Welcome to the second installment of Astronomy Club's exclusive column, A World Outside the Globe! This week, read on for an exploration of two universal mysteries by Vincent Xin '22.
Dark Energy and Dark Matter are two of the greatest unsolved mysteries in the universe: the little we already know of them has already changed our entire perception of the universe and what is unknown has even more potential. Hopefully, in the near future, some brilliant scientist (it might be one of you guys!) will solve this mystery, but here’s what we know as of now…
History
In the early 1990s, it was believed that the constant expansion of the universe would slow down as time went on due to the attractive force of gravity pulling matter together. However, in 1998, Hubble Space Telescope observations of very distant supernovae (which showed them as they were millions or even billions of years ago due to the time it takes light to travel) showed that back then the universe was actually expanding more slowly than it is today. So, contrary to what everyone thought, the expansion of the universe has actually been accelerating, but what is causing it? Over the years, three theories have emerged.
Theories
One theory was the idea of a “cosmological constant”, a variable representing the energy density of space that was proposed by Einstein in his theory of relativity but which was shortly thereafter abandoned by other scientists and Einstein himself for being unsatisfactory and redundant. Einstein realized that empty space is not nothing but rather has some amazing properties, one of them being that it is possible for more space to come into existence. Einstein then made a second prediction alongside his cosmological constant: that “empty space” can possess its own energy, which means that as more space comes into existence, more of this energy-of-space would appear, thus causing the universe to expand faster and faster.
The second theory hypothesized the existence of some strange kind of energy-fluid that filled space but whose effect on the expansion of the universe is the opposite of that of normal matter and normal energy. This fluids has been named “quintessence”, after the fifth element of the Greek philosophers.
The third explanation was that there is a flaw in Einstein’s theory of gravity and over time a new theory may be developed. But this new theory would have to correctly describe the motion of the bodies in the Solar System, just like Einstein’s theory of gravity, as well as explain the accelerating expansion of the universe.
Theorists still don’t know what the correct explanation is: none of the above theories are supported by any compelling evidence. But they have given the solution a name: dark energy.
Dark Energy and Dark Matter
Dark energy is the unknown force behind the acceleration of the universe’s expansion. It has been calculated from theoretical models and observations that approximately 68% of the universe is dark energy, with dark matter making up about 27% and the remaining 5% being “normal” matter - what we can actually observe with instruments.
As for dark matter, similar to dark energy, we have little idea what it actually is. But we know that it is not in the form of stars and planets that we can see, that it is not dark clouds of normal matter, that it is not antimatter, and that it is not solely caused by black holes. The most common view of the makeup of dark matter is that it is made up of exotic particles like axions or WIMPS (Weakly Interacting Massive Particles), whose existence have not yet been proven.
Pandora’s Cluster, a collision site of two galaxy clusters. Total mass concentration is shown in blue, with most of it being dark matter
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