What if the universe is smaller then the observable universe

[LordOfTheGeeks]

I have read that this is a possiblility, and it really intriges me. I know that one of the effects would be mirror images of galaxies that are close by, but alot futher away and alot younger. What other effects will be observed, what does this mean about the expension of the universe and is there any evidence to show that the universe is definatly larger then the observable universe...

[speedfreek]

Hi there LOTG, welcome to the BAUT forums!

The idea that the whole universe might be smaller than the observable universe is indeed an intriguing possibility. It involves some perhaps counter-intuitive concepts concerning the topology of the universe, or the "shape of space".

To start with, have a look through all the sections at the link below, it is a short, easy introduction to the subject:

The Shape of Space

In section 7 we see a short description of how we might detect "the shape of space", based on an analysis of our current map of the Cosmic Microwave Background Radiation (CMBR).

Since that article was written, the WMAP project has given us a pretty good map to work with and the analysis has begun! Here is a description of the ideas involved, from the team of scientists working on this problem:

While it is certainly possible that the Universe extends infinitely in each spatial direction, many physicists and philosophers are uncomfortable with the notion of a universe that is infinite in extent. It is possible instead that our three dimensional Universe has a finite volume without having an edge, just as the two dimensional surface of the Earth is finite but has no edge. In such a universe, it is possible that a straight path in one direction could eventually lead back to where it started. For a short enough closed path, we expect to be able to detect an observational signature revealing the specific topology of our Universe.

In simple terms, if the whole universe has one of a certain set of topologies, and is small enough, light would have been able to "circumnavigate" the universe, and we would be looking at the same regions of space when we look in different directions! If that were the case, then the whole universe is smaller than the observable universe.

But what is the observable universe then? Well, it is a concept, based on where cosmologists think everything was, when the light we see today was originally emitted. They then take that information and extrapolate from it where they think everything would be today.

The light that has taken the longest time to reach us is the CMBR which was released throughout the whole universe, around 13.7 billion years ago. At that time, our observable universe was small - something around 40 million light-years in radius. What this means is that we think that all the CMBR we detect today, coming in from all directions, was originally released at a distance of only 40 million light-years.

The rate of expansion was thought to be so fast at that time, that anything that distance away would have been receding much faster than light. The expansion was so fast that light itself would have been receding, from our point of view, as it travelled towards us from the point of view of its source. You can think of it as if space was expanding faster across that distance, than those photons could move towards us. As the rate of expansion slowed over billions of years, those photons were able to make progress towards us and finally reach us 13.7 billion years later.

As, out of all the light we see, this is the light that has been travelling the longest time to reach us, we can consider the distance where that light was released to be where the edge of our observable universe was, at that time. Since then, due to the expansion of the universe, we think that place will have receded to a distance of around 46.5 billion light-years, giving the observable universe a diameter of 93 billion light-years.

But, what if the whole universe today actually has a diameter smaller than 93 billion light-years, and light has had time to "circumnavigate" the universe? This would mean that those CMBR photons we currently detect were not originally released 40 million light-years away, they came from a place originally closer than that, but headed away from us! The CMBR from that place that headed towards this location would have already reached us in sometime in the past, and today we are receiving CMBR that was going in another direction.

If the topology were spherical, we could make an analogy with the Earth. Imagine two countries almost opposite to each other, the UK and Australia for example. If we send out a signal from the UK to Australia, and it travels in different directions around the globe, then, as Australia is closer to us in one direction than it is in another, the Aussies would detect that signal coming in from different directions at different times. If they only detected the signals that took longer routes (perhaps the detector wasn't switched on when the first signals were coming in), and tried the work out the distance from Australia to the UK based on their data, they would arrive at a distance that is essentially correct, but not the shortest route. If they had all the data, they could work out all the distances in different directions, and might be able to work out the shape of the thing those signals were travelling across.

But back to that team of scientists working on this problem. The results based on the WMAP data were published in 2006, and they could find no matching patterns in the data. Even the WMAP data is not of a sufficiently high enough resolution to get a complete picture, but what they found was that at least 78 billion light-years of our 93 billion light-years diameter observable universe, is comprised of unique space. With better data and analysis techniques they expect to be able to extend their search for "matching circles" out to the full diameter of 93 billion light-years.

So, as it stands, cosmologists think the whole universe is probably larger than our observable universe, as there is no evidence to the contrary.

Here is a link to the relevant paper:

Extending the WMAP Bound on the Size of the Universe