Question: Expansion of the Universe & Time Shift?


#1

I understand that light waves are red shifted do to the expansion of the universe but are they also time shifted? For example, assume the New Horizons probe sent a radio burst of exactly x seconds long in duration towards Earth. Would we see that bust as x+(some extra amount of time) or would the duration still be exactly the same?

Thanks in advance!


#2

Hi Josh. Full disclosure - I’m not a scientist, let alone an astrophysicist. So while I feel I may be able to answer your question, maybe someone can butt in and correct me if I’ve made errors.

The cosmological redshift of electromagnetic waves is due to the apparent movement of the source traveling away from us, similar to the doppler effect. I say ‘apparent’ because, when talking about the expansion of the universe, it’s worth noting that the objects aren’t actually moving from their coordinate, but rather the expansion of space everywhere increases the distance between objects, giving the appearance of everything moving apart. This increased distance between things means that light takes longer to travel between them.

As far as I know, the extra time it takes for light to travel due to expansion is not a relativistic effect, such as time dilation (wasn’t sure if this is what you meant when you said “time shifted”). It is merely a matter of greater distance = greater time to traverse it. As an example, a signal sent to us from an object that is initially 1,000 light years away will take 1,000 years + a bit more due to the extra distance from expansion by the time is reaches us. So it is not that we are perceiving radio signals from the New Horizons probe as taking longer from our reference frame, because the expansion/increasing distance is also seen from the perspective of the New Horizons probe.
I’m not sure, as I don’t know the math (there are equations to calculate expansion, as the rate of expansion increases for objects the further away they are), but the expansion between us and the New Horizons probe would probably be extremely small, and perhaps negligible for practical reasons.

A cool tidbit is that, from the perspective of the light (radio signal), it would not experience any difference from expansion as light does not experience time. From the perspective of light, everything is contracted so that there is no distance at all!

Edit: Grammar.


#3

The way I was thinking about it is in the classic balloon metaphor. If you draw two dots on the balloon, one representing the probe and one representing Earth. When the balloon is inflated the dots get further apart due to expansion.

Now instead of just the two dots, draw a short dash of a line representing a pulse of light signal from the probe to Earth. When the balloon is inflated the dash will lengthen. Does this modified metaphor still hold up?


#4

The balloon analogy can be a bit misleading and, as is the case with many scientific analogies, falls apart when you look at it in more detail. The aspect of the balloon analogy that I do like is that the fabric of a balloon stretches, giving a somewhat visual concept of space expanding from all points, rather than just 2 points moving apart. I also like that dots drawn on the balloon maintain their position on the fabric, conveying the concept that they are not actually moving from their coordinate, but instead the space inside and around them is expanding.

The analogy is lost when you look at the fact that the edge of the balloon is growing, invoking questions like: “what is the universe expanding into?”. When, as far as we know, the universe is infinite and is actually expanding with comoving coordinates. A difficult concept to understand, as it’s a kind of motion that we never experience as humans and therefore is not intuitive.

But I think that you’ve got the right idea. Although I wouldn’t think of the pulse of light as a dashed line, as to me that implies that the light exists across all of the distance simultaneously and is being stretched. Rather the distance is increasing as it travels, taking it longer to traverse. Imagine driving a car on a highway that is being extended as you drive on it. It’s not your car that’s being stretched, but the road. As a result, your journey ended up taking longer than what you would have originally measured at the beginning.
Your dashed line would be space, not the pulse of light that travels through it.

The astrophysicist Jean-Pierre Luminet wrote a very accessible article about this concept on his blog that helped me understand it a bit better.