AUM is the vibration of the universe. When the Rishis were in deep meditation they were able to control the senses and block out all sounds from the outer world. The only thing they could hear was this single sound which sounds similar to AUM. This is all they could hear in deep meditation. So when they came out of deep meditation this is the only sound they made a comparison to. According to science the universe started with the big bang theory. This sound according to Hinduism is the sound AUM(OM). This is why it is sacred and it is chanted during meditation.
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