Introductory Review of Cosmic Inflation

Introductory Review of Cosmic Inflation Shinji Tsujikawa (辻川信二 ) hep-ph/0304257 Research Center for the Early Universe, University of Tokyo Yue Zou, Tsinghua University

Content • Ingredients for standard big-bang cosmology • What is inflation • Problems of the standard big-bang cosmology • Introducing the scalar field • Scalar field dynamics for inflation • Density perturbation and the origin of large scale structure • Reheating after inflation Yue Zou, Tsinghua University

Ingredients for Standard big-bang cosmology • Our tool: General Relativity • Based on the cosmological principle • Metric: Friedmann-Robertson-Walker metric • Energy-momentum tensor for perfect fluid: Yue Zou, Tsinghua University

Ingredients for Standard big-bang cosmology • Then the Einstein equations yield is the Planck energy hereafter =c=1 • The equation of state for radiation p=0 for matter • When K=0 (flat infinite universe), the solution for radiation dominant:matter dominant: • In these simple cases the universe is expanding deceleratedly. Scalar field Horizon Monopole Yue Zou, Tsinghua University

Ingredients for Standard big-bang cosmology • Introducing Hubble parameter and critical density we can rewrite the Friedmann equation as • And we can further define , thus we have Flatness Problem Yue Zou, Tsinghua University

What is “inflation”? • Inflation means a stage in the early universe, with accelerated expansion • It is also equivalent to r+3p<0 : negative pressure! • It is also equivalent to where the Hubble parameter • Without inflation, the standard big-bang cosmology would suffer from several severe problems. A.H.Guth first noted that introducing inflation would provide an efficient solution to these problems (A.H.Guth, Phys. Rev. D 23, 347, 1981). Yue Zou, Tsinghua University

Problems of the standard big-bang cosmology - Flatness problem • Friedmann equation can be rewritten aswhere . In standard big-bang, for either radiation or matter dominant, always decreases. • is unstable: it tends to shift away from unity with the expansion of the universe • WMAP: , very close to one. • We require . This is an extremely fine-tuning of initial conditions. Yue Zou, Tsinghua University

Problems of the standard big-bang cosmology - Flateness problem • With inflation: since term increases during inflation, W rapidly approaches unity. As long as the inflationary expansion is sufficient, W stays of order unity even in the present epoch. Yue Zou, Tsinghua University

Problems of the standard big-bang cosmology - Horizon problem • Particle horizon: is the largest distance that can have casual contact at time t • Radiation dominant: matter dominant:therefore the horizon is much smaller in the past. • In fact the causally contacted surface of the last scattering surface only corresponds physical scale to the angle of order • Observationally, however, we see photons which thermalize to the same temperature horizon in all regions in the CMB sky. Yue Zou, Tsinghua University

Problems of the standard big-bang cosmology - Horizon problem • If there is an inflation period in the early universe, the scale factor a(t) would grow drastically, while the particle horizon would nearly stay unchanged. Then before inflation, the scale could be much smaller than horizon. • Therefore the isotropy in the CMB spectrum and the large scale structure can be solved. Yue Zou, Tsinghua University

Problems of the standard big-bang cosmology - Horizon problem • Another form of the Horizon problem: the origin of large scale structure. • Comoving wavelength: l. Physical wave lenghth: al • Perturbation scale larger than horizon can not be amplified and therefore can not form structure. • Larger scale perturbations enter the horizon later, and has less time to evolve, to form structure. Yue Zou, Tsinghua University

Problems of the standard big-bang cosmology - Horizon problem • Therefore it is practically impossible to generate a scale-invariant perturbation spectrum between the big bang and the time of the last scattering in the standard big-bang cosmology • COBE and WMAP have seen nearly scale-invariant perturbation spectrum . • WMAP: Yue Zou, Tsinghua University

Problems of the standard big-bang cosmology - Horizon problem • If there is inflation… • Early stage of inflation, the scale of perturbations is smaller than horizon, which form the seeds of large scale structure. • Perturbations grow out of horizon Scale of perturbation and are frozen. • After inflation: standard big-bang stage. The perturbations enter horizonhorizon again. Then the frozen perturbations continue to evolve into structure. spectrum Yue Zou, Tsinghua University

