Primordial Gravitational Waves Sirichai Chongchitnan (with George Efstathiou) Institute of Astronomy University of Cambridge 25th September 2006 Is it worth the search? and Black Holes:

Questions Does inflation predict directly detectable gravitational waves? What will we learn about inflation? e.g. Energy scale Shape of potential Small-scale physics … Is it worth all that ?

Primordial Gravitational Waves Energy spectrum Inflation predicts a *small* stochastic background of GW, but how small?

The search for tensor modes Planck, BICEP, CLOVER, QUIET etc. WMAP3 + SDSS : r < 0.2 (95%) at k = Mpc -1 [Peiris, Easther]

Direct detections Joseph Weber 1969 Nautilus (Rome)

What does inflation predict? Modelrω gw ∼ 0.1 ∼ ∼ 0.2 ∼ Hybrid ∼0∼0 ∼0∼0 But V(Φ) is unknown!

Hamilton-Jacobi Formalism ✱ Dynamics well-understood [Chongchitnan and Efstathiou PRD’05] Specify initial Taylor coefficients of H(Φ) Evolve “flow equations” till inflation ends ≡ a potential V) ≡ a potential V(Φ) ✱ H(Φ) determines ω gw directly [Hoffman&Turner’01] [Kinney’02]

Generic constraint on ω gw for all models of single field inflation [Chongchitnan and Efstathiou PRD’06] 0.92 ≲ n s ≲ ≲ ≲ 0.03 At f ≃ Hz, [Viel et al’04],[Seljak’05]

High ω gw models Directly detectable GW ⇔ “pathological” potential (r= few x )

Energy scale of inflation Future GW experiments will probe a tiny range of energy scales e.g., r ≃ , V 1/4 ≃ GeV [Efstathiou and Chongchitnan Prog.Theor.Phys.’06]

Primordial Black Holes Formed by great overdensities during inflation Slow-roll underestimate PBH abundance. Precise calculations needed “Absence” of PBH rules out very pathological models. [Chongchitnan and Efstathiou, astroph/0610…]

Conclusions Large scales, Planck/CLOVER can rule out large-field (chaotic) inflation models Small scales, BBO/DECIGO will improve limit of energy scale of inflation by only a few. Will test only pathological models. Worth looking at other signatures of inflation : Non-gaussianity from multi- field inflation or distinctive signatures of cosmic strings from brane inflation.