In mathematics and theoretical physics, the Eguchi–Hanson space is a non-compact, self-dual, asymptotically locally Euclidean (ALE) metric on the cotangent bundle of the 2-sphere T^*S². The holonomy group of this 4-real-dimensional manifold is SU(2). The metric is generally attributed to the physicists Tohru Eguchi and Andrew J. Hanson; it was discovered independently by the mathematician Eugenio Calabi around the same time in 1979.^[1][2]

The Eguchi-Hanson metric has Ricci tensor equal to zero, making it a solution to the vacuum Einstein equations of general relativity, albeit with Riemannian rather than Lorentzian metric signature. It may be regarded as a resolution of the A₁ singularity according to the ADE classification which is the singularity at the fixed point of the C²/Z₂ orbifold where the Z₂ group inverts the signs of both complex coordinates in C². The even dimensional space C^d/2/Z_d/2 of (real-)dimension ${\displaystyle d}$ can be described using complex coordinates ${\displaystyle w_{i}\in \mathbb {C} ^{d/2}}$ with a metric

${\displaystyle g_{i{\bar {j}}}={\bigg (}1+{\frac {\rho ^{d}}{r^{d}}}{\bigg )}^{2/d}{\bigg [}\delta _{i{\bar {j}}}-{\frac {\rho ^{d}w_{i}{\bar {w}}_{\bar {j}}}{r^{2}(\rho ^{d}+r^{d})}}{\bigg ]},}$

where ${\displaystyle \rho }$ is a scale setting constant and ${\displaystyle r^{2}=|w|_{\mathbb {C} ^{d/2}}^{2}}$.

Aside from its inherent importance in pure geometry, the space is important in string theory. Certain types of K3 surfaces can be approximated as a combination of several Eguchi–Hanson metrics since both have the same holonomy group. Similarly, the space can also be used to construct Calabi–Yau manifolds by replacing the orbifold singularities of ${\displaystyle T^{6}/\mathbb {Z} _{3}}$ with Eguchi–Hanson spaces.^[3]

The Eguchi–Hanson metric is the prototypical example of a gravitational instanton; detailed expressions for the metric are given in that article. It is then an example of a hyperkähler manifold.^[2]

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