What is the story of Brahma taking birth from the navel of Vishnu?

 

Actually this is a parable first mentioned in the Rig Veda, which was expanded in Puranas, duly deifying the ABSTRACT advaitic concept into physical deities Vishnu and Brahma.

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The basic advaitic concept, in brief, is as follows:

The Universe came into existence from/being governed by the formless, invisible ENERGY or BRAHMAN or Atma. The Modern-day scientists are calling that formless, invisible ENERGY or BRAHMAN or Atma as the Dark Energy.

Rig Veda says this as the source of everything - nābhi - navel, where the BRAHMAN resides.

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The BRAHMAN is spread into the entire Universe.

Rig Veda 10.129.3

तम आसीत्तमसा गूळ्हमग्रेऽप्रकेतं सलिलं सर्वमा इदम् । तुच्छ्येनाभ्वपिहितं यदासीत्तपसस्तन्महिनाजायतैकम् ॥
tama āsīt tamasā gūḻham agre 'praketaṃ salilaṃ sarvam ā idam | tucchyenābhv apihitaṃ yad āsīt tapasas tan mahinājāyataikam ||

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Every being that came into existence will be carrying that BRAHMAN or formless, invisible ENERGY, remained concealed within it.

When the human being is able to experience at the culmination of spiritual practice that BRAHMAN, one will find that one is the same DIVINITY as the BRAHMAN.

This is called one is getting connected to that BRAHMAN or nābhi - navel or source of everything.

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Rig Veda 10.82.6

तमिद्गर्भं प्रथमं दध्र आपो यत्र देवाः समगच्छन्त विश्वे । अजस्य नाभावध्येकमर्पितं यस्मिन्विश्वानि भुवनानि तस्थुः ॥
tam id garbham prathamaṃ dadhra āpo yatra devāḥ samagacchanta viśve | ajasya nābhāv adhy ekam arpitaṃ yasmin viśvāni bhuvanāni tasthuḥ ||

English translation:

Just this first embryo did the waters receive, where all the gods gathered together: the one fitted upon the navel of the unborn (BRAHMAN), that upon which all living beings have taken their places.

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This subtle spiritual concept, as mentioned in the Rig Veda, was adopted in the Puranas, as the birth of Brahma from the navel of Vishnu.

In Puranas, the individual experiencing the DIVINE consciousness, realising that One is nothing but the same BRAHMAN, and that one is connected to the BRAHMAN, is converted into a story, replacing the REALISED SOUL with Brahma and the formless ENERGY or BRAHMAN with Vishnu, and Brahma taking birth from the navel of Vishnu.