What happens at the end of each day of Brahma?

At the end of the creation, all universes are destroyed and all the souls enter into the body of Lord Maha-Vishnu. However, what exactly happens at the end of each day of Brahma?

At the end of the creation, all universes are destroyed and all the souls, as well as the material energy, enter into the body of Lord Maha-Vishnu following His process of inhalation. However, what exactly happens at the end of each day of Brahma?

"At the end of the day, under the insignificant portion of the mode of darkness, the powerful manifestation of the universe merges in the darkness of night. By the influence of eternal time, the innumerable living entities remain merged in that dissolution, and everything is silent.

When the night of Brahmā ensues, all the three worlds are out of sight, and the sun and the moon are without glare, just as in the due course of an ordinary night. (SB 3.11.28-29)

At the end of each day of Brahma, there is a partial destruction of the universe. As a result, all the planetary systems at the bottom of the universe, up to Svargaloka are completely destroyed and all living entities, with the exception of great sages who live in the higher planetary systems (Janaloka, Tapoloka, and Satyaloka), go to sleep in the body of Garbhodakasayi Vishnu. This division happens because the sages living in these higher planetary systems are involved in advanced spiritual activities, while the ordinary living entities living in the lower planetary systems are engaged in material activities using gross material bodies.

It is described that the sun and the moon are not destroyed, but they stop providing illumination, and thus everything becomes dark, in a night that continues for 4.32 billion years.

It's important to note that the Earth's satellite, the piece of rock that sits about 238,000 miles from Earth and is called "the moon" in modern astronomy is different from the moon that is described in the Vedas. The moon described in the Vedas is a celestial planet that is presided over by the demigod Soma. It's larger than the sun and is situated even further from our planet, hundreds of millions of miles away. The moon described in the Vedas also emits light, just like the stars. This is explained in the Bhagavad-Gita (10.21) when Krsna says "nakṣatrāṇām ahaṁ śaśī" (amongst the start, I'm the moon).

The planetary system of Svargaloka includes many stars that are situated relatively close to our planet on the cosmic scale. These stars have a nature that is also different from what is believed in modern astronomy, emitting a modest volume of light, instead of being powerful like the sun. The moon described in the Vedas is the chief among such stars and shares their luminous nature, instead of simply reflecting the light of the sun. Because of these differences, Srila Prabhupada argued that the Apollo missions didn't actually reach the moon, since they came to a place that was deserted and filled with rock and sand, and not to the celestial planet described in the Vedas. They went somewhere, but not to the same place described in the Vedas as the celestial moon.

This makes the point that the satellite orbiting our planet and the celestial moon described in the Vedas are two separate things. The moon around our planet could be a gross manifestation of the celestial moon, the way it appears in our gross dimension, or could even be Rahu, as some believe. The main point is to understand that these are two different objects. The moon that orbits our planet is destroyed at the end of the day of Brahma, together with the planet itself, while the celestial moon remains.

Apart from these stars that form Svargaloka, or universe includes many other stars that are situated much farther from our planet, all the way to the edge of the universe. These stars form the higher planetary systems of Maharloka, Tapoloka, Janaloka, and Brahmaloka. Many of them may not be visible to us. These higher stars also survive the devastation.

How does the destruction take place?

At the end of the day of Brahma, there is a great fire, and thus the sages living in Maharloka are forced to relocate to Janaloka. In the Srimad Bhagavatam, it is mentioned that this great fire comes from Lord Ananta, situated at the bottom of the universe, while the Vayu Purana (2.38) gives a slightly different description, mentioning that the sun becomes red, like an incandescent ball of iron, and its size increases seven times, scorching Bhu-Mandala and the heavenly planetary systems, destroying all forms of life in these places. These two descriptions are not contradictory, since the Srimad Bhagavatam emphasizes the main factor (Lord Ananta), while the Vayu Purana emphasizes the secondary factor, the changes in the sun.

This description of the Vayu Purana matches the current theory that our sun will become a red giant in the future, destroying most of the planets in our solar system. Modern estimates calculate that this process will start between 3.5 and 5 billion years. According to the Vedas, the destruction at the end of the day of Brahma will happen at about 4 billion years of our planet (taking into consideration Sridhara Swami's factor of two), so we can see that the dates given in the Puranas fit the range given by modern studies.

The Vayu Purana also mentions that after this process the sun will disintegrate, ejecting gigantic luminous clouds in all directions. This also matches the modern theory that after about 150 million years of the red giant phase, the sun will explode into a large planetary nebula, ejecting clouds of hot plasma in all directions and becoming a white dwarf. Again, this matches the description of darkness during the night of Brahma.

The main difference between modern theories and the Puranas is that modern theories predict that the universe will continue indefinitely until the sun and all stars exhaust their fuel and the universe just becomes a dark and cold wasteland. The Puranas give a more optimistic revelation, showing how not only the stars will continue shining for a much longer period (311,032 trillion earthly years, until the next complete devastation), but our solar system will be re-created at the start of the next day of Brahma.