Being the occasionally interesting ramblings of a major-league technophile.
There's a lot out there we don't see. This isn't surprising. We can only perceive distant objects by the light they emit themselves or reflect from other sources. (Here using "light" to include the complete electromagnetic spectrum.) For some bodies we also detect particles they emit. Except for such nearby bodies as the Moon and Venus (both of which have been radar scanned from Earth) we can't supply this illumination.
Digression: Well, we can't supply the amount needed from Earth. Not with any practical application of current technology. The inverse square law sees to that. Typically, if you double the distance you need four times the energy sent for the same energy delivered to the target, because the radar wavefront spreads as it travels. Then subtract what the target absorbs. Then only a tiny fraction of what's reflected actually heads back towards the transmitter, and even that is again weakened by distance on the return trip.
Years ago there was a serious proposal to launch a probe well out of the plane of the ecliptic. Once far enough from what we already know about (especially including us) it would detonate a gigaton warhead. This would act as a flashbulb, illuminating the solar system out to perhaps hundreds of astronomical units. This "snapshot" would reveal many new objects in the Kuiper Belt and Oort Cloud.
Also, some space probes have had radar. Others have transmitted their radio data signal as they passed around the limb of a planet or moon, giving us information on the makeup of any atmosphere present.
Coming back on target: Even the most sensitive telescopes can only detect what bodies reflect or emit themselves, and even that must be above a certain threshold, due to system noise in the instrumentation. That threshold gets a little lower every year, but it's still there. So, when looking out at the universe, we know we're missing a lot.
In the Thirties, brilliant (if eccentric) physicist Fritz Zwicky used data from astronomers to demonstrate that we could not see enough material in other galaxies to explain how they held together against their own spin. There just wasn't enough visible mass. He coined the phrase "dark matter" ("dunkle Materie" in his native Swiss) as a label for this substance we can't see. Like the Force, dark matter holds the galaxy together. (Oh, so many jokes to resist...)
There are many potential candidates for this matter we aren't seeing. One is numerous massive objects out in the cometary halos around star systems. For our own system, these are detectable through occultation (that is, they get in the way of light from stars) or by direct observation if they reflect enough light from the Sun. We don't know how many there are, but considering we've already discovered several of them the number is probably large. However, even using maximum estimates for their total mass, there aren't enough. There has to be more. Much more. Two classes of what that more might be are the origin of this column's title.
MAssive Compact Halo Objects (MACHO) are massive bodies such as dead stars (including neutron stars and black holes) and small, dim stars; "free" or "rogue" planets which wander between stars; brown dwarfs and a few other things. They could be imaged through occultation or - for the most massive of these - gravitational lensing. These massive bodies would produce a specific lensing signature when passing in front of a star. That is, there would be a smooth, gradual rise to a peak then a gradual falloff symmetrical with the rise, over many days to several months, depending on the geometry and relative speeds. These are normal bodies, made of normal matter (that is, baryonic matter, which is what we are made of). They are just too small or too old to glow detectably on their own. (A black hole doesn't glow at all, but matter falling into one does. However, if there's no matter falling in...)
Weakly Interacting Massive Particles are similar to neutrinos in that they are incredibly difficult to detect. However, those ghostly particles have barely any mass at all. WIMP particles do... theoretically. If they exist, they are likely the most significant part of whatever makes up dark matter.
There are even GNACHOs: Gravitationally Negative Anomalous Compact Halo Objects. These are also detectable through gravitational lensing, but with a different and distinct signature. (Gradual rise in intensity, sharp cutoff to zero, sharp rise to a second peak, gradual falloff. Again, the time scale depends on geometry and relative speeds.) These are very hypothetical objects which are the results of matter pouring through a wormhole and producing a negative gravity effect. Which makes my head hurt just thinking about, so let's move on.
A similar exotic concept is negative matter. While antimatter has a reversed electrical charge with respect to what we're made of, negative matter has a reversed gravitational charge. If it exists, it would collect between galaxies, and might provide an inward repulsive force which would counter the centripetal acceleration which would otherwise tear apart galaxies and other large collections of stars. It could also be all or a major component of the mysterious "dark energy" (you'd think they'd call it "dark force" but noooo...) which is expanding the universe. However, it also is still very hypothetical.
So, the WIMP concept seems the most likely source of unseen mass. Multiple detectors around the world are seeking the WIMP population, using several different methodologies. If they exist theory calls for a whole family of different particle types, to symmetrically balance with visible matter. Some physicists (and others) have imagined that these aren't just particles flying around on their own. That there would be at least dark matter atoms, and likely molecules. There could possibly be entire planets and star systems, existing alongside us unseen and undetectable except for their gravity. Actually, the individual particles, atoms and molecules have so little mass they could be passing right through us continuously, and we'd never notice. One wonders if WIMP stars produce an analog to electromagnetic radiation, and how that might be detected.
We see the presence of dark matter on a galactic scale, but what about within our own solar system? No matter what is responsible for the extra gravity, the effects would be subtle in something as small as a star system. Inside the orbit of Jupiter, even light pressure from the Sun would overwhelm the effect. However, there are signs of the influence of an unexpected force on some of our most distant probes. There is something causing the Pioneer 11 and 12 probes to slow more quickly than expected as they move outward from the Sun. The Voyager probes can't show this effect, since they are not spin stabilized and fire their thrusters to maintain or change orientation. That overwhelms this miniscule effect. The given explanation is that thermal photons (IR heat emissions) from radiothermal generators is providing enough directional thrust to cause the differential. However, some say this doesn't completely account for the deceleration.
One wonders whether the piano-sized New Horizons probe - at the time this is written over halfway to Pluto - will show this effect. New Horizons has both spin-stabilized (cruise) and three-axis stabilized (during science operations, using thrusters) modes. In the spin stabilized mode it should be subject to the same effect as the Pioneers, once it gets far enough from the Sun for that effect to be detectable.
An alternate explanation to dark matter is MOdified Newtonian Dynamics, or MOND. It claims that gravity does not behave exactly as one would expect according to the inverse square law. That over very large distances, gravity decreases less and less. This makes gravity stronger at great distances than if it operated strictly according to the Newtonian mechanics.
MOND does explain galaxies holding together in spite of their spin. It also explains the Pioneer anomaly. However, there is no experimental evidence to support it. We know there are MACHO out there; we've seen some of them. We have actually observed lone planets between stars. We've seen large objects in our own cometary nebula, too. Given our current state of the art and the number so far detected, there must be a *lot* of both 'em we aren't seeing... at least yet. Most people familiar with MOND dismiss it as an interesting intellectual exercise which is almost certainly not applicable to the real universe.
There are no confirmed findings of WIMP matter, but the concept fits so well with existing models of subatomic particle physics it is probably real. Besides the detectors - which look for existing WIMP particles - there is hope that the Large Hadron Collider - which already found the Higgs Boson even while operating at a reduced power level due to equipment problems - will produce detectable WIMP particles once at full power.
There's a lot to look for out there. Even if we can't see it.
This document is Copyright 2014 Rodford Edmiston Smith. Anyone wishing to repost it must have permission from the author, who can be reached at: email@example.com