That sounds just like the anecdote Don Wilhelms tells in "To a Rocky Moon"! When will NASA learn stop throwing its treasures out with the trash?

Sorry, but what is this anomaly about?
( I'm a Newbie asking too many questions )

Google searches will answer lots of questions -- try here for starters.
The Pioneer Anomaly

--Emily

Cool! I hope they attempt it and it's successful. Later!

J P

I have an image in my head of driving around the M25 ( the london 'orbital' motorway ) and yelling at a service station "BIG MAC AND FRIES PLEASE"...then doing another lap of the motorway, only to have a burger land on my windscreen about 80 miles later once I was back in the same place

Doug

Here is a concise summary by the three principles:
http://arxiv.org/PS_cache/gr-qc/pdf/0411/0411077.pdf

And if you want to see how exhaustively this phenomena has been explored, here is another 55 pages:

http://arxiv.org/PS_cache/gr-qc/pdf/0104/0104064.pdf

NASA's unwillingness to fund a follow-up mission, or even preserve historical data that may shed additional light is disheartening. We know our physical models are close, and we also know exceptions that may require new models, if they exist, must be hiding in the fringes. Anderson & company have tried every avenue they can imagine to null this anomaly as an artifact, and have been unable to do so.

From a risk assessment basis, I don't find it disheartening that no dedicated mission is on the books. For one, it's possible (even if unlikely, and I mean no disrespect to Anderson et al) that future analysis will lead to a forehead-slapping resolution to the anomaly. That head-slap would be a lot harder if we had a $150 million mission already in interplanetary cruise for no good reason when the anomaly were understood. It's also possible that a future mission could investigate this completely in line with its other, primary goals.

There is no way we can investigate various outer solar system objects in great detail without visits from spacecraft. There's no chance that an ingenious look at old data will suddenly give us maps of the unseen hemispheres of the uranian moons. But in this case, there is a chance, and there's no urgency to resolving the mystery before the ground on which it rests has been plowed into dust by means much, much cheaper than a spacecraft.

The disregard for the existing data is rather more disheartening, but simply goes to show how things can be neglected for their due worth by a boxology-driven bureaucracy like NASA.

I agree with this, especially with the conclusion. THE EXISTING DATA MUST BE KEPT SAFE and analyzed completelly. The results must be confirmed by other flights (of which it can be a secondary goal) before sending a dedicated mission.

If the unmodeled acceleration is indeed, as the evidence suggests, about 0.00000008 cm-per-second per second, then since 1980 (when the effect first definitely appears) there would be a cumulative unmodeled delta-v of about 17 metres-per-second towards the Sun, and a cumulative unmodeled spatial displacement of about 269,000 km toward the Sun.

Given the upcoming opportunity for a com session with one of the Pioneers, this would result in a round-trip time almost two seconds shorter than what it 'should' be. Although it is still about 25 hours.

Bill

Is there any way to tell if small planetoids or comets are undergoing the same effect? Or are they just too big and their orbits - especially the ones farther out - just not known well enough?

Heck, while I'm at it - has anyone investigated to see if Dark Matter exists not just outside galaxies but in interstellar space as well? Perhaps closer than we think?

I personally am less than convinced of the existence of so-called Dark Matter.

The problem for me is that:

1) New physics is needed to account for its existence.

2) In order for it to account for the observed galactic rotation curves (which is why it was originally proposed), the distribution of 'Dark Matter' in each galaxy must be carefully adjusted by radius from the galactic center in order to match the needed gravitational potentials. This distribution needs to vary wildly from one galaxy to the next, in order to produce rotation curves that are far more similar to each other than they 'should be'.

3) No candidate Dark Matter particles have ever been detected.

Compare this with Mordehai Milgrom's MOND theory, which was also designed to explain galactic rotation curves:

1) New physics is needed to account for the deviations from Newtonian physics. Okay, so far the two explanations are a wash.

2) MOND naturally explains galactic rotation curves with little or no special adjustment, by assuming that the visible matter (including gas and dust) is in fact all there is. The theory was used to make predictions of rotation curves for classes of galaxies (i.e. low surface-brightness galaxies) that were unknown when it was introduced, that have panned out exactly. Dark Matter, on the other hand, made wildly inaccurate predictions, that must be corrected ex post facto.

