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?
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.
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.
What the article is referring to is that de Sitter spacetime, which is an idealized universe that only contains vacuum energy, has no initial singularity; it extends infinitely far back in time (and infinitely far forward as well). This is the basic model (at the classical level--see below) used to describe inflation, so in that sense I think the answer is that the "no singularity" result applies to all inflation theories.
That's not quite the end of the story, because the idealized model I just referred to is a classical model; it doesn't include quantum effects. I think the general opinion among physicists is that, if we extrapolate inflation back far enough, we reach a point where quantum effects become important, and the classical model above no longer applies. I don't think anyone expects a quantum model with an initial singularity to replace the classical model in this regime; but at this point we don't know what the correct quantum model is.
Since the very earliest days of Big Bang cosmology there has been a debate over the initial singularity, because nobody likes singularities.
There was a debate about black holes for the same reason. The original Thorne-Hawking bet (scroll down past the more recent one on black hole information: https://en.wikipedia.org/wiki/Thorne%E2%80%93Hawking%E2%80%9...) was driven in part by discomfort with singularities. There was and is a feeling that something ought to make them go away, but we remain unsure if they really will, and until we have a genuine theory of quantum gravity our accounts of what happens at very high densities is going to be a bit hand-wavy.
So this article's suggestion that the current situation is particularly new or different isn't all that accurate. There have always been two senses of the meaning of "the Big Bang" and the degree of comfort people have had with singularities has varied over time (lower in the '60's and '70's, higher in the '80's and '90's, lower again today.)
Nor is it accurate to say that inflationary cosmology is completely uncontroversial and so well understood that we have no choice but to discard the initial singularity. Inflationary cosmologies are popular for good reasons, but they are not the only game in town--we recently saw a thing on Bohmian mechanics go by on HN that suggested an alternative to inflation, for example (I'm far from sold on the work, but use it simply as a way of pointing out that alternatives to inflation exist and can't quite be discarded completely just yet.)
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?