When Universes Collide, How Would We Know?

As far as we can tell, the universe is about 93 billion light years big and about 14 billion years old.

That’s a head scratcher for cosmologists. In 14 billion years, light can travel…errr…14 billion light years. So how did the universe get so big, so quickly?

The best explanation is a mysterious process called inflation. The general idea is that soon after it was born, the universe rapidly increased its size by many orders of magnitude in an instant.

Cosmologist love to ponder the way in which inflation was triggered. Short answer: nobody really knows, although there’s no shortage of speculation.

A somewhat lesser known problem is what might have stopped inflation. Why doesn’t the cosmos continue to expand at an exponential rate?

One of the most curious answers is this: that the universe is still expanding and that we exist in a tiny region of stability, a cosmic bubble in a mighty maelstrom.

Of course, our cosmic bubble would be just one among countless others.

But how could we ever see these other bubbles given that they must be beyond the edge of the visible universe?

Today, Anthony Aguirre at the University of California, Santa Cruz and his buddy Matthew Johnson at Caltech review this scenario and give an answer of sorts.

They say the only way that we could see evidence of another cosmic bubble is if it had collided with our universe in the distant past.

Interesting idea but not one without a few challenges. The main problem is that in mosts cases collisions would destroy the spacetimes in both bubbles, thereby ensuring that we couldn’t be here to observe the aftermath.

However, Aguirre and Johnson identify a class of cosmic collisions that preserve the three dimensions of space and one of time that we need for our existence. These are not so much cosmic collisions as glancing blows.

So what would the aftermath of such a cosmic scrape? Aguirre and Johnson say that evidence of a negative curvature to the universe we be compatible with the idea that we exist in a cosmic bubble while positive curvature would rule it out.

Beyond that, a cosmic prang would have left its mark in the form of various symmetric features in the cosmic microwave background.That’s something we could see in the data from telescopes such as Planck.

All tantalising stuff. But the trouble is that none of this would provide definitive, unambiguous evidence of a collision and that means we’ll probably never know for sure.

But cosmologists are not to be deterred, this is the kind of speculation they love.

Aguirre and Johnson finish with this statement:

“With some luck, the discovery of ‘other universes’, a concept seemingly out of science fiction, may be just around the corner!”

If you believe that, you have a fruitful career ahead of you in cosmology

Ref: arxiv.org/abs/0908.4105: A Status Report on the Observability of Cosmic Bubble Collisions