The Pioneer Anomaly Options

[remcook]

http://www.planetary.org/news/2005/pioneer_anomaly_faq.html

The planetary society may be checking it out...

The Planetary Society has committed to raise the funds to preserve the priceless Pioneer data from destruction.


After years of analysis, but without a final conclusion, NASA, astonishingly, gave up trying to solve the "Pioneer Anomaly" and provided no funds to analyze the data. The Pioneer data exists on a few hundred ancient 7- and 9-track magnetic tapes, which can only be read on "antique" outdated computers. The agency is going to scrap, literally demolish, the only computers able to access and process that data in the next few months!

[Jeff7]

Concerning dark matter, I remembered some article that had a theory for why dark matter need not exist at all - that we only need to use relativity properly, and apply it to gravity. Something along those lines.
Link - it has a summary of this theory, and a link to the full abstract. It seems to say that, when looking at something small, like a single star, Newtonian physics may be an adequate approximation, but this doesn't work when you're looking at the way an entire galaxy behaves.

[Mongo]

Concerning dark matter, I remembered some article that had a theory for why dark matter need not exist at all - that we only need to use relativity properly, and apply it to gravity. Something along those lines.
Link - it has a summary of this theory, and a link to the full abstract. It seems to say that, when looking at something small, like a single star, Newtonian physics may be an adequate approximation, but this doesn't work when you're looking at the way an entire galaxy behaves.

Here are the direct links for the original paper:

http://xxx.lanl.gov/abs/astro-ph/0507619

and for its follow-ip:

http://xxx.lanl.gov/abs/astro-ph/0512048

These two paper are interesting indeed! Their argument is that galactic rotation curves have always been modeled using Newtonian Gravity (NG) rather than the full General Relativity (GR), because GR is so much harder to model for something like a galaxy. In fact, even today it is impossible to accurately model a galaxy with GR, the two papers describe highly simplified models. Everyone assumed that there would be little difference between the results using NG and GR, but the authors show that there is actually a huge difference in the results.

Models using NG predict, from the visible matter, rotation curves that fall far short of observations, requiring new physics as a result: either invisible 'Dark Matter', that does not fit into the Standard Theory of physics derived from direct experiment, or a theory of MOND that modifies either gravity or inertia. The (highly simplified) model using GR, on the other hand, predicts rotation curves that closely match observations, obviating the need for 'Dark Matter' or MOND.

I should point out, however, that MOND was always a phenomenological theory, and its predictions using visible matter would closely match the predictions using GR. In other words, MOND was describing the effects of GR on galaxies, without realising it!

Will these papers have the influence on cosmology that they seem to deserve? The problem is that almost all of modern cosmology is based on the assumption that 'Dark Matter' exists. Most current cosmologists have based their careers on this assumption. They would be VERY reluctant to throw so much of their professional work away.

I would like to see follow-ups to these papers, but right now I am inclined to think that galactic rotation curves are indeed accurately described by GR, without the need for 'Dark Matter'.

Bill

[Guest_Richard Trigaux_*]

Will these papers have the influence on cosmology that they seem to deserve? The problem is that almost all of modern cosmology is based on the assumption that 'Dark Matter' exists.  Most current cosmologists have based their careers on this assumption.  They would be VERY reluctant to throw so much of their professional work away.


Bill

However an explanation of galactic behaviour without extra hypothesis would be fine.


A note is that dark matter is suggested, not only by the rotation curve, but also by gravitationnal lens effects, which suggest a mass 10 to 100 times more that of visible stars.


An idea of mine was to derive the state equation of the "dark matter" from the rotation curve. (the rotation curve gives the mass as a function of the distance to center, which gives density. From all the mass "above", we get the pressure). Easy at a first glance, but there is a differential equation to solve, a bit beyond my possibilities. And the result may be very different if we consider the matter is in a disk or in a sphere. Also the equation can be properly solved only if we have an upper limit to the mass repartition. So I was not sure of the result.

Recently, the observation of gravitationnal lens effects allowed to build density profiles in far galaxies and clusters. So the differential equation can be solved numerically, and I had hints it was done. But I never see any result. However this would give serious clues about what is dark matter. For instance finding that the state equation has a power law of 1.4 would make quasi-certain that the dark matter is neutral molecular hydrogen. Other candidates, dust, stars or subatomic particules would give other signatures.

[Mongo]

A note is that dark matter is suggested, not only by the rotation curve, but also by gravitationnal lens effects, which suggest a mass 10 to 100 times more that of visible stars.

Another effect is the apparent 'extra mass' in galaxy clusters suggested by the observed excess galactic velocities.

However, the authors point out that the virial theorem, which is the basis of this supposed extra mass, is itself based on Newtonian Gravity. As far as I know, nobody has tried to model galaxy clusters using General Relativity. Given the huge apparent difference that GR versus NG makes in modeling galactic rotation curves, it would not surprise me at all if GR eliminated the 'extra mass' (beyond the already-known hot gas, etc.) in galaxy clusters.

As for the magnitude of the gravitational lens effect, I am not in a position to comment. Were those mass calculations done using full GR modeling of a rotating galaxy, or they done by treating the galaxies as a 'point source' of gravitational distortion? It sounds like galaxies as a whole have a larger second-order gravitational effect under GR than had been expected.

Bill