We know that the effect considered in Section 4.4, the modification of gravity by the presence of a background Sµ with a rest frame, is present in nature, because the electromagnetic radiation in the CMB has a conserved Poynting 4-vector:
Pµ= 1 8π E
2+B2,2E×B
. (4.41)
This background Pµ modifies gravity because gravitons can couple to acoustic pertur- bations in it, as shown in Fig. 4.2. This effect is, however, completely negligible, since the characteristic energy scale of the CMB is TCMB ∼2.7 K, which means that this effect is suppressed by a factor of TCMB MPl 2 ∼10−64 . (4.42)
The question remains, however, whether there might be some other background that, unlike the CMB, couples strongly to gravity (and only to gravity, so as to explain why it has not been otherwise detected). The Goldstone bosons of spontaneous Lorentz violation would correspond to the sound waves in this background, and the modification to gravity comes, as it did in Fig. 4.2 from the mixing of the gravitons with these acoustic modes.
In [73], the authors find the propagation velocities of the five graviton polarizations in vector-tensor theories with the unit constraint. In our language, these are the velocities of the two usual gravitons plus the three acoustic modes in the Lorentz-violating background:
2 transverse traceless metric vtt = 1/(1−c13)→1 ,
2 transverse Goldstones vtrv = (c1−c21/2 +c23/2)/(c14)(1−c13)→c1/(c14) ,
1 longitudinal Goldstone vlgt =c123(2−c14)/c14(1−c13)(2 +c13+ 3c2) →c123/c14 ,
(4.43)
whereci...k≡G(ci+. . .+ck) and where the limits correspond to vanishing ci’s. Since, for
general ci’s, there are two distinct sound speeds, one for the longitudinal and one for the
transverse modes, our Lorentz-violating background fulfills the canonical definition of a solid.8 The transverse sound speed is associated with a shear mode (a deformation which alters the shape but not the volume of a body). Linear shear waves are absent in a fluid (see, for instance, Chapters III and VI in [83]).
In Section 4.4 we emphasized the difference between our model, which we may now refer to as the “cosmic solid” model, and the “ghost condensate” of [81]. In [81], Lorentz invariance is broken by a VEV for a spin-0 vector field Aµ=∂µφ with a single degree of freedom, whereas in the cosmic solid model the Lorentz invariance is broken by a spin- 1 vector field Aµ with three degrees of freedom. Therefore the ghost condensate has a single Goldstone boson, with non-relativistic dispersion relationsE ∝ |k|2, and it gives the
graviton a mass when minimally coupled to it, whereas the cosmic solid has three Goldstone bosons, with dispersion relationsE∝ |k|, and it does not give the graviton a mass. It turns out that if the ghost condensate is gauged (i.e., if the ghost condensate fieldφis minimally coupled to a U(1) gauge field Aµ), then the two polarizations of the gauge field provide the two extra degrees of freedom, and the resulting model is equivalent to the cosmic solid ([84]). Whether the ghost condensate itself admits a high-energy completion is unresolved (see [85, 86]).
It can be seen from Eq. (4.43) that the speeds of the Goldstone bosons can be made su- perluminal without introducing ghosts or other obvious problems in the low-energy effective action. As pointed out in Section 4.4, if the Goldstones are required to be subluminal, then
α2 no longer gives the strongest constraint on the size of theci’s because a far more strin-
8
gent limit applies, from the gravitational ˇCerenkov radiation of the highest-energy cosmic rays. Superluminal Goldstone bosons would evade that constraint. Whether superluminal- ity could result from a reasonable high-energy completion, and whether the initial value problem in the low-energy effective action is well-posed in the presence of superluminal modes, remain open questions.
