Present vs Past in Cosmological time

Question

My question is in regard to “seeing back in time” when observing objects in space. I understand the concept that light takes a certain amount of time to reach the earth and we can determine the distance to a very distant object via red shift. Certainly this allows us to see and understand how objects were and the progression of the universe. However aren’t we blind as to how the universe looks like at the present. If an object is a million light years away we have no idea what it’s present state is. In fact we have no idea what it’s state was 500,000 light years ago. If the universe presents nothing but the past how can we be sure what is really happening right now in the present? Maybe this has a simple answer but it has me stumped.

Answer 1

We assume that the universe is the same everywhere. This assumption is known as the cosmological principle. If this assumption is correct then distant regions of the universe right now look like the near regions, and the differences that we see are only due to time.

The cosmological principle is just an assumption, but it is a reasonably well founded one. When we look out at the universe we see it is the same in all directions. The only way this can be true is if either the universe is the same everywhere or we just happen to be right at the centre. The latter seems unlikely as it would be a remarkable coincidence that the planet we live on just happened to be in the centre of the universe, so we assume the former is true i.e. the universe is the same everywhere.

Answer 2

Your worries about our knowledge of the Universe is quite common, and can be categorized under "How can we be sure that things are like this?".

In essence, it's the same question as "How can we be sure that this stone falls to the ground when I let go of it?".

We model the world

That answer to both questions is "We can't". But through experience we can convince ourselves that this is probably how it is, and through constant experimentation and challenging of our current understanding and perception of the world, we can verify our models and theories (or perhaps at some point falsify them, in which case our worries were justified, and we must start over).

In physics, one can never be sure about anything. But at some point, we can be so convinced that, for all practical purposes, it doesn't make sense to keep being suspicious. For instance, you shouldn't worry that the stone will fly upwards and hit your face when you let it go. But that doesn't mean that the law of gravity isn't questioned anymore; indeed it is (sometimes perhaps a bit too much, in my opinion).

So, to answer your question: We "know" what's going on in a galaxy far, far away today, even though we see it in the past, because we have constructed a model for the Universe which numerous observations over the past centuries have verified, and which allows us to make sensible predictions about things we cannot hold in our hands.

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To address your specific concern about where an observed has moved since it emitted the light we see, consider first the Andromeda galaxy: Its distance is $d = 2.5$ million lightyears (Mlyr) and it approaches us at $v\sim100\,\mathrm{km}\,\mathrm{s}^{-1}$. In the $t = 2.5$ million years (Myr) that has passed since it emitted the light we see today, it has hence traveled $d = v t \simeq 900\,\mathrm{lyr}$, or 0.04% of its current distance, an insignificant amount.

More distant galaxies have traveled farther since they emitted the light we see (though away from us rather than toward us), and sufficiently distant galaxies are so far away that we will never be able to see the light they emit today. But that doesn't really matter, because by observing galaxies at various distances — and hence at various times throughout the history of the Universe — we can say something meaningful about the typical (as well as the less typical) fate of galaxies.

So I guess the anser to your question is this; we don't know, and don't really care, about what has happened to some particular distant galaxy, since the time it emitted the light we see. What we do care about, and actually know, it what happens "on average" to galaxies.

Answer 3

To observe the universe at the present time, we look not far away- just in our local neighborhood of galaxies that are no more than a few million light-years away. This furnishes us with a baseline against which we chart differences between what's near (and in our recent past) and what's far (and hence in our deep past).

Answer 4

Just to say you've got some good answers here, and it's mainly about lack of complete certainty but we make reasonable assumptions about what is likely to be going on. It's not so very different from everyday life. I don't know for sure what my cat is doing right now (it's not in the room with me at the moment) but I'm pretty sure it is not peeling a banana or riding on a bike. Very likely it is asleep.

However, when it comes to the universe at large there is an interesting observation to make, which is that we receive light from only a tiny fraction of the whole. We gather information arriving at Earth from our past light cone, and that's it: a tiny sliver from the whole history of the cosmos. You can often find statements referring to 'the observable universe' and then the speaker goes on to describe things way way out of our past light cone, sometimes without realising that they are doing so. It's a reasonable hypothesis that a galaxy we can see at a large red shift has, since then, evolved in a similar way to other galaxies of similar make up. But such a hypothesis is only good to the extent that there is uniformity as opposed to richness and variety in the universe. In practice the universe combines both. It is not a desert, but neither is it a jungle.