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When people calculate the forces inside the universe at a large scale, gravitational force is often focused, while the radiation pressure forces of stars are often neglected. This article reveals after calculations that, at a large distance, the radiation pressure force is larger than the gravitational force inside the universe. This will cause the universe to expand at increasing speeds. In addition, calculation also shows that cosmic rays pressure force is a stronger force than radiation pressure force in accelerating the expansion of the universe. Other factors including gamma ray bursts can also be important causes of growing universe expansion. The causes of universe expansion provided by this article is original and innovative. This scientific discovery can be used to replace the space expansion theory to explain the accelerating expansion of the universe. Moreover, The relationship between the total radiation pressure force (F) of a star and the power (P) of the star can be described by equation: F=P/C. Thus we can roughly calculate the overall power the universe needs to sustain an accelerating expansion.