**Cosmology, gravitational waves and fundamental physics**

Our
standard model of the universe can describe accurately a wide range of
astronomical observations, spanning its nearly 13.8 billion years of
history. However, this remarkably simple model requires 3 pieces of new
fundamental physics: 1) cosmic inflation, an initial era of accelerated
expansion that sets the universe’s initial conditions, 2) Dark matter,
an abundant yet undetected form of non-atomic matter that drives the
dynamics of galaxies and formation of large-scale structure in the
universe and 3) Dark energy, a form of energy that causes the expansion
of the universe to accelerate, overcoming the attractive nature of
gravity. All current evidence for dark matter and dark energy stems from
their gravitational effects and when combined they represent 95% of the
energy in the universe today.

My research aims at combining
theoretical and observational insights to better understand the
properties of dark matter and dark energy. I initially trained as a
theoretical cosmologist but eventually understood the immense potential
of gravitational wave (GW) astronomy and its synergies with
“traditional” cosmological observations (like supernovae or the
large-scale structure of the universe) in answering some deep, long
standing questions:

- What is the nature of dark matter and dark energy?

- Is Einstein’s theory a correct description of the universe?

- How can we optimally combine cosmological and GW observations to advance fundamental physics?

My work is structured along 4 inter-related themes:

__Cosmology beyond the standard paradigm__

Understanding
dark energy and dark matter is a major goal of observational and
theoretical cosmology. My work explores the viability of non-standard
scenarios in light of current and data upcoming cosmological data. I am
the main developer of the hi_class code, an accurate, fast and flexible
code to obtain cosmological predictions in general dark energy models,
publicly available to the community (www.hiclass-code.net). I use
hi_class to test gravity theories and dark energy models. I am
particularly interested in scenarios that address potential issues in
the standard model, such as the discrepancy in the universe’s expansion
rate based on different methods. I am also a member of the Euclid
mission, a satellite to study dark energy and fundamental physics by
mapping the large-scale structure of the universe.

__Gravitational Waves beyond General Relativity__

The
rise of gravitational wave astronomy provides an decisive advantage in
the study of gravity and dark energy. I work towards a deeper understand
of how GWs propagate in theories beyond Einstein Gravity, exploring new
effects that may be used to test those theories. One such effect is the
anomalous GW speed, which was spectacularly constrained by the
detection of gravitational and electromagnetic radiation from the first
neutron-star merger, GW170817, thus ruling out many gravitational
theories. GW propagation effects also include anomalous amplitude,
phase, and even oscillations into additional polarizations (analog to
flavor oscillations for massive neutrinos). I recently became involved
with the LISA mission, a future low frequency gravitational wave
observatory in space, and the effort to define the science case for 3rd
Generation detectors on the ground.

__Dark matter and gravitational lensing__

Despite
a very precise knowledge about its abundance, the nature of dark matter
has eluded decades of dedicated experiments, and many models with
vastly different properties remain as viable alternatives. I am
exploring how gravitational lensing, the bending of light rays by
gravitational effects, can be used to test the properties of dark
matter. An example is how the (lack of observation) of magnified
supernovae helped rule out stellar-mass black holes, such as the ones
detected by LIGO/Virgo, as the primary dark matter component. I am also
interested in how GW observations can be used to further probe these and
other dark matter candidates.

__Theories of gravity beyond Einstein__

Insights
into dark energy often stem from theoretical considerations, which may
reveal new possibilities or obstructions to known scenarios. I am
interested in exploring ideas leading to new viable gravitational
theories, which may be latter applied to construct interesting
cosmological scenarios. To this end, I use methods such as redefinitions
of the fundamental fields, a technique that produced the first examples
of viable theories beyond Horndeski, previously thought to be the most
general viable theory of its class. New ideas for gravitational theories
are now more necessary than ever, given the stringent limitations
placed on the theory landscape by recent gravitational wave
observations.

__Selected Publications__

[1] “Limits on stellar-mass compact objects as dark matter from gravitational lensing of type Ia

supernovae”, M. Zumalacárregui and U. Seljak, arXiv:1712.02240 Phys. Rev. Lett. 121 (2018) 141101

[2]
“Dark Energy after GW170817: dead ends and the road ahead” J. M.
Ezquiaga and M. Zumalacárregui, arXiv:1710.05901 Phys.Rev.Lett. 119
(2017) 251304

[3] “hi class: Horndeski in the Cosmic Linear
Anisotropy Solving System” M. Zumalacárregui, E. Bellini, I. Sawicki, J.
Lesgourgues and P. Ferreira arXiv:1605.06102 JCAP08 (2017) 019

[4] “Transforming gravity: from derivative couplings to matter to second-order scalar-tensor theories

beyond the Horndeski Lagrangian” M. Zumalacarregui, J. Garcia-Bellido, arXiv:1308.4685. Phys.Rev. D89 (2014) 064046

[5]
“Galileon Gravity in Light of ISW, CMB, BAO and H 0 data” J. Renk, M.
Zumalacarregui, F. Montanari and A. Barreira, arXiv:1707.02263 JCAP10
(2017) 020

[6] “Screening Modifications of Gravity through
Disformally Coupled Fields” T. Koivisto, D. Mota, and M. Zumalacarregui,
arXiv:1205.3167, Phys.Rev.Lett. 109 (2012) 241102.

[7] “Dark Energy
in light of Multi-Messenger Gravitational-Wave astronomy” J. M.
Ezquiaga, M. Zumalacárregui arXiv:1807.09241 Front. Astron. Space Sci.
(2018) 44

__All Publications__

Here are links to all my publications: from INSPIRE, from the ADS database and on the Arxiv.