Source Raj Vedam
Poor Kakudmi. He couldn't find a suitable match for his highly accomplished daughter Revathi, and so off he went with Revathi to ask Brahma the Creator, if he could recommend a groom. On arriving in Brahmaloka, they saw the Creator enjoying the music of the divine musicians, and waited for the Gandharvas to finish their concert. Brahma laughed on hearing Kakudmi's request and said in the interval they were waiting for the music to be done, 27 Chatur Yugas had passed on Earth, and every kingdom and person they knew was dead and long gone. Brahma consoled the desolate Kakudmi and told him to return to Earth, in time for Bhagavan Krishna and Balarama to incarnate on Earth, and that Balarama would be the ideal match for Revathi. Ref: Srimad Bhagavatam 9.3, verses 29-33.
Readers can take away various metaphysical meanings from these passages. It is fascinating to read these passages thru the prism of our current understanding of cosmology.
A Chatur Yuga has 4 Yugas (Satya, Treta, Dwapara, Kali) in a 4:3:2:1 ratio, equivalent to 1,728,000 + 1,296,000 + 864,000 + 432,000 Earth years, amounting to 4.32 million years. Thus 27 Chatur Yugas involve 116.64 million years.
Assuming Kakudumi and his daughter traveled at 99.99999999999999 percent the speed of light, they would have traveled just a couple of years of dilated time with respect to time on Earth, which would have amounted to 116.64 million years.
Lets get some scale to appreciate this. Light takes 8 minutes to reach us from the Sun. The center of the Milky Way galaxy is 25,000 light years away. The nearest galaxy to us, Andromeda, is 2.5 million light years away. We are part of a local cluster of galaxies (see the picture, radius of 8 million light years from Earth), and the neighboring Virgo supercluster is 65 million light years away. Several clusters of galaxies form the Laniakea Supercluster of galaxies, approximately within 200 million light years of us.
Thus 116.64 million light years would put Kakudmi and Revathi somewhere in the Laniakea Supercluster of galaxies (see the picture below), which is home to our Milky Way galaxy, and 100,000 other galaxies. This was Brahmaloka, or the known universe to the ancient Indians. This is far more than any other civilization could even grasp, even in the 20th century!
Lets see this in a different way. Time dilation also occurs in the presence of super-large gravitational fields when compared with a reference. If we assume that Kakudmi and Revathi traveled close to the event horizon of the super-massive Black Hole at the center of our Milky Way galaxy, then too, they would have experience time-dilation with respect to a reference on Earth. I don't have the calculations to show how close they need to be to the event horizon to experience such time-dilation. If the mass of the black hole (4.3 million solar masses) is insufficient, one would have to look for an even larger mass object where such time-dilation could have been possible (see the note below).
Were Kakudmi and Revathi time-travellers? We cannot say without more evidence. But Rishi Veda Vyasa who recounted the Srimad Bhagavatam obtained in meditative states, surely had the divine insight to embed many such startling, amazing facts.
Note: This is just a simplistic reading of SB, which says nothing about the time Kakudmi took to travel - it just talks about the duration of the concert. Working the numbers thus would lead to far greater diameter of Brahmaloka OR an estimate of the entire mass of the universe (Brahman manifested over all creation).