Hacker News Reader

Comment by dak1 | Hacker News Reader

This is probably a rather simplistic question to answer, but I've been confused how we go from "on average, the farther away a galaxy is, the faster it’s moving away from us" to "the expanding Universe".

If every galaxy was observed in its present day state, then that conclusion would seem to be valid. However, when we're looking at a galaxy 5 billion light years from us, we're looking at it as it was 5 billion years ago (including the light shift).

Therefore, it seems at least plausible that expansion is slowing rather than accelerating, since galaxies were moving away faster the further back you go in time.

I'm sure the above can be explained, although it may require a lot more time/effort to refute than it took for me to make the argument, but could somebody with more training in the subject point out the flaw in my logic or explain why the expansionist interpretation fits better with observation?

View on HN

The history of astronomy is filled with people figuring out how to measure apparently-impossible-to-measure things. For example, how would you go about measuring the distance to the sun? (Keep in mind that you're not allowed to use things we derived from that measurement and take for granted now.)

When you look at a galaxy from 5 billion years ago, you're not looking at it as it was including the light shift. The red-shift they're talking about is not from the initial difference in velocity, it's from the wave length being stretched by the expansion of space as the light travels.

For example, the wikipedia article about the accelerating expansion [1] mentions things like:

> the distance-redshift relation deviates from linearity, and this deviation depends on how the expansion rate has changed over time

If the universe decided to suddenly increase in size by 1% over the next hour, all the already-travelling light would arrive with a 1% larger wavelength. If it had suddenly increased in size by 1% a billion years ago, the already-travelling light would get just that 1% larger wavelength but also 1% more travel time for stretching out to occur over. So if you have an independent way of determining distance... a lot of information falls out of that red-shift-vs-distance plot.

1: http://en.wikipedia.org/wiki/Accelerating_universe

Thanks. I was apparently making the mistake of assuming the red shift was due to velocity instead of the expansion of space itself.

For example, how would you go about measuring the distance to the sun?

So, what's the answer? Where can I read more about that?

A clever chap figured out a method a couple of thousand years ago: http://en.wikipedia.org/wiki/Aristarchus_of_Samos

This is a question that google is good at answering.

http://physics.stackexchange.com/a/29366

Imagine a balloon being blown up. Two points close to each other move apart much slower than points further apart.

I understand that model as the interpretation for inflation, but why do we presume the "balloon" is continuing to inflate at the same rate or faster today than it did in the past?

It also makes me wonder how we can equate distance to time in a 1:1 ratio. If space is expanding exponentially, then it almost seems like the distance a photon can travel in a given time is decreasing (from the perspective of the observer). I'm probably just confusing myself though.

Reply on news.ycombinator.com