Bubble universes

Ken Tapping, 1st March, 2016

In the sky this week…

  • Jupiter rises about 7pm, Mars at 1am and Saturn at 3am.
  • Venus rises at 6am, lying low in the dawn glow.
  • The Moon will reach Last Quarter on the 1st and will be New on the 8th.

The structure of our universe, and all the things going on in it, is due to just a small set of parameters. If at the beginning just one of them had been even slightly different, the universe today would be drastically different. There might be no stars. Even if there were stars, they might not make all the elements needed to form planets and people. It could be that if the elements formed, they could not react to produce the chemicals necessary for life to begin. Clearly the right numbers turned up at least once.

If these important numbers arise at random, and there are other universes with other values, there might be universes that are void, dark and cold, or filled with stars that become unstable or explode before life might arise. Maybe the values these special parameters can have are not random?

This puzzle touches on another. If our universe began just under 14 billion years ago as something small, hot and dense, which then expanded and cooled into the universe we live in today, what was going on 15 billion years ago, or before that?

Picture our universe as a sort of four-dimensional balloon, which is expanding. All points on the balloon are moving away from each other as the balloon gets bigger. Creatures living on its surface would see all other points on the surface getting further away. If they can perceive only what lies on the surface, how can they ever find out if there are other balloons nearby? What would those inhabitants see if their balloon is touching another?

If you have ever blown bubbles you will have noticed that isolated bubbles are completely spherical. However, when two bubbles touch or are joined together, the interface between them is curved differently, or flat. On the large scale, we expect our universe to be uniformly curved, like the surface of a sphere. Are there parts of space that are curved differently? How would we find out?

On the really large scale, in the space far beyond our galaxy and its nearby neighbours, we would expect space to look the same in all directions. One test of this is to count distant galaxies, to see whether the most distant galaxies are distributed differently in some parts of the sky. The problems with this are firstly that we are not able yet to see the most distant galaxies, and secondly, that the ones we see are not distributed uniformly anyway; they are distributed in a three-dimensional network, like a sponge. They form filaments and surfaces enclosing great, empty voids.

We could look carefully at the cosmic microwave background radiation. In the beginning the universe was hot, dense and opaque. Then, about 380,000 years after the Big Bang, and the temperature had fallen to about 3,000 Celsius, hydrogen atoms separated out and the universe became transparent to light and heat radiation. As the expansion continued, the universe continued to cool, until today the temperature has fallen to -270 Celsius. This whisper of heat, known as the Cosmic Microwave Background, or CMB, has been detected and mapped.

The maps of the CMB show tiny temperature variations that are the fossil remains of the beginnings of formation of the first galaxies. A key attribute of the CMB is that it should look the same in all directions: a background with the same sorts of irregularities scattered over it. Is there a direction in which the CMB looks different? The idea that our universe may be just one bubble in a cosmic foam of universes is attracting increasing scientific interest. If we are just one bubble in a foam of universes, then 15 billion years or longer ago, the foam existed, but our bubble was yet to form. One step in that direction is to find if our bubble is touching any others.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
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