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Multifield Inflation After Planck: Isocurvature Modes From Multifield Inflation after Planck: Isocurvature Modes from Nonminimal Couplings Katelin Schutz, Evangelos I. Sfakianakis and David I. Kaiser∗ Center for Theoretical Physics and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 USA (Dated: March 19, 2014) Recent measurements by the Planck experiment of the power spectrum of temperature anisotropies in the cosmic microwave background radiation (CMB) reveal a deficit of power in low multipoles compared to the predictions from best-fit ΛCDM cosmology. If the low-` anomaly persists after additional observations and analysis, it might be explained by the presence of primordial isocurvature perturbations in addition to the usual adiabatic spectrum, and hence may provide the first robust evidence that early-universe inflation involved more than one scalar field. In this paper we explore the production of isocurvature perturbations in nonminimally coupled two-field inflation. We find that this class of models readily produces enough power in the isocurvature modes to account for the Planck low-` anomaly, while also providing excellent agreement with the other Planck results. PACS numbers: 04.62+v; 98.80.Cq. Published in Physical Review D 89: 064044 (2014) I. INTRODUCTION field models, including the production and amplification of isocurvature modes during inflation [12{19]. Inflation is a leading cosmological paradigm for the Unlike adiabatic perturbations, which are fluctuations early universe, consistent with the myriad of observable in the energy density, isocurvature perturbations arise quantities that have been measured in the era of preci- from spatially varying fluctuations in the local equa- sion cosmology [1{3]. However, a persistent challenge has tion of state, or from relative velocities between vari- been to reconcile successful inflationary scenarios with ous species of matter. When isocurvature modes are well-motivated models of high-energy physics. Realistic produced primordially and stretched beyond the Hub- models of high-energy physics, such as those inspired by ble radius, causality prevents the redistribution of en- supersymmetry or string theory, routinely include mul- ergy density on super-horizon scales. When the pertur- tiple scalar fields at high energies [4]. Generically, each bations later cross back within the Hubble radius, isocur- scalar field should include a nonminimal coupling to the vature modes create pressure gradients that can push en- spacetime Ricci curvature scalar, since nonminimal cou- ergy density around, sourcing curvature perturbations plings arise as renormalization counterterms when quan- that contribute to large-scale anisotropies in the cos- tizing scalar fields in curved spacetime [5{8]. The non- mic microwave background radiation (CMB). (See, e.g., minimal couplings typically increase with energy-scale [11, 20].) under renormalization-group flow [7], and hence should The recent measurements of CMB anisotropies by be large at the energy-scales of interest for inflation. We Planck favor a combination of adiabatic and isocurvature therefore study a class of inflationary models that in- perturbations in order to improve the fit at low multi- cludes multiple scalar fields with large nonminimal cou- poles (` ∼ 20 − 40) compared to the predictions from the plings. simple, best-fit ΛCDM model in which primordial per- It is well known that the predicted perturbation spec- turbations are exclusively adiabatic. The best fit to the tra from single-field models with nonminimal couplings present data arises from models with a modest contribu- produce a close fit to observations. Following conformal tion from isocurvature modes, whose primordial power transformation to the Einstein frame, in which the gravi- spectrum PS (k) is either scale-invariant or slightly blue- tational portion of the action assumes canonical Einstein- tilted, while the dominant adiabatic contribution, PR(k), Hilbert form, the effective potential for the scalar field is is slightly red-tilted [11]. The low-` anomaly thus might stretched by the conformal factor to be concave rather arXiv:1310.8285v2 [astro-ph.CO] 18 Mar 2014 provide the first robust empirical evidence that early- than convex [9, 10], precisely the form of inflationary po- universe inflation involved more than one scalar field. tential most favored by the latest results from the Planck Well-known multifield models that produce isocurva- experiment [11]. ture perturbations, such as axion and curvaton models, The most pronounced difference between multifield in- are constrained by the Planck results and do not im- flation and single-field inflation is the presence of more prove the fit compared to the purely adiabatic ΛCDM than one type of primordial quantum fluctuation that can model [11]. As we demonstrate here, on the other hand, evolve and grow. The added degrees of freedom may lead the general class of multifield models with nonminimal to observable departures from the predictions of single- couplings can readily produce isocurvature perturbations of the sort that could account for the low-` anomaly in the Planck data, while also producing excellent agree- ment with the other spectral observables measured or ∗ [email protected], [email protected], [email protected] constrained by the Planck results, such as the spectral 2 index ns, the tensor-to-scalar ratio r, the running of the A. Einstein-Frame Potential spectral index α, and the amplitude of primordial non- Gaussianity fNL. We perform a conformal transformation to the Einstein Nonminimal couplings in multifield models induce a frame by rescaling the spacetime metric tensor, curved field-space manifold in the Einstein frame [21], 2 and hence one must employ a covariant formalism for g~µν (x) = Ω (x) gµν (x); (3) this class of models. Here we make use of the covariant formalism developed in [22], which builds on pioneering where the conformal factor Ω2(x) is related to the non- work in [13, 18]. In Section II we review the most rele- minimal coupling function via the relation vant features of our class of models, including the formal 2 machinery required to study the evolution of primordial 2 I Ω (x) = 2 f φ (x) : (4) isocurvature perturbations. In Section III we focus on a Mpl regime of parameter space that is promising in the light of the Planck data, and for which analytic approxmations This transformation yields the action in the Einstein are both tractable and in close agreement with numerical frame, simulations. In Section IV we compare the predictions Z "M 2 # from this class of models to the recent Planck findings. 4 p pl 1 µν I J I S = d x −g R − GIJ g @µφ @ν φ − V (φ ) ; Concluding remarks follow in Section V. 2 2 (5) where all the terms sans tilde are stretched by the confor- II. MODEL mal factor. For instance, the conformal transformation to the Einstein frame induces a nontrivial field-space metric [21] We consider two nonminimally coupled scalar fields φI fφ, χg. We work in 3+1 spacetime dimensions with 2 Mpl 3 the spacetime metric signature (−; +; +; +). We ex- GIJ = δIJ + f;I f;J ; (6) 2f f press our results in terms of the reduced Planck mass, −1=2 18 Mpl ≡ (8πG) = 2.43 × 10 GeV. Greek letters and the potential is also stretched so that it becomes (µ, ν) denote spacetime 4-vector indices, lower-case Ro- man letters (i, j) denote spacetime 3-vector indices, and M 4 V (φ, χ) = pl V~ (φ, χ) capital Roman letters (I, J) denote field-space indices. (2f)2 We indicate Jordan-frame quantities with a tilde, while (7) M 4 λ g λ Einstein-frame quantities will be sans tilde. Subscripted = pl φ φ4 + φ2χ2 + χ χ4 : commas indicate ordinary partial derivatives and sub- (2f)2 4 2 4 scripted semicolons denote covariant derivatives with re- spect to the spacetime coordinates. The form of the nonminimal coupling function is set by the requirements of renormalization [5, 6], We begin with the action in the Jordan frame, in which the fields’ nonminimal couplings remain explicit: 1 2 2 2 f(φ, χ) = [M + ξφφ + ξχχ ]; (8) Z 2 4 p I 1 µν I J I S~ = d x −g~ f(φ )R~ − G~IJ g~ @µφ @ν φ − V~ (φ ) ; 2 where ξφ and ξχ are dimensionless couplings and M is (1) some mass scale such that when the fields settle into their 2 where R~ is the spacetime Ricci scalar, f(φI ) is the non- vacuum expectation values, f ! Mpl=2. Here we assume minimal coupling function, and G~ is the Jordan-frame that any nonzero vacuum expectation values for φ and IJ χ are much smaller than the Planck scale, and hence we field space metric. We set G~IJ = δIJ , which gives canon- ical kinetic terms in the Jordan frame. We take the may take M = Mpl. Jordan-frame potential, V~ (φI ), to have a generic, renor- The conformal stretching of the potential in the Ein- malizable polynomial form with an interaction term: stein frame makes it concave and asymptotically flat along either direction in field space, I = φ, χ, λφ g λχ " !# V~ (φ, χ) = φ4 + φ2χ2 + χ4; (2) M 4 λ M 2 4 2 4 I pl I pl V (φ ) ! 2 1 + O I 2 (9) 4 ξI ξJ (φ ) with dimensionless coupling constants λI and g. As dis- cussed in [22], the inflationary dynamics in this class of (no sum on I). For non-symmetric couplings, in which models are relatively insensitive to the presence of mass λφ 6= λχ and/or ξφ 6= ξχ, the potential in the Einstein terms, m2 φ2 or m2 χ2, for realistic values of the masses frame will develop ridges and valleys, as shown in Fig. 1. φ χ p that satisfy mφ; mχ Mpl. Hence we will neglect such Crucially, V > 0 even in the valleys (for g > − λφλχ), terms here. and hence the system will inflate (albeit at varying rates) 3 1.0 s n 0.5 0.0 0.01 0.1 1 10 100 k FIG.
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