3) The same physics that seems to be behind MOND can also explain so-called 'Dark Energy' and, in certain formulations, even the Pioneer Effect,

Here are some papers about MOND:

The arXiv papers of the originator, Mordehai Milgrom, regarding MOND:

http://xxx.lanl.gov/find/astro-ph/1/AND+au...D/0/1/0/all/0/1

TeVeS, the currently most popular relativistic formulation of MOND:

http://xxx.lanl.gov/find/astro-ph/1/abs:+TeVeS/0/1/0/all/0/1

Bill

This site gives the general history and background on MOND, including links to other sites:

http://en.wikipedia.org/wiki/MOND

Astrophysics, abstract
astro-ph/0503368

From: Dario N\'u\~nez [view email]

Date (v1): Wed, 16 Mar 2005 19:54:33 GMT (409kb)
Date (revised v2): Mon, 9 Jan 2006 17:37:29 GMT (421kb)

Pioneer anomaly? Gravitational pull due to the Kuiper belt

Authors: Jose A. de Diego, Dario Nunez, Jesus Zavala

Comments: 5 figures, final version, accepted for publication at Int. J. of Mod. Phys. D

In this work we study the gravitational influence of the material extending from Uranus orbit to the Kuiper belt and beyond on objects moving within these regions. We conclude that a density distribution given by $\rho®=\frac{1}{r}$ (for $r\geq 20 UA$) generates a constant acceleration towards the Sun on those objects, which, with the proper amount of mass, accounts for the blue shift detected on the Pioneers space crafts. We also discuss the effect of this gravitational pull on Neptune, and comment on the possible origin of such a matter distribution.

http://arxiv.org/abs/astro-ph/0503368

Anderson & Co. wrote a response to this - Essentially this type of mass could NOT produce the measured linearity - the probe should be accelerating at an increasing rate (if the Kuiper belt has a high enough density), and/or the density would have to extend to near the orbit of Saturn and therefore be easily be detectable.

MOND is not theoretically based - it is purely a phenomological explanation of a rather confusing family of observations.

http://arxiv.org/PS_cache/gr-qc/pdf/0104/0104064.pdf


Four independent data point should lead to a little bit more than head scratching. True, an independent mission is prohibitively expensive, but piggy-backing a frizbee on the New Horizon's probe: spin stabilized, with a Doppler repeater similar to the Pioneer probes - that could have been a reasonably cheap and lightweight add-on with enough parametric control to nail this puppy down.

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.

Anomalous Acceleration of Pioneer 10 and 11: Dust Density in the Kuiper Belt

http://www.newtonphysics.on.ca/Anomalous/Acceleration.html


And this BBC item from 1999 may be of interest:

http://news.bbc.co.uk/1/hi/sci/tech/460095.stm


Relevant information on the Kuiper Belt:

http://www.lpl.arizona.edu/faculty/malhotr.../ISP_Nov04.html

http://www.lpl.arizona.edu/faculty/malhotr...-TPF_Darwin.pdf

Thank you ljk4-1, that first link was interesting indeed.

So it might simply be dust?
Well its the simplest explanation, so im inclined more to that one than some of the other proposals.

As for the final destiny of the spacecrafts, that they would aquire so much dust eventually was a surprise to me:

"Pioneer spacecraft will become the nucleus of asteroids flying away from the solar system with the interstellar dust. "

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

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.

Judging by these two new news items, dark matter/energy cosmology is indeed the "in" topic for astronomers these days.

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


Gamma-ray burst study may rule out cosmological constant

Dark energy - the mysterious force that drives the acceleration of
the universe - changes over time, suggest controversial new
calculations.

If true, the work rules out Einstein's notion of a "cosmological
constant" and suggests dark energy, which now repels space, once
drew it together.

Read the full story on New Scientist Space:

http://www.newscientistspace.com/article/dn8566

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

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

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.

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

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).






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.

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)

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.

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.





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.

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

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.

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)

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

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.

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?

Probably a very long stick with an RTG at one end and a HGA at the other

Pioneer 10 or 11 basically

Doug

Actually, this might be a significant and useful chunk of data, if the vector is known, and it is in either the same or opposite direction as the solar wind.

Amoung the possible explanations for the Pioneer enomally is a solar field effect that diminishes as i/r - which would be much more pronounced between 1 and 8 AU than >20. So a magnitude of three greater at closer distances is actually in-family. At distances greater than 20 AU, a 1/r attenuation reduces to a nearly constant rate, as observe by the Pioneer probes.

