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The background of this post lies in these two articles published long ago.
Readers can find entire Hiranyagarbha Sukta posted and discussed in second link provided above. The recurring phrase in Hiranyagarbha Sukta goes like this -
कस्मै देवाय हविषा विधेम ? (to) which god, (our) oblation offered/goes/given?
The phrase comes after description OR characteristic OR act which pertains to creation OR existence of this world (I will not use the word Universe, simply because I do not know whether it is a "uni"verse or not)...
हिरण्यगर्भः समवर्तताग्रे भूतस्य जातः पतिरेकासीत | स दाधार पर्थिवीं दयामुतेमां कस्मै देवायहविषा विधेम ||
Now, the verb used for Hiranyagarbha's entry in our frame of inquiry is "Sama+vartana" - Sama + vartana - vartana (rotation), Sama refers to even, symmetrical, equal.. Samavartana means during course OR point of a symmetrical cycle (or existence, sustenance and dissolution, perhaps). Which point? Beginning (Agre) Samavartataagre - At the beginning of a symmetrical cycle.
So हिरण्यगर्भः समवर्तताग्रे means Hiranyagarbha (one with a golden womb - something like sun) first arose upon the samaavartana (of? perhaps world). This is origin of our concept of Pralaya and new genesis. But the clause is not complete. Hiranyagarbha itself is an adjective. A description. But of whom? This is answered in second clause of the first line -
भूतस्य जातः पतिः एक आसीत One Lord of all beings who exist.
So, in my understanding, sages are saying that the "One lord of all beings that exist, who is a Hiranyagarbha, came into being (aasit) at the beginning of this symmetrical cycle."
What did he do?
स द आधार पृथ्वी द्युः उत् इमा He gave support to earth and skies (thus making this world which is fit for life to exist).
So the entire verse comes to mean as following:
हिरण्यगर्भः समवर्तताग्रे भूतस्य जातः पतिरेकासीत | स दाधार पर्थिवीं दयामुतेमां कस्मै देवायहविषा विधेम ||
"One lord of all beings that exist, who is a Hiranyagarbha, came into being at the beginning of this symmetrical cycle. He (then) gave support to earth and skies (thus making this world which is fit for life to exist). Who is such a radiant being (deva) to whom we should offer our havi (offerings, oblation in yagna)"
They are asking this question to Prajapati. Prajapti is another name OR earlier name of Brahmadeva (creator). He is perhaps the most supreme force described in Vedic literature. Yet, he is not this "lord of all that exists" and who "supported earth and heavens". They are asking Prajapati this question, so they are perhaps sure that Prajapati is not the one.
This entire Sukta (known as Hiranyagarbha Sukta, Rigveda 10.125) is question inquiring who this eka Bhootasya Jaata pati (one lord of everything that exists) is?
The answer comes 4 suktas later in Naasadeeya Sukta (RV 10.129). In the last verse, where they doubt whether even Prajapati knows the answer to this question or not. Perhaps even Prajapati (who is ultimate repository of all knowledge of this world because he created this world) does not know because perhaps there IS NO such "Eka Bhootasya Jaata pati (One lord of everything that exists).