Present vs Past in Cosmological time

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.

• "what is really happening right now in the present? Maybe this has a simple answer" - It does: time is a local concept. "Right now over there" has no meaning. – safesphere Mar 6 at 7:38
• And if the person sitting across from you has a heart attack, you won't know it until a fraction of a second after it happens. Get used to it. – WillO Mar 11 at 22:43

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.

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.

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.

• Sorry I haven't been replying/commenting, but I was thinking of another way of asking the question. Consider this... We know that how we see Proxima or Andromeda (I picked Andromeda for a reason) is how they were in the past. How would these objects look if we could see them as they are right now? For instance I picked Andromeda because it is actually moving towards the Milky Way while most objects are moving away. So by projecting its speed towards us and then subtracting the amount it is moving away (hubble's constant) where is its present location. Is that even worth considering? – Robert Fallows Mar 19 at 5:14
• @RobertFallows Those are physically valid questions, but since the speed of the objects you mention is much less than the speed of light, the answer is not too interesting: Andromeda is 2.5 million lyr away and approaches us with 100 km/s. When the light emitted today reaches us in 2.5e6 yr, Andromeda will be less than 1000 lyr closer to us, i.e. it will have decreased its distance by 0.04%. Similarly, Proxima Centauri is 4 lyr away and moves with some 20 km/s toward us. In four years, it will be 0.007% closer. – pela Mar 19 at 14:26
• If, on the other hand, you consider very distant galaxies, then there may be a huge difference in how they look to us, and how they are today. For instance, the most distant galaxy observed, GN-z11, is seen as it were less than 100 million yr after it formed, and is quite small. But today, it is probably an evolved galaxy like the Milky Way. It may have (and probably has) merged with other larger and smaller galaxies, so we can't say anything meaningful about that particular galaxy's fate [cont.] – pela Mar 19 at 14:31
• […] But that doesn't matter, because by observing that and many other galaxies at various distances — and hence at various times throughout the history of the Universe — we can say something meaningful about the typical, and also about 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 it emitted the light we see. What we do care about, and actually know, it what happens "on average" to galaxies. – pela Mar 19 at 14:35
• Thank you for the clarifications. Seeing it in actual numbers is what I hoped for. – Robert Fallows Mar 20 at 19:08

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).

• Since I asked the question/concern originally I’ll reply that I appreciate the feedback & am in agreement with all who have responded. The speed of light allows us to look back & gain knowledge of the universe’s evolution. BUT what is the universe like in the present? We are beginning to look for exoplanets & believe there are some that maybe like earth. However we are seeing these planets as they were. In the time that light from them has reached us they may no longer exist or, on the other hand, developed intelligent life. Maybe they are looking at our planet & see it before humans existed. – Robert Fallows Mar 7 at 15:08
• @Robert We're looking for exoplanets in our own galaxy, and the ones we've found are a few thousand light years away, at most. – PM 2Ring Mar 12 at 1:12

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.

• Your reference to a “light cone” is something I hadn’t considered but very interesting. In addition I have been thinking about how objects are accelerating away from everything else and the acceleration is increasing. I guess it is just the nature of the universe that we may never know what is actually presently occurring in the farthest areas of the universe. We may at some point be able to see “first light” after the Big Bang but not know the actual present structure of the universe. (Yes we can theorize but that’s about it). A little ironic. A bit of a cosmic joke. Sorry, couldn’t resist. – Robert Fallows Mar 12 at 1:44