The low-energy dynamics of a generic self-gravitating media can be studied by using effective field theory in terms four derivatively coupled scalar fields and naturally gives rise to an interesting model of dark energy. Depending on internal symmetries, the theory describes fluids, superfluids, solid and supersolids. Dynamical and thermodynamical properties are also dictated by internal...

The physical reason for the observed acceleration of the Universe is one of the most important mysteries in cosmology. This is also one motivation for the next generation of large galaxy surveys like Euclid, LSST or SKA that will observe billions of galaxies to provide galaxy number counts and weak lensing measurements.

In the first part of my talk, I'm going to show a systematic extension of...

Perturbations on a static and spherically symmetric solution in the Horndeski theory were studied by previous researches. Their radial stability conditions were calculated but angular stability condition of the even-parity mode, which has more complicated form of perturbative equations of motions than the odd-parity mode, was not obtained. We have calculated it with high multipole limit and...

In my talk, I will present the impact of general, model independent conditions of theoretical stability and cosmological viability on the analysis of scalar-tensor theories with cosmological data. These conditions account for the avoidance of ghost and gradient instabilities as well as exponential growth of the scalar perturbations in the Dark Energy sector.

As an example, I will show the...

There is a well known degeneracy between the enhancement of the growth of large-scale structure produced by modified gravity models and the suppression due to the free-streaming of massive neutrinos at late times. This makes the matter power-spectrum alone a poor probe to distinguish between modified gravity and the concordance ΛCDM model when neutrino masses are not strongly constrained....

We present a complete analysis of the observational constraints and cosmological implications of our Bound Dark Energy (BDE) model aimed to explain the late-time cosmic acceleration of the universe. BDE is derived from particle physics and corresponds to the lightest meson field $\phi$ dynamically formed at low energies due to the strong gauge coupling constant. The evolution of BDE is...

We discuss the embedding of the Horndeski model into supergravity. In the case of linearly realized supersymmetry, higher derivative interaction often leads to ghost and propagating auxiliary field problems. Therefore, supergravity realization of the Horndeski model has not been known so far.

These issues can be circumvented in the recently proposed framework, called pure de Sitter...

We discuss the ability of a dark fluid becoming relevant around the time of matter radiation equality to significantly relieve the tension between local measurements of the Hubble constant and CMB inference, within the $\Lambda$CDM model.

We show the gravitational impact of acoustic oscillations in the dark fluid balance the effects on the CMB and result in an improved fit to CMB measurements...

In this talk I present a novel model of a unified dark sector, where late-time cosmic acceleration emerges from the dark matter superfluid framework. We will start by reviewing the dark matter superfluid model and show how it describes the dynamics of dark matter in large and small scales. Then we will show that if the superfluid consists of a mixture of two distinguishable states with a small...

I will present current observational bounds on general Horndeski scalar-tensor theories of gravity, using data from the Planck, SDSS/BOSS and 6dF surveys. Using such theories as an example, I will also show how combining these observational bounds with insights from theoretical particle physics (e.g. stability criteria and positivity bounds) can drastically improve constraints and therefore...

In this talk I will discuss a scalar field model of dark energy that exhibits essentially no sensitivity to initial conditions and possesses a naturally suppressed effective mass and interactions in the late Universe. The magnitude of dark energy today is generated via an intricate conspiracy of numbers related to inflation, gravity and electroweak physics. arXiv:1905.00045

We consider a subclass of degenerate higher-order scalar-tensor (DHOST) theories in which gravitational waves propagate at the speed of light and do not decay into scalar fluctuations. The screening mechanism in DHOST theories evading these two gravitational wave constraints operates very differently from that in generic DHOST theories. We derive a spherically symmetric solution in the...

Astrophysical tests of the stability of fundamental couplings such as the fine-structure constant $\alpha$ and the proton-to-electron mass ratio $\mu$ are a key probe of fundamental physics and cosmology. A new generation of high-resolution spectrographs and improved statistical analysis techniques are enabling tests with improved sensitivities and larger redshift ranges. I will present new...

In this talk, I will discuss the decay of gravitational waves (GWs) into dark energy fluctuations $\gamma \rightarrow \pi\pi$ taking into account the large occupation numbers. We study the decay due to the $m_3^3$- and $\tilde{m}_4^2$-operators in the context of the effective field theory (EFT) of dark energy. It turns out that, in the regime of small GW amplitude corresponding to narrow...

In this talk I will discuss the classical decay of gravitational waves into dark energy fluctuations $\pi$ in the context of the EFT of Dark Energy. For cubic Horndeski and beyond Horndeski theories, the gravitational wave acts as a classical background for $\pi$ and thus modifies its dynamics. In particular, for a sufficiently large amplitude of the wave, the kinetic term of $\pi$ becomes...

Although quadratic curvature terms are well-motivated by leading quantum corrections to gravity and can be responsible for inflation, they generally lead to a massive spin-2 ghost. In this talk, instead of Riemannian geometry, we study quadratic curvature theories in four-dimensional Riemann-Cartan geometry where the torsion tensor does not vanish and can carry new degrees of freedom....