Hiraṇyagarbha (Sanskrit: हिरण्यगर्भः ; literally the 'golden womb' or 'golden egg', poetically translated as 'universal germ') is the source of the creation of universe or the manifested cosmos in Vedic philosophy, as well as an avatar of Vishnu in the Bhagavata Purana. It finds mention in one hymn of the Rigveda (RV 10.121), known as the Hiraṇyagarbha Sūkta, suggesting a single creator deity (verse 8: yo deveṣv ādhi devā eka āsīt, Griffith: "He is the God of gods, and none beside him."), identified in the hymn as Prajāpati. The concept of the "golden womb" is again mentioned in the Vishvakarman Sūkta (RV 10.82).
The Upanishad calls it the Soul of the Universe or Brahman, and elaborates that Hiraṇyagarbha floated around in emptiness and the darkness of the non-existence for about a year, and then broke into two halves which formed the Svarga and the Pṛthvi.
In classical Purāṇic Hinduism, Hiraṇyagarbha is the term used in the Vedanta for the "creator". Hiraṇyagarbha is also Brahmā, so called because it is said he was born from a golden egg (Manu Smṛti 1.9), while the Mahābhārata calls it the Manifest.
What follows is my exchange with a devotee of Sathya Sai Baba, who wrote in response to my June 21 post More on Sathya Sai Baba and the Golden Heart Lingam (Hiranyagarbha), and which had followed up my June 1 post, Saving power of photo of Golden Heart Lingam - Sathya Sai Baba.