In 1998, astronomers made a startling announcement. Space-time, the unified phenomenon that comprises our universe and that was previously understood to be expanding, was actually not just growing, but growing faster and faster as time went on. In other words, its expansion was accelerating. This was the birth of the cosmic acceleration problem: what was causing this acceleration? It seemed to be literally coming from nowhere – from the vacuum.
From the point of view of general relativity, cosmic acceleration could be explained by saying that empty space-time has energy that drives this expansion, that it isn’t completely empty. This energy is called the cosmological constant. Happily, from the point of view of quantum mechanics, empty space-time always has an energy associated with it, which seems to offer a source for the cosmological constant. Quantum vacuum energy can be calculated.
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But there is a mismatch: the number we get when quantum physics is taken into account is astronomical compared with the number that explains things from the astrophysics point of view of relativity. In other words, cosmic acceleration suggests there is a vacuum energy, but the value for this that we get from astrophysical observation doesn’t match the figure from quantum theory.
Cosmic acceleration was such an exciting discovery that I wrote about it in my university applications, including one to the California Institute of Technology. For this, I cut out part of a Scientific American article about the discovery and glued it into my submission. Below it I wrote: “I’m going to solve this problem.”
Twenty-seven years later, the problem isn’t solved, but I don’t even work on cosmic acceleration! Not anymore, anyway. In 2010, I defended my PhD dissertation, . My premise was that the mismatch between the apparent observed value of the cosmological constant and our calculations was our first experimental hint about the nature of quantum gravity.
My younger self is vindicated. Cosmic acceleration may indeed be a quantum gravity phenomenon!
You might recall that quantum gravity is a hypothetical theory that merges quantum physics with general relativity. These two very different frameworks for reality are our most fundamental theories. But they don’t work well together. In general relativity, space-time is a smooth, continuous phenomenon. In quantum physics, almost nothing is smooth and continuous, but involves individual units that we call quanta. It is like the difference between your favourite quilt and an object made of individual Lego bricks.
There are other differences: in general relativity, given certain information about a system, we can definitively calculate its past and future. In quantum physics, we can only speak of probabilities for the system. This is one reason the quantum vacuum is expected to have an energy – probability means particles are constantly appearing and disappearing, so it is never really empty.
My PhD research proposed that the reason for conflict between the astrophysically observed vacuum energy and the one predicted by quantum theory is that we were getting a first hint about the need to modify how we calculate the quantum vacuum energy. I believed in this deeply, but at the time we weren’t getting much data to help test the model. I was also interested in the idea the energy was dynamic and changing in time (in this form, it is called dark energy), but it would be a long time before this could really be tested.
Just months ago, the Dark Energy Spectroscopic Instrument (DESI) collaboration announced evidence that, in the past, the value of the cosmological constant was different than it is now. If so, it would mean cosmic acceleration truly is caused by something like a dark energy, a phenomenon changing in time.
I hope that for today’s 16-year-old physics nerds, this news is as exciting as cosmic acceleration was for me. If it holds true, it will be just as significant. Because the US leadership is proposing to devastate the funding that is needed to follow up on this result, researchers here may not be able to investigate further. But I hope the rest of the world will – and maybe create opportunities for US scientists to join them.
Even as I worry about ensuring the next generation of stargazers has the opportunities that I did, I am heartened to see that my colleagues keep writing papers full of interesting ideas. After the DESI announcement, I started seeing researchers asking how the results could be explained using quantum gravity theories. Though I have moved on to other questions, my younger self is vindicated. Cosmic acceleration may indeed be a quantum gravity phenomenon! What a lesson in how science is multi-generational, requiring patience from those who work in it and the public that funds us.
Chanda’s week
What I’m reading
I am getting into Sex is a Spectrum: The biological limits of the binary by Agustin Fuentes.
What I’m watching
I have seen the film Sinners three times now – one of the greatest movies ever made.
What I’m working on
Trying to save US science while onboarding a new PhD student, to ensure she has a future in it.
Chanda Prescod-Weinstein is an associate professor of physics and astronomy, and a core faculty member in women’s studies at the University of New Hampshire. Her most recent book is The Disordered Cosmos: A journey into dark matter, spacetime, and dreams deferred
