with Evangelos Sfakianakis and David Kaiser
Adiabatic fluctuations in my nutella sandwich (left) vs. isocurvature fluctuations in my PB&J (right).
This paper was about a period in our universe known as inflation. In a sense, inflation is the mechanism puts the “bang” in “big bang,” so this project was really about the beginning of our universe. Super casual. ;)
Inflation is an incredibly clever mechanism that was thought of by one of my mentors, Alan Guth in what he deemed a “spectacular realization” in his personal notes as he was figuring out (in real time, back in 1980) how great inflation is. What it boils down to is that the universe expands in volume by a factor of 10^75 (approaching a googol!) in the first 10^-36 seconds of the universe. During this time, the universe is a very different place, very hot and full of strange particles that are well above an energy scale that we can access at the Large Hadron Collider. Inflation is a very generic mechanism that can be realized in any one of many ways (like it’s very easy to think of different theories which support inflation) and it solves lots of puzzles about why the universe looks the way it does. First, it explains why our universe is very uniform on the largest scales (I’m talking on the scale of a billion light years.) In other words, if you look at one direction of the night sky, it doesn’t look radically different from any other direction. In fact, on the largest scales, our universe is uniform to one part in 10,000. It would be an extremely weird coincidence if that happened just by chance. Another thing that inflation explains is why our universe has just the right amount of stuff in it so that the universe doesn’t end. What I mean by that is that if the universe has too much or too little stuff in it, then it either collapses on itself quickly or expands so quickly that galaxies don’t have time to form. In order to have the kind of universe we see (and that we can survive in), we would need to fine tune the amount in stuff in our universe to roughly one part in a billion billion. Again, it would be very strange if this happened by coincidence. Finally, inflation explains why we don’t see magnetic monopoles (i.e. an electron but with a magnetic charge rather than an electric one) because inflation specifically thins them out so much because of how heavy they are.
Another thing that is explained by inflation is how galaxies were seeded by small fluctuations in the amount of matter: those were actually quantum fluctuations in the field responsible for inflation. It was those fluctuations that I was concerned with in this project. Specifically, I was looking at fluctuations in the relative amount of different kinds of matter -- isocurvature perturbations -- that would be generated by a type of inflation. This type, known as non-minimally coupled multifield inflation, is a very plausible inflationary mechanism because of how we think low-energy particle physics has to generalize to higher energies. Isocurvature perturbations are relevant because (in contrast to adiabatic perturbations aka fluctuations in the total amount of stuff at any given position) they can account for why there seems to be an absence of structure on some of the largest scales relative to what we would have predicted using our simplest theories.
We explored this kind of inflation, creating formalism and gaining intuition that will be useful in general when you have multiple fields and funky kinetic terms in your inflationary Lagrangian. What we found is that there is a sweet-spot which gives an isocurvature fraction that can definitely account for the "low-multipole" anomaly in the CMB while also completely satisfying other CMB constraints on inflationary parameters. This sweet spot of parameter values in the Lagrangian is actually the kind of thing you would expect if you had a high energy symmetry which gets softly broken at the scale of inflation. So in addition to being observationally motivated and interesting, these kinds of couplings are also theoretically viable and make sense in the context of other stuff we know about particle physics. Pretty cool eh? Some of my collaborators on this project are still working on figuring out all the different aspects and observational signatures of these models and it's very interesting work. If you're interested in learning more, I encourage you to check it out!
The dip that you see in the Planck CMB data relative to the red theory curve (around l of 20) can be explained by our inflationary model! In the future, there will be lots more probes of inflation by looking at CMB lensing, so we'll see if the viability of our model can stand the test of time!