Why I choose dark matter over dark energy – for now at least

Dark matter is my focus nowadays, but the difficult problems of dark energy and cosmic acceleration are still in my mind, says Chanda Prescod-Weinstein

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April 13, 2022

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A PART of what makes my intellectual life interesting is that I do research not only in physics and astronomy, but also in the social sciences. On the social science side, I am particularly interested in how race and gender shape how physics takes place, and when I think about this I often come across the question of language: how does it affect the way people from different societies relate to science? Believe it or not, but this is what first came to mind when a reader wrote in and asked why I work with dark matter instead of dark energy.

Scientifically, it is a fair question, but I wondered if the compilation 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 researchers cannot see it and do not know what is happening”.

As a quick reminder, cosmologists now believe that about 95 percent of the energy-matter content in the universe consists 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 not only expands, but that expansion picks up speed. It’s completely wild! So far, our simplest explanation is that there is an energy associated with empty space that makes this expansion go faster. This has come to be known as dark energy.

At the same time, dark matter behaves very differently from dark energy: it gravitates just as we expect matter to gravitate. But, like dark energy, we have never seen it or interacted with it. We know it must be a different type of particle than all the ones we’ve seen in the lab, and right now we’re trying to find out exactly what it is.

The social science side of my brain is interested in how these names work, how scientists first stuck to the name dark matter and then apparently adapted it when the problem of cosmic acceleration came up. This curiosity perhaps runs parallel to the one who asked me why I spend so much time on one and not the other. Like I said, that’s a good scientific question.

“For whatever reason, space-time is not just expanding, but expanding. It’s completely wild!”

I’m not sure I have a good scientific answer. However, I have a personal one. I came of age with the cosmic acceleration problem. As a teenager, I cut out a figure from one Scientific American article on the new discovery that space-time expansion is accelerating, and I glued it to my application to the California Institute of Technology. I wrote below: “I want to solve this problem.” For the next 11 years or so, that was exactly what I was trying to do. During my first year as a doctoral student, I stayed up late at night, with the impression that if I just read enough newspapers and thought carefully enough, the solution would come to me.

I eventually graduated with my PhD, after defending “Cosmic acceleration as quantum gravity phenomenology” as my dissertation. I thought – and still do – that cosmic acceleration is our first experimental tip on how to solve the great problem of quantum gravity, the question of how to combine quantum mechanics with gravity. This perspective is still not very fashionable, but here I am, clinging to it.

This is probably also why I came to see dark energy as a kind of difficult problem for me to personally work to solve. On the one hand, I have a gut feeling (it may be wrong) about how it fits in with other issues we have. On the other hand, there are physicists who think that the rest of us are thinking about it. In their view, only a constant energy is known as the cosmological constant, and the value of the cosmological constant is an accident.

This prepares us for a philosophical confrontation that I find very painful. If the cosmological constant had a different 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 it does not feel like an explanation so much as abandonment.

Eventually I stopped working on trying to solve the cosmic acceleration problem because I had no 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 made it possible for me to draw from ideas in atomic physics that I have always found fascinating.

Dark matter is a big open cosmological issue and, at least for the time being, it’s more fun to work with. It is also possible that my subconscious mind is working on a good explanation for dark energy – so maybe when it’s done I’ll be back.

Chanda’s week

What I read
This month I am very fond of Aria Hallidays Buy Black: How Black Women Transformed American Pop Culture.

What I’m looking at
Like many here in the United States, I am completely in love with both Abbott Elementary and the American version of Ghosts.

What I work with
I’m giving a TED talk on dark matter, and it’s a lot of work to prepare!

Next week: Graham Lawton

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