More about the origin of the large scale structure • Hubble radius: 1/H~t. Comoving Hubble length: 1/aH. • Hubble radius (Hubble length) can be a good estimator of the particle horizon, both being ~ t • Later we shall not distinguish ‘horizon’ and ‘Hubble length’ • Standard big-bang cosmology: aH always decreases, then comoving Hubble length increases all the time. • Inflation: aH increases, comoving Hubble length decreases. Yue Zou, Tsinghua University

More about the origin of the large scale structure • Early stage of inflation: l<1/aH, causality works to generate small quantum fluctuations, which form the seeds of large scale structure. • Then l>1/aH, perturbations are frozen • After inflation: standard big-bang stage. 1/aH increases. l<1/aH again, then causality works again. Then the frozen perturbations continue to evolve into structure. • The small perturbation imprinted during inflation appears as large-scale perturbations after this ‘the second horizon crossing’. Yue Zou, Tsinghua University

Problems of the standard big-bangcosmology-- Monopole problem • According to the view of particle physics, the breaking of supersymmetry (SUSY) leads to the production of many unwanted relics such as monopoles, cosmic strings, and topological defects. • String theory: gravitinos, Kaluza-Klein particle, etc. • Their energy density decrease as a matter component, much slower than radiation energy density. In radiation-dominant era, these massive relics could be the dominant materials in the universe, which contradicts with observations. Yue Zou, Tsinghua University

Problems of the standard big-bangcosmology-- Monopole problem • If there is inflation… • Provided that these unwanted relics are produced before inflation, their energy densities would decrease drastically with the fast increase of scale factor ‘a(t)’. Thus unwanted relics can be red-shifted away. • We still have to worry about those relics produced after inflation. Generally if the reheating temperature is sufficiently low ,the thermal production of unwanted relics, such as gravitinos, can be avoided. Yue Zou, Tsinghua University

Scalar fields in particle physics and cosmology • To obtain inflation, we need materials with the unusual property of a negative pressure. • It is normally imagined that inflation begins at the Planck scale. Therefore we have to seek for a quantum theory to describe the materials for inflation: scalar field (spin-0) • Because of Planck scale, it is most suitable to adopt a quantum theory of gravity to describe inflation. • Unfortunately this theory has not come yet. • Our approach is a semi-classical one: we do not quantize the gravity field. Quantum Field Theory + Classical background Yue Zou, Tsinghua University

Scalar fields in particle physics and cosmology • As yet, there has been no direct observation of a fundamental scalar particle (such as Higgs), but they play a crucial role in particle physics theory in bring about mass through spontaneous symmetry broken (SSB). • Earlier inflationary models simply use the Higgs field for the Grand Unified Theory (GUT) such as SU(5) and first order transitions. • However they can not meet the requirements of cosmology. Yue Zou, Tsinghua University

Scalar fields in particle physics and cosmology • In inflationary cosmology scalar fields are introduced in a more phenomenological way. • Anyway, we look for guidance about the likely form of the scalar field potential in particle theory, hoping that in the end, it will belong to a complete ‘Theory of Everything (TOE)’. • Recent trend is to construct inflationary models based on superstring or supergravity models. • The scalar field responsible for inflation is often called ‘inflaton’. Yue Zou, Tsinghua University

Scalar field dynamics • The standard way to specify a particle theory is via its lagrangian. The lagrangian of a single scalar field with potential V is • Then the energy-momentum tensor can be written as • Assuming the f is spatially homogeneous, or noting the fact that spatially gradient terms ~ , the energy-momentum tensor take the form of a perfect fluid with Yue Zou, Tsinghua University

Scalar field dynamics • Substituting the expression of r and p into the basic two equations, we have • During inflation we require r+3p<0, which yields . Therefore a flat potential is required. • Once inflation gets under way, then the curvature term in the Friedmann equation becomes less and less important. Normally it is assumed negligible from the start. • Different inflationary models give different potentials. Chaotic Yue Zou, Tsinghua University

Slow-roll approximation spectrum • The standard technique for analyzing inflation is the slow-roll approximation: • Defining the so-called slow roll parameterone can verify that slow-roll approximation are valid whene<<1, |h|<<1 • eand hare functions of V, therefore it is easy to see where inflation might occur. Inflation ends when eand hgrow >~1 Yue Zou, Tsinghua University