A higher magnitude effect in Cassini is consistent with the magnitude and attenuation of 'unmodeled forces' experienced by both Galileo and Ulysses. Since the effective 'force' of these peculiarities is in the same direction as the solar wind, it is impossible to disentangle the parameters. (Using the solar wind to model Galileo and Ulysses accelerations requires a secondary term that attenuates as 1/r. (According to Anderson & Nieto.)

Paper: astro-ph/0601422

Date: Thu, 19 Jan 2006 10:51:50 GMT (4kb)

Title: Upper limits on density of dark matter in Solar system

Authors: I.B. Khriplovich and E.V. Pitjeva

Comments: 4 pages
\\
The analysis of the observational data for the secular perihelion precession
of Mercury, Earth, and Mars, based on the EPM2004 ephemerides, results in new
upper limits on density of dark matter in the Solar system.

\\ ( http://arXiv.org/abs/astro-ph/0601422 , 4kb)

Paper: astro-ph/0601431

Date: Thu, 19 Jan 2006 17:08:29 GMT (79kb)

Title: Modified gravity without dark matter

Authors: R.H. Sanders

Comments: 28 pages, 10 figures, lecture given at Third Aegean Summer School,
The Invisible Universe: Dark Matter and Dark Energy
\\
On an empirical level, the most successful alternative to dark matter in
bound gravitational systems is the modified Newtonian dynamics, or MOND,
proposed by Milgrom. Here I discuss the attempts to formulate MOND as a
modification of General Relativity. I begin with a summary of the
phenomenological successes of MOND and then discuss the various covariant
theories that have been proposed as a basis for the idea. I show why these
proposals have led inevitably to a multi-field theory. I describe in some
detail TeVeS, the tensor-vector-scalar theory proposed by Bekenstein, and
discuss its successes and shortcomings. This lecture is primarily pedagogical
and directed to those with some, but not a deep, background in General
Relativity

\\ ( http://arXiv.org/abs/astro-ph/0601431 , 79kb)

Why is it that the Pioneer and Voyager probes had to have their science instruments as far from the RTGs as possible, yet that does not seem to be the case with Cassini and New Horizons?

Better shielding? More advanced/protected instruments? We don't fear radiation as much as we used to?

Well shielded RTG should radiate very little, shouldn't it?

It will still radiate heat, the RTG itself isn't particularly efficient so most of the decay energy is ultimately expressed as heat which is radiated out (hence the kwatt or so referred to in the sci.space.history post) from the RTG module. A significant portion of this IR energy "hits" the main body of the spacecraft but is reflected away in arbitrary directions from it by the thermal insulation, hence the complexity of analysing the effect.

The RTGs on the Pioneer probes were symetrically mounted on booms, so that the (waste) thermal energy should be symetrically distributed. One of the still plausible scenarios is that the solar wind has caused substantial aging or discoloration of the sun-facing side of the boom(s), causing the energy to be dispersed asymetrically.

This may sound like an absurdly small effect, but that it is the ballpark of the Pioneer anomalies (1x10^-9 m/s^2), and this is why it is so difficult to propose a definitive test - very, very small force.

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

replaced with revised version Fri, 20 Jan 2006 16:04:50 GMT (37kb)

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, 13 pages, 3 tables, 4 figures, 14 references. References
added. Stressed the fact that, even by assuming errors in the planetary
orbital elements 30 times larger that those published by Pitjeva, the
anomalous Pioneer effects on Uranus, Neptune, Pluto still remain well larger
and, thus, detectable if present

Subj-class: General Relativity and Quantum Cosmology; Space Physics

\\ ( http://arXiv.org/abs/gr-qc/0601055 , 37kb)

This is an important constraint, under the tested conditions:


I think this highly constrains MOND-like, Dark Matter-like, or Dark Energy-like candidates.

These results do not constrain 1) non-linear effects, 2) systemics that may cause us to incorrectly calculate the mass and/or positions of the planets, or 3) linear effects that only act upon small conductive, and/or radioactive bodies.

Although the measured Pioneer accelerations appear to be roughly linear, it is important to remember that over great distances: 1/r, 1/r^2 and 1/z^4 - these normal distant field scaling factors can be reduced to nearly linear approximations (over relatively short distances)

I wonder if the magnitude of the 'Pioneer Effect' depends upon the radial velocity of the object, such that objects moving away from the Sun appear to have a force acting upon them toward the Sun, and objects moving toward the Sun appear to have a force acting upon them away from the Sun, with the magnitude of the 'force' proportional to the radial velocity of the object.

