by Les Johnson
Figure 1. Shown are the galaxies making up galaxy cluster 1E 0657-66. On either side of the center, the pink clumps show hot gas detected by NASA’s Chandra x-ray observatory. Just outside this gas are regions where the bulk of the matter resides (shown in blue). This matter, detected via gravitational lensing, shows where the dark matter that makes up most of the mass of the cluster lies.
(Image courtesy of NASA.)
In the "eye" of a hurricane, the sea level rises precipitously. Your ears "pop" as you rapidly rise in the elevator to the top of the Empire State Building in New York City. Air pressure.
The compass needle spins and settles pointing north – an unseen force acts to align the iron-containing needle in a northerly direction no matter where you are in the northern hemisphere. Magnetism.
The water runs down the hillside toward the pond below. The Earth flies through space at about 67,000 miles per hour tracing a circular path around the Sun and doesn’t fly off into deep space. Gravity.
Two pieces of uranium are brought together under controlled conditions and they begin to grow warm and give off heat. They are brought rapidly together in a unique geometry and they explode, producing a "nuclear explosion". Radiation.
The velocities of stars orbiting the centers of galaxies, rather than decreasing as a function of distance as would the velocities of planets orbiting a star, remain constant out to the edge of the galaxy. Gravity bends space-time, causing light to focus when it passes by a massive object, allowing scientists to measure of the mass of the object bending the light in a process called Gravitation Lensing; The bending of light is much, much larger around celestial objects than expected. dark matter.
In our everyday lives we are used to working with things our senses don’t directly perceive. You don’t have trouble believing that the lights will come on in a room when you enter and flip a switch on the wall. Do you see the electricity flow through the wires to the light over your head? Or do you see the effects of the electricity acting on the components in the fixture that produces the light? You get in your car and expect it to move once the engine is started and you press the gas pedal. Do you see the chemical bonds in the gasoline breaking and reforming into carbon dioxide and water as they release energy? No, you see the effects of the chemical reaction as heat and motion. I could go on and on. We directly perceive very little of the physical world around us, yet we have a fairly good understanding of it by observing the secondary or tertiary effects of its interactions with the parts of the world we do perceive.
So why should it be different with dark matter? Is it because of its unfortunate name? “Dark” implies mystery and, perhaps something evil and malevolent. Would we have the same visceral skepticism of its existence if it had been originally steeped in scientific sounding jargon and called “non-baryonic matter?” Or is our reaction to the currently ill-understood science of dark matter rooted in the fact that is not yet fully understood nor easily explainable? After all, in our everyday lives, modern science and engineering has done a pretty good job of explaining and making use of gravity, air pressure, magnetism and electricity to give us the modern conveniences that define the 20th and 21st centuries: air travel, computers, medicine, cell phones, etc. and we don’t yet have a link between dark matter and the latest gizmo that someone is trying to sell us? Personally, as a physicist, I think my initial skepticism of the existence of dark matter is rooted in the sheer amount of the dark matter thought to exist and a faint hope that perhaps we don’t yet have a good understanding of physics. Perhaps explaining dark matter will turn our understanding of physics on its head and allow us to find a way to cheat the laws of physics and travel faster than light. Yes, that is a stretch and a faint hope, but it was nonetheless at the core of my initial skepticism.
Scientists now believe that dark matter accounts for about 84 percent of the matter in the universe. In other words, when you look into the sky and add up the mass of the planets, the Sun, all the stars (many of which, we now know, have planets circling them), the black holes, pulsars and quasars you will have only accounted for about 16% of the mass that should be there – if our understanding of gravity and its effects are correct. Something different (not made of protons, neutrons, electrons and their subatomic constituents) and unseen appears to be here with us normal matter types. We cannot see it, hear it or touch it, but we can see its effects just as we can see the effects of magnetism and radiation – though we cannot see them directly either.
Before delving into the history of dark matter, I would like to address the caveat I made above, “– if our understanding of gravity and its effects are correct.” There is always a chance that our understanding of gravity is fundamentally wrong. The history of science is one of ‘final theories’ being found to be incomplete or incorrect and a new "final Theory" coming along to explain what others before had failed to explain. It would be arrogant to say that we know, beyond a shadow of doubt , that we now have a final-for-all-time theory and understanding of anything, let alone gravity. What we can say is that in virtually all other areas where gravity’s effects are measureable, our current theories seem to explain them very well and there is no other reason (other than the observation of dark matter, that is) to doubt that our understanding of gravity is either or wrong or incomplete.*
Being in the “space business,” I of course look toward our experiences in spaceflight to find evidence of in support of my assertion that we really do have a very good practical understanding of how gravity works. We’re able to launch spacecraft from the Earth and successfully have them land on other planets within a few miles of where we intended – across distances of hundreds of millions, if not billions of miles between launch and landing. For example, the Mars Curiosity Rover launched at 10:02 am Eastern Standard Time on November 26, 2011. It then flew approximately 350 million miles to reach Mars and landed at 1:31 Eastern Daylight Time on August 6, 2012 -- within 1.5 miles of where it was supposed to land. Another example is the Gravity Probe B mission, launched into Earth orbit in 2004, which successfully measured predictions resulting from Einstein’s General Theory of Relativity. To do so, it measured very small changes in the spin direction of four gyroscopes resulting from the Earth warping space-time and the planet’s rotation "dragging" space-time around with it.
