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We point out that, due to the nonlinearity of the Einstein equations, ahomogeneous approximation in cosmology leads to the appearance of an additionalterm in the Friedmann equation. This new term is associated with the spatialinhomogeneities of the metric and can be expressed in terms of densityfluctuations. Although it is not constant, it decays much slower (as t^(-2/3))than the other terms (like density) which decrease as t^(-2). The presence ofthe new term leads to a correction in the scale factor that is proportional tot^2 and may give account of the recently observed accelerating expansion of theuniverse without introducing a cosmological constant.Comment: 4 pages RevTeX, 2 figures, significant new material is adde

ACCELERATING EXPANSION OF THE UNIVERSE MAY BE CAUSED BY INHOMOGENEITIES