2.2 Generalizations and extensions
2.2.1 Finite temperature and AdS black holes
At finite temperatureTany quantum mechanical system can be found in one of the possible states of energyEwith the probability distribution in equilibrium described by the density matrix
ˆ
ρ= e−βH
Tre−βH , (2.44)
whereβ = 1/T is the inverse of the temperature andHis the Hamiltonian of the system. Thestatisticalpartition function of the ensemble of systems at temperatureT can then be defined as
Zstat= Tre−βH =X
n
hφn|e−βH|φni, (2.45)
where the|φniform a basis of the state space of the system. The statistical partition function defines the weight of each state that contributes to ensemble averages. Expectation values of some observableAin a thermal ensemble are calculated with respect toZstatby
hAi= Tr ( ˆρA) = Tr Ae−βH Zstat
. (2.46)
In the quantum mechanical formalism of path integrals, transition amplitudes are given by
hφf(tf)|φi(ti)i=hφf(tf)|e−i(tf−ti)H|φi(ti)i=
φ=φf
Z
φ=φi
Dφ eiS[φ]. (2.47)
The idea is to sum up all possible paths φ(t) that evolve from the initial configurationφi to the finalφf. The complex phases give the weight for each possible configuration that contributes to the evolution. If we would not only consider one initial and one final state but sum over an ensemble of many possible states, we should therefore recover the sum of all weights, the partition sum. The actionS[φ]above is defined as
S[φ] =
tf
Z
ti
dt Z
dd−1xL(t, x, φ). (2.48) In quantum mechanics the standard method to obtain expectation values is the evaluation of functional derivatives of generating functionals, which are
commonly also referred to as partition functions. They are defined for some functionalSE[φ]by
Zgen = Z
Dφ e−SE[φ], (2.49)
where we need to specify which functionsφwe have to integrate over and what the functionalSE[φ]is. Theimaginary time formalismgives a prescription which exactly reproduces the thermal equilibrium probability weights with the Boltzmann factor given in (2.44). The prescription is to analytically continue the time coordinate into the complex plane, such thattin(2.48)integrates over complex times. Additionally, we introduce a new time coordinateτ =itas a Wick rotation oft. If we now restrict the system to such field configurationsφ that are periodic (in fact fermionic fields would have to satisfy anti-periodicity) along the imaginary axis in complex timetwithtf −ti =−iβandβ ∈R between the initialtiand the finaltf, we can reproduce the Boltzmann weights by setting
SE[φ] =
β
Z
0
dτ Z
dd−1xLE(τ, x, φ). (2.50) The indexE refers to the fact that we use the Euclidean, i. e. the Wick ro-tated version, of the action. Then the integration from0toβ translates into integration over complex timesti to tf and therefore introduces the factor tf −ti =−iβin(2.47). As we restrict to periodic states on the integration in-tervall, the final stateshφf|match the initial states|φii. Thus the path integral resembles a trace[27],
Zgen = Z
allβ-periodic states
Dφ e−SE[φ]= X
allβ-periodic states
hφβ|e−βH|φβi=Zstat. (2.51)
Adding source terms to the action, which are set to zero after functional derivation, yields the Boltzmann factors as weights from the Euclidean gen-erating functional. The imaginary time formalism in this way trades time for temperature and imposes boundary conditions. The result no longer depends on a real valued time interval but only on the purely imaginary time inter-valtf −ti =−iβ, which we interpret as the temperatureT by identifying T = βk1
B. Abandoning time dependence is in accordance with the fact that we investigate a system in equilibrium, where expectation values do not change with time. Another consequence of the periodic boundary conditions is that any solution admits a discrete spectrum in its Fourier transformation. A propagator G(τ)can be decomposed according to
G(τ) = 1 β
X
n
e−iωnτG(ωn). (2.52)
The frequenciesωnare calledMatsubara frequencies.
The link between field theory at finite temperature and gravity is known to be related to black hole physics, which has many parallels to thermodynamics.
