It starts with Einstein
Albert Einstein is known by most for the equation E = mc2. Although the equation is not entirely his own —he borrowed ideas of mass-energy equivalence from Friedrich Hasenöhrl and Henri Poincaré—he was the first to work out the idea of mass-energy equivalence in light of the theory of relativity. E = mc2 means that any amount of mass has an amount of energy that is its equal, and vice versa. After Einstein, mass and energy are different forms of the same thing.
Predicted by Einstein’s general relativity theory, the observation of light bending under the force of gravity is the best proof yet of the existence of that mysterious stuff known as dark matter.
—Albert Einstein
Fear of the dark
Dark matter is described as having an unknown composition and observable only through indirect means. The reason that scientists cannot directly see dark matter is that it does not reflect or give off enough electromagnetic radiation (electricity, visible light, radio waves, etc.) to be detected with current technology. It would be easy to dismiss dark matter as a needle-in-a-haystack exercise if it weren’t for one startling detail—it is estimated that only four per cent of the total energy and mass of the universe is the visible type. The remaining 96 per cent is a combination of unseen dark matter and dark energy.
This notion of an indirectly identifiable force that makes up a vast majority of the universe borders on the philosophical. If we can’t prove directly that it exists, is it even there? There are many reasons to believe it does exist. Ironically enough, they have to do with how this invisible dark matter and energy affect visible matter.
Calculations on the Fritz
Fritz Zwicky, a Swiss astrophysicist working out of Caltech in 1933, offered up the fi rst evidence of the existence of dark matter. He estimated the mass of the Coma cluster of galaxies using the observed motion of the galaxies on the edge of this cluster. After comparing the value he obtained to another value gauged by counting the number of galaxies in the cluster and its relative brightness, a surprising result surfaced— the mass of the cluster estimated using the first method was 400 times greater than the value given by the second method. Zwicky reasoned that there had to be some form of invisible matter that was providing enough mass (and therefore gravity) to hold the giant galaxy cluster together.
Further study of galaxies yielded the galactic rotation curve, which predicts the velocity of rotation of stars around a galaxy’s centre based on their distance from that centre. Using only visible matter, the observed values do not match up against predicted values. This again suggests that there may be dark matter that produces greater-than forecast gravitational forces and is the source of the discrepancy.
Number one with a Bullet
Dark matter has been one of the biggest unsolved problems in astrophysics— and perhaps in all of science— from the moment it was first described. Last year, some of the best evidence to date of its existence was published in the Astrophysical Journal, in which Marusa Bradac of the Kavli Institute for Particle Astrophysics and Cosmology revealed her findings in her research paper, “A direct empirical proof of the existence of dark matter”. Bradac studied a collision between two galactic clusters that occurred 150 million years ago, known as the Bullet Cluster. Because the different types of matter in the clusters exhibited distinct behaviour during the collision, it offers a convenient instance to study those different types separately.
The dark matter component of this astronomical collision was detected using a process known as gravitational lensing. When light from a far away and highly luminous object (for example, a quasar) bends around an object with a large mass (such as a galaxy), it creates a gravitational lens: what looks to an observer is on the other side of the galaxy as a ring surrounding the quasar or as multiple distorted copies of the object, depending on the positioning. Such gravitational lensing happened to objects behind the collision of the Bullet cluster, but no visible mass source could be seen to account for the bending light. The researchers in the study concluded that dark matter was the guilty party.
There’s a hole in my universe dear liza
As if things weren’t complicated enough, scientists recently found what they describe as a “hole in the universe.” The area is almost one billion light years across and contains nothing—no galaxies, stars or, as far as they can tell, dark matter. When scanning the skies for cosmic microwave background radiation (leftover noise from the Big Bang that gave birth to the universe) using the Wilkinson Microwave Anisotropy Probe satellite, the University of Minnesota team led by Lawrence Rudnick found a “cold spot”. Upon closer investigation using the Very Large Array radio telescope, the team found there was literally nothing there. Scientists were completely surprised at the find, as no void that large was ever expected to exist. No one has any possible explanation for the phenomenon at this point in time.
WIMPs versus MACHOs
One of the most frustrating aspects of dark matter is that we are currently unable to detect it directly using current technology. If scientists were able to determine the subatomic composition of dark matter in laboratories, its behaviour in far off environments might be better understood. Conclusions regarding sub-atomic dark matter particles using data gleaned from galaxies millions of light years away may be leaps of faith.
A commonly held view of dark matter is that it is made up of atypical elementary particles (particles unlike electrons, protons or neutrons). Some examples include weakly interacting massive particles (WIMPs) and sterile neutrinos that do not interact with other particles using any fundamental interactions. Since gravitational, electromagnetic, strong or weak interactive forces cannot be measured from these hypothetical atypical particles, finding them becomes a seriously daunting task.
A theory to help explain the missing- matter problem lies in the figurative opposite of the tiny WIMP: a massive compact halo object (MACHO). These objects are comprised of typical matter (known as baryonic matter), but emit no detectable radiation and coast through space as free agents, not tied to any solar system. Neutron stars, black holes and brown dwarfs can potentially fall under this category. The foremost problem with this theory is that there simply isn’t enough typical matter in the universe to make up the gravitational effects felt by dark matter without seriously changing the relative abundances of the elements. There may still be some MACHOs that remain undetected and exert gravitational forces, but they cannot account for all the mass attributed to dark matter
For sale: increasing galactic real estate
Another piece of the puzzle lies with what is now known about the structure of the universe and, once again, Einstein. He believed that the universe was neither expanding nor contracting, but was in stable equilibrium. In order to reconcile this idea with his general relativity equations, he introduced the cosmological constant known as lambda. Famously, he later said that introducing this constant was “the biggest blunder of my life.”
It was Edwin Hubble, after whom the incredibly useful Hubble space telescope is named, that first found evidence that the universe is expanding. When he compared the distances of galaxies from Earth, he found that the galaxies farther away were moving at a quicker pace. Not only is the universe expanding, it is doing so at an accelerating pace.
History has been kind to Einstein in the end, as some astrophysicists believe that his constant, lambda, is in fact approximately equal to the value of dark energy. This dark energy is believed to be driving the expansion of the universe, since through Einstein’s mass-energy equivalence, a given amount of energy would have a certain amount of gravity associated with it. Dark energy is a formidable force as it is estimated to make up 74 per cent of the mass of the known universe.
There are always alternatives
Progress is being made on the subjects of dark matter and dark energy, but clearly there is still much to learn. Alternatives to the theory of dark matter and its exertion of gravitational force propose that dark matter may not exist; instead, it may be that we simply do not adequately understand the workings of gravity. Reconciling these modified models with Einstein’s widely-accepted relativity framework has proven complicated.
It will most likely be a very long time until the missing mass of the universe is fully described. Perhaps another genius of Einstein’s calibre is the necessary catalyst. Or perhaps an accidental discovery in the realm of particle physics will be the key. What is certain is that dark matter and dark energy are two of the greatest puzzles ever faced by the scientific community. By unravelling its complicated web, incredible discoveries—maybe an infinite energy source or the ability to harness gravity for near lightspeed space travel—would no doubt be the next step. The future will be dark and that may not be a bad thing.
