3 Connections on principal fiber bundles
The advantage of working directly with principal fiber bundles (PFB’s) is that structures
are simpler. For example, it is easier to work out particular features of associated vector
bundles, or features of gauge-fixed structures directly from the general formalism of PFB’s
than vice-versa. The standard example of a PFB is the bundle of linear frames over a
spacetime manifold M, with structure group GL(n).
We will try as much as possible to adopt notation which can be naturally extended to
the infinite-dimensional functional context.
PFB sections. A principal fiber bundle P , is a smooth fiber bundle, for which a Lie
group G has an action from the right G × P → P ,
6
which we denote by R
g
p =: p g, for
g ∈ G, p ∈ P . The action is assumed to be free, so that p g = p iff g = id
G
, the identity
of the group. The quotient of P by the equivalence relation given by the group, that is
p ∼ p
0
iff p
0
= p g for some g ∈ G, is usually identified with the spacetime manifold, i.e.
M = P/G. We denote the projection map by pr : P → M. where R
g
denotes the right
action of g on P (which should be distinguished from the action of g on the group G itself).
The set O
p
= {p g | g ∈ G}, is called the orbit through p, or the fiber of [p] = pr(p). The
action of pr projects O
p
to [p] ∈ M .
Let U ⊂ M be an open subset of M , then a smooth embedding σ : U → P is a
section (or a “gauge-fixing”) if pr ◦ σ = id
M
. The section then gives rise to a trivialization
G ×U ' pr
−1
(U), given by the diffeomorphism F
σ
: U × G → pr
−1
(U), ([p], g) 7→ σ([p]) g .
Generically, there are no such U = M and the bundle is said to be non-trivial. In the
functional, i.e. field space, case, the base manifold is a modular space and the obstruction
to triviality is known as the Gribov ambiguity [16, 17]. We briefly review that in section 7.
PFB connections. Before going on to define a connection, we require the concept of a
vertical vector in P . Let exp : Lie(G) = g → G be the group exponential map. Then by
dragging the point with the group action we can define a vertical vector at p, related to
X ∈ g, as
X
#
p
:=
d
dt
|t=0
p exp (tX)
∈ T
p
P (3.1)
The vector field X
#
∈ Γ(TP ) is called the fundamental vector field associated to X. The
vertical space V
p
is defined to be the span of the fundamental vectors at p.
From (3.1) one has that fundamental vector fields are ad-equivariant, in the follow-
ing sense:
(R
g
)
∗
X
#
p
= (Ad
g
−1
X)
#
p/g
(3.2)
where R
g
∗
: TP → TP denotes the pushforward tangential map associated to R
g
and
Ad
g
: g → g, X 7→ gXg
−1
is the adjoint action of the group on the algebra. This notation
emphasizes that the vector field at p ∈ P is pushed-forward to p g ∈ P . Both sides of the
equality, however, are evaluated at the same point, p g ∈ P . Now, using the fact that the
Lie derivative of a vector field Y
#
along X
#
is defined as the infinitesimal pushforward by
6
This means that in the coordinate construction of P the transition functions act from the left.
– 6 –