From the moment of the discovery of theAdS/CFT correspondence it was expected that the finite temperature description of the field theory must be given by gravity in theAdSblack hole background[6]. We will now introduce theAdS5 black hole background and give arguments for the relation between the horizon radius and the temperature of the dual field theory along the lines of arguments in refs. 28, 29. The generalization of theAdS5×S5metric(2.21) to the black hole solution with a horizon atr=r◦is given by,
ds2 = r2
R2 −f(r) dt2+ dx2 +R2
r2 1
f(r)dr2+R2dΩ25, f(r) = 1−r◦4
r4.
(2.53)
The signature of the metric again depends on our choice of working in either Lorentzian or EuclideanAdSspace. To draw the connection to finite tempera-ture imaginary time formalism we again work in the Wick rotated coordinate τ =it, where the metric has Euclidean signature. Euclidean signature however is only given outside the horizon, inside we would introduce negative signs fromf(r)(in the Lorentzian case the signs oftandrwould change). On the other hand it is known that the spacetime can be continued beyond the horizon and therefore the spacetime can be regularized atr◦.
The idea is to show that periodicity in the Euclidean timeτ that leads to thermal probability distributions in field theory corresponds to a regularization of the Euclidean spacetime at the horizon. The period β = 1/T is then identified with the inverse temperature as in field theory. We concentrate on theτ andrcoordinates in the above metric and observe how they behave near the horizon atr≈r◦,
ds2= 4r◦
R2 (r−r◦) dτ2+R2 4r◦
(r−r◦)−1dr2. (2.54) We know that the Euclidean spacetime is well defined only on and outside the horizon and therefore introduce a new coordinate%2=r−r◦, which casts the metric into the form
ds2= R2 r◦
d%2+%2 4r◦2 R4 dτ2
. (2.55)
The factor in front is merely a constant. The metric in parentheses is the metric of a plane in polar coordinates,ds2 = d%2+%2dθ2. The angular variable in our case isθ=τ2r◦/R2. The space in polar coordinates, however, is regular only for an angular variable that is periodic with period2π, otherwise a conical singularity is located at% = 0, which is the horizon of theAdSblack hole.
Periodicity ofθwith period2πthen translates into periodicity ofτ with period βby
2r◦
R2 τ + 2π ∼ 2r◦
R2 (τ +β). (2.56)
Where we used the symbol∼to denote the equivalence relation of identified points. Regarding identified points as equal and usingβ = 1/T we obtain the relation between the temperature of the field theory and the horizon radius on the gravity side,
r◦ =T πR2. (2.57)
This result exactly reproduces the expression for the Hawking temperature of theAdSSchwarzschild black hole described by the metric (2.53).
TheAdS black hole background allows to investigate strongly coupled gauge theories at finite temperature by performing calculations in the gravity theory. Moreover, finite temperature defines an energy scale (or length scaler◦
on the gravity side) in the theory and therefore breaks global scale invariance, which will have consequences for the property of (de)confinement of the field theory, which we discuss in section 2.3. Despite the detour to a background of Euclidean signature with timeτ, the field theory we want to use as a model for real world QCD is defined on a background of Minkowski signature with timet, which we will use predominantly from now on. Some subtleties and consequences regarding Minkowski signatureAdS/CFT are discussed in the following.
Thermal real time Green functions
TheAdS/CFT correspondence was originally formulated and successfully applied in backgrounds of Euclidean signature, i. e. in the imaginary time formalism. Results in real time can be derived by subsequent Wick rotation.
However, many common situations require the formulation of a problem and its solution in real time. One mathematical reason for the need of a real time formulation arises from simplifications which are often introduced by deriving solutions only for certain limits of the parameter space. In many cases for in-stance, solutions are obtained in the hydrodynamic limit of low frequency/long distance physics. In this case only the low Matsubara frequencies are known and therefore analytic continuation of results to real time is somewhere be-tween difficult and impossible. Physical arguments against the imaginary time formalism arise whenever deviations or even far from equilibrium scenarios are considered. We discussed that the imaginary time formalism mimics ther-mal equilibrium probability distributions. For systems out of equilibrium the restriction of the path integral to periodic paths is not justified. Moreover the solutions considered in the imaginary time formalism are periodic, it is doubtful whether such solutions can model long time evolutions. Summing up,
FIGURE2.2: Integration con-tour in the complex time plane for theimaginary timeand the Schwinger-Keldyshformalism for finite temperature field the-ory.
