In Universal Extra Dimension models, the lightest Kaluza-Klein (KK) particle
is generically the first KK excitation of the photon and can be stable, serving
as particle dark matter. We calculate the thermal relic abundance of the KK
photon for a general mass spectrum of KK excitations including full
coannihilation effects with all (level one) KK excitations. We find that
including coannihilation can significantly change the relic abundance when the
coannihilating particles are within about 20% of the mass of the KK photon.
Matching the relic abundance with cosmological data, we find the mass range of
the KK photon is much wider than previously found, up to about 2 TeV if the
masses of the strongly interacting level one KK particles are within five
percent of the mass of the KK photon. We also find cases where several
coannihilation channels compete (constructively and destructively) with one
another. The lower bound on the KK photon mass, about 540 GeV when just
right-handed KK leptons coannihilate with the KK photon, relaxes upward by
several hundred GeV when coannihilation with electroweak KK gauge bosons of the
same mass is included.Comment: 38 pages, 4 figure