Courtesy of Andrew W. Steiner, Associate Professor
For millennia, astronomy was limited to what can be learned about stars from photons. Over the past few decades, we have begun to be able to observe stars with either neutrinos or gravitational waves. The promise of multi-messenger astronomy is that, by observing objects using different messengers, we can expand our understanding of the universe.
We have been analyzing photon-based observations of neutron stars for over a decade, and in 2015 we predicted the neutron star tidal deformability (squishiness). This prediction was confirmed via gravitational waves by LIGO ("Laser Interferometer Gravitational-wave Observatory") in 2017. Unfortunately, this prediction had limited accuracy: the observational data still left quite a bit of room for different theoretical models of the neutron star interior.
The next step is to combine information from both messengers and thereby obtain a better picture of neutron star interiors. In an article published in Physical Review Letters this month, we did just that. The collaboration included Graduate Student Mohammad Al-Mamun, Associate Professor Andrew Steiner, and seven other researchers.
We started by confirming that the photons and gravitational waves were both telling the same story: they both give the same typical radius for a neutron star. We then used the observations to determine the full neutron star mass-radius curve. The mass-radius curve gives the relationship between mass and radius for all neutron stars in the universe.
Finally, we used photon and gravitational wave observations to determine the nature of dense matter. The interactions between neutrons and protons are perniciously difficult because the theory which describes them, quantum chromodynamics, has proven intractable at these densities. Now, with multi-messenger observations of neutron stars, we are able to provide the best current constraints on the pressure of dense matter as a function of density, bypassing the difficulties of quantum chromodynamics, and furthering our understanding of how neutrons and protons work on earth.
Graduate Student Mohammad Al-Mamun
Associate Professor Andrew Steiner