Galactic Accelerometers Probe Milky Way’s Dark Side

Jim1348
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Topic 224765

Binary pulsars can serve as sensitive accelerometers that probe the gravitational forces in our Galaxy, which could help in building a detailed picture of the dark matter distribution.

https://physics.aps.org/articles/v14/18

Mike Hewson
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Interesting, but then I'm not

Interesting, but then I'm not a fan of dark matter. I'd be wondering about whether one could ignore the ( tremendously strong ) magnetic field of a pulsar. It's a magnet moving in the magnetic field of the galaxy so it'll suffer a force due to that. Is that comparable to some alleged 'dark matter' component ? That's a problem with the definition of dark matter - it is the bit left over when you account for all that you can think of.  Maybe dark  matter is no more mysterious than a failure of imagination.

[back of the envelope partial calculation]

The magnetic field of the galaxy is of the order of 10 uG ie. 10-5 Gauss. But a neutron star has a magnetic field of order 1014 Gauss, and it's the main reason we 'see' these things at all : from radiation processes that are proportional to massive induced currents ie. the pulsar is a dynamo. But it's hard to estimate the magnitude of those currents if you don't have a good idea of neutron star matter properties .... and if one knew the current then one could gauge the force on the star.

A quick/slick guess would use F = B * I * L, where F is the force on the neutron star, B is the galaxy's magnetic field strength*, I is the 'effective' current in question, and L is the 'length of the conductor' ( a neutron star's diameter is ~ 20km ). If Mn is the mass of the neutron star** then F/Mn gives the acceleration of interest. Hmmmmm .....

.... or you could flip the question and estimate the current from the measured residual acceleration ( call it An ) using the methods in the paper :  

I = (MnAn)/(B * L ) ~ 1030 * An /(10-5 * 10-5 * 2 * 104) = An * 5 *  1037 

and then decide if I was a 'reasonable' measure. An epic number of amperes for every 1 m/s2 of acceleration. Now given "Over a year, they may amount to a few centimeters per second" gives

A~ 10-2/(seconds per year) ~ 10-2/(3 * 107) ~ 3 * 10-10 m/s2

Thus I = 3 * 10-10 * 5 *  1037 = 15 * 1027 ~ 1028 Amps

Now I'm not saying this is right, but it gives a feel for the tremendous numbers that follow from my assumptions. That's a staggering amount of current. To restate : to account for the alleged dark matter effect on the acceleration as not due to gravity but good old EM effects then a neutron star has to carry ~ 1028 Amps. If I'm wrong then we at least know by how much ! :-)

* not forgetting that 105 Gauss is one Tesla = 1 volt second per square meter, or 1 Gauss = 10-5 Tesla 

** Call it one solar mass = 1030 kg

[/back of the envelope partial calculation]

Cheers, Mike.

( edit ) Numbers that beggar belief are common when one studies neutron stars. Black holes too. But one can only measure three things about an isolated black hole : mass, angular momentum and charge. Neutron stars are nearly black holes but have ever so many measurable things, especially if they are in company.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

tullio
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In my opinion dark matter is

In my opinion dark matter is not made up from particles, Wimps or other. Protons and neutrons, our standard matter constituents, are made up from quarks, which are never seen alone and therefore are not particles. So dark matter may be made up from something similar to quarks, such as a quark-gluon plasma. Of course this is only my humble opinion.

Tullio

Jim1348
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Mike Hewson wrote:I'd be

Mike Hewson wrote:
I'd be wondering about whether one could ignore the ( tremendously strong ) magnetic field of a pulsar. It's a magnet moving in the magnetic field of the galaxy so it'll suffer a force due to that. Is that comparable to some alleged 'dark matter' component ?

I did my master's thesis in the area of magnetic materials.  We always said that he who rules magnetism rules the world.

Mike Hewson
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Magnetism certainly rules

Magnetism certainly rules pulsars. What was your thesis ?

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Jim1348
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Mike Hewson wrote: What was

Mike Hewson wrote:
 What was your thesis ?

Ancient stuff.  Rolled nickel-iron alloy tapes (permalloy).  It was a question of how the domains form and move in various magnetic fields.  I don't know that you could even buy the stuff today. 

Jim1348
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There is a discussion of Dark

There is a discussion of Dark Matter over at Milky Way, in case anyone wants to join in (here or there).  It is beyond me anyway.

https://milkyway.cs.rpi.edu/milkyway/forum_thread.php?id=4736

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