Our observable Universe is the region of space that we can see today — all points in space where light has had enough time to reach us if it left at the Big Bang (13.7 billion years ago) or later. The multiverse is made up of our observable universe as well as all of space outside of it — in other words, everything “out there.”
One of the most common interpretations of “other universes” is “bubble universes,” which are predicted by cosmological inflationary theory. They are areas of space where inflation has been particularly successful, and these areas grow incredibly fast. These bubble universes may have observers similar to ours, but they are usually at great distances from us (much larger than the radius of our observable universe, which is about 50 billion light-years).
Any future collision between our observable universe and a bubble universe will most likely take place on cosmological timescales — tens of billions of years or more. So we shouldn’t expect a collision next month. In fact, we hope it won’t come because the collision would happen at the speed of light, destroying our habitat at the same moment we see it.
It is possible, however, that our observable universe collided with another bubble universe in the past. If this is the case, the cosmic microwave background (CMB) radiation could contain remnants of such a collision. Current research is focused on predicting the shape of these relics and searching for them in the CMB, with some scientists claiming to have discovered such evidence. More information in the video bellow.
READ MORE: Our Place in the Multiverse[s].