By Ignacio Gonzalez
The past 18th of October Carolina, Josue and me were on a mission. We knew that if we were to successfully accomplish it then our bodies would pay the consequences dearly, undoubtedly our feet would take the highest toll. We managed to coincide in New York City after more than a year and a half from our last encounter. I was flying from Germany and they both from Mexico, not a small deal of organization, money and will had been put into the effort and we had to make the whole event count. The plan for the day was to visit not one, not two but three museums, and, we had to reach the third before 6:45 pm!… and none of us was wearing particularly comfortable shoes! It was too late to back off, the targets were set and the route planned, time had come to man-up, but much more importantly, to nerd-up. At 10:00 am sharp the doors were open: the American Museum of Natural History welcomed us!
And what a welcoming sight that was! You have not yet completely dragged yourself through the entrance door and you already need to pick up your jaw that has dropped to the floor while simultaneously your neck arches backwards in an effort to fully contemplate the figure of a towering sauropod skeleton (a Barosaurus) rising to its full frame. Luckily, if you already had the chance to walk around skyscraper-crammed Manhattan, maybe by then your anatomy is used to this uttermost anti-ergonomic posture. So imposing is the intimidating stance of the Barosaurus that one almost misses the Allosaurus which is provoking the defensive reaction in the first place. But when one does spot the (wannabe) predator, one cannot help to feel a bit of sympathy mixed with pity towards it. No matter how many dozens of razor-shaped teeth you can squeeze into your mouth, there’s no chance you’re going to take down such a leviathan. If the Allosaurus had rationalized that it surely wouldn’t have acted so cocky. But wait, not so fast! maybe the intended prey was never the gigantic long-necked dinosaur but instead its considerably smaller offspring that shies away behind the protective colossal parent. The Allosaurus was not that bold (or blatantly stupid) after all, it was just trying to get very very lucky.
We had only as much time as it took us to get our entries to contemplate that fossilized scene of Jurassic drama. Why the rush? cause we had less than 30 minutes before the big show began. You see, the American Museum of Natural History is a large place, so big that you could fit, say, a planetarium in it. And that is exactly what whoever is in charge of taking such decisions did. Thanks to an enlightened choice the Museum also serves as home for the Hayden Planetarium and it was there where we were heading. The featured space show was (and it still is) the “Dark universe” and I will be telling you in more detail about it in a moment but first allow me a slight deviation. The show is narrated by Dr. Neil deGrasse Tyson. Chances are that the reader knows who Mr. deGrasse Tyson is, if not from his prolific scientific work as a physicist (mainly in the fields of cosmology and general relativity), or his contributions as an educator for the masses (he has authored or co-authored at least ten popular science books and he is the heir of Carl Sagan as the host of the totally refurbished version of the classic Cosmos TV series), then, at least because he has been meme-fied (or meme-rized, as you prefer) for posterity. As of right now myriads of hand-sketched deGrasse Tysons in an it-was-not-me sort of gesture swarm through the internet (open your facebook, he might be there… more than once). Tyson also happens to be the director of the Hayden planetarium but it is not that office what makes him the obvious candidate to guide us through the dark universe, probably not even his appropriate credentials were the decisive factor, surely if he had lacked that characteristic baritone voice of his then the job would have gone to another astrophysicist. On our way to the planetary dome I kept scanning the corridors looking for a sign saying something like “Dr. deGrasse Tyson office. Knock twice for an autograph. Thrice for free books” followed by an accusatory arrow but such a sighting never materialized. In a snap my hidden agenda simply vaporized, even being fully conscious of the naivety of my childish little plot I was not quite able to shake a tiny feeling of failure. However, I knew that as soon as the tour through the “Dark universe” began I would be more than compensated.
