Back when the early humans started to wonder and contemplate the night sky, it would have been far easier than in the current times to appreciate the beauty of the universe with a naked eye.
The murky sky would have been brimming with countless stars, some in the backdrop of the Milky Way, and some in the other patches of the sky. A thought might have occurred to one of the primates – Why don’t these stars fly away?
That string of thought, passing through the generations, voyaging the scales of time, finally led to an indirect discovery of the invisible glue that holds everything in the universe.
What Exactly Is Dark Matter?
Dark matter is a hypothetical form of matter that makes up around 85% of the matter in the universe and governs the remaining 15% (galaxies, stars, planets, etc.). The word ‘dark’ is actually not an appropriate prefix, it should be rather ‘invisible’ as researchers have not been able to detect its presence directly to date.
Also, based on cosmological observations, scientists have devised a theoretical model for the composition of the universe according to which normal matter accounts for ~5%, dark matter ~27%, and dark energy is ~68%.
Dark energy is in itself a topic of greater importance and deserves an individual article. However, for the sake of this write-up, dark energy can be thought of as a mysterious entity, a driving force, that is predominantly accounted for the accelerated expansion of the universe.
What are the properties of Dark Matter?
Even though dark matter cannot be inferred straightaway, it does make its presence felt through the gravitational effect it has on visible matter. Dark matter has the following properties:
- It does not emit, reflect, or absorb any form of electromagnetic radiation.
- It rarely interacts either with itself or with normal matter.
- Its density dilutes with the inverse volume, i.e., as the universe expands, its effect diminishes.
Probable candidates of Dark Matter
Scientists have much more confidence about what dark matter is not than what it is. Let’s see some of the probable candidates:
- Dead stars or rogue planets – A rogue planet is a lone wanderer in the cosmos that does not orbit its parent star. Both dead stars and rogue planets do not have enough mass that can account for the required 27% dark matter distribution.
- Neutrinos – At first glance, neutrinos are the ideal dark matter candidate. A neutrino is a neutral subatomic particle that rarely interacts with normal matter (matching one of the properties of dark matter), but it has too little mass and high energy. Due to its high energy, it moves at nearly the speed of light, but observations have shown that dark matter is mostly cold, i.e., it moves relatively slow.
- Baryonic clouds – These are the dark clouds of normal matter composed of particles called baryons. However, it is possible to detect the baryonic clouds as they absorb radiation passing through them. This property prohibits them from being a potential candidate for dark matter.
- Black Holes – Though black holes are no longer just the prediction of Einstein’s theory of relativity, the number of gravitational lenses we observe is far too less to suggest that black holes can make up the required contribution of dark matter to the universe’s composition.
Why do we believe that dark matter exists?
Cosmological observations have led to the presence of something that we can’t see but has gravitational effects on other things or structures in the universe. Let’s discuss some of them in detail below:
Coma Cluster study
It was historically the first evidence for dark matter. The Coma cluster is a group of hundreds of galaxies that are held together by their own gravity.
In the 1930s, Fritz Zwicky, a Swiss astronomer, was studying the great Coma cluster of galaxies. During the study, he inadvertently found a notable discrepancy between theory and observation.
According to the Virial Theorem, the total kinetic energy of a self-gravitating body on account of the motion of its components is related to the gravitational potential energy of the body. And, we have a direct relationship between energy and mass from Einstein’s mass-energy equivalence principle (E=mc^2).
The average velocity of galaxies within a cluster is dependent upon its total mass. Zwicky calculated the total mass of the Coma cluster based on his observation of the velocity of galaxies moving in it. Additionally, based on the total light output from all the galaxies, he calculated the total mass in the form of luminous stars.
There was a remarkable difference between the calculation of total mass from both methods – the mass of the cluster based on the velocity of the galaxies was about 10 times more than that calculated based on the total light output.
This led to an incredible conclusion of the presence of some form of matter, totally invisible, that creates enough gravity to hold the rapidly moving galaxies intact and prevent them from flying away.
Gravitational Lensing
Predicted by the General Theory of Relativity, gravitational lensing is a phenomenon due to which space-time distorts around massive objects, thereby causing light to bend around them.
It can be thought of like this – if there is a massive foreground object between a source and an observer, multiple/magnified/distorted images of the source form around its sides as if some kind of a lens exists between the source and the observer.
A single dense galaxy can cause a simple type of gravitational lensing. The light coming from a distant galaxy traveling in a straight line gets redirected around the dense foreground galaxy, thereby producing multiple images of the background galaxy. In cases where the lensing approaches perfect symmetry, a circular ring-like structure is formed, known as an Einstein Ring.
