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Alternatives To Elusive Dark Matter

from KHouse Newsletter January 2014

The Universe is made up primarily of a mysterious substance called dark matter, a mesh, a spider web of space. That’s what popular science says, at least. Astrophysicists insist that dark matter is there; the indirect evidence is substantial. Yet, after multiple millions of dollars have been spent on trying to track down the actual physical particles that make up dark matter, science continues to come up empty. Maybe the astrophysicists need to try another approach in order to finally detect the elusive substance, or maybe they just need to adjust their current models about the nature of light, time, and the Universe itself.
It all started with the spinning of distant galaxies. A Swiss astrophysicist named Fritz Zwicky postulated in the 1930s that invisible stuff he called “dunkle Materie” hid inside the galaxies he was studying, because they spun too fast to contain only the visible stars and gas he could account for. Scientists observe the same puzzling phenomenon today. Based on spectral line data, it appears that the outer rims of spiral galaxies are moving at the same rate as the insides of the galaxies – and that doesn’t make any sense. The galaxies should fly apart from spinning that fast.
This problem caused Zwicky to hypothesize the existence of dunkle Materie—large amounts of invisible material that provides the gravitational pull to hold the galaxies together. It’s what physicists think dark matter is – neutral, uncharged particles that interact with visible material by massive gravitational force.
There is also the matter of gravitational lensing. Starlight through space if often seen to bend and warp around unseen massive objects. The Hubble space telescope can often produce two or three images of the same galaxy in one single picture. The individual images may be different sizes but contain the same features, as though space were a hall of mirrors. As beams of light from the same galaxy bend around objects in space, they reach the earth from slightly different angles, giving the appearance of coming from different locations. Clumps of invisible dark matter between us and these galaxies are blamed for causing the distortions.
Cosmologists have a variety of reasons for embracing the idea of dark matter. The problem is that its existence is inferred from physicists’ current interpretations of data; nobody has been able to directly detect the stuff yet. The physicists are confident that dark matter comes in the form of a particle, a weakly interactive massive particle (WIMP) that creates gravitational effects but otherwise ignores normal visible particles. The trick is to get it to get some WIMPs to show themselves by hitting visible matter into them and making them say, “Ow!”
Rick Gaitskell of Brown University has been hunting for dark matter for 24 odd years and heads the team that turned on the Large Underground Xenon (LUX) experiment in South Dakota. A mile underground in the Homestake Gold Mine, the LUX particle accelerator shoots xenon particles past ultra-sensitive detectors. If the xenon particles smack into one of these WIMPs, it should give off a little flash of electricity that the detectors can catch and record.
So far, though, the LUX hasn’t found anything. Gaitskell told Popular Science this past autumn, “Every experiment has reported essentially negative results. No one even knows for sure if the d- stuff really exists.” If dark matter really does make up five-sixths of the matter in the Universe, it certainly does an excellent job of hiding itself.
A Dark Herring
Of course, dark matter may not exist after all. In his own PowerPoint slides on dark matter posted on the Brown University website, Gaitskell tells his students, “It has been a Problem in Cosmology that astrophysical assumptions often need to be made to interpret data/extra parameters.” It’s true. Scientists create models they use to interpret the information that space gives them. The models are based on certain assumptions, and if those assumptions are incorrect, the data gets interpreted wrongly.
Possible Alternatives
If dark matter is just an illusion, though, what is causing the observed phenomena? What does hold spinning spiral galaxies together and cause the bending of light through allegedly empty space?
First of all, it is odd that so many spiral galaxies appear to have the same issue – the matter across their diameters all appear to be rotating at the same rate – all without flying apart. It may be that that the light information coming from them is interpreted incorrectly. The redshifts that are treated as a sort of Doppler effect – light appearing to lengthen as its source moves away from us – may have another explanation.
In the 1970s, William Tifft at the University of Arizona noted that his redshift measurements didn’t show gradual, smooth shifting to the red. Instead, they were quantized – the measurements made small jumps as though going up a flight of stairs. Two astronomers from Scotland, Guthrie and Napier, tried to disprove Tiffts quantized redshift ideas in the 1990s, but they finally confirmed his results.
Professor José Senovilla, Marc Mars and Raül Vera of the University of the Basque Country, Bilbao, and University of Salamanca, Spain proposed in 2011 that the redshift isn’t caused by a Doppler shift but by the slowing of Time itself. Dark energy supposedly permeates the Universe, causing the outer edges of space to expand at an accelerating rate. That’s the wrong way to interpret the light wave data, suggest these scientists. Senovilla and Vera argue that the better explanation is the opposite, that Time has been slowing down and we see its effect in the apparent stretching of light waves. The light reaching the Hubble telescope from distant galaxies might not tell us as much about the rate the galaxies are spinning as about the nature of Time itself.
The speed of light itself may be slowing. Physicists insist that light speed is a constant, but they may have made that determination prematurely. The speed of light may not be dropping very quickly, but a variety of papers have been written in the past several decades that suggest light speed is not a constant after all. Paul Davies, currently of Arizona State, argued in 2002 that the speed of light may be slowing down, and physicist Barry Setterfield has written extensively on the subject.
Yves-Henri Sanejouand from the University of Nantes in France in 2010 showed a possible slowing of the speed of light by about 0.02–0.03 m/s per year. That’s not much, but it demonstrates the real possibility of a much faster speed of light in the past. “The constancy of the speed of light is one of the fundamental pillars of contemporary physics,” explains Sanejouand, “so the possibility that it may instead vary (even at a slow rate) has far reaching consequences (although mostly on the theoretical side).”
It may also be that spiral galaxies haven’t had time to fly apart. If the speed of light has been slowing, methods for dating the age of the Universe might be way off. The age of the Universe itself may have been overestimated.
While dark matter is credited with causing gravitational lensing, Anirudh Pradhan of Hindu P. G. College in India suggests that the observed bending of light might be caused by the refraction of light as it hits the gasses around various astronomical bodies. We see the refraction of light all the time in everyday life. The fisherman who goes to stab a fish in the water cannot aim directly at the image of the fish, because the light changes direction as it leaves the denser water and hits the less dense air. The refraction of light makes the fish look like it’s in a spot that it isn’t. The same thing can happen in space. As light shoots through the vacuum of space, it hits clouds of gasses that cause it to change direction so that when it reaches us, multiple images of various sizes are produced – and we can’t be certain of where they actually originated.
The nature of the Universe is an involved mystery, a deep subject that requires a great deal more study. Yet, the hunt for dark matter highlights the importance of examining one’s assumptions in the pursuit of scientific truth. Assumptions are required to interpret data, but a great deal of time and money can be spent to prove incorrect interpretations when the underlying assumptions are faulty.
Further Reading
• Is Light Slowing Down?
— Optics and Photonics Focus

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