[Ian Hutt]

Ian Hutt asked the Naked Scientists:

I believe it is now established that the universe is expanding at an ever increasing speed. I also understand that time slows as speed increases.

Is it therefore possible that at some in the very far future that the expansion of the universe will reach the speed of light and therefore time will come to a standstill which will in effect be the end of the both the universe and time?

What do you think?

[DoctorBeaver]

The way the universe is expanding is not that easy to grasp at first. If you take any 2 objects in the universe (e.g. galaxies) then although it appears these objects are moving away from each other, it is actually the space between them that is expanding. The objects themselves are not moving in space.(Actually they are, but we can ignore their other motions when considering expansion)

The further away any 2 objects are from each other, the more space there is in between them to expand and, hence, the greater the amount of expansion and the greater the apparent rate at which they appear to be moving apart.

Think of it like this. If we start with a distance of 100 units between 2 objects and the expansion is 10% per year (the real expansion rate is nowhere near that; I use those figures for illustration only) then after 1 year the objects are 100+10%=110 units apart. After the 2nd year they are 110+10%=121 units apart, after 3 years it is 121+10%=133.1 and so on. As you can see, the increase in separation was 10 in the 1st year, 11 in the 2nd, 12.1 in the 3rd. The greater the distance between them, the greater the increase.

There comes a point when the rate of separation increases beyond the speed of light. However, as the objects themselves are not moving, the speed of light is not a problem; no laws of physics are being broken.

So, to answer the question - it would seem that there is actually no upper limit to the speed of recession of the most distantly separated objects.

[Alan McDougall]

If the universe continues to accelerate like it present is, the expansion would become exponential and even out toward infinity,

Hard to contemplate!!

Alan

[DoctorBeaver]

Alan - the universe is not accelerating. The rate at which distant galaxies are receding is accelerating because as the space between them expands, so there is more space between them to expand. That gives the impression of acceleration.

However, the rate of expansion appears to be increasing and this has given rise to the Big Rip theory.

[ianhutt]

Thank you for the responses. Whilst I appreciate that space itself is expanding surely this still means that relative to us the recession of some far distant Galaxy's will be, or will eventually be, in excess of the speed of light. Clearly we would have considerable difficulty in either observing this or measuring the speed of the recession. If this is an instance of the speed of light, which I had always understood to be absolute, being exceeded are there other examples. I believe there is also some evidence that the speed of light may have varied over time rather than being a contant. 

[DoctorBeaver]

Ian - you are both right and wrong. You are right insofar as there will be galaxies receding from us faster than the speed of light. At least, that's what theory says and what observations appear to show. However, if they are indeed receding faster than light then we will never know for certain.

With regard the speed of light being absolute - yes(ish). The speed of light does vary depending on the medium it is passing through, but that is irrelevant to this discussion. Although extremely distant galaxies may be receding faster than the speed of light, they are not actually travelling at that speed. Their apparent movement is due to the expansion of space betweem them & us. As their recession is not due to their velocity, light speed is not exceeded.