In observational cosmology, a supernova Ia is used as a standard candle in order to extend the Hubble diagram to a higher redshift range. Astrophysicists found that the observed brightness of high redshift supernovae Ia is dimmer than expected. This dimming effect is considered observational evidence for the existence of dark energy in the universe. It should be noted that this conclusion is based on an assumption that the mass density of the cosmic plasma is very small. Therefore, the dimming effect caused by the Compton scattering of free electrons in cosmic plasma can be neglected. X-ray observations suggest that the mass density of the cosmic plasma may be very large. In theory, the observed dimming effect of high redshift supernovae Ia may be caused by the Compton scattering of free electrons in the cosmic plasma. In this paper it will be shown that this idea is reasonable. Therefore, there is no need to introduce the confusing concept of dark energy into cosmology.

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