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## Quantum Spacetime and Cosmic Inflation

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**Quantum Spacetime and Cosmic Inflation**Jerome Martin CNRS/Institut d’Astrophysique de Paris Probing Quantum Spacetime with Astrophysical Sources: the CTA era and beyond LPNHE, 29 and 30 November 2017**The talk**Outline • The theory of cosmic inflation in brief: basic principles • Cosmological fluctuations of quantum-mechanical origin • Quantum Mechanics in the sky? Opens issues • Conclusions**The talk**Outline • The theory of cosmic inflation in brief: basic principles • Cosmological fluctuations of quantum-mechanical origin • Quantum Mechanics in the sky? Opens issues • Conclusions**Lambda/CDM=Four epochs**Inflation is a phase of accelerated expansion taking place in the very early Universe. It solves the fine tuning puzzles of the standard model of cosmology Dark energy dominated era Radiation dominated era Matter dominated era Scale factor: a(t)/a(tini)**Inflation in brief**Inflation is (usually) realized with one (or many) scalar field(s) If the scalar field moves slowly (the potential is flat), then pressure is negative which, in the context of GR, means accelerated expansion and, hence, inflation takes place.**Inflation in brief**Inflation (usually) stops when the field reaches a part of the potential which is no longer flat enough to support inflation; this happens in the vicinity of the minimum of the potential The field oscillates, decays and the decay products thermalize …Then the radiation dominated era starts …**The talk**Outline • The theory of cosmic inflation in brief: basic principles • Cosmological fluctuations of quantum-mechanical origin • Quantum Mechanics in the sky? Opens issues • Conclusions**Structure formation**In order to have a convincing scenario for structure formation, we need to answer two questions**Structure formation**In order to have a convincing scenario for structure formation, we need to answer two questions How do they grow? Gravitational instability**Structure formation**In order to have a convincing scenario for structure formation, we need to answer two questions What is their origin?**Structure formation**In order to have a convincing scenario for structure formation, we need to answer two questions What is their origin? Quantum fluctuations during inflation According to inflation, this is the quantum spacetime which is at the origin of CMB anisotropies and large scale structures For a review, see J. Martin, Lect. Notes Phys. 669 (2005), 199, hep-th/040611**Perturbations of quantum-mechanical origin**If inflation is correct … … These are probes of quantum spacetime**Perturbations of quantum-mechanical origin**1- In the early Universe, the fluctuations are small and, hence, a linear approach is possible**Perturbations of quantum-mechanical origin**1- In the early Universe, the fluctuations are small and, hence, a linear approach is possible 2- The small fluctuations on top of an expanding spacetime are of two types: a- scalar characterized by curvature perturbations v(t,x) and b- tensor (or primordial gravitational waves)**Perturbations of quantum-mechanical origin**1- In the early Universe, the fluctuations are small and, hence, a linear approach is possible 2- The small fluctuations on top of an expanding spacetime are of two types: a- scalar characterized by curvature perturbations v(t,x) and b- tensor (or primordial gravitational waves) 3- The evolution of the quantum perturbations is controlled by the Hamiltonian**Perturbations of quantum-mechanical origin**1- In the early Universe, the fluctuations are small and, hence, a linear approach is possible 2- The small fluctuations on top of an expanding spacetime are of two types: a- scalar characterized by curvature perturbations v(t,x) and b- tensor (or primordial gravitational waves) 3- The evolution of the quantum perturbations is controlled by the Hamiltonian Free term**Perturbations of quantum-mechanical origin**1- In the early Universe, the fluctuations are small and, hence, a linear approach is possible 2- The small fluctuations on top of an expanding spacetime are of two types: a- scalar characterized by curvature perturbations v(t,x) and b- tensor (or primordial gravitational waves) 3- The evolution of the quantum perturbations is controlled by the Hamiltonian Free term Interaction term between the quantum fluctuations and the classical background**Perturbations of quantum-mechanical origin**1- In the early Universe, the fluctuations are small and, hence, a linear approach is possible 2- The small fluctuations on top of an expanding spacetime are of two types: a- scalar characterized by curvature perturbations v(t,x) and b- tensor (or primordial gravitational waves) 3- The evolution of the quantum perturbations is controlled by the Hamiltonian Free term Interaction term between the quantum fluctuations and the classical background 1- Pump field ~ time dependent coupling constant. 