Large and massive galaxy clusters lead to more complex gravitational lensing. As the matter is not centered around a particular area in the case of galaxy clusters, the lensing does not achieve a perfect symmetry causing irregularities. Images of the background galaxies in such cases usually appear as small, thin arcs around the boundaries of the cluster.
The analyses of these lensed images help in probing matter distribution in the galaxy clusters. The results show that a larger portion of the matter cannot be attributed to the visible galaxies or the gas and dust clouds around them, rather most of the matter does not emit light and is invisible, pointing evidence to the dark matter.
Mathematical models of these results provide us with the characteristics of the lensing material, both visible and dark. The dark matter comes out to have approximately 5 times more presence than the normal matter.
Simulations lead to a web-like network of dark matter filament structures in the universe that has cultivated and spread over time. Wherever these filaments intersect, we have lumps of visible matter in the form of galaxy clusters.
Galactic Rotation Curves
A galactic rotation curve is a 2-D plot of the stellar rotational velocity of a galaxy versus the stars’ respective distance away from the center of the galaxy.
The stars and gas in a galaxy disk move in an almost circular orbit. The inward acceleration necessary for the circular motion is provided by the gravitational field of the galaxy. To a reasonable approximation, we have the rotational velocity given by Newton’s gravity equation as below:
v^2 = GM/r
where the rotational velocity (v) is directly proportional to the total mass (M) and inversely proportional to the distance from the center (r). G is the gravitational constant.
As evident from the equation, because of the inverse relationship between ‘v’ and ‘r’, the rotational velocity should gradually decrease for stars farther away from the galactic center.
Surprisingly, the observations were contrastingly different. While studying multiple galaxies, it was found that the stellar rotational velocity remained fairly constant even for the stars with increasing distance away from the center. In terms of a plotted rotational curve, the velocity was almost a ‘flat’ line.
The above observation is extremely counterintuitive. The flat curve points to evidence of a presence of some form of matter around every galaxy that is invisible and contributing to the total mass of the galaxy, thus balancing the effect of increasing distance from the galactic center.
It is now widely accepted that dark matter is present in the roughly halo spheres that envelopes every galaxy.
So why does Dark Matter matter?
Despite the invisible nature of dark matter, it has dominated the universe for a significant duration of time. Scientists believe that dark matter has been influencing the cosmos since its beginning, and for the initial 9 billion years or so, it has been laying the foundation for building the structure of the universe.
Perhaps the most vital aspect of dark matter’s existence is the very existence of us. The way it has been critical to the evolution of our universe, the time-variant structure of the cosmic web has yielded the emergence of galaxies, stars, planets, and even life.
Without dark matter, the galactic structures would not have been able to hold on for such a long period, eventually wiping out whatever normal matter was present in the universe.
The conditions essential for the formation of our galaxy, our solar system, the Earth, and our life was formed during the Big Bang only – and all have been due to the existence of dark matter.
The Future: Further studies and experimental discovery of dark matter particles
For a theoretical physicist, or even for an experimentalist, the pursuit of dark matter is an ongoing area of research. It exists, that is for sure, but what it is made up of, we do not know yet.
Weakly Interacting Massive Particles (WIMPs) are the strongest candidate for dark matter particles. WIMPs are hypothetical particles that do not interact strongly with normal matter or electromagnetic forces and are believed to generate the current abundance and composition of dark matter in the universe. The inert neutrinos fall under the category of WIMPs.
Scientists in the Large Hadron Collider (LHC), the world’s largest and highest-energy particle collider, are in continual pursuit of hunting down the WIMPs. According to the LHC’s ATLAS experiment collaboration member Caterina Doglioni,
“..Placing the LHC results in the context of the global WIMP search that includes direct- and indirect-detection experiments has also been a focus of discussion in the dark-matter community, and the discussion continues to date on how to best exploit synergies between different experiments that have the same scientific goal of finding dark matter.”
https://home.cern/news/series/lhc-physics-ten/breaking-new-ground-search-dark-matter
We certainly live in an inspiring and exciting time. The more we feel certain that we have reached the end of understanding the fundamental reality, another topic such as dark matter pops up, leaving us with so many questions to answer.
This is also an inherent characteristic of science itself – to continue searching for the most fundamental understanding of our existence, our reality, and to push the boundary even further when it seems we have reached a wall.
Areas such as dark matter are extremely important to our history. It gives us hints of new physics or new forces that are even more fundamental to our understanding and knowledge.
And what could be more enthralling and philosophically soothing for us as a species to know about the underlying structure, the first principles of our very own existence?