By Yuan-Sen Ting
I've always been the quiet kid in school, and even nowadays, it's awkward for me to make friends. This is what usually happens:
"I'm an accountant. How about you?
"Uhm, I'm an astrophysicist."
Generally, there would be an abrupt silence at this point of the conversation. The space would become so quiet that you could hear a pin drop. Fortunately, as an astrophysicist, I'm equipped with three handy icebreakers: aliens, black holes, and dark matter. Whenever the atmosphere gets too awkward, I would whip my trump card out and turn the tables by saying:
"Have you heard of dark matter?"
Illusion vs reality
The Matrix is one of my all-time favorite movies. It talks about how the main character, Neo, notices that things are contradictory in the simulated world created by an evil computer to prison humans. This is how he realizes that what one derives from one's five senses merely gives an illusion of reality. He proceeds to uncover the true nature of reality. The slow-motion fight scenes in the movie are timeless indeed, but The Matrix probably became a classic courtesy of the philosophy behind it. The ancient Greek philosopher Plato's allegory of the cave echoes this philosophy.
Plato spoke of a group of prisoners who were held captive in a dark cave. Their necks and bodies were shackled to a wall, and the exit of the cave was behind this wall above them. The world out there was bright, and small animals passed by the exit every now and then. However, the prisoners' field of view was limited because they were restrained. All they could see was the shadows of these small animals. Most prisoners were primed to believe that these shadows were the sole reality. It was only when like Neo, one of the prisoners freed himself of the shackles of his mind and realized that the shadows were merely an illusion of reality did they discover that there was a vast, bright world behind them.
And this was how humanity discovered the existence of dark matter too.
The discovery of dark matter
Around the year 1970, there were starting to be telescopes that were better at detecting the movements of the stars and gases in galaxies. This technological breakthrough was a dream come true for astrophysicists. Last issue, we talked about how the movement of stars is affected by the gravitational pull of the galaxy, and that by measuring the trajectory of stars, we can theoretically infer the mass of a galaxy. At that time, everyone thought that stars at the periphery of galaxies ought to move slower compared to stars closer to the center of galaxies because the mass density of the galaxy is lower at the periphery. For example, in the Solar system, the planets that are further away from the Sun such as Uranus orbits the Sun slower compared to the Earth. But astrophysicists were in for a shock. The observed data was the opposite of what they had theoretically predicted. Stars at the periphery of galaxies were moving much faster than expected!
There is a large amount of dark matter in galaxies
This is a little like how a merry-go-round works. For a merry-go-round to operate at a high speed without causing any havoc, the riders must fasten their seat belts very tightly. In the context of galaxies, the gravitational pull of galaxies is like seat belts. But what everyone can't understand is that for stars to move at such a high speed, their seat belts need to be fastened a lot tighter. To be exact, the mass needed to keep the stars in place while rotating so quickly is ten times higher than the total mass of the observable matter in galaxies! In other words, galaxies look like they're very skinny, but when you measure their mass, they're actually gigantic! This contradiction in the appearance of reality forced astrophysicists to renew the perception of the true reality. And the only reasonable explanation is that aside from observable matter, there is a large amount of matter in the galaxies that we cannot observe. And this, my friend, is what we call dark matter.
Even though this explains the movement of stars, the spirit of science required this theory to be able to withstand the test of time before it became a well-established fact. And among the vast variety of researches in the field of astronomy that had been conducted so far, whether they're about how various elements were formed during the Big Bang or the distribution of galaxies in the cosmos, dark matter had again and again proven to be an indispensable part of the Universe. There is no doubt that dark matter exists. I won't go into details here.
Dark matter is incredibly helpful
There is something pretty interesting though—the gravitational lensing effect caused by dark matter. In short, based on Einstein's general theory of relativity, when one observes a galaxy, if the light emitted by that galaxy passes through a large aggregate of dark matter, the huge gravitational pull from the dark matter would bend the light and cause a warp in space-time. As a result, the galaxy would appear distorted and magnified. This is a little like looking at a light source from the bottom of a wine bottle. The light source would become distorted due to the refraction caused by the glass. Even though such large aggregates of dark matter are rare, this interesting phenomenon had been observed again and again by the Hubble Space Telescope.
It turns out that this gravitational lensing effect is extremely vital when it comes to studying certain ancient galaxies in the Universe. Most of these ancient galaxies cannot be directly studied because they're too far away from us. This is when dark matter would sometimes make a cameo and bend the light emitted by these ancient galaxies so that they are magnified, making it possible for astrophysicists to observe and study them. It is through dark matter's unintentional help, we're able to get a glimpse of how the early stage of the Universe.
With these said, what exactly is dark matter? What subatomic particle (or particles) is it? Unfortunately, there isn't an answer for that. Right now, even though we know that dark matter is omnipresent and visibly affects the way celestial bodies move, its exact subatomic composition remains a mystery. In fact, understanding dark matter is still a very active field of research that many researchers have been dedicating their lives to.
But unanswered questions aren't really a bad thing, are they? They serve as a warning for humanity, a reminder of how insignificant we are. We're just like those prisoners in Plato's cave. But there's something great about humanity— even though we're restrained, we're still able to utilize our boundless imagination to delve into the secrets of the universe.
We're not just prisoners. We're also Neo.
(Yuan-Sen Ting is an astrophysicist. Yuan-Sen obtained his Ph.D. in astrophysics from Harvard University in 2017. He is currently a researcher working at the Institute for Advanced Study in Princeton, funded through a NASA Hubble Fellowship. Yuan-Sen is also an incoming faculty member at the Australian National University.)