The big-bang theory is arguably one of the most fascinating and significant theories in modern physics. However, an expanding space-the major notion of the big-bang cosmology-is also one of the most confusing concepts in modern physics. In this thesis, we first explore various models of expanding universe by virtue of three cosmological horizons, i.e. the particle horizon, the event horizon, and the Hubble radius. We then discuss some of the most perplexing features of the big-bang cosmology, such as cosmological redshifts and the receding velocity of distant galaxies. Finally, based on the gedanken experiment of the “tethered galaxy” proposed by Harrison in 1995, we investigate the very nature of space expansion and its consequences in different cosmological models.