Well, if you define “few” as 150…
JS Princeton:
Technically we do not know what is going on now in a galaxy several million light years away from us; for that matter, we do not know what is going on now in the vicinity of a star several dozen light years away from us either. But it is my understanding that most of the existing theory concerning the origins and evolution of the universe are based on the assumption that what we see when we look at quasars and galaxies ten or more billion light years away is representative of what the universe in general (including “here”) was like ten or more billion years ago, and that it (the universe, both “here” and “way the heck out there”) did over the intervening eons become a universe of which the local environment is representative. No astronomical theorist with whom I am familiar suggests that we are at the center of a sphere the outer shell of which is densely populated with quasars which are rare-to-nonexistent here at the core; or that cosmic youth occupies the outer perimeter and the older part of the universe is where we are found, etc.
Good catch. Indeed, I should not have said “ago” which is entirely incorrect (unless a few is defined as 150). The dangers of uncareful editting are legion.
This is correct, however we need to make a distinction between what is observed and what is practically assumed. For all intents and purposes, we are talking about looking at systems which are statistical probes of the universe at a given epoch. Since those are the only things we can see, that is the way the universe is. Handy, huh? The problem I have with your statement is the “including here” part. “Here” is defined with position but whenever you’re dealing with the universe we deal with position AND time. There’s really no way to separate the two of them. So when I say “here”, for all cosmological purposes, I’m talking about here and now (now, in fact being roughly the same as 5 billion years ago!). We can extrapolate backwards on our worldline to a time-like separation between two events, but in actuality our ability to talk about absolute positions in space and time is limitted by the laws of special relativity and our past light cone. Here now and alpha centauri “now” are separated space-like. Here now and alpha centuri in “five years” time are actually separated time-like. Those are the only real meaningful distinctions we should make as honest scientists. This is why astronomers just speak about the observable universe and don’t worry about the silly effects of propagation through space and time (which are not absolutes). What we see is what we get… and we keep in mind the space AND time that is separating us from the events we witness.
Right, except in the limit where one side of the sky is out of causal contact with the other, then there’s no reason to assume that there should be any sort of uniformity. This is one of the reasons why we have inflation models.
I should hope not, because we don’t live exclusively in three dimensions! However, you have illustrated, to some extent, exactly what is observed. The observable universe, you see, is not as pleasant a physical object as say, an electron, or the Earth, or even the next-door neighbor galaxy cluster. All of these objects are effected by processes other than simply the transferance of information. Talking about the “observable” universe neccessitates talking about what information has been transferred to the observere. There’s really no other decent definition. The observable universe itself is spread in both space and time, and so speaking about the way the observable universe is “now” really doesn’t make much sense. “Now” in which reference frame? Why, the one you’re in, of course! That means we observe the observable universe from Earth, the further out we go, the earlier it is. That’s just the way the observable universe is set up!
In case you didn’t notice, in the last paragraph I’m being rather silly in keeping the adjective “observable” in front of all my universes. Often this is adjective isn’t included, but unless we are told otherwise it is assumed that “observable” comes along for the ride whenever an astronomer talks about “the universe”.
I certainly wouldn’t go that far. We have some perfectly good foggy clues as to where “that” galaxy is “now”. We don’t know it of an absolute certainty, but then, we don’t know anything of an absolute certainty. Your statement is comparable to me saying that I haven’t the foggiest idea whether my mother is currently alive. I just talked to her last night, and at that time she was in good health and at a reasonably safe place, but I haven’t observed her now.
I get the impression that this is all off-topic, though, since I suspect that the OP is confusing “galaxies” with “solar systems”. A galaxy consists of, in the immortal words of Carl Sagan, Billions and Billions of Stars (actually a huge understatement, but I don’t feel like looking up the number). When you get to numbers like that, it’s very probable indeed that you have life somewhere in any given galaxy.
A solar system, by contrast, consists of one or a few stars, together with any planets, comets, asteroids, etc. that may orbit it/them. As of right now, we have only discovered a very small number