I travel to Cambridge, MA tomorrow to participate in the workshop Rethinking the Dark Matter Paradigm (I had nothing to do with the choice of title). I went to college at MIT in the ’80s, so is a bit back to the future for me in space as well as time. There is a lot to rethink, or nothing at all, depending on who you ask. I’m curious to see if any of us are willing to think beyond I was right all along!
One of the compelling notions that emerged in the ’80s was non-baryonic dark matter. Baryons are the massive particles (protons & neutrons) of which normal stuff is made. It was well established by that time that the light elements were produced in the early universe by Big Bang Nucleosynthesis (BBN). It became clear in the ’80s that the mass density of normal stuff produced by BBN did not add up to the mass we needed to explain a whole host of astronomical observations, in both cosmology and galaxy dynamics. In short, Einstein’s General Relativity plus the baryons we could see did not suffice to explain the universe.
There were two obvious paths forward. Modify Einstein’s theory, or invoke unseen non-baryonic matter. The latter course seems by far the more plausible. No one had a compelling reason to challenge Einstein’s highly successful theory. On the other hand, there were plenty of reasons in particle physics to imagine new particles outside the standard model, particularly in the hypothesized supersymmetric sector.
It was quickly realized that large scale structure would only grow if this new stuff were composed of slow moving, non-relativistic particles – a condition summarized as dynamically “cold.” Hence Cold Dark Matter (CDM) was born. Weakly Interacting Massive Particles (WIMPs) from supersymmetry were a good candidate to be the CDM.
Thus began the marriage of astronomy and particle physics, two fields divided by a common interest in dark matter and cosmology. The heated embrace of the honeymoon has long since worn off, to the point that some of us are ready to rethink the whole paradigm.
This is no small step. Though I’ve come to doubt the existence of CDM, I still feel very comfortable with it. First love, and all. More importantly, it has been the one essential item in cosmology that has remained unchanged through the turbulent ’90s and on to today. But that is a longer story that will take many posts to tell.
For now, we’ll go see how much rethinking we’re willing to do.