The Pioneer Anomaly Options

[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

[Mongo]

Those first results from the project, known for short as RAVE, confirm that
dark matter dominates the total mass of our home galaxy, the Milky Way, team
members at The Johns Hopkins University and elsewhere said. The full survey
promises to yield a new, detailed understanding of the origins of the
galaxy, they said.
http://www.newscientistspace.com/article/dn8566

Of course it proves nothing of the sort. This is a perfect example of circular reasoning: they assume that dark matter exists and analyse the results given that assumption. All that this survey shows is that the local stars have orbital velocities higher than that predicted using Newtonian Gravity -- which has been known for decades. It is true that this survey covers more stars, with more accuracy than earlier surveys, but nothing really new has been discovered.

Unless it is disproven, I will go with the idea that the excess orbital velocities are an illusion, due to using Newtonian Gravity rather than General Relativity in the models.

Bill

[Jeff7]

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!

Something else, with modeling something like a galaxy, and failing to take general relativity into account - with a few billion individual stars to model, and if Newtonian physics introduces a tiny amount of error with each star in the model, those small deviations will really compound each other. Each star interacts with others, and if those interactions themselves are in error because of initial errors, etc etc etc - billions of tiny errors add up to one big problem.


It sort of surprised me when I read about this, like "well yeah, duh, general relativity. Why aren't you using it already?" I just think it makes sense, now that we have a theory of relativity, that we actually use it, rather than invent a kind of matter that we just can't directly observe in any fashion.
Relevant image. (I can't take credit, I just found it online somewhere awhile ago.)

But really, we can't disprove dark matter. Star Trek Voyager proved that it existed when they encountered a dark matter asteroid.

[The Messenger]

It sort of surprised me when I read about this, like "well yeah, duh, general relativity. Why aren't you using it already?" I just think it makes sense, now that we have a theory of relativity, that we actually use it, rather than invent a kind of matter that we just can't directly observe in any fashion.
Relevant image. (I can't take credit, I just found it online somewhere awhile ago.)

Three problems with using GR to account for missing galactic mass:

1) GR theoriest did not predict the missing mass - tweaking GR parameters so that they effectively model overaggressive rotational velocities is just curve fitting.

2) Dark matter and energy are needed to literally fill the gaps in the Cosmic Microwave Background. Without dark matter, the CMB power function falls why short of BB model expectations.

3) No gravity waves. Unless and until GWs are detected, all the curve fitting in the world cannot solidify relativistic theory. To date, it can reasonably be argued that we have lacked the needed sensitivity, but this is no longer true. The constraints are such that if LIGO does not detect anything in the next half decade, we are looking down the barrel of another Michelson Morley null.

[Mongo]

Three problems with using GR to account for missing galactic mass:

1) GR theoriest did not predict the missing mass - tweaking GR parameters so that they effectively model overaggressive rotational velocities is just curve fitting.

2) Dark matter and energy are needed to literally fill the gaps in the Cosmic Microwave Background. Without dark matter, the CMB power function falls why short of BB model expectations.

3) No gravity waves. Unless and until GWs are detected, all the curve fitting in the world cannot solidify relativistic theory. To date, it can reasonably be argued that we have lacked the needed sensitivity, but this is no longer true. The constraints are such that if LIGO does not detect anything in the next half decade, we are looking down the barrel of another Michelson Morley null.

1) Since it appears that nobody had ever actually created a GR model of galaxy rotation before last year -- due to the computational difficulty of using full GR -- it is no surprise that no predictions were made. On the other hand, once a GR model was made, it produced results close to observations. No 'tweaking' was required.

2) There are four (2x2) possibilities: GR is accurate or not, and BB models use full GR or not.

If the BB models don't assume full GR, then they are flawed from the beginning, given the results described in the rotation curve papers. If the BB models assume full GR, but GR is inaccurate, then the BB models are worthless. On the other hand, if GR is accurate, then it predicts galactic rotation curves that match observations closely enough that there is no place for dark matter anywhere near a galaxy. Given a choice between GR and dark matter, I will choose GR every time. The CMB power function may well be explained by 'new physics' -- possibly related to the Pioneer Anomoly -- but it does not have to be the currently popular CDM + Lambda.

3) Once gravity waves are conclusively disproven, then we can talk. Until then, I will continue to accept that GR is a closer fit to 'reality' than Newtonian Gravity. In any case, even if GR is disproven, its replacement could very well show similar effects on a galactic scale.

Bill

[Guest_Richard Trigaux_*]

The density of galaxies was determined using the local lens effect, so that it allows to draw a density map of the galaxy. It is as if the galaxy was a glass lens: observing the distortion of the background through it allows to derive a map of the glass lens thickness.

And this allows us to have numeric data on the density of galaxies, not a formula in the style 1/R2 or 1/R3. This in turns allows to solve the differential equation numerically, not algebrically (using numbers and computer calculus, not formulas).