Relation between inflation and slow-roll • Slow-roll approximation is a sufficient condition for inflation. This can be qualitatively seen in the first approximated equation of motion. • Another way to see this explicitly: slow-roll requiresconsequently • One the other hand we havethe definition of inflation is recovered. Yue Zou, Tsinghua University

Amount of inflation • We need sufficient amount of inflation to solve the flatness problem, horizon problem, ets. • Usually we define • To solve the flatness problem, we require N>~70 • Similar value of N is required to solve the horizon problem. Yue Zou, Tsinghua University

A simple example – Chaotic inflation • This model is described by the quadratic of quartic potential • Substituting the form of V into previous equations, we have • We have a exponentially expanding solution: Yue Zou, Tsinghua University

A simple example – Chaotic inflation • The slow-roll parameter reads therefore the inflationary period ends around ,after which the universe enters a reheating stage. • The total amount of inflation is approximately • In order to lead to sufficient inflation N>~70, we require the initial value to be Yue Zou, Tsinghua University

A simple example – Chaotic inflation • Detailed analysis should have more dependence on Quantum Field theory (QFT). • The detailed form of the inflation potential V should be corrected by loop correction in perturbation theory and renormalization theory. • The inflaton mass m can have dependence on f, whose form can be calculated from the Renormalization Group Equation (RGE). • Another important type of inflationary model is using multi-field, such as hybrid models of A.D.Linde (Phys. Lett. B, 259, 38, 1991) Yue Zou, Tsinghua University

Basic picture of density perturbation and the origin of large scale structure • At early stage of inflation, vacuum fluctuation of the inflaton field is generated. – Quantum Field Theory • After the first horizon crossing, the fluctuation grows as classical one, which forms the origin of large scale structure • After the second horizon crossing, the fluctuation evolve fully classically, which forms today’s universe. • Here I shall briefly show the first two stages: how quantum fluctuation forms the origin of large scale structure. • Inflationary models have most predictive power in this aspect. Therefore the observation can kill many inflationary models through observing the large scale structure. Yue Zou, Tsinghua University

Vacuum fluctuation in quantized scalar field • The lagrangian of a scalar field in arbitrary spacetime (metric) is • The field equation can be obtained by • For a flat spacetime, we adopt the Lorentz metric, then we have • For non-interacting, or freefield, we havethen the field equation is , namely the Klein-Gordon equation Yue Zou, Tsinghua University

Vacuum fluctuation in quantized scalar field • Inflaton: we need the Robertson-Walker metric instead of the Lorentz metric. • Then the field equation for the inflaton issplit the field into an unperturbed part and a perturbation: f(x,t)=f(t)+df(x,t) • and given a Fourier component, we have Yue Zou, Tsinghua University

Vacuum fluctuation in quantized scalar field • After canonical quantization for , we have • where and are the annihilation and creation operator for an inflation with momentum k, respectively. • satisfies • Because annihilation operator annihilates the vacuum, we have the form of the vacuum fluctuation,which is purely a quantum effect. Yue Zou, Tsinghua University

Vacuum fluctuation in quantized scalar field • From slow-roll approximation, we can deduce that H varies very slowly during inflation. • Then we can seek a solution ignoring the variation of H: • Here L is the comoving box size for normalization. And k is the wavenumber. Remembering that 1/k and 1/aH means the comoving wavelength and the comoving Hubble length respectively, the epoch k=aH just means the crossing of horizon. Yue Zou, Tsinghua University

The spectrum of perturbation • A few Hubble times after horizon-crossing we have k<<aH therefore • The spectrum of density perturbation can be defined as • The spectrum of primordial curvature perturbation is given by • Because we are dealing with slow-roll inflation, H varies very slowly, the above expressions can be evaluated at the epoch of horizon exit k=aH Yue Zou, Tsinghua University

The spectrum of perturbation • Using slow-roll approximation, we can express as where e is the slow-roll parameter. • Now we can define the ‘effective spectral index’ n(k) asthis is equivalent to the power-law behavior that is assumed when defining the spectral index in the normal way, Yue Zou, Tsinghua University

The spectrum of perturbation • A simple calculation evaluated at k=aH can show that • Because slow-roll requires e<<1 and h<<1, we draw an important conclusion:inflation predicts the spectrum is close to scale-invariant. • WMAP: strong support for the inflation scenario. Yue Zou, Tsinghua University

