Why I’m choosing dark matter over dark energy – for now at least

Dark matter is my focus these days, but the intractable problems of dark energy and cosmic acceleration are still on my mind, says Chanda Prescod-Weinstein

By Chanda Prescod-Weinstein

PART of what makes my intellectual life interesting is that I do research not only in physics and astronomy, but also in social studies of science. On the social studies side, I’m particularly interested in how race and gender shape how physics happens, and while thinking about this, I often run into the question of language: how does it influence the ways that people from different communities relate to science? Believe it or not, this is what first came to mind when a reader wrote in to ask why I work on dark matter instead of dark energy.

Scientifically, it is a fair question, but I wondered whether the juxtaposition only existed in the reader’s mind because both contain the word “dark”. In some ways, that is pretty much the only thing they have in common: the use of the word “dark” to say “we as scientists can’t see it and don’t know what’s going on”.

As a quick reminder, cosmologists now believe that about 95 per cent of the energy-matter content in the universe is comprised of dark energy and dark matter. Dark energy is a term that describes a probable answer to an open question in observational cosmology: why is the expansion of the universe accelerating? Yes, for whatever reason, space-time is not only expanding, but that expansion is picking up speed. It is completely wild! So far, our simplest explanation is that there is an energy associated with empty space that is causing this expansion to go faster. This has come to be known as dark energy.

Meanwhile, dark matter behaves very differently to dark energy: it gravitates exactly the way we expect matter to gravitate. But, like dark energy, we haven’t ever seen it or interacted with it. We know it has to be a different type of particle to all of the ones we have seen in the lab, and right now we are trying to figure out exactly what it is.

The social studies side of my brain is interested in the way these names work, the way that scientists first clung to the name dark matter and then evidently adapted that when the cosmic acceleration problem came along. This curiosity maybe runs in parallel with that of the person who asked me why I spend so much time on one and not the other. Like I said, it is a good scientific question.

“For whatever reason, space-time is not only expanding, but that expansion is picking up speed. It is completely wild!”

I’m not sure I have a good scientific answer. I do have a personal one, though. I came of age with the cosmic acceleration problem. As a teen, I cut out a figure from a Scientific American article about the new discovery that space-time’s expansion is accelerating, and I glued it to my application to the California Institute of Technology. I wrote underneath: “I want to solve this problem.” Throughout the next 11 years or so, that is exactly what I tried to do. In my first year as a PhD student, I stayed up late at night, under the impression that if I just read enough papers and thought hard enough, the solution would come to me.

I eventually graduated with my PhD, having defended “Cosmic acceleration as quantum gravity phenomenology” as my dissertation. I believed – and still do – that cosmic acceleration is our first experimental hint at how to solve the great quantum gravity problem, the question of how to merge quantum mechanics with gravitation. This perspective is still not particularly fashionable, but here I am, clinging to it.

This is probably also why I came to see dark energy as a kind of intractable problem for me to personally work towards solving. On the one hand, I have a gut feeling (that could be wrong) about how it fits in with other questions we have. On the other, there are physicists who believe that the rest of us are overthinking it. In their view, it is just a constant energy known as the cosmological constant, and the value of the cosmological constant is an accident.

This sets us up for a philosophical confrontation that I find to be very distressing. If the cosmological constant had another value, the universe would have evolved differently, and we might not be here. Variations on this theme are often known as the anthropic principle. I hate it, because that doesn’t feel like an explanation so much as resignation.

Eventually, I stopped working on trying to solve the cosmic acceleration problem because I wasn’t having any ideas that were better than fans of anthropics, and anthropics made me sad. In the meantime, I have developed an expertise in dark matter and my contribution has allowed me to draw from ideas in atomic physics that I always found fascinating.

Dark matter is a major open cosmological question and, at least for the moment, it is more fun to work on. It is also possible that my subconscious is working away at a good explanation for dark energy – so maybe when it is done, I’ll be back.