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Absence of Ghost Images Excludes Large Values of the Cosmological Constant
ABSENCE OF GHOST IMAGES EXCLUDES LARGE VALUES OF THE
COSMOLOGICAL CONSTANT
J. M. Eppley1 and R. B. Partridge
Department of Astronomy, Haverford College, Haverford, PA 19041
ABSTRACT
We used the 1.4 GHz NRAO2 NVSS survey to search for “ghost”
images of radio sources, expected in cosmologies with a positive
cosmological constant and positive space curvature. No statistically
significant evidence for “ghost” images was found, placing constraints on
the values of Λ, the space curvature or the duration of the radio-luminous
phase of extragalactic radio sources.
1. INTRODUCTION
Cosmologists now accept the possibility of a non-zero value for the cosmological
constant, Λ, which implies a small vacuum energy density in the Universe. A small
positive value of Λ would help to resolve the age problem (some measures of the age of
the Universe appear to exceed the time since the Big Bang); to explain the apparent
acceleration of the expansion of the Universe (Perlmutter et al., 1997; Reiss et al., 1998;
Garnavich et al., 1998), or to ensure zero space curvature as favored by inflationary models
even if the density of matter in the Universe is below its critical value, ρc. Values of Λ close
to 2 H02 = (8-13) × 10-36 sec-2 have been suggested (see Carroll et al., 1992, for a review),
corresponding to ?Λ = 0.67 where ?Λ ≡ ρΛ/ρc.
If Λ is somewhat larger, however, so that ? + ?m 1, the space curvature of the
Universe becomes positive, even for very low matter densities, ?m. We investigate one
consequence of values of Λ close to or exceeding 3 H02: light from a source can reach an
observer from two opposite directions in a spatially closed Universe, as sketched in Fig. 1.
1 Current address: Art Technology Group, 101 Huntington Ave., Boston, MA 02199.
2 The National Radio Astronomy Observatory is a facility of the National Science Foundation operated
under cooperative agreement by Associated Unive
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