How would we know if our bubble universe is colliding with another?
There are a few conditions, that I can think of at the moment, which would alter the outcome.
“When and where did the collision is taking place?” (If collision is really happening).
Although according to Cosmic Inflation (which predicts Multiverses) bubble universes can collide only during the early phases. And not in later stages.
But who is stopping us from stress testing the idea of later collision possibility?!
That been said, let’s understand all the scenarios.
CASE 1: Before First Light (During the Early ages of Universe)?
Let's assume it did collide with another, during the time when there was no light (Before CMB). It should leave scars on the CMB. Something like highlighted below.
But we don’t.
This is what we see. We see voids, but we have explanations for them.
This indicates that no collision during the early universe.
But what if the collision happened after the formation of first light.
CASE 2: After First Light?
We can’t use CMB data in that case, but a collision in the later ages should lead to some other consequences that would be measurable.
Clue # 1 | Uni-directional Expansion of the Universe: If our bubble universe was colliding with another bubble universe. One side of the universe should have been expanding slightly different than another side.
Which would lead to some specific alignments of galaxies that would reveal such fact.
NOTE: Please note, above image is not directly related to what I am talking about, but it can very nicely give you an idea of how we could know if such a universal event was happening.
We are still getting better in capturing the better and better images of the deep space, so we may not have seen this yet. Be ready for results from Vera Rubin telescope (One of the most powerful and revolutionary telescopes so far), in next 10 years it has plans to complete the scan the entire space from our perspective.
Clue # 2 | Unexpected Gravitational Wave Background: Just like CMB, we also expect a Gravitational Wave Background that would have generated, not by the merger of Blackholes but at the time of Big Bang. When we detect that, expect to see waves of breaking news: “Yet another proof of Big Bang”.
It expected to be so miniscule that its very hard to detect. LISA (expected to launch by 2035) is expected to detect that.
Just like that, merger of universe should also create a great continuous Gravitational wave background
Clue # 3 | Inconsistency in Physical Laws: Multiverse theories predict the presence of different universe with slight variations in physical laws. Upon universal level collisions we would have noticed unnatural inconsistency in physical laws which were impossible to explain.
Currently we do see inconsistencies between theories and observations but we have solid explanations for them.
Some say that the Dark Matter exerts gravitational effects because its the gravity being exerted by matter in another universe. But cross-universe gravity models do not match data quantitatively. Hence all such speculations can be dismissed.
CASE 3: Recently? (Which means the collision would be happening in unobservable universe)
If the universe starts colliding now or recently, it would mean the collision would be happening beyond our observable universe.
Above is the chart of universe size (Current and Future) that I created previously for reference purposes.
CASE 3.1: Between 46.5 Bn LY - 62 Bn LY
This is the region, from where light has not yet reached us but would reach in future.
In this scenario, we would see above shared clues in coming ages.
CASE 3.2: Beyond 62 Bn LY
This is the causal boundary of our universe. Anything happening beyond this region would never reach us. In this scenario, even the fact of universe colliding would only be a hypothecation for us, forever.