John Joseph Murphy Carrasco

Research Associate

Stanford Institute for Theoretical Physics

John Joseph M. Carrasco JJ Carrasco

Info

  • office: 362 Varian Laboratory
  • e-mail: jjmc@EXPECTED_UV_DIVERGENCE_stanford.edu (cancel the EXPECTED_UV_DIVERGENCE_)
  • (Physics) Articles: [INSPIRE]
  • CV: [pdf]
  • Statement of Research [pdf] (What's this?)
  • Teaching Philosophy: [pdf] (What's this?)


    Fun

  • IMDB: [Bringing my Bacon+Erdos number to 6 (through a director :-) )...]
  • Google Scholar: [Includes some machine-learning/pattern-classification + patents]
  • Follow me on Twitter: [Better to micro-blog than never to have blogged at all...]


    Current Research:

    I'm trying to to find a path to answering some of the "Big Questions" by thoroughly understanding the predictions of quantum Yang-Mills and Gravity theories. I've also recently gotten excited about Cosmological Large Scale Structure. I'll attempt to provide a readable broad description of these activities below. Actual details can be found by clicking on the publications link above. Email me if you would like to discuss getting involved in any of these projects.

    Emergence and evolution of the structure of space and time

    A gauge theory called Yang-Mills describes the strongest force we have observed -- the one holding nucleons together in the center of atomic nuclei. Gravity is the weakest force we have observed, even though it seems to couple to all energy we can see. It turns out they're intimately related in many surprising ways.

    One of our most exciting discoveries is that the sheer combinatorial explosion of graphs contributing to quantum corrections to the scattering of gluons in gauge theories may be fantastically redundant -- beautifully algorithmic algebraic structures relate all of their contributions to the information associated with a very few. Making this structure manifest (the duality between color and kinematics) allows for efficient calculation, but spectacularly also simple and natural expressions for graviton-scattering predictions in terms of double-copies of gluonic behavior -- even perturbative quantum corrections at the integrand level! How strange and wonderful, for all the information relevant to our description of the (perturbative, at least) evolution of the structure of space and time to be so compactly encoded in the interactions of gluons. Exploring the consequences of this duality, and the symmetries behind it, is very much a subject of rich active research. A completely open question at the moment is its relation to holography -- a robust idea from the early 1990s -- the complementary notion that strongly interacting gravitons in the volume of certain special space-times can be completely described by weakly interacting gluons on its surface.

    One of the most surprising and important results we found stood in stark contrast to popular wisdom at the time; quantum field theory may admit perturbatively consistent and probability-conserving quantum descriptions of gravity in four dimensions. Previously physicists believed in the irreconcilability of (point-like) quantum field theory and Einstein's general theory of relativity, even under the protection of maximal supersymmetry. Calculating amplitudes where consensus predicted divergence, and discovering, instead, surprisingly good behavior in the ultraviolet, suggests that these assertions of incompatibility may be overly pessimistic. Rather, we find evidence of cancellations that propagate to all loop orders as can be seen using the unitarity method. We still have no proof of perturbative finiteness -- even if finite, may not prove to be UV-complete non-perturbatively. Even so, through explicit calculation we have exposed mysteries about the quantum behavior of gravity, that almost nobody suspected, which are quite exciting to pursue. I should mention that this has invigorated fields of formal field-theoretic inquiry to explain the observed cancellations -- forming an ongoing dynamic interplay between the amplitudes community, string theorists, and supergravity experts.

    Effective Field Theory of Cosmological Large Scale Structure

    There is a vast amount of precision data to be uncovered in upcoming maps of large scale structure in the universe. This should allow us to make incredibly detailed measurements of the parameters which governed the evolution of our early universe. To extract this information it is crucial to arrive at analytic methods that allow us to complement the incredibly powerful simulations that can clarify the regime where gravitational collapse overtakes expansion. We have established -- through explicit calculation -- the predictive reach of an Effective Theory of Large Scale Structure -- demonstrating the emergence of fluid-like parameters that can be measured observationally. In essence we are integrating away the strongly-coupled small scale physics resistant to perturbative description, and allowing the construction of a rigorously predictive analytic understanding. Our latest calculation -- the EFT prediction for the power-spectrum through 2-loops, has demonstrated percent precision against simulation to a factor of 6 higher in the UV than the linear scale. This translates into an additional volume of modes of over 200 times the regime accessible by traditional perturbative methods.

    This is a research program its early stages, with much important work to be done to eventually get this in the hands of experimentalists. Beyond explicit measurements, this is definitely an intriguing theory in its own right -- a wonderful place for quantum field theorists to explore the consequences of some of their most familiar concepts (renormalization, running, etc.) in a completely classical (stochastic) domain that promises to be very data-rich in the near future.


    (*) What are research/teaching statements? Fairly formal, space-constrained documents aspiring academics are asked to generate when applying for jr faculty positions. Feel free to email with feedback/questions.