How much do we actually know about dark matter and energy? Photo credit: NASA Hubble Space Telescope via Unsplash
The universe must be much heavier than it appears to be. Observable matter is estimated to make up barely 5% of the components of space whilst the other 95% is accounted for by substances known as dark matter and dark energy. Despite this discovery fundamentally reshaping the way scientists view the structure of our universe, the secrets of dark matter and its energy counterpart have remained largely hidden. Though their effects are evident on nearby stars and galaxies, physicists still grapple with defining the true nature of dark matter and energy.
…the other 95% is accounted for by substances known as dark matter and dark energy.
In 1933, a physicist named Fritz Zwicky stumbled across an astounding discrepancy between theory and observation while surveying a cluster of galaxies called the Coma Cluster. According to the calculations, there was not enough mass to hold this cluster together and the galaxies were moving far too fast to remain bound to it. Yet, they remained clustered. The stars at the periphery of our galaxy behaved similarly; instead of flying away and unravelling the milky way like a spool of thread, they remained, orbiting an invisible mass. Fritz proposed this mass took the form of “Dunkle Materie”, or dark matter. Unsurprisingly, this theory was slow to take off in the field. To hypothesise not only a completely new type of matter, but also to claim that it dominated the presence of regular matter was unconventional to say the least.
Though Fritz is known as the father of dark matter, a scientist named Vera Rubin is crucial to the verification and public acceptance of his theory. Working with updated technology, she studied the rotation of spiral galaxies including the Andromeda galaxy. Working with her partner Kent Ford, they found that all matter within spiral galaxies rotates equally as quick, regardless of whether it is closer to the middle, or strewn around the edges. This observation directly contradicts Newton’s second law of motion which would only hold true if a halo of dark matter is present around each galaxy to compensate for the speed at which the stars orbit. Rubin initially did not know what her results described, stating that ‘months were taken up in trying to understand what I was looking at … one day I made sketches on a piece of paper and suddenly I understood it all’. However, despite Rubin’s clear-cut evidence cementing the place of dark matter in the cosmos, she has never been recognised for it. She was a ‘people’s choice’ for the Nobel prize but was overlooked by the committee.
This observation directly contradicts Newton’s second law of motion…
Since dark matter was first proposed, it has been categorised into objects known as MACHOs and WIMPs. MACHOs stands for “massive compact halo objects”, such as low luminosity stars and black holes. These bodies often evade detection and thus are deemed dark matter. Black holes have gravity so strong not even light can escape from their field. While the black hole itself is invisible, it can be observed via its telltale event horizon. This boundary, where light bends and barely escapes the immense gravitational pull, forms a bright, ring-like structure surrounding the dense centre. The first photo of a black hole was taken by the Event Horizon Telescope (EHT) in 2019.
The black hole at the centre of the M87 galaxy imaged by the EHT
Though MACHOs are good candidates for dark matter, there are not nearly enough of them to account for its predicted total mass. This gave rise to a second category, WIMPS, or “weakly interacting massive particles” which neither emit nor absorb light. These particles are currently purely hypothetical and have a compilation of eccentric names such as squarks, winos, gluinos and higgsinos. WIMPS are thought to be stable and slow moving, and so may have been around since the formation of the universe. They interact with very little, so any experiment attempting to detect them must be extremely sensitive.
One clever method for seeing the invisible is gravitational lensing. Gravity can be conceptualised as the curvature of spacetime itself, like contour lines on a map. Heavy bodies distort the very fabric of the universe so even rays of light get bent as they follow the shortest path. Astronomers can thus observe stars as either in the wrong place or much brighter than they should be because of this effect. The mass of the body that is effectively acting as a lens, magnifying or shifting the image of the star, can be measured from the image distortion. The Hubble telescope has recently managed to map dark matter using this method.
