BRANE-WORLD COSMOLOGY Porto 2004University of Portsmouth Roy Maartens Revent reviews: Brax, Davis, vd Bruck, hep-th/ RM, gr-qc/

standard cosmology highly successful standard cosmology highly successful concordance model = excellent fit but – still a paradigm seeking a fundamental theory but – still a paradigm seeking a fundamental theory deep puzzles - inflation, deep puzzles - inflation, dark energy, dark energy, dark matter, … dark matter, … more to come more to come

GR breaks down GR breaks down need quantum gravity in the early universe high-precision cosmology is probing the limits of GR no QG theory as yet no QG theory as yet but M theory is a promising candidate M theory needs extra dimensions + branes M theory needs extra dimensions + branes can also lower the Planck scale can also lower the Planck scale explain ‘weakness’ of gravity explain ‘weakness’ of gravity

why don’t we see the extra dimensions? conventional Kaluza-Klein idea: conventional Kaluza-Klein idea: internal extra dimension too small to be seen internal extra dimension too small to be seen discovery of D-brane discovery of D-brane matter fields restricted matter fields restricted to lower dimensional brane to lower dimensional brane external bulk felt only external bulk felt only through gravity through gravity extra dimension bigger extra dimension bigger 4D spacetime small extra dimension large extra dimension gravity

small extra dimensions = moduli fields m A (t,x) large extra dimension = radion R(t,x) visible brane shadow brane THE BIG IDEA: M THEORY WILL SOLVE PUZZLES * dilaton/radion = natural source of brane inflation? * more radical: no inflation - brane-brane collision = brane-brane collision = big bang (cyclic)? big bang (cyclic)? * moduli = varying constants? * shadow matter = dark matter? dark matter? dark energy? dark energy? dilaton D(t,x)

GR phenomenology QG the harsh reality … the harsh reality … still no string/ M theory cosmology still no string/ M theory cosmology even inflation/ dark energy hard to get even inflation/ dark energy hard to get 2 parallel approaches 2 parallel approaches * hunt for string/ M theory cosmology * hunt for string/ M theory cosmology * use braneworld phenomenology * use braneworld phenomenology braneworld cosmology braneworld cosmology use M theory ideas in cosmology use M theory ideas in cosmology use high-precision data to test & constrain models use high-precision data to test & constrain models

2 key features 2 key features braneworld gravity brings new features braneworld gravity brings new features KK modes of the graviton KK modes of the graviton new fields (moduli, dilaton,…) new fields (moduli, dilaton,…) shadow matter shadow matter holography, … holography, … precision data constrains extra-dimensional gravity precision data constrains extra-dimensional gravity dynamics – BBN, SNe dynamics – BBN, SNe perturbations – CMB and LSS perturbations – CMB and LSS

start with simple models start with simple models similar to inflation and dark energy models similar to inflation and dark energy models simplest models with predictive power simplest models with predictive power focus on one or two key aspects at a time focus on one or two key aspects at a time KK modes of graviton KK modes of graviton scalar fields – radion, dilaton, moduli scalar fields – radion, dilaton, moduli …. …. Randall-Sundrum braneworlds 5D models 5D models gravity itself is central gravity itself is central – warped extra dimension – warped extra dimension

stringy/quantum curvature corrections Gauss-Bonnet (early universe) induced gravity (late universe)