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Saha Ionization Equation in Early Universe Draft version October 17, 2019 Typeset using LATEX default style in AASTeX62 Saha ionization equation in the early Universe Aritra Das,1 Ritesh Ghosh,2 and S. Mallik,3 1HENPP Division, Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhan Nagar, Kolkata 700064, India. 2Theory Division, Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhan Nagar, Kolkata 700064, India. 3Theory Division, Saha Institute of Nuclear Physics, 1/AF Bidhan Nagar, Kolkata 700064, India. Submitted to ApJ ABSTRACT The Saha equation follows from thermal equilibrium of matter and radiation. We discuss this problem of equilibrium in the early Universe, when matter consists mostly of electrons, protons and hydrogen atoms. Taking H-atoms in their ground state only and applying the real time formulation of thermal field theory, we calculate the difference of ionization and recombination rates, which controls the equilibration of H-atoms. By comparing with realistic calculations including the excited states of H- atom, we conclude that the presence of excited states lower the equilibrium temperature from 5000 K to 4000 K. 1. INTRODUCTION A century ago Saha ( 1920) used the thermodynamics of chemical equilibrium to find the degree of ionization of atoms in a thermal bath. Since then it has been vigorously applied to investigate the spectrum of the sun and other stars. More recently, it has been used to find the epoch of hydrogen recombination involving the reaction + − γ + H p + e ; (1) in the early Universe, leading to an understanding of the cosmic microwave background (CMB) radiation observed today (Penzias & Wilson 1965; Dicke 1965). (There was an earlier epoch of Helium recombination, which can be treated separately in a first approximation.) In the early Universe the equilibrium condition is not guaranteed a priori; it depends on the reaction rate and cosmic expansion rate. Also the Saha equation neglects the excited states of the H-atom, which is a very good approximation as long as thermal equilibrium prevails. But away from equilibrium the excited states may be important in the process. Accordingly a number of authors (Peebles 1968; Zeldovich & Sunyaev 1969) have investigated the hydrogen ionization and recombination in the realistic case, without assuming equlibrium and including the excited states (2s, 2p) of the H-atom along with the ground (1s) state. The discovery of CMB anisotropies prompted a resurge of the calculation arXiv:1812.10686v2 [hep-ph] 16 Oct 2019 of recombination including non-leading effects. Thus the earlier (effective) three level calculation was replaced with a multi-level one (Seager et al 1999) including also Helium and their higher excited states. The effect of Raman scattering along with the related two-photon emission (Chluba & Thomas 2011) was incorporated in the calculation. A review of all such effects is contained in the Karl Schwarzschild lecture by Sunyaev and Chluba ( 2018). In this note, we attempt an indirect but easy way to study the effect of the excited states in attaining equilibrium. We calculate the ionization and recombination rates in a simplified model, where we exclude the excited states, taking only the H-atom in the ground state. Comparing these reaction rates with the cosmic expansion rate, we may know Corresponding author: S. Mallik [email protected] [email protected] [email protected] [email protected] 2 at which temperature the equilibrium is lost in the simplified model. On the other hand, comparing the fractional hydrogen ionization from the Saha equation with that of the realistic calculation (including the excited states), we can find when equilibrium is lost in the real world. Now comparing the two results for the loss of equilibrium, we may see the role of the exited states of H-atom. We use the reaction rates to write the Boltzmann equation for an arbitrary (nonequilibrium) distribution of H- atoms. It has a simple analytic solution consisting of two terms (Weldon 1983). The first term gives the equilibrium distribution, while the second term vanishes exponentially with time. It is the coefficient of time in the exponential of this term, which is identified with the reaction rate tending the distribution to equilibrium. This rate must be compared with the expansion rate of the Universe, given by the-then Hubble parameter. Though the problem is non-relativistic, we shall use relativistic expressions to evaluate the rates and then apply non-relativistic approximation. We use units with }(= h=2π) and c taken to be unity, where h and c are Planck's constant and the velocity of light respectively. Also we write β = 1=kBT , where kB is the Boltzmann constant and T the temperature. 