The spectrum of perturbation • Similarly we have • Different models have different e and h, therefore sufficiently precise measurement of the spectrum, n(k) and would discriminate different models. • WMAP: Yue Zou, Tsinghua University

Reheating: Recovering the Hot Big Bang • Reheating: the period of inflationary expansion gives way to the standard Hot Big Bang evolution • Typically reheating would have little impact on the predictions on density perturbation from the inflationary scenario. • However, reheating is crucial to our understanding of: whether baryogenesis can be brought about successfully; whether gravitinos might be over produced;whether topological defects can be produced after inflation. Yue Zou, Tsinghua University

Reheating: Recovering the Hot Big Bang • There are typically three periods of the reheating process1. non-inflationary scalar field dynamics,2. decay of inflation particles,3. thermalization of decay product. • The theory of the second stage has recently gained important developments, which led to a significant change of view since books such as Kolb and Turner’s ‘The Early Universe’ (1990). Yue Zou, Tsinghua University

Reheating: Recovering the Hot Big Bang • Once inflation is over, slow-roll approximation is no longer valid. Recalling the general equation of motion for inflaton f: • The scalar field begins to oscillate about the minimum of the potential. • Then the equation of motion can be rewritten as the equation for the time-average energy density : Yue Zou, Tsinghua University

Reheating: Recovering the Hot Big Bang • If the particle decay is slow (e.g. if the only decay channels are into fermions), one can insert a phenomenological term directly into the above equation: such an equation can be used to describe the coherent oscillation of inflaton, slowly producing fermions • Recently it was found that the inflaton may decay into bosonic particles, allowing a decay by parametric resonance. This permits an extremely rapid decay of the inflaton particles. • This dramatically rapid decay has been termed preheating to distinguish it from the old scenario of reheating, which is now believed to happen later than preheating. Yue Zou, Tsinghua University

Reheating: Recovering the Hot Big Bang • The occupation number generated by parametric resonance (preheating) are huge, so that bosons created are far from thermal equilibrium. • Fermions and the Pauli exclusion principle. • Decay and thermalization: the bosonic particles produced in preheating should decay, interact, and finally reach thermal equilibrium. The details will be strongly dependent on the field theory adopted. • After reheating the universe is on its way of standard big-bang cosmology again. Yue Zou, Tsinghua University

Summary and discussion • Success of inflation: it solves a number of cosmological problems such as flatness, horizon, and monopole problems, and at the same time it generates the seed for nearly scale-invariant large scale structure. • Cosmological scenarios alternative to inflation:pre-big-bang (M.Gasperini and G.Veneziano, Astropart.Phys. 1, 317, 1993)and cyclic model (P.J.Steinhardt and N.Turok, Phys.Rev.D, 65, 126003, 2002) • Problem: the origin of inflaton? What is the state of the universe before inflation? • Future: high-precision observation is expected to reveal the detailed nature of inflation; from theoretical side extensive works are consctructing viable models based on string and supergravity theories. Yue Zou, Tsinghua University

Recommendation for references • J.V.Narlikar and T.Padmanabhan, Gravity, Gauge theories, and Quantum Cosmology. D.Reidel, 1986.This book provides a careful introduction to the details of quantum cosmology. To do so the authors have described the ingredients of Gauge Field Theories and General Relativity before begin the discussion for quantum cosmology, including the inflationary scenario. Yue Zou, Tsinghua University

Recommendation for references • E.W.Kolb and M.S.Turner, The Early Universe, Addison-Wesley, 1990This is classic book described ideas across the whole range of what had become known as particle cosmology or particle astrophysics, including such topics as topological defects, inflationary cosmology, dark matter, axions, and even quantum cosmology. Yue Zou, Tsinghua University

Recommendation for references • A.R.Liddle and D.H.Lyth, Cosmological Inflation and Large-Scale Structure, Cambridge University Press, 2000.As a recent textbook, it provides a modern and unified overview of the inflationary scenario and the origin of density perturbation. Its discussion is very clear, and carefully compares predictions with observations. Yue Zou, Tsinghua University

Thank you very much for your attention! Yue Zou, Tsinghua University

Introductory Review of Cosmic Inflation