Johns Hopkins Univ. news release dated 1/11/06

SCIENTISTS "RAVE-ING" ABOUT MOST AMBITIOUS STAR SURVEY EVER

An international team of astronomers today announced the first results from
the Radial Velocity Experiment, an ambitious all-sky spectroscopic survey
aimed at measuring the speed, temperature, surface gravity and composition
of up to a million stars passing near the sun.

Those first results from the project, known for short as RAVE, confirm that
dark matter dominates the total mass of our home galaxy, the Milky Way, team
members at The Johns Hopkins University and elsewhere said. The full survey
promises to yield a new, detailed understanding of the origins of the
galaxy, they said.

The results were released at the American Astronomical Society's 207th
meeting in Washington, D.C.

An image is available at http://www.jhu.edu/news/home06/jan06/wyse.html

Very interesting indeed, but they assume that all the stars in the galaxy have elliptic orbits. This is simply not true, and may account with many difficulties.
If a majority of the mass is concentrated into the galaxy plane, stars into inclinated orbits will have chaotic trajectories.
Even stars in the disk plane will not have elliptic orbits, if the mass is into the disk.
The only stars with regular orbits are those in a circular orbit around the center of the galaxy.
Why? because they "feel" the mass "under" them (closer to the centre) as a point mass, and the mass "above" (further from the centre) has no influence. (and even this is not sure, it is true only if the galaxy has a spherical symmetry.)

So until now I even don't know if a spiral galaxy like ours has a spherical symmetry or a disk symmetry. Maybe all the galaxies are more or less elliptic.

However only a precise density map of the supposed dark matter can lead to its state equation, and from there to its physical nature: molecular hydrogen, atomic hydrogen, baryonic objects (ranging from dust to small stars and black holes) or other subatomic particules. If the state equation points at none of these things, so we will have to admitt that there is no dark matter, but some geometric effect.

[ljk4-1]

Paper: astro-ph/0601247

Date: Wed, 11 Jan 2006 20:05:34 GMT (7kb)

Title: Alternative proposal to modified Newton dynamics (MOND)

Authors: Juan M. Romero and Adolfo Zamora

Comments: 4 pages. Accepted for publication in PRD
\\
From a study of conserved quantities of the so-called Modified Newtonian
Dynamics (MOND) we propose an alternative to this theory. We show that this
proposal is consistent with the Tully-Fisher law, has conserved quantities
whose Newtonian limit are the energy and angular momentum, and can be useful to explain cosmic acceleration. The dynamics obtained suggests that, when
acceleration is very small, time depends on acceleration. This result is
analogous to that of special relativity where time depends on velocity.

\\ ( http://arXiv.org/abs/astro-ph/0601247 , 7kb)

[ljk4-1]

I don't want this turning into the Dark Matter/Energy Topic (unless of course that is what is affecting the Pioneer probes), but I wanted to share this news item while we are still on the subject:

Dark Matter Galaxy?

Summary - (Thu, 12 Jan 2006) Astronomers think they might have found a "dark galaxy", that has no stars and emits no light. Although the galaxy itself, located 50 million light years from Earth, is practically invisible, it contains a small amount of neutral hydrogen which emits radio waves. If astronomers are correct, this galaxy contains ten billion times the mass of Sun, but only 1% of this is hydrogen - the rest is dark matter.

http://www.universetoday.com/am/publish/pp...hi.html?1212006

If there is life in that galaxy, just try to imagine how utterly different it probably is from ours.

[Guest_Richard Trigaux_*]

I don't want this turning into the Dark Matter/Energy Topic (unless of course that is what is affecting the Pioneer probes), but I wanted to share this news item while we are still on the subject:

Dark Matter Galaxy?

Summary - (Thu, 12 Jan 2006) Astronomers think they might have found a "dark galaxy", that has no stars and emits no light. Although the galaxy itself, located 50 million light years from Earth, is practically invisible, it contains a small amount of neutral hydrogen which emits radio waves. If astronomers are correct, this galaxy contains ten billion times the mass of Sun, but only 1% of this is hydrogen - the rest is dark matter.

http://www.universetoday.com/am/publish/pp...hi.html?1212006

If this is confirmed, it rules out any alternative explanation about dark matter, such as geometric effects, MOND theory, etc and left us only to accept the fact that there is an unknown mass.

If there is life in that galaxy, just try to imagine how utterly different it probably is from ours.

We have no idea until now about what is dark matter. This observation also weakens any explanation based on dwarf stars, black holes, dust, etc... as these objects would be visible in a way or another, at least colectively.

The better explanation left is about weakly interactive particulres (neutrinos or other) which are not a good life support (they don't interact with each other).

But we really don't know, and there is perhaps some kind of alternate universe, with its own stars, or very novel structures, supportive for life.

But I don't believe too much to "dark stars", as if so there would be black holes formed by dark matter, and then ordinary matter spiraling around them, which would be visible.

[Mongo]

So this team has observed a cloud of neutral hydrogen with an estimated mass of 100 million suns, which has a much too large rotational velocity for its mass. This is the logical end point of the trend from high-surface-brightness elliptical galaxies, which have only slightly too large rotational velocitys, through normal spiral galazies, which have larger excess rotational velocitys, through low-surface-brightness galaxies, which have extreme excess rotational velocities, and finally this lowest-surface-brightness galaxy (which is what this object really is), which has the highest excess rotational velocity of all.