Objects in bound orbits, such as the planets, would end up having the two effects cancel out over each full orbit. The effect of such a 'force' would be to make their orbits somewhat less eccentric than they would otherwise be, but since the effect would be small (due to the low eccentricity of all the large objects with well-known orbits, resulting in low radial velocities) and would act over only half of an orbital cycle before being reversed (as the radial velocity changes from outward to inward and vice versa), the difference between the actual orbit, forced to lower eccentricity under the Pioneer Effect, and a non-Pioneer Effect orbit of sufficiently lower eccentricty to match, might well be too small to observe with current technology.

The 'Pioneer Effect' would only be easily visible in unbound trajectories such as Pioneers 10 and 11, where the effect is larger due to the larger radial velocity, and can accumulate over a much longer period of time.

Bill

From New Scientist -> http://www.newscientistspace.com/article.ns?id=dn8631

Gravity theory dispenses with dark matter

A modified theory of gravity that incorporates quantum effects can explain a trio of puzzling astronomical observations – including the wayward motion of the Pioneer spacecraft in our solar system, new studies claim.

The work appears to rule out the need to invoke dark matter or another alternative gravity theory called MOND (Modified Newtonian Dynamics). But other experts caution it has yet to pass the most crucial test – how to account for the afterglow of the big bang.

Astronomers realised in the 1970s that the gravity of visible matter alone was not enough to prevent the fast-moving stars and gas in spiral galaxies from flying out into space. They attributed the extra pull to a mysterious substance called dark matter, which is now thought to outweigh normal matter in the universe by 6 to 1.

But researchers still do not know what dark matter actually is, and some have come up with new theories of gravity to explain the galaxy observations. MOND, for example, holds that there are two forms of gravity.

Above a certain acceleration, called a0, objects move according to the conventional form of gravity, whose effects weaken as two bodies move further apart in proportion to the square of distance. But below a0, objects are controlled by another type of gravity that fades more slowly, decreasing linearly with distance.

But critics point out that MOND cannot explain the observed masses of clusters of galaxies without invoking dark matter, in the form of almost massless, known particles called neutrinos.


http://www.newscientistspace.com/article.ns?id=dn8631

Sounds like a version of the theory that general relativity needs to be applied to gravitational models, instead of simpler Newtonian physics. That theory hclaims to be able to do away with dark matter entirely and still explain both small-scale and very large-scale systems.
I just imagine the thought of doing away with dark matter is unappealing to some people, as I'm sure some have made the study of dark matter their life's work. Another way of looking at it though is that they're just inadvertently referring to "the effects of general relativity" as "dark matter".

Paper: astro-ph/0601581

Date: Wed, 25 Jan 2006 21:28:29 GMT (15kb)

Title: Globular Clusters as a Test for Gravity in the Weak Acceleration Regime

Authors: Riccardo Scarpa, Gianni Marconi, and Roberto Gilmozzi

Comments: Comments: To Appear in the proceeding of the "First crisis in
cosmology" conference, published in the American Institute of Physiscs'
proceedings series, Vol. 822. (includes 3 pages and 1 fig)
\\

Non-baryonic Dark Matter (DM) appears in galaxies and other cosmic structures
when and only when the acceleration of gravity, as computed considering only
baryons, goes below a well defined value a0=1.2e-8 cm/s/s. This might indicate
a breakdown of Newton's law of gravity (or inertia) below a0, an acceleration
smaller than the smallest probed in the solar system. It is therefore important
to verify whether Newton's law of gravity holds in this regime of
accelerations. In order to do this, one has to study the dynamics of objects
that do not contain significant amounts of DM and therefore should follow
Newton's prediction for whatever small accelerations. Globular clusters are
believed, even by strong supporters of DM, to contain negligible amounts of DM
and therefore are ideal for testing Newtonian dynamics in the low acceleration
limit. Here, we discuss the status of an ongoing program aimed to do this test.
Compared to other studies of globular clsuters, the novelty is that we trace
the velocity dispersion profile of globular clusters far enough from the center
to probe gravitational accelerations well below a0. In all three clusters
studied so far the velocity dispersion is found to remain constant at large
radii rather than follow the Keplerian falloff. On average, the flattening
occurs at the radius where the cluster internal acceleration of gravity is
1.8+-0.4 x 10^{-8} cm/s/s, fully consistent with MOND predictions.

\\ ( http://arXiv.org/abs/astro-ph/0601581 , 15kb)