On a more everyday level, we demonstrate a thorough understanding of gravity’s effects in the way we build houses, skyscrapers, bridges and just about every other construction in the world. It affects our choice of building materials, overall design and the way we landscape so as to avoid flooding (water runs downhill – due to gravity – after all!). Physicians model the effects of gravity on the human body, and NASCAR drivers must take it into account when they decide to accelerate on the racetrack. There is scarcely an activity performed by humans that doesn’t rely on an implicit or explicit understanding of gravity’s effects. Our science and engineering seems to work pretty well here on Earth and, by extension, to the rest of the universe we observe when we look through our telescopes. And then there’s the problem of the missing mass…
Figure 2 The Coma Cluster consists of thousands of galaxies within a 20 million light year volume of space.
(Image courtesy of NASA.)
In the 1930s, an astronomer named Fritz Zwicky noted that the velocity of the outermost galaxies in cluster of distant galaxies was much higher than it should have been. So high, in fact, that the galaxies should have long-ago broken free of their gravitational orbits around the central galaxies and flown off into space. When he looked at the brightness of the stars in the cluster and estimated the amount of mass within them, it didn’t add up. He estimated that there must be at least four hundred times more mass than was visible to account for the velocities he measured. We’ve since narrowed the gap by taking more accurate measurements and by taking into account the black holes, neutron stars and other exotic objects unknown in Zwicky’s time – and the discrepancy is now down to a factor of five. A factor of two could just be the result from a measurement or calculation error. A factor of five is simply too large to ignore. Since then, there have been numerous other measurements of many different astronomical objects and all of them show a discrepancy between the amount of matter this is directly observed and what must be there according to the observed gravitational effects. The plot has thickened as scientists looked in other ways for the missing mass.
While dark matter does interact gravitationally, its effects are not noticed unless we are making observations on scales the size of galaxies or larger. It is thus either so diffuse that the effects are not noticeable on the scale of a single solar system (not to mention the scale of a planet), or it represents some phenomenon which comes into play on a galactic scale, or some combination of the two. The plot thickens.
Most regular (non-"dark") matter interacts gravitationally and electromagnetically. We’ve discussed how dark matter’s existence is inferred by its gravitational effects, so the obvious next question is "can we see it in other ways?" Regular, everyday matter is observable electromagnetically, especially in large quantities. It reflects, absorbs or alters in some way the electromagnetic radiation that impinges upon it. When I say electromagnetic radiation, it is okay to think of ordinary light, though I am referring to frequencies of light across the spectrum, including not only visible light, but also radio, microwaves and infrared. Usually, indeed almost all the time, matter will affect electromagnetic radiation in some way. You’d think that if there were five times the mass in a galaxy or group of galaxies out there, we would see it. We don’t. The light of more distant galaxies isn’t dimmed as it passes by, as it would be if it were passing through clouds of gas in deep space. The light from the galaxies themselves appears unattenuated, and it most certainly would be reduced in brightness if it were passing through that amount of ordinary matter. Clearly, dark matter is radically different from the matter than that we used to seeing and working with. What’s up with that?
For a while, some scientists thought that the missing mass might be in form of neutrinos. Neutrinos are exotic particles to be sure, but there simply aren’t enough of them to account for the missing mass of the universe. Neutrinos are very small particles (small meaning that they have a very small, nearly unmeasurable mass) that travel near the speed of light and don’t interact with matter electromagnetically. It is the last feature that had some thinking they might account for the missing mass. Dark matter doesn’t interact electromagnetically and neither do neutrinos, therefore they must be the same thing. They were wrong. Neutrinos do interact with matter via the weak nuclear force and it is through this interaction that they have been detected and counted. Deep underground (about 8600 feet down) in Canada, researchers at the Sudbury Neutrino Observatory measure the flux of neutrinos coming from the Sun and other, more distant objects. Since they only interact with matter in one way, and since the likelihood of a neutrino interacting with the detector is very small, researchers had to place the detector deep under the mass of the Earth in order to reduce the background signals from cosmic rays – which are comprised of normal matter and would overwhelm their instruments as noise. Based on measurements from Sudbury and other, similar observatories, we now know that about 65 billion solar neutrinos pass through every square centimeter of space near the Earth each and every second. The key here is that though neutrinos only weakly interact with normal matter, they do interact with it. If you then add up the estimated number of neutrinos in the universe and add their mass to the balance sheet, you still come up five times short of what must be there to account for the gravitational observations. Even the exotic neutrino is, in fact, made from normal matter.
dark matter isn’t made from protons, neutron, electrons or the myriad subatomic particles from which they are made (neutrinos, muons, quarks, gluons, etc.). No, dark matter must be something else entirely – something that we cannot see in any way other than through its interaction with normal matter via gravity.
