A new experiment to understand dark matter

Is dark matter a source of a yet unknown force in addition to gravity? The mysterious dark matter is little understood and trying to understand its properties is an important challenge in modern physics and astrophysics.

Read more at: https://phys.org/news/2018-06-dark.html#jCp

Tau.Neutrino said:

A new experiment to understand dark matter

Is dark matter a source of a yet unknown force in addition to gravity? The mysterious dark matter is little understood and trying to understand its properties is an important challenge in modern physics and astrophysics.

Read more at: https://phys.org/news/2018-06-dark.html#jCp

> At present, there are various experiments setting tight limits on such a fifth force originating from dark matter. One of the most stringent experiments uses the Earth-Moon orbit and tests for an anomalous acceleration towards the galactic center, i.e. the center of the spherical dark matter halo of our galaxy. The high precision of this experiment comes from Lunar Laser Ranging, where the distance to the Moon is measured with centimeter precision by bouncing laser pulses of the retro reflectors installed on the Moon.

Good old LLR. I mentioned it on this Wikipedia page in connection with looking for deviations from General Relativity. https://en.m.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism

Binary pulsars make an appearance in tests of General Relativity as well. You can see both binary pulsars and Lunar Laser Ranging in the table. The difference of the new proposal is that in the past, binary pulsars have been used to confirm the GR preciction of the strength of gravitational waves. Now the proposal is to use it to test a different PPN parameter.

By the way, I note that there has been no significant editing of this table since I added it to Wikipedia ten or so years ago. Surely there have been more accurate tests of GR since then.

Let me know if you find one.

I’m not sure, but this could be the original technical article.

Symmetries play an important role in modern theories of gravity. The strong equivalence
principle (SEP) constitutes a collection of gravitationalsymmetries which are all implemented
by general relativity. Alternative theories, however, are generally expected to violate some
aspects of SEP. We test three aspects of SEP using observed change rates in the orbital period
and eccentricity of binary pulsar J1713+0747: 1. the gravitational constant’s constancy as part
of locational invariance of gravitation; 2. the post-Newtonian parameter α ³ in gravitational
Lorentz invariance; 3. the universality of free fall (UFF) for strongly self-gravitating bodies.
Based on the pulsar timing result of the combined dataset from the North American Nanohertz
Gravitational Observatory (NANOGrav) and the European Pulsar Timing Array (EPTA), we
find GÛ/G = (−0.1±0.9) ×10^−12 yr^−1
, which is somewhat weaker than Solar system limits, but
applies for strongly self-gravitating objects. Furthermore, we obtain the constraints |∆| < 0.002
for the UFF test and −3 × 10^−20 < α ³ < 4 × 10^−20 at 95% confidence. These are the first direct
UFF and α ³ tests based on pulsar binaries, and they overcome various limitations of previous
tests.

Let’s just check that α ³ from wikipedia shall we.

That’s already in there, < 4 × 10^−20 from pulsar spin down tests. Excellent, all that’s missing from wikipedia is a cross reference to the technical paper. And that doesn’t matter.

Just what does that value of α ³ rule out? It rules out three alternatives to GR and puts a tight Occam’s Razor type “fine tuning” constraint on a fourth.

mollwollfumble said:

Tau.Neutrino said:

A new experiment to understand dark matter

Is dark matter a source of a yet unknown force in addition to gravity? The mysterious dark matter is little understood and trying to understand its properties is an important challenge in modern physics and astrophysics.

Read more at: https://phys.org/news/2018-06-dark.html#jCp

> At present, there are various experiments setting tight limits on such a fifth force originating from dark matter. One of the most stringent experiments uses the Earth-Moon orbit and tests for an anomalous acceleration towards the galactic center, i.e. the center of the spherical dark matter halo of our galaxy. The high precision of this experiment comes from Lunar Laser Ranging, where the distance to the Moon is measured with centimeter precision by bouncing laser pulses of the retro reflectors installed on the Moon.

Good old LLR. I mentioned it on this Wikipedia page in connection with looking for deviations from General Relativity. https://en.m.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism

Binary pulsars make an appearance in tests of General Relativity as well. You can see both binary pulsars and Lunar Laser Ranging in the table. The difference of the new proposal is that in the past, binary pulsars have been used to confirm the GR prediction of the strength of gravitational waves. Now the proposal is to use it to test a different PPN parameter.

The most quoted alternative to General Relativity that includes dark matter in its formalism is f ( R ) gravity, which is the modern version of MOND.

It’s difficult to check this with https://en.m.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism because those tests assume that the formalism exactly matches Newtonian mechanics in the non-relativistic limit, which f ( R ) gravity doesn’t.

I’ve just found several papers of f ( R ) gravity that derive PPN parameters γ and β. Because the function f ( R ) is arbitrary, it can be chosen to set γ and β to be exactly the same as those of GR. But in general they will differ. γ will differ from the GR value of 1 by an amount that depends on the distance over which gravity acts (distance R). So that can be tested.

I didn’t find any papers that derive the other PPN parameters for f ( R ) gravity. So the new value of a ³ from the article in the original post can’t be used for the purpose for which it is intended, to check whether “Is dark matter a source of a yet unknown force in addition to gravity?”

mollwollfumble said:

> At present, there are various experiments setting tight limits on such a fifth force originating from dark matter. One of the most stringent experiments uses the Earth-Moon orbit and tests for an anomalous acceleration towards the galactic center, i.e. the center of the spherical dark matter halo of our galaxy. The high precision of this experiment comes from Lunar Laser Ranging, where the distance to the Moon is measured with centimeter precision by bouncing laser pulses of the retro reflectors installed on the Moon.

Random thought in response to that:

Is “centimeter precision” really the best we can do.

I mean, I know it’s a long way to the Moon (on Earthly scales), but given the near incredible precision of distance measuring for gravity wave detection, surely we can do much better than cm precision for measuring the distance to the Moon?