Steven Gratton

Kavli Institute Fellow

Kavli Institute for Cosmology and Institute of Astronomy,
Cambridge, UK.

01223 765849


I am a Kavli Institute Fellow based at the Kavli Institute for Cosmology at the University of Cambridge, with interests in theoretical cosmology and cosmic microwave background (CMB) analysis.

A lot of my time has been spent working on the cosmological analysis of data from the Planck satellite. In March 2013 we released our first cosmology papers, based on temperature data from the 14-month nominal mission. The second release followed in 2015, this time using both temperature and polarization data from the full mission. The third and final collobaration release occurred in 2018, with the accepted papers appearing in 2020. They are available from the ESA website:

Our 2020 "CamSpec" likelihood is available here.

Planck and the earlier WMAP satellite have given us exquisite views of the microwave sky at different wavelengths, along with ancillary data. We find out a wealth of information about the universe by carefully studying the intensity and polarization of the cosmic microwave background (CMB) radiation reaching us today.

Unfortunately there are some complications. A major one (particularly for polarization) is contamination from "foreground" signals, typically from the galaxy. These signals have different spectral and spatial correlation properties than the CMB, enabling us to extract the cosmological information nontheless. Before Planck launched, I spent a lot of time developing ways to do this and to fold in our incomplete knowledge of the foregrounds and their properties into uncertainties on cosmological predictions.

I have done work on quantum cosmology, inflation and the landscape measure problem and continue to think about such matters. I looked again at eternal inflation from a novel standpoint, partially inspired by a minigrant from the Foundational Questions Institute. I was able to put a measure on fluctuating inflationary histories and to see how volume weighting (favouring the trajectories that inflated more) affected the way inflation might end. See the paper on the arXiv.

That minigrant enabled me to begin to investigate doing computing for cosmology on graphics cards, as GPGPU computing was just coming online. Click here for more details, now mainly for historic interest: Graphics Card Computing for Cosmology.

Another FQXi minigrant supported a fun meeting in 2009 entitled "New Thoughts About The Universe". See its webpage.

Another FQXi minigrant supported a follow-on meeting in 2015, "More New Thoughts About The Universe". See its webpage.

A third meeting, "New Thoughts 3: About the Universe and More...", took place in 2017, again supported by an FQXi minigrant. See its webpage.

In an effort to contribute to the understanding of the COVID-19 outbreak, I have investigated how path integral techniques might be useful in understanding uncertainties in "SIR" (susceptible-infectious-recovered)-type models of disease spread. It turns out that by solving additional differential equations one can calculate unequal time correlators for fluctuations around the background solution. This is of use in the construction of likelihoods for comparing parametrized models to data. See the preprint at arXiv:2006.01817.