2- Only depends on the scale factor and its derivatives. 3- Vanishes if a’=0**Perturbations of quantum-mechanical origin**1- In the early Universe, the fluctuations are small and, hence, a linear approach is possible 2- The small fluctuations on top of an expanding spacetime are of two types: a- scalar characterized by curvature perturbations v(t,x) and b- tensor (or primordial gravitational waves) 3- The evolution of the quantum perturbations is controlled by the Hamiltonian Free term Interaction term between the quantum fluctuations and the classical background Corresponds to creation of pair of particles k -k**Perturbations of quantum-mechanical origin**This is similar to the Schwinger effect: interaction of a quantum field with a classical source J. Martin, Lect. Notes Phys. 738 (2008), 195 arXiv:0704.3540 Inflationary cosmological perturbations Schwinger effect - Electron and positron fields - Inhomogeneous gravity field - Background gravitational field: scale factor - Classical electric field • Amplitude controlled by the Hubble parameter H • Amplitude of the effect controlled by E See also dynamical Schwinger effect, dynamical Casimir effect etc …**Perturbations of quantum-mechanical origin**Under the previous Hamiltonian, Schroedinger evolution gives Two mode squeezed states**Perturbations of quantum-mechanical origin**Under the previous Hamiltonian, Schroedinger evolution gives Two mode squeezed states • - Squeezed states are very well-known in other field of Physics, e.g. Quantum Optics -”Classical” coherent state: equal dispersion on x and p - Squeezed state: not the same dispersion (rsqueezing parameter)**Perturbations of quantum-mechanical origin**Under the previous Hamiltonian, Schroedinger evolution gives Two mode squeezed states • - Squeezed states are very well-known in other field of Physics, e.g. Quantum Optics • - This is the strongest squeezed state ever produced in nature, r=O(100)!**Perturbations of quantum-mechanical origin**Under the previous Hamiltonian, Schroedinger evolution gives Two mode squeezed states - Squeezed states are very well-known in other field of Physics, e.g. Quantum Optics - This is the strongest squeezed state ever produced in nature, r=O(100)! - A squeezed state is an entangled state. In the limit of very strong squeezing, it is equivalent to an Einstein Podolsky Rosen (EPR) state (~ r → ∞)**The talk**Outline • The theory of cosmic inflation in brief: basic principles • Cosmological fluctuations of quantum-mechanical origin • Quantum Mechanics in the sky? Opens issues • Conclusions**Open issues**The fact that the state of the perturbations is “very quantum” raises some issues and hopes with regards to the quantum-mechanical nature of spacetime • Quantum to classical transition of the perturbations • What is the importance of decoherence? (D.Polarski, A Starobinsky, CQG 13 1996) • Reduction of the wave packet: should we go beyond Copenhagen? • Unambiguous observational signature of the quantum origin of the galaxies? Can we see quantum correlations in the sky? (J. Maldacena, Fortsch. Phys. 64 2016; J. Martin & V.Vennin, PRD96 2017 063501, PRA94 2016 052135) time**The talk**Outline • The theory of cosmic inflation in brief: basic principles • Cosmological fluctuations of quantum-mechanical origin • Quantum Mechanics in the sky? Opens issues • Conclusions**Conclusions**Recap • According to cosmic inflation, the CMB anisotropies and the galaxies are a result of quantum spacetime. Planck data is a way to probe quantum spacetime! • Inflation is the only situation in Physics where GR & QM are used and where, at the same time, we have high accuracy data • Inflation pushes QM in unexplored regimes, eg high energies, very large • length scales, QM without observers etc … • Take away message: inflation is not only a successful scenario of the early Universe, it is also a very interesting playground for foundational issues of quantum mechanics and (maybe) quantum gravity