Ret Imt
ti ti+β
tf
−iβ 0
it is desirable to have a real time prescription for computations of correlation functions in Minkowski space.
In field theory such a prescription is given by the Schwinger-Keldysh for-malism[27]. The difference to the imaginary time formalism is a modification of the time integration in the complex time plane. The time coordinate is still defined on the complex plane. However, instead integrating fromtitoti−iβ along the negative imaginary axis, this time a detour along the real axis is taken. The path first proceeds along the real axis fromtitoβ, which marks the end of the physical real valued time interval of interest. Then, the integration contour enters the negative half plane arbitrarily far and leads back below the real axis toRet= Retito continue parallel to the imaginary axis and finally end intf =ti−iβ. Figure 2.2 shows the integration paths of the imaginary time and Schwinger-Keldysh formalisms. We are interested in the analog of the latter prescription in the gauge/gravity duality.
A recipe for the derivation of holographic Minkowski space Green func-tions in real time was derived by Son and Starinets together with Herzog and Policastro[22, 23, 30]. The resulting prescription is concise and amounts in only small changes from the prescription given by(2.35). The difference is given by the way we compute the on shell actionSsugra. The action is commonly obtained by writing the solution of a fieldφsuch that the “bulk contributions”
in radial directionrfactorize from the “boundary contributions” along the field theory directionsxon the boundaryrb. Usually we work in momentum space with momentumkinstead of position space with coordinatexand write
φ(r, k) =f(r, k)φbdy(k), with lim
r→rbf(r, k) = 1. (2.58) In the coordinates introduced so far the boundary was located atrb =∞. The on shell action can then be written as
Ssugra=
Z d4k
(2π)4 φbdy0(−k)F(r, k)φbdy(k)
r=rb
r=r◦
. (2.59)
Here, we carried out the integration over radial coordinates from the black hole horizon to the boundary. The twoφ0arise from the kinetic term and the
functionF(r, k)collects the remaining factors off(r, k)and∂rf(r, k)and possibly other factors appearing in the action under consideration. A detailed and explicit calculation can be found in chapter 3 where we apply the recipe, and in refs. 22, 23. The two point Green function of the operator dual toφ would then be given by the second functional derivative of the action,
G=−F(r, k)
rb
r◦
− F(r,−k)
rb
r◦
. (2.60)
So far we followed the method we already introduced for zero temperature.
The only difference is that nowr◦ 6= 0. The evaluation of this expression is mathematically possible, but gives physically wrong answers. For example, the resulting Green functions would be real functions, opposed to physical solutions, which are in general complex valued. This behaviour is due to the boundary conditions that have to be imposed on the fields. In the Schwinger-Keldysh formalism they arise from the periodicity of the fields and from the orientation of the integration contour. This introduces a contour ordering prescription, which translates to time ordering in physical processes. Loosely speaking, a causal propagator in theAdSblack hole background describes propagation of a field configuration that has to obey the infalling wave bound-ary condition at the black hole horizon. This boundbound-ary condition imposes the physically given fact that at the horizon, positive energy modes can only travel inwards, while negative energy modes only travel outwards. It can be shown that upon imposing these boundary conditions, the time ordered retarded part of the propagator in momentum space is determined by the boundary behaviour of the fields alone[30]. The prescription of Son and Starinets for fields obeying the infalling wave boundary condition then reads
GR=−2F(r, k)
rb. (2.61)
The contributions from the horizon are neglected. This method was used to great extent in subsequent publications. At zero temperature it agrees with the analytic continuation of Euclidean results[23].