Much as there is talk about 鈥渢he scientific method鈥�, the reality is that there are many scientific methods. For example, nuclear and particle experiments tend to rely on collisions in a laboratory. These happen because people set up experiments where they will occur. Astrophysics, by contrast, looks at phenomena that happen to have occurred or be occurring. Those of us in this field don鈥檛 prompt the experiment. Instead, we watch the unfolding experiment that is the universe.
This creates unique challenges because we can鈥檛 decide to recreate results. If we see a galaxy with a strange shape, we can鈥檛 simply go out and get another one. We can look and look, but there is no guarantee we will find one. Similarly, if we are dealing with something we can鈥檛 see, that means some effort is required to understand what we are (not) looking at. This is the case with dark matter.
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A quarter of the way into the 21st century and we have never been more confident that we have detected dark matter through gravitational means. When we look at how stars move in galaxies, we can鈥檛 make sense of their motions without adding in some sort of matter that isn鈥檛 visible to us. When we look at the way massive clusters of galaxies bend light around them 鈥� gravitational lensing 鈥� we can鈥檛 explain the paths that light follows without adding in some invisible matter.
Our best evidence for dark matter鈥檚 existence could arguably be called non-gravitational evidence. There is a form of light that has been travelling through the universe since it first became transparent to light. This cosmic microwave background radiation (CMB) started moving a little under 400,000 years after space-time came into existence.
We are able to observe the CMB. Looking at how strong it is in different frequencies gives us information about the conditions at the light鈥檚 moment of origin. When we compare theory with these observations, we get one of the most beautiful fits between theory and experiment in all of physics. But the match only works if we assume the existence of some type of invisible matter that outnumbers visible matter.
Naturally, the first question one must ask about something like dark matter is how we know it is actually invisible and distinct from all the forms of matter we know about. For example, it has long been known that cosmic dust isn鈥檛 very hot and therefore doesn鈥檛 radiate much. Are we certain that there isn鈥檛 just a lot more dust than we thought?
Are we certain that dark matter truly exists or could there just be a lot more cosmic dust than we thought?
This question, which a reader sent me, is an important one that astronomers have been compelled to explore. At this point, we are confident that dark matter isn鈥檛 dust. The reason has to do with experiments we do here on Earth. Cosmic dust is mostly (though not entirely) made of hydrogen. Hydrogen, like all atomic elements, has a nucleus with electron orbits beyond that. Quantum mechanics teaches us that the energy levels of these orbits are discrete 鈥� they come in whole numbers like 1 and 2, not 1.5 and 2.5. Another feature of quantum mechanics is that the difference in energy between these levels corresponds to a characteristic light wavelength. What this means is that when an atom is in a particular energetic state, it will uniquely absorb and emit light at a wavelength that corresponds to that energy level.
That is a lot of physics, so let me put it more concisely. Hydrogen dust, though it tends not to be very bright, does, in fact, both emit and absorb light. We are able to observe dust because of this. Dark matter, on the other hand, doesn鈥檛 emit or absorb light, at least not to a level that we can see.
This is one of the reasons I think it should really be called invisible matter, not dark matter. The word 鈥渄ark鈥� can be misleading, because it suggests there is some kind of absorption happening, when, really, there is simply a complete absence of interaction with light, which seems to go right through it. The only dark matter scenario where this isn鈥檛 the case is the one where dark matter is comprised of primordial black holes that formed in the early universe. In that case, dark matter really would be absorbing light!
The fact that dark matter is invisible hasn鈥檛 stopped us learning about it. We can use its impact on visible matter to study where dark matter is and how much of it there is. In the research group I lead, we are investigating a hypothetical class of dark matter particles called axions. Our shows that, depending on what quantum properties we give the axion, it creates slightly different structures on scales that are astrophysically observable. This indicates that we may be able to use galaxies to distinguish between dark matter models.
Even though dark matter may be impossible to see, we can still learn a lot about it from looking.
Chanda鈥檚 week
What I鈥檓 reading
I am really liking Park Seolyeon鈥檚 A Magical Girl Retires, translated into English by Anton Hur.
What I鈥檓 watching
I鈥檓 sorry to report to the people who are horrified by my soap habits that I鈥檓 into EastEnders now.
What I鈥檓 working on
I am attending a workshop entitled 鈥淲hat is particle theory?鈥� and trying to answer the question!
Chanda Prescod-Weinstein is an associate professor of physics and astronomy, and a core faculty member in women鈥檚 studies at the University of New Hampshire. Her most recent book is The Disordered Cosmos: A journey into dark matter, spacetime, and dreams deferred
