# Théorie

# Observations with LHAASO-1/2 and the Construction Status

**This is a joint seminar with the Institute of High Energy Physics in Beijing.**

# Composite and holographic axions

The idea that low energy fields such as Gravitons or Axions can be thought of as composite particles has been entertained since a long time. We revisit this idea motivated by the AdS/CFT correspondence. We focus in the case of axions and show that hidden sectors coupled to SM fields may provide an emergent axionic field that is a composite of the hidden fields (instanton density). This is a more general phenomenon beyond holographic theories. We study the general properties of such an "emergent axion", without a PQ symmetry. The construction goes beyond

# Euclidean Wormholes and Holography

# Inflation and Geometry

# Dark energy after gravitational wave observations

The observed accelerated expansion of the Universe opens up

the possibility that general relativity is modified on cosmological

scales. While this has motivated the theoretical study of many

alternative theories that will be tested by the next generation of

cosmic large-scale structure surveys, I will show that the recent

observations of gravitational waves by LIGO/Virgo has dramatic

consequences on these theories.

# Horndeski and the Sirens

*standard sirens*, by analogy with electromagnetic standard candles, because their waveform directly gives access to their distance. When an electromagnetic counterpart is observed, such sources thus allow us to construct a

*Hubble diagram*, just as supernovae. Recently, the gravitational-wave Hubble diagram has been argued to be a key probe of alternative theories of gravity, such as

*Horndeski models*.

# BMS flux-balance laws

# Vacuum stability without Supersymmetry: Brane dynamics, bubbles and holography

n this talk I will present some recent results on instabilities of anti-de Sitter flux compactifications in effective field theories arising from non-supersymmetric string models, namely the USp(32) and U(32) orientifold models and the SO(16) x SO(16) heterotic string. We focus on non-perturbative instabilities, framing the vacua in terms of brane stacks, analyzing their back-reacted geometry and reproducing AdS its near-horizon limit.