To understand the premise of the show first consider all what you can perceive with your own senses, all of that already encompasses a considerable fraction earthly and cosmic stuff. Now add to that bunch everything that you can detect aided by whichever piece of gadgetry or machinery that has ever been conceived, then your list has become immensely improved. Now you can also count electromagnetic radiation (or photons) of essentially any frequency you choose (radio waves, infrared, X-rays, gamma rays…) and a dazzling zoo of subatomic particles as entities that are perceivable. We have worked out how all those ingredients can cluster to produce a rich plethora of matter-energy states (matter and energy being ultimately interchangeable according to the über-famous Einstein’s equivalence E = mc2, where E stands for energy, m for mass and c is the speed of light). We have come to understand how liquids, solids, gases, plasmas, laser beams, rays of particles, Bose-Einstein condensates, and other rarities come into existence. And more recently we started to peer into fancy new states of matter-energy like “molecules of light“,”dropletones” and the first observed Majorana fermion. Indeed mankind has amassed a lot of physics over the last couple of centuries and with all that knowledge we have forged two powerful theoretical frameworks that together (but so far not united) span an umbrella under which we can accommodate almost every phenomenon ever registered: we have general relativity to explain gravity and the standard model of particles physics to deal with electromagnetism, the weak, the strong force and the whole cornucopia sub-atomic particles (in principle gravity could be included within the standard model but for that we still need to detect the yet unseen graviton that works as carrier for the gravitational force). The picture of the universe painted by all these couple of theories is quite impressive, but, and here comes the devastating blow to our ego as a species, it is also utterly and miserably incomplete.
Turns out that of all the stuff (matter and energy) that makes up our universe only around 5% of it can be explained in terms of our beloved theories, the remaining 95% is in a form which is totally unaccounted for by either the standard model or by general relativity. More precisely, to the best of our knowledge that 95% is made of around 27% of something called “dark matter” and 68% is an even more mysterious thing denominated as “dark energy”. One can almost hear the echo of the Nobel laureate physicist I. I. Rabi exclaiming “Who ordered that?!” as he did when he was informed about the discovery of the muon. Well, dark matter and dark energy is what the “Dark universe” show is all about, however, in this entry I will refer only to the former.
The complains start with the term “dark matter” itself. It is commonly regarded as a staunch reminder of the dull naming abilities of physicists (consider the examples: standard model, black holes, or the quarks which come in up, down, strange, charm, bottom and top versions… not exactly pearls of creativity). Dark matter is dark because we cannot see it, that is, because it does not emit or reflect light. Nevertheless, I cannot get rid of the feeling that “dark” does not quite work as a good label mainly because dark matter does not even absorb light which is the single most transcendental characteristic of dark objects. Other than deflecting light due to its gravitational pull, dark matter remains supremely oblivious to the existence of photons. I think the name “transparent matter” would have been more accurate though perhaps with a weaker marketable punch, but then who asked my opinion?
The million dollar question that is begged at this point is: if we cannot “see” it then how do we even know that it is there? Well, I sort of let a spoiler slip through the last paragraph. We can be unmistakably aware of its presence due to its gravitational effects over “conventional” (or baryonic) matter as well as over light.
The first solid hints of the presence of dark matter came from the abnormal dynamics of stars within galaxies, more specifically, their rotational speed around the center of their respective galaxy. Just as the gravitational pull of the Sun causes the planets to go around it, all the stars contained within a galaxy describe elliptical orbits around their galactic center where most of the galaxy’s matter was thought to be concentrated. Furthermore, this distribution of matter should make the stars nearer to the center to go around faster than those located at the outskirts, however, this is not what is observed. The astronomer Vera Rubin cataloged several galaxies where the rotational speed of the stars is essentially the same regardless of their distance to the galactic center. That is an astonishing fact on its own right but there were yet mor surprises in store. It turns out that the overall speed of rotation of the stars was simply too high, at those velocities the stars should get projected into the void of intergalactic space! The galaxies were spinning so fast that their gravity should be insufficient to hold them together unless: 1) Our theories of gravity had been embarrassingly wrong all along, an option too scary to contemplate or 2) There is more matter lurking in the galaxies than meets the eye (actually much more than that which we can see). It is via the option No. 2 how dark matter enters the picture and saves gravity.
More evidence. Matter and energy can bend the space around it (one of the central consequences of general relativity) and this bending can be measurable to a high degree of accuracy by a phenomenon called gravitational lensing where the light passing nearby a massive object (or collection of objects) suffers an apparent deflection due to the curved space around caused by the object itself. It is not that the light rays actually curve their path, they continue travelling in straight lines, it is that the flatness of space itself has been disrupted due to gravity! (curiously, Einstein was fully aware of this implication of his theory but he deemed the effect as to be too weak to ever be measured experimentally). Astronomers have measured the gravitational lensing due to large clusters of galaxies and determined that the amount of deflection of the light that they cause is much too large to be provoked by only the luminous matter seen in the clusters. Again, most of the matter is missing… or not, it is there but it refuses to be seen, it is dark. The list of experimental results demonstrating the existence of dark matter is very rich but I better move on rather than going through them all.
