## How can observations of the distant universe prove that the expansion is accelerating *now*? (Advanced)

I believe I've heard that more recent observations of the universe indicate that the expansion of the universe is excelerating, and that given what is believed to be the mass of the universe that it is doubtful it will ever start to contract. My question is relative to the observations. If the observations are based on, say, very deep space (really far out) measurements of clusters or groups then how can we know TODAY that the exceleration is increasing when, in fact, we also know that the further out we look the further back in time we are seeing. My simple mind wants to tell me that the velocity I'm seeing at the fringes of the known universe are what those velocities were many billions of years ago, not today. If I can equate the big bang to a bullet shot from a rifle then I would expect the velocity to be higher at the beginning; which is where we are seeing the higher velocity of the universe way out there near the beginning.

It's precisely *because* we can observe the rate of the expansion of the universe at different epochs that we can figure out that it's accelerating. You can't determine that it's accelerating just by observing the expansion locally. The acceleration is way too slow for you to see it happening in an individual galaxy. However, as you point out, if you observe the velocities of galaxies that are billions of light-years away, you're measuring the expansion rate of the universe as it was billions of years ago. Since we observe that the expansion rate billions of light-years away--billions of years ago--is slower than the expansion rate locally--at the present time--we infer that the expansion of the universe is accelerating.