Throwing Light on the Secrets of the Cosmos: Dark Matter and Dark Energy
In 1933, an astrophysicist named Fritz Zwicky calculated the mass required to exert enough gravitational force to hold a certain galaxy cluster together. To his surprise, he found that this required sum was nearly 100 times the actual mass of all the stars in said cluster. Further observations and investigations of nearby galaxies seemed to suggest the same: the visible matter in the respective galaxies was far from sufficient to hold them together. The discovery seemingly violated all known laws of physics at the time, and this “missing mass” is what we know as dark matter today.
What is dark matter? It would be easier to answer the question of what dark matter is not, rather than what it is, because of our collective lack of knowledge on the phenomenon. Here’s what we do know, however:
Dark matter does not interact with the electromagnetic force. This implies that it does not absorb, reflect or emit photons, which accounts for its supposed invisibility. Moreover, we know that dark matter outweighs regular matter in the universe, with recent estimations predicting that it comprises 27% of the universe (as opposed to the 5% that regular matter does), thus maintaining a ratio of roughly 6:1 with regular matter. We are also certain that it is neither matter nor antimatter, due to our observation of their absorption and emission of radiation. However, we do know that it interacts with the gravitational field, given its ability to cause gravitational lensing, and its primary function to hold whole galaxies together (despite the exponentially expanding universe). But what exactly makes the universe’s expansion accelerate?
In 1924, astrophysicist Edwin Hubble gazed up into the cosmos to discover the recession of galaxies and other celestial bodies. He also noted that the more distant galaxies showed signs of faster recession, leading him to believe that the universe was expanding faster. This was confirmed in 1998, upon the investigation of light emitted by Type IA supernovae (a supernova that occurs when one of two stars in a binary system is a white dwarf). It was observed that the stars were dimmer than they should’ve been if the universe’s expansion was slowing down. This suggested an exponentially expanding universe (i.e. one where the acceleration of the universe’s expansion itself was increasing), fuelled by some sort of energy invisible to the naked eye. This is now known as dark energy, with the effects of this energy seeming to reflect that it had a naturally repulsive pressure. The theory has been consolidated by further investigations that prove the same, with recent studies confirming the hypothesis upon the observation of neighbouring galaxies. The culmination of calculations and research has approximated dark energy to comprise 68% of the universe, and proves that its concentration does not dilute with time. This implies that it could potentially lead to a Big Rip, a scenario in which the universe expands fast enough to tear apart galaxies and clusters, and soon enough, even matter itself.
Of course, there is a possibility that these occurrences may be explained by something entirely different. The terms dark energy and dark matter are simply speculative, created in order to account for a gap in calculations. Some believe that there is no dark matter or dark energy at all, and the phenomena we observe is a result of divine intervention, while some scientists seem to think that it is a result of inconsistent densities in regions throughout the universe. Hopefully, we will one day be able to throw light on the shadowed secrets of the cosmos and make theories with as much certainty as Heisenberg allows us.
Binary system: a system of two celestial bodies that orbit each other as a result of the gravitational attraction exerted by each in close proximity.
White dwarf: An extremely dense stellar remnant; it is a star that has exhausted its nuclear fuel and expelled most of its outer material.
Gravitational Lensing: The distortion of light from galaxies or celestial bodies due to the gravitational attraction of (usually large) masses/objects.
Antimatter: Matter that consists of the antiparticles of regular matter (particles with opposite charge, i.e. positrons, antiprotons and antineutrons)