 

[ohwilleke]

It is stated in literature, that dark matter is distributed in a halo (sphere), yet it is only needed in the peripheries of galaxies. . . . Currently, there is no relativistic theory of MOND.

Dark Matter Halo Shapes

The most precise estimates infer that dark matter halos are prolate (i.e. rugby ball or ellipsoid shaped) rather than spherical, although a spherical approximation is sometimes used. See, e.g. Hayashi and Chiba (2014).

"Halo" as used in dark matter astronomy means something closer to "a diffuse cluster of particles in three dimensions around a central point", than it does to its narrower definitions of something with a strictly circular or spherical shape. Definition 2(b) in "The Free Dictionary" states that a halo is (emphasis added):

A roughly spherical region of relatively dust-free space surrounding a galaxy and extending beyond the visible parts of the galaxy. Galactic halos contain stars (often located in globular clusters), gas, and dark matter.

For what it is worth, a sphere and an ellipsoid are topologically equivalent.

Relativistic Modified Gravity Theories

"Currently, there is no relativistic theory of MOND"

Actually there is at least one (non-unique) direct generalization of MOND made by the late Jacob Bekenstein in 2004 called TeVeS. It is TeVeS that was allegedly falsified by the GW170817/GRB170817A neutron star and black hole collision event observations. But, that paper expressly states that:

While not falsifying MOND per se, GW170817 severely constrains relativistic extensions of MOND to theories that do not rely on additional matter-coupling fields but rather upon modified field equations for one universal gravitational and physical metric. Here I mention a simple preferred-frame theory as an example.

There are also some relativistic modified gravity theories that approximate MOND other than TeVeS (e.g. John Moffat's MOG theory), that aren't actually relativistic theories of MOND, although they are quite similar in their predictions and approach.

Moffat's MOG theory was reviewed in light of observations of the neutron star merger and gamma ray burster event GW170817/GRB170817A in an October 2017 pre-print with three co-authors that has not yet been published, and was found in that review to be consistent with that event.

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