It was cosmology that drew Irene Sendra from Valencia to the Basque Country. Cosmology alsogave her the chance to collaborate with one of the winners of the 2011 Nobel Prize for Physics on one of the darkest areas of the universe. And dark matter and dark energy, well-known precisely because so little is known about them, are in fact the object of the study bySendra, a researcher in the Department of Theoretical Physics and History of Science of the UPV/EHU’s Faculty of Science and Technology.

“Observations tell us that about 5% of the universe is made up of ordinary matter; 22% corresponds to dark matter, which we know exists because it interacts gravitationally with ordinary matter; another 73% is dark energy, which is known to be there because otherwise one would not be able to account for the accelerating expansion of the universe,” explains Irene Sendra; “We are trying to find out a bit more about what dark energy is,” she adds.

If dark energy did not exist, the gravitational pull exerted by matter would slow down the expansion of the universe, but observations have concluded that the opposite is the case.Dark energy is what makes the universe expand in an accelerating way, and contributing towards understanding its nature is the basis of the research Sendra has done as part of her PhD thesis entitled: “Cosmology in an accelerating universe: observations and phenomenology”.

The research starts with the hypothesis that dark energy could be dynamic.The most widely accepted model, known as the Lambda-CDM, explains the acceleration of the universe by means of the cosmological constant, whose equation of state would have a value of -1, constant throughout the whole evolution of the universe.However, there are observations which this model cannot account for.“We look for a dynamic, dark energy that would vary over time; we apply various models to the observable data, we play around with small disturbances, and we see whether they adapt better than a constant," explains Sendra.

Making use of mathematical and statistical tools, the values that the observation proposes for the parameters studiedare compared with those proposed by the model.“So,throughmany iterations, we can see which values would take the constants of our model.The equation of state of dark energy is worth practically -1 now, but it appears to have evolved from different values in the past; however, there is still a high percentage of error in determining these values.”According to Sendra’s calculations, these data are consistent with dynamic dark energy, which would vary with the redshiftobserved in the universe.Results that have yet to be published and obtained in collaboration with Adam Riess, the 2011 Nobel Prize Winner for Physics, go further in that direction.

In this PhD thesis, besides studying the equation of state of dark energy, a new model has been proposed and it would unite dark energy with dark matter.As Sendra explains, “They could be the same thing that is manifested in a different way depending on the context; we have explained the effect of both of themthrough one single component, and the observations give better results in this model than in others that try to unite matter anddark energy.”

Finally, Sendra has peered at the oldest universe by means of the study of its cosmic microwave background.“It is the most distant proof we have of the universe,” she comments, “and the study of it tells us that the actual number of neutrinos is higher than three.Nevertheless, what we know for a fact, through the standard model of particles, is that there are three kinds of neutrinos.We have ended up with a somewhat strange value, so we are trying to account for that excess in the number of neutrinos”. Sendra's proposal is heading in the direction of the string theory. According to her results, this neutrino excess could be interpreted as the contribution of primordial gravitational waves, caused by the interaction of cosmic strings at the time when the cosmic microwave background was produced.

#### References

DBI models for the unification of dark matter and dark energy. L. P. Chimento, R. Lazkoz, I. Sendra, Gen. Rel. Grav. 42 (2010) 1189-1209.

Oscillations in the dark energy EoS: new MCMC lessons. R. Lazkoz, V. Salzano, I. Sendra, Phys. Lett. B694 (2010) 198-208.

SN and BAO constraints on (new) polynomial dark energy parametrizations: current results and forecasts. Sendra and R. Lazkoz, Mont. Not. Roy. Astron. Soc. 422 (2012) 776-793.

Improved limits on short-wavelength gravitational waves from the cosmic microwave background. I. Sendra and T.Smith, Phys.Rev. D85 (2012) 123002.