Induced Gauge Transformations of the Instantaneous Fluxes

Chapter VI: Comparison of 3PM Fluxes to PN Literature

6.2 Induced Gauge Transformations of the Instantaneous Fluxes

will compare it to the isotropic-gauge HamiltonianH^iso by first expanding the PM expression from [56] (reproduced in Eqs. (3.37), (3.38)) in𝑣 <<1 to 3PN order. In ADM gauge, there are three independent degrees of freedom: the relative separation 𝑟, the radial momentum 𝑝_𝑟 = 𝒑· 𝒓/𝑟, and the momentum-squared 𝑝² = |𝒑|². In isotropic gauge, 𝑝_𝑟 = 0. At order 𝐺³, we may ignore back-reaction from the radiation of𝐸 and 𝐽 on the trajectory, and so𝐽 = 𝐽¯=𝑟

√︁

𝑝²−𝑝²_𝑟. We constructed ansätze forG𝐶 as a power series in the ADM variables,

G𝐶[ ®𝑎] =∑︁

𝑖 𝑗 𝑘

𝑎_{𝑖 𝑗 𝑘}𝜂^{2𝑖+2𝑗+𝑘}(𝑟) 𝐺 𝑚

𝑟 ^𝑖

𝑝^2𝑗𝑝^𝑘

𝑟. (6.12)

We use this generator to construct ˆΦ[ ®𝑎] according to Eq. (6.6), and then use Eq. (6.11) to construct the transformation from ADM to isotropic coordinates.

Because we have definedΦas the (nonscaling) map from isotropic to ADM coordi- nates,Φ, notΦ⁻¹will appear in the ADM to isotropic Hamiltonian transformation.

The resulting matching equation reads H^iso =K^iso ≡ 1 𝜇 𝜈

×Φ[ ®ˆ 𝑎]

H^ADM◦𝜆

. (6.13)

𝜇and 𝜈 are simply thrown on the pile of undetermined coefficients 𝑎®to be deter- mined.

As expected, these constraints admit solutions. Using the 𝐺³, 3𝑃 𝑁 Hamiltonian approximations, these fix the generator to 𝐺³,2.5𝑃 𝑁 order, which is more than sufficient to match all fluxes at the orders considered in this work. Expressions for the Hamiltonians, canonical generator, and derived𝑞, 𝑝 transformations are all provided in the ancillary file.

97 in the spacetime. It follows that there is no unique extension of a point 𝑥 along the trajectory with time coordinate𝑡 to a Cauchy surface defining a time-slice𝑡 for the entire spacetime. However, a particular choice of Cauchy surface at one time 𝑡 may be extended to a foliation of the spacetime1 by the flow of a timelike vector field, so it is sensible to speak of the instantaneous fluxF𝑄(𝑡) as a function of all 𝑡 ∈ R. The point is that different foliations lead to different time dependencies for the fluxes associated to a single physical scattering process. The various Cauchy foliations are generally related by diffeomorphisms of the gravitational theory, and so the associated ambiguity in the instantaneous fluxes as functions of time may be understood as a manifestation of gravitational gauge freedom.

The fluxes defined in Chapter 5 as phase-space functions2 of the effective theory thus carry two residual dependencies on the diffeomorphisms of the full theory.

On one hand, a diffeomorphism acts directly on the spacetime as an active coordi- nate transformation. In the effective theory, such transformations induce canonical transformations of the binary phase-space. We will slightly abuse notation by using the symbol Φ to represent both the spacetime diffeomorphism in the full theory, Φ:𝑥↦→ 𝑥˜ = Φ(𝑥), and the induced canonical transformation in the effective theory, Φ : 𝜒 ↦→ 𝜒˜ = Φ(𝜒), where the maps are distinguished by their arguments. The gauge transformation of the fluxes thus includes the active canonical transformation (in terms of 𝜒)Φ:F𝑄(𝜒) ↦→ F𝑄 ◦Φ⁻¹(𝜒).

On the other hand, by deforming the Cauchy surface S𝑥 through which the binary trajectory passes at spacetime point𝑥(𝑡), the diffeomorphism has altered the value of the instantaneous fluxassociated to the the original trajectory point𝑥(𝑡). Explicitly, Φ:F𝑄(𝑥) ↦→W˜𝑄(𝑥) =W𝑄 ◦Φ⁻¹(𝑥) ≠ F𝑄◦Φ⁻¹(𝑥). (6.14) Figures6.2,6.2, and 6.2 depict example trajectories in a 1+1 spacetime, and will hopefully clarify this point. LetF𝑄(𝑥(𝑡))denote the flux obtained by integrating the Noether current over the Cauchy surface at time𝑡in Figure6.2. The diffeomorphism relating the surfaces of Fig.6.2to those of Figure6.2is simply a global shift in the

1Provided the spacetime is globally hyperbolic, which we will always assume.

2We have been somewhat careless about specifying thedomainof the flux functions. When we write F𝑄(𝑡), we are referring to the function obtained by substituting the phase-space trajectory 𝜒(𝑡) into our phase-space flux expressions,F𝑄(𝑡) =F𝑄(𝜒) ◦𝜒(𝑡). If we apply the induced canonical transformation (not the full gauge transformation) to both the phase-space functionF𝑄(𝜒) and the trajectory 𝜒(𝑡)and then compose them,F𝑄(𝑡)will, of course, be left invariant, provided the canonical transformation is not explicitly time-dependent (which we always assume). In this section, we will directly compare our phase-space expressions to corresponding expressions in the literature, without any need for trajectories. For this reason, we must account for canonical transformations.

spatial direction. Points on the trajectories of the constituents transform asΦ:𝑥 ↦→

𝑦 =𝑥+𝑎, but the flux at any given time is unaffected because the Cauchy surfaces are stabilized by the shift. Therefore Φ : F𝑄(𝑥) ↦→ W˜𝑄(𝑥) = W𝑄 ◦Φ⁻¹(𝑥) = F𝑄 ◦Φ⁻¹(𝑥), so the flux transforms as a scalar. By contrast, the diffeomorphism relating Figure6.2to Figure6.2deforms the Cauchy surfaces, altering the associated fluxes as well as applying the usual active coordinate transformation. Therefore, Φ:F𝑄(𝑥) ↦→W˜𝑄(𝑥˜) =W𝑄 ◦Φ⁻¹(𝑥) ≠ F𝑄 ◦Φ⁻¹(𝑥).

In terms of the phase-space coordinates 𝜒,

Φ:F𝑄(𝜒) ↦→W˜𝑄(𝜒) =W˜𝑄 ◦Φ⁻¹(𝜒) ≠ F𝑄 ◦Φ⁻¹(𝜒) (6.15) in general. As in the case of the Hamiltonian/Kamiltonian transformation, both the function and the point at which it is evaluated have changed. The relationship between F and W, however is generally much more complicated than the simple scaling relatingH toK.

Figure 6.2.1: A binary system (red and blue) emitting gravitational radiation (green) through Cauchy surfaces (black). This diagram depicts only a finite spacetime patch, not the entire spacetime.

99

Figure 6.2.2: The same binary system shown in Figure 6.2, but with a different Cauchy foliation, related to the original by a global spatial shift.

Figure 6.2.3: The same system shown in Figure6.2, but now with a Cauchy foliation related to the original by a non-trivial small diffeomorphism.