Hubble Telescope’s picture of galaxy cluster Cl 0024+17 in which the suggested location of dark matter is shaded a lighter blue
Dark matter, despite outweighing regular matter, makes up only 27% of the composition of the universe. As laid out in the most famous equation in physics: E=mc2, energy and mass are intrinsically related, and so much of this unaccounted-for mass is thought to be taken up by “dark energy”. Physicists know even less about this form of energy than they do about dark matter, but one theory prevails above all others. Empty space has a baseline energy because at any given time, tiny fundamental particles are popping in and out of existence. To put this energy into perspective, each cubic metre of space only possesses enough of this “vacuum” energy to power a lightbulb for one trillionth of a second.
This dark energy leads to a repulsive force that counterbalances the pull of gravity, causing the expansion of the universe to accelerate. This force appears in the laws of physics as a number remarkably close to zero, known as the cosmological constant. Classically, it factors into equations that calculate the true age of the universe, as well as in the observation of emission spectra from luminous celestial objects known as quasars. In terms of its quantum interpretation, the energy that comes from fundamental particles being constantly created and destroyed means the cosmological constant is more than just a fudge factor used in astrophysics equations.
The cosmological constant has continued to cause problems in physics since then.
Though now understood to exactly equal dark energy, the cosmological constant first appeared to fudge Einstein’s equations of general relativity. His description of cosmology was a static universe, which given the lack of evidence suggesting otherwise in the 1900s, was not an unreasonable assumption. Here, the role of the constant was to oppose any expansive forces and so upon discovering that that the universe actually was expanding, he later called it his ‘biggest blunder’. The cosmological constant has continued to cause problems in physics since then. When dark energy is measured from its effects on the expansion of the universe, the answer is absurdly different from its predicted value. Summing all of the quantum effects of empty space gives an answer many orders of magnitudes higher than the observed value of 10-52 m2. Echoing Einstein’s earlier sentiments, the cosmological constant has now been renamed ‘the worst theoretical prediction in the history of physics’.
The value of the cosmological constant may appear arbitrary, but physicists often puzzle over why its value is so close to—and yet not—zero. The thought process behind this is that these numbers, no matter how small, are a fundamental property of the universe. Stephen Hawking described this problem as ‘based on the self-evident truth, that if the universe had not been suitable for life, we wouldn’t be asking why it is so finely adjusted.’ The discussion of fine tuning refers to the parameters in equations in physics that appear to be fixed by an external mechanism. Our universe would look drastically different if the charge of the electron, or the relative strength of gravity, differed even marginally. The best argument that we have for why any constant is associated with a specific number is the anthropic principle, stating that the laws of physics (and the constants that balance our universe) allow for the existence of life to observe them. If the constants were different, our universe would not have evolved to contain intelligent life
Anthropic reasoning may seem trivial since it holds little predictive power, but it can be applied to the problem of constants that seem to have been fine-tuned for the existence of life. A scientist named Weinberg formalised its application to the cosmological constant. Weinberg reasoned that if the constant were too large, the universe would have expanded too rapidly for structures of matter to form. If it were too small, space would have collapsed back in on itself before any matter had formed anyway. Weinberg imagined many different observers in many hypothetical universes, each with a different value for the cosmological constant. From this, he found a slim range of possible values that dark energy could take. According to this logic, the value of fundamental constants becomes a question of the probability of observers existing in a universe exactly right for life.
Dark energy and matter seem to spawn problems throughout physics. They represent the ultimate discrepancy between theory and experiment, detected indirectly and theorised through the unconventional thinking of scientists such as Zwicky and Rubin. Einstein appears to have been wrong about his ‘blunder’ since his fudge factor has since been able to empirically describe the baseline dark energy of the entire universe. Its value becomes a Goldilocks enigma of satisfying the conditions for life that are ‘just right’. Together, these dark forces form a delicate balance that shape the cosmos. Dark matter must exist to glue the universe together and dark energy must exist to sustain its expansion rate. In the past decade, sophisticated computer science such as machine learning has been applied to the detection and mapping of dark matter in the hopes that the more MACHOs and WIMPs that are found, the more can be learnt about the composition of space. For now, however, dark matter and energy stand as a reminder of just how much of the universe remains hidden.