Chapter 5
Some considerations on the
cosmological constant problem
5.1
Introduction
Consider Einstein-Hilbert gravity as an effective theory, containing all the terms compatible with its symmetries:
S = Z d4x√−g Lmatter(φ, gµν)−2Λ +MPl2R+· · · , (5.1) whereMPl≡ p 1/8πGand Tµν ≡ 1 √ −g δSmatter δgµν . (5.2)
The equation of motion for the metric gµν is
Rµν−
1
2gµνR−Λgµν = 8πGTµν . (5.3)
We would naturally expect that
Λ∼MPl4 ∼ 10 28 eV4 . (5.4) If we let gµν =ηµν+ 1 MPl hµν , (5.5)
Figure 5.1: Feynman diagram of the coupling of a single graviton to the cosmological constant Λ in Eq. (5.1). The blob may also be thought of as a collection of vacuum-to-vacuum quantum processes.
wherehµν is the graviton field, then the Λ term in Eq. (5.1) gives
−√−g(2Λ) =−2Λ− Λ MPl
hµµ− O(h2). (5.6)
The first term in the right-hand side of Eq. (5.6) is an irrelevant constant, but the second gives a tadpole diagram for the graviton, as shown in Fig. 5.1. By Eq. (5.4) we would therefore expect this tadpole interaction to be of orderMPl3.
Alternatively, one can think of this tadpole diagram, shown in Fig. 5.1, as the coupling of a single graviton to the quantum-mechanical vacuum energy. This corresponds to moving the Λgµν in Eq. (5.3) from the left-hand to the right-hand side and thinking of it as the
contribution to the matterTµν from the quantum-mechanical vacuum energy. In quantum
field theory, each frequency mode of the free field is a simple harmonic oscillator. Therefore, each mode has a zero-point energy E=ω/2. We clearly have to cut off the sum at some scale, but the successes of quantum field theory so far suggest the cut off scale can’t be much smaller than∼1 TeV.
In any case, we get a positive value of Λ (the “cosmological constant”) far, far in excess of what observation allows. To see qualitatively the effect of large positive Λ, imagine vacuum energy inside a piston. Its energy density, ρ, is constant. If the piston is pulled out, as shown in Fig. 5.2, the total energy must increase: dE=ρdV. By energy conservation, we must have supplied that energy when we pulled on the piston: dW =F d`=pdV =−dE. Therefore the piston wouldresist being pulled out: Pressure is negative,p=−ρ.
The Newtonian limit of GR for a test mass on the edge of a uniform sphere of radius r0
gives an acceleration
g= 4π
3 G(ρ+ 3p)r0 . (5.7)
F
dl
vacuum
energy
Figure 5.2: Consider a piston filled with vacuum energy, whose density is constant. By energy con- servation, the piston must resist being pulled out, and therefore the vacuum energy exerts negative pressure.
what we would expect on effective theory or quantum mechanical grounds would rip apart the universe, preventing it from developing any structure. It was long presumed that some unknown symmetry of quantum gravity would forbid the Λ term in Eq. (5.1), thus naturally making the cosmological constant zero. In Chapter 3 we discussed one such idea: that the graviton was a Goldstone boson of spontaneous Lorentz violation, so that the broken Lorentz invariance protected it from getting any potential at all.
Data on the accelerated expansion of the universe, however, has recently shown that there is a small but non-zero anti-gravitating term.[87, 88] Two possible approaches to this cosmological constant problem that will be of interest to us here are:
• to imagine that the true Λ is zero, but that the universe contains some other field, coupled only to gravity, which accounts for the accelerated expansion.
• to imagine that the value of Λ varies over some landscape of possible universes, and that we naturally happen to live in one where Λ is small enough that structure (and therefore intelligent life) may form.
The first line of thought will lead us in Section 5.2 to consider whether a cosmological scalar field can have a pressure more negative than −ρ. In Section 5.3 we will consider how the ghost condensate of [81] would behave if it were responsible for the accelerated expansion of the universe. In Section 5.4 we will re-examine the second line of thought in light of the proposal that other parameters besides Λ vary over the landscape of possible universes.