But, isn’t it also possible that our understanding of how nature works is flawed? After all, prior to the twentieth century, some scientists thought they understood most things fairly well. Then along came the weird world of the atom and its constituents, leading to the development of quantum theory – a radically new way of understanding nature that appears to explain far more than could ever be explained using the theories that pre-dated it. At about the same time physicists were grappling with the very small using quantum theory, astronomers were trying to understand seemingly inexplicable things about the very large (the universe!) using the laws originally formulated by Isaac Newton – they were many times unsuccessful. Along came Einstein with his theories of Special and General Relativity. Now, almost all of those pesky observations that they couldn’t quite understand were no longer mysterious. Except, of course, for dark matter and Dark Energy (the subject of a future essay). So, yes, it is possible that we’re missing something in our scientific theories of the universe. If so, then whatever we’re missing would have to be able to explain the dark matter problem and not significantly alter anything else – after all, our understanding of "everything else" is pretty good and there doesn’t seem to be much else out of line.
What next? Are there experiments planned that could help us better understand dark matter? Yes, as a matter of fact, there are.
Figure 3. NASA's Alpha Magnetic Spectrometer (AMS) onboard the International Space Station may detect dark matter.
(Image courtesy of NASA.)
Like the Sudbury Neutrino Observatory, Italy’s Xenon100 dark matter experiment is looking. The Xenon100 is a steel tank containing liquid xenon that is constantly observed for a telltale flash indicating that a particle of dark matter has interacted with the xenon therein. With a density of about three times that of water, liquid xenon is ideally suited to stop an atom of dark matter, if it exists. Europe’s Large Hadron Collider (LHC) will also be used in the hunt. The LHC consists of a 16.5 mile ring of superconducting magnets used to accelerate charged particles, atoms, to near the speed of light. Two beams of atoms, traveling in opposite directions, are then made to interact with each other, causing the atoms to smash into each other with collision energies not seen since the Big Bang. Any dark matter particles created at the accelerator might be visible as the collision products are studied and categorized. Finally, NASA’s Alpha Magnetic Spectrometer, now flying onboard the International Space Station (ISS), is catching and characterizing cosmic rays – looking for signs of dark matter as it does so. Despite recent press releases touting possible dark matter detection, none of these experiments or facilities have yet produced a compelling scientific case supporting their claims.
Something massive, unseen and nearly undetectable is all around us and acting on us gravitationally. We see its effects. We measure them. But we don’t have a clue as to what this “dark matter” actually is. I suspect that in a hundred years we’ll have solved the problem and uncovered equally enigmatic new ones to be solved. That’s the history of science and, as a scientist, that’s what makes it all worthwhile.
To learn more about the search for dark matter on the International Space Station: http://science.nasa.gov/science-news/science-at-nasa/2013/14apr_ams/
Several teams are taking their detectors deep underground in their search for dark matter. Among them are the Cryogenic dark matter Search (CDMS) and the Chicagoland Observatory for Underground Particle Physics.
Other researchers are looking at common, everyday objects, like the Sun, for evidence of dark matter: http://www.sciencedaily.com/releases/2010/07/100721132407.htm
*I should comment here on quantum gravity. Virtually all areas of physics, except gravity, are very nicely explained using quantum mechanics. Quantum mechanics assumes that every facet of nature is ultimately composed of small, discrete elements – including the particles associated with the fundamental forces of nature such as electromagnetism (the photon). Our current best theory of gravity, General Relativity, is not "quantized." This causes many scientists pause for thought. How is it that every fundamental force and element of nature is discrete (broken down into very small, single parts and not continuous) yet gravity is not? Nature seems to be consistent in this regard and many therefore believe there is something fundamentally incomplete in our theory of gravity.
Copyright © 2013 by Les Johnson
Baen author Dr. Les Johnson, coauthor with Travis S. Taylor of Back to the Moon, and coeditor of science and science fiction anthology Going Interstellar, is also Deputy Manager for the Advanced Concepts Office at the NASA George C. Marshall Space Flight Center in Huntsville, Alabama.