I work on different angles of the problem of cosmic acceleration and the possibility to use cosmology to test fundamental physics. I am also commited to the interpretation of observations within non-standard scenarios, both to test physical ideas and to improve the model independence of data analysis strategies.
For an overview you can take a look at my scientific publications:
or look at the brief descriptions of my work in dark energy and tests of gravity theoretical aspects of gravity and cosmic voids.
Einstein's theory of gravity has been very successful at explaining gravitational phenomena, including cosmology. Yet, many alternative theories have been proposed, both as candidates to explain cosmic acceleration and as framewoks to test gravity in different regimes, including cosmology.
My work has dealt with different ways in which to test these theories using cosmological and other data. I am the main developer of Hi_CLASS (Horndeski in the Cosmic Linear Anisotropy Solving System), an accurate, fast and flexible code to obtain cosmological predictions in general dark energy models. Hi_CLASS does not rely on approximations valid only on small scales, and I am therefore using it to test gravity in new regimes such as the early universe or ultra-large scales.
Research in non-standard cosmologies is useful to revise model independendt assumption of data analysis techniques. In this spirit, I examined the non-linear evolution of the baryon acoustic scale (measured by galaxy surveys) in the pressence of modified gravity. I found that the departure from the standard ruler behaviour was considerably larger than in the standard case, but yet not enough to be relevant for future surveys.
I have also worked on different models for cosmic acceleration, including tensor modes in theories with massive gravitons and models based on disformal couplings. I am also interested in screening mechanisms , non-linear effects that hide modified gravitational dynamics in regimes where gravity is very well tested, such as the Solar System.
Publications:
Nonlinear evolution of the baryon acoustic oscillation scale in alternative theories of gravity, E. Bellini, M. Zumalacarregui PRD92 063522
Surfing gravitational waves: can bigravity survive growing tensor modes?, L. Amendola, F. Koennig, M. Martinelli, V. Pettorino, M. Zumalacarregui JCAP 1505 052
Screening Modifications of Gravity through Disformally Coupled Fields, T. Koivisto, D. Mota, M. Zumalacarregui PRL 109 241102
Disformal Scalar Fields and the Dark Sector of the Universe, M. Zumalacarregui, T. Koivisto, D. Mota, P. Ruiz-Lapuente JCAP 1005 038
Constraining Entropic Cosmology, T. Koivisto, D. Mota, M. Zumalacarregui JCAP 1102 027
Important insight into dark energy is often gained from theoretical considerations, which may reveal new possibilities or obstructions in known models. This has been exemplified by the recent discovery of ghost-free massive gravity and the myriad of problems found in applications to cosmology.
An important tool in examining theories of gravity is the possibility of redefining the fundamental fields. By using one such redefinition, I was able to show the existence of viable scalar-tensor theories beyond the Horndeski Lagrangian, which was thought to be the most general ghost-free theory of its class. Earlier I used field redefinitions to show the equivalence between disformally coupled theories and DBI Galileons, providing a simpler framework to understand their properties and generalizing the notion of the Einstein and Jordan frame.
General theories of gravity also exhibit growing levels of kinetic mixing between their degrees of freedom. This property is useful to characterize different families of theories and their phenomenology.
Publications:
Transforming gravity: from derivative couplings to matter to second-order scalar-tensor theories beyond the Horndeski Lagrangian, M. Zumalacarregui, J. Garcia-Bellido PRD89 064046
Shaken, not stirred: kinetic mixing in scalar-tensor theories of gravity, D. Bettoni, M. Zumalacarregui PRD 91 104009
DBI Galileons in the Einstein Frame: Local Gravity and Cosmology, M. Zumalacarregui, T. Koivisto, D. Mota PRD 87 083010
Cosmic voids are regions with a low matter content that fill up most of the space in the universe. Their properties and distribution are sensitive to the cosmological expansion, and they have been proposed as a test of dark energy. I am currently developing the VOS (Voids Over the Sky) code to compute the effects of voids in the propagation of light and how these affect observable quantities such as gravitational lensing.
Voids also constitute a "theoretical low-cost'' alternative to a cosmological constant. This requires for our galaxy to be located at the center of a huge void in order to satisfy supernovae observations. My work showed additional distance information from baryon acoustic oscillation is incompatible with supernova, making the model inviable and observationally supporting the Copernican Principle. This has a nice geometrical interpretation as the incompatibility between standard candles and standard rulers in the presence of sizeable inhomogeneity.
Publication:
Tension in the Void: Cosmic Rulers Strain Inhomogeneous Cosmologies, M. Zumalacarregui, J. Garcia-Bellido and P. Ruiz-Lapuente, JCAP 1210 009