Is Vishwamitra also a creator of a new Universe like Brahma?
Yes, Vishwamitra did his own creation. This is described in the Bala Kanda of the Ramayana. The solar dynasty king Trishanku wanted to go to Swarga in his own body, so he asked the sage Vasishta to help him. Vasishta refused, so Trishanku did a variety of bad things to Vasishta in response. Vasishta was outraged and cursed Trishanku to become a Chandala. But then the sage Vishwamitra decided to help Trishanku, out of hatred for Vasishta. Vishwamitra performed a Yagna which sent Trishanku to Swarga in his own body. As soon as he arrived in Swarga, Indra threw him down from there. But then Vishwamitra created a new Loka, called Trishanku Swarga, for Trishanku to rule over. The Devas were concerned about this, and requested Vishwamitra not to allow a sinner like Trishanku to enjoy such prosperity. Vishwamitra compromised with them and made Trishanku hang upside down in the Loka he had created, as described in this chapter of the Bala Kanda of the Ramayana:
Like the other Creator that resplendent Vishvamitra, himself staying among sages, has replicated Southerly Ursa Major in southerly direction. And still remaining amidst of sages that highly reputed sage Vishvamitra further started to replicate the stereotyped stocks of stars sequentially, resorting to the southern hemisphere, as he is convulsed in anger at Indra. On replicating the stereotyped stocks of other galaxies and stars, and while fury continued to blemish him Vishvamitra said, 'I will now clone an alternative Indra, or let that realm created by me remain without any Indra,' and when he is about to proceed to clone even gods in his wrath, the gods are startled.
The gods are highly startled at Vishvamitra's creation, and they along with demons and with observances of hermits addressed the great-souled Vishvamitra with placatory words. 'Oh great-fortunate Vishvamitra, this king Trishnaku is damned by his mentor, hence oh, ascetically wealthy sage, he is not at all eligible to go to heaven with his mortal body.' So said gods to Vishvamitra. On hearing that sentence of those gods, the eminent saint Kaushika spoke this highly laudable sentence to all of the gods. 'Let safety betide you all. I have promised to this king Trishanku that he will ascend to heaven with his mortal body, and I am not interested to make it untrue. Let there be eternal heaven to Trishanku with his mortal body. Next, as along as the worlds remain, let all of these stars and galaxies I have created also remain eternally in their places as my creation. It will be apt of you all gods to accede to this.' So said Vishvamitra to gods.
When all the gods are addressed thus they replied the eminent saint Vishvamitra saying, 'so be it! Safe you be! Let all the created objects prevail in their respective places. Those amazing and numerous stars you have created will remain in firmament, but outside the path of stelliform of Cosmic Person. Trishanku will also remain in the circle of stars you created, but upside-down, for Indra's indict cannot be annulled, and he will be gleaming like a star and similar to any celestial. According to their wont to circumambulate someone who has gone to heavenly worlds, all the stars will suppliantly circumambulate this best king Trishanku, who has achieved his ends and who has become an acclaimed one with your tour de force.' So said gods to Vishvamitra in deterring his further cloning of a simulative Universe. Even the benign-souled Vishvamitra, when reverenced by all gods, that great- resplendent sage without stirring himself from among the sages said to all gods, 'Agreed!'
In any case, the outcome of this compromise is that Trishanku Swarga is still there to this day. Wikipedia says "Trishanku corresponds to the collection of three crosses around Crux, also known as the Southern Cross." By the way, it's interesting that Trishanku Swarga is described as outside the "the path of stelliform of Cosmic Person" or Vaishvanara Patha, which I assume is the Milky Way. (By which I do not mean the entire galaxy, but the part of the galaxy that's visible as a colorful blur in the night sky in places with little light pollution.)
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