Mysteries exist to be solved, and, the more puzzling the mystery the higher the motivation to clear it out for good. Dark matter is one of the biggest mysteries of modern science, thus, it is not a surprise to have a surplus of theories competing for the “final answer” status. Physicists seem to have picked their favorite theory, one stating that dark matter is made of WIMPs! I do not mean that, over ridden by despair, physicists have resorted to loudly accuse dark matter of having a feeble anatomy so as to at least vent out their frustration, WIMPs is actually an acronym that stands for Weakly Interacting Massive Particles. Surely the reader (as myself) cannot escape the feeling that, stated like this, this answer sounds like no answer at all. It is like living in a universe where corkscrews have not been invented and stumbling upon a bottle of wine, then, the stumbler accepts that he cannot think of any kind of known machine that would allow him to open the bottle and then theorizes that a WIBOD or Wine Bottle-Opener Device is just the type of machine that would solve the problem and walks away feeling elated for his cleverness. This is to me the same way in which WIMPs solve the problem of dark matter. WIMPs are particles with mass thus they can give rise to the gravitational attraction and they interact so weakly with ordinary matter as to almost obliterate our chances to detect them (they could be neutrinos, axions or another kind of supersymmetric particle. Only neutrinos have been detected so far), but is this not kind of reformulating the problem of dark matter but this time pretending that the act of restating it provides in itself a solution?
Then string theory (or a family of string theories that have been amplified and re-baptized as M-theories) comes along and offers that gravitons are what actually constitute dark matter, quadrillions of them. The issue then is obvious, why if they are so numerous not a single one of them has been yet detected? String theory attempts to get away with this enigma by assuming two things: 1) Our three-dimensional spatial universe is actually a so-called brane embedded in a higher dimensional space which in principle can host many more branes, and 2) gravitons are tiny pieces of strings forming closed loops such that they do not have open ends which might anchor them to a particular brane. In this way gravitons are fundamentally higher dimensional entities that are free to travel from between branes. The only footprint they leave during their fleeting path across our brane-universe is their gravitational attraction. Interesting premise undoubtedly, but there is a deep stigma that prevents one from investing too much faith in string theories, none of them has been able to make predictions that can be experimentally tested. For a group of theories that have been around for more than 30 years that is hardly an invigorating health signal (see Lee Smolin’s “The trouble with physics“).
As I said earlier, we do not suffer from a famine of ideas regarding the dark matter puzzle, nevertheless, I sense a strong imbalance or bias in the arguments of such theories. Briefly, what all of them basically do is to pick a particle (known or unknown) and hold it as their champion suspect for it being the stuff of dark matter. To the best of my knowledge there is not a single theory which dismisses the particle hypothesis altogether, which, leads me to ask the very obvious (and probably equally ingenuous) question: What if dark matter is actually not made of matter at all? and more importantly, if that is the case, then, of what else can dark matter be made of? Well… what about space itself? would that be a too crazy/stupid suggestion? maybe, but give it a chance for a moment.
Imagine that space itself has an intrinsic and particularly intricate “curvature pattern” at a cosmic scale. How could this come to be? consider the possibility that during the very early period of expansion of the universe some regions of space “flattened” at a slower rate than others, and that the places where those regions would be located were somehow determined by the positions where the earliest clusters of matter got nucleated. Afterwards, as the universe continued to grow, the regions where matter agglomerated to form stars and galaxies co-evolved forever entangled with those primordial regions of “crumpled” space, creating a super structure of unevenly stretched space that survives nowadays. Of course, this is wild speculation. I do not attempt to provide a mechanism explaining how the clustering of matter promoted a network of spatial “wrinkles” additional to the amount of space curvature induced by virtue of the clusters’s own mass, nor I can explain why those regions have not been yet totally flattened by further cosmic expansion. I would only like to provide a new perspective from which to look at the problem of dark matter that does not consist solely on hunting for the right kind of particle, and at the same time, raise speculation about the potentially crucial role that space itself might play in this puzzle.
Well, this is a juicy topic and it is impossible to do justice to it in a single blog entry, undoubtedly these dark matters will be routinely addressed in this space. Before finishing this post I would only recommend that if a visit to the American Museum of Natural History and the Hayden Planetarium within it is within your reach, then, do not miss the chance! Go watch “Dark Universe” and enjoy the show… oh, and the dinosaurs, enjoy the dinosaurs, they are forever cool!