2. IONIZATION AND RECOMBINATION RATES In atomic physics the processes represented by (1) are well-known. Recombination (or recapture) is the capture of an electron in the continuum by the atomic nucleus with the emission of photon. Ionization (or photoeffect) is the inverse process, where a photon is absorbed by an atom accompanied by ejection of an electron. These transitions are caused by the electric dipole operator in quantum mechanics (Bethe 2008; Weinberg 2013). It is given by the potential function, V = −(e=m)A · p, where e; m and p are the electric charge, mass of the electron and its momentum operator and A is the radiation field. The transition amplitudes for the above processes are given by the matrix elements of V between two atomic states, where one is discrete and the other in the continuum. Instead of using the above method to calculate the reaction rates, we use the elegant method of thermal quantum field theory for the problem (Semenoff 1983; Niemi 1984; Mallik & Sarkar 2016), where we directly get the recombination and ionization probabilities multiplied by appropriate factors involving distribution functions for particles in the medium. We first construct the interaction Lagrangian involving all the particles in (1). Let the photon, electron and proton fields be respectively Aµ(x), e(x) and p(x). We take H-atoms in the ground state only (ignoring its excited states), when it can be represented by an elementary scalar field Φ(x)(Weinberg 1995; Dashen 1974). Then the required effective interaction Lagrangian is µ Lint = g(Φ pγ e + h:c:)Aµ : (2) It describes an electromagnetic interaction and g must have mass dimension −1. So we take e g = ; (3) m to within some uncertainty. k1 γ q q − k2 e H H p+ k3 Figure 1. A self-energy graph for H-propagator The calculation of the relevant reaction rate in thermal field theory can be best approached by considering the self- energy graph for the H-atom shown in Fig.1. It includes in the intermediate state all other particles which appear in (1). The elements of real-time thermal field theory is sketched in AppendixA. These elements are used in AppendixB to calculate this graph, from which we obtain the different imaginary parts, which are collected in (B24) in a compact 3 0 00 000 form. Any particular imaginary part may be isolated by integrating the variables k0, k0 and k0 in (B24) over the appropriate delta functions in the spectral functions, such as in (A3). Before we write the desired imaginary part, we establish our notation. We shall not use the four-momentum notation anymore; instead, we now denote the magnitudes of three-momenta of H-atom (of mass MH ), photon, electron (of p 2 2 p 2 2 mass m) and proton (of mass M) by q; ki (i = 1; 2; 3) and energies by ! = q + MH ;!1 = k1;!2 = k2 + m and p 2 2 !3 = k3 + M respectively. We are interested in the imaginary part shown in Fig2, corresponding to processes in (1) with the photon (and H-atom) incoming and electron and proton outgoing for ionization, when the reverse process (recombination) will automatically be given by the second term in bracket in (B24). Accordingly we choose the delta 0 00 000 0 00 000 functions in k0; k0 and k0 variables as δ(k0 + !1), δ(k0 − !2) and δ(k0 − !3). On using (A6) and (A11) to convert f and fe to n and ne, we get for the required processes (denoted by subscript 1) from (B24), Z d3k d3k 1 ImΣ = 16πg2mM 1 2 [n (1 − n )(1 − n ) − (1 + n )n n ]δ(! + ! − ! − ! ) ; (4) (1) 3 3 1 e2 e3 1 e2e3 1 2 3 (2π) 2!1 (2π) 2!2 2!3 where n1(!1); ne2(!2) and ne3(!3) are the equilibrium distribution functions for the photon, electron and proton respectively. It resembles the unitarity relation for the S-matrix in vacuum for the two-particle states of photon and H-atom. Compared to that relation, we now have the difference of two terms. Dividing it by !, we convert it to rates (Weldon 1983) ImΣ (1) = Γ − Γ ; (5) ! d i where Γd and Γi are the first and the second term in (4), representing respectively the decay and inverse decay rates of H-atom. γ 2 γ 2 ⃗ k1 ⃗ − − k2 e e ⃗ H q p+ H + ⃗ ⃗ p q ⃗ − k1 − k2 Figure 2. The relevant imaginary part of Fig1 3. BOLTZMANN EQUATION The rates Γd and Γi are related. To see this, we write the equilibrium distribution functions explicitly as exp (−β!1=2) exp (β!1=2) n1 = ; 1 + n1 = exp(β!1=2) − exp(−β!1=2) exp(β!1=2) − exp(−β!1=2) and similarly for factors involving ne2 and ne3. Taking into account the energy conserving delta function in (4), we get M Γ ;Γ = 16πg2m fexp[β(! − µ − µ )=2] ; exp[−β(! − µ − µ )=2]g × L; (6) d i ! 2 3 2 3 where µ2 and µ3 are the chemical potentials for the electron and proton and L is given by Z d3k d3k 1 δ(! + ! − ! − ! ) L = 1 2 1 2 3 ; (7) 3 3 Y (2π) 2!1 (2π) 2!2 2!3 fexp[β(!i − µi)=2] ∓ exp[−β(!i − µi)=2]g 4 where the product runs over i = 1; 2; 3.
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