The multiverse, also known as an omniverse or meta-universe, is a group of multiple universes. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, and the physical laws and constants that describe them. The different universes within the multiverse are called "parallel universes".
The mystery of the Parallel Universe
As per string theory, there are more than some 10500versions of spacetime, each with their own laws of physics. It is an almost the infinite number of parallel universe. Mathematicians says that parallel universe covers the all possible outcome of a result. For example, you might be enjoying eating crabs in this universe but in other parallel universe Crabs with height, more than 10 feet enjoy eating us.
Parallel Universe in Hinduism
Concept of parallel universe is derived from Hindu Sanatan Dharma. All parallel universe float on Karana cosmic ocean which is consistently observed by Cosmic Maha Vishnu. Brahma is a creator of each parallel universe. Single Brahma cannot do it so there is an infinte number of Brahmas responsible for the creation. Some Brahmas usually have 3 heads, some have 10, some have even more than 1000. In Hindu Sanatan Dharma, there was one sage who created a parallel universe for his devotee only. To keep his words, he built a separate universe for his follower which even shocked the Brahma at that time.
As per Bala Kanda of the Ramayan, the solar dynasty king "Trishanku" wanted to go to Swarglok in his own material body. Trishanku asked sage Vashisht to help him. Vashisht refused and asked to leave him alone. Hence, Trishanku started doing bad things to him. Sage Vashisht got frustrated, he cursed him to become a Chandala. Now Trishanku physical appearance became so scary that even Rakshas started getting scared from him.
Further, Trishanku went to Sage Vishwamitra. Sage Vishwamitra decided to help Trishanku, out of hatred for Vasishta. Vishwamitra performed a Yagna which sent Trishanku to Swarglok in his own body. When Trishanku reached the abode of Indra, the Devtas, seeing the terrible Chandala-like appearance of Trishanku, spoke out to Indra:-- “Who is this person coming like a Devta with a violent speed in the air? Why does he look like a Chandala".
Indra vs Trishanku
Indra explained the curse of Sage Vashishta and went to stop the Trishanku. Indra came in between the path of Trishanku. Indra said - "You are a Chandala, quite unfit for the Devaloka; so where are you going? You ought not to remain here; so go immediately back to the earth. O Destroyer of the enemies"!
Indra speaking thus, the King dropped from the Heavens and, like a Deva whose merits had been exhausted, fell down immediately. While coming back to earth from heaven Triśaṅku cried and said to Vishwamitra.
“O Viśvāmitra! O Viśvāmitra! Being displaced from the Heavens I am now falling very violently; so save me from this trouble.”
Cosmic creation of new Universe
Upon hearing his cry and seeing him getting down, Vishwamitra by virtue of the Tapas kept stationed him in midway. And started creating a new universe. Balakanda explains :
O, King! Hearing his cry and seeing him getting down, Viśvāmitra said:-- “Wait, wait.” Though displaced from Heaven, the King by virtue of the Muni’s Tapas, remained stationed at that place in the middle of the air. Viśvāmitra then began to do Ācaman (sip water) and commenced his great Sacrifice to create another new creation and a second Svargaloka (Heaven).
Seeing his resolve Indra became very anxious and came down to Vishwamitra and said to Vishwamitra."There is no necessity to create another new creation". But Vishwamitra was determined one. And asked Indra to take Trishanku in his own abode. Indra accepted that.
Indra gave him a divine body and sent a beautiful divine chariot to the Trishanku to take him to Swarglok. The whole incident shows the power of Vishwamitra's merits (Tapas) which made him create cosmic creations i.e. Grahas, Nakshatras, lokas, etc. including Swarglok (Heaven). In the new universe, Vishwamitra created Buffalo as a replacement of cow. He created crow as a replacement for the sparrow. etc. Later on, both universe were merged together to maintain natural equilibrium.
Sarga 60 of Bala Kanda of Valmiki Ramayana given below explains the complete story.