Looks like a trend to me.

I fail to see that this proves that dark matter exists, since the same MOND-like physics (which apparently describe full General Relativity) that were postulated to explain other cases would presumably apply to this object as well. The whole point of MOND-like theories is that they apply under conditions of galactic distances but low gravitational acceleration, which is why the difference between Newtonian models and observation increases with declining surface brightness (which tracks mass and hence gravitational acceleration). Something like this gas cloud would be expected to have remarkably high rotational velocitys.

I think that we should wait to see from a refereed paper if this object is explainable under MOND or GR before proclaiming that dark matter exists.

Bill

[The Messenger]

I think that we should wait to see from a refereed paper if this object is explainable under MOND or GR before proclaiming that dark matter exists.

Bill

I think we better wait longer that that - there are many refereed papers that all-but-insist Dark Matter is a done deal. Sorry - I don't believe in the Easter Bunny, and I don't believe theories that cannot be demonstrated using local observables and principles are scientifically valid.

There has been an intense campaign in the last four decades to identify the baryons responsible for altering galactic rotations, and these careful seaches have turned up naughta. Most of the conjecture I have seen about why these searches have failed; and how Dark Matter can best be explained have involved hypotheses that simply cannot be tested - and yes, this includes redistributing galactic masses so that General Relativity fits the bill.

The Pioneer anomalies are observational events that we can sink our teeth into. This is where the trail should be picked up - in our own backyard. This is where we can either support or null a hypothesis.

[ljk4-1]

Paper (*cross-listing*): gr-qc/0601055

Date: Sat, 14 Jan 2006 16:11:23 GMT (36kb)

Title: What do the orbital motions of the outer planets of the Solar System
tell us about the Pioneer Anomaly?

Authors: Lorenzo Iorio

Comments: Latex2e, 12 pages, 3 tables, 4 figures

Subj-class: General Relativity and Quantum Cosmology; Space Physics
\\
In this paper we investigate the effects that an anomalous acceleration as
that experienced by the Pioneer spacecraft after they passed the 20 AU
threshold would induce on the orbital motions of the Solar System planets
placed at heliocentric distances of 20 AU or larger as Uranus, Neptune and
Pluto. It turns out that such an acceleration, with a magnitude of about 8 X
10^-10 m s^-2, would affect their orbits with secular and short-period signals
large enough to be detected with the present-day level of accuracy in orbit
determination. The absence of such anomalous signatures in the latest data
analyses rules out the possibility that in the region 20-40 AU of the Solar
System an anomalous force field inducing a constant and radial acceleration of
that size is present.

\\ ( http://arXiv.org/abs/gr-qc/0601055 , 36kb)

[mchan]

Below is an excerpt from The Planetary Society's website of Merek Chertkow's report on the 2005 Pioneer Anomaly Conference:

On using New Horizons to investigate the Pioneer Anomaly, here is a comment from a poster on sci.space.history --

begin quote

Unfortunately, New Horizons is a rotten design for this particular study,
even though it will be spin stabilized. The problem is that the RTG heat
will be radiated very unevenly. The Pioneer effect is equivalent to a few
tens of watts more being radiated antisunward than sunward. Pioneer was
quite good for this since the RTGs are on booms, and hence are radiating
mostly into free space. Still, one of the biggest questions is how much
IR bounces off the spacecraft, and which way it bounces.

On New Horizons, the RTG is very close to the spacecraft body, and radiating
a kW or so of IR, so hundreds of watts of IR will hit the spacecraft.
Figuring out where this will go exactly is very hard. The spacecraft is
very un-symmetrical in the direction of motion, so the IR reflections will
certainly favor some directions rather than others. Also, the spacecraft
is covered with thermal blankets, making reflections even harder to predict.

In general, the systematics are predicted to be several times the Pioneer
effect, and of an unknown magnitude. This was exactly the problem with
Cassini, which also had the RTGs mounted close in. (They spent a month
in cruise using only the reaction wheels, which is as good as spin stabilized,
and the measured acceleration was about 3x the Pioneer effect, and well
off the pre-flight predictions, and so was useless for studying this
effect. )

Lou Scheffer

end quote

[Guest_AlexBlackwell_*]

On using New Horizons to investigate the Pioneer Anomaly, here is a comment from a poster on sci.space.history --

[...]

Lou Scheffer

Well, Scheffer certainly has a "dog in the hunt." Go to the LANL arXiv preprint server and run an "Author" search on Louis K. Scheffer.

[ljk4-1]

Will it be possible to design a KBO and/or Interstellar Medium probe that can "look" for the Pioneer Anomaly while still being able to properly explore the Kuiper Belt and beyond?

Cool as it would be, I cannot imagine anyone getting serious funding for a strictly PA mission at this stage.

Would would an "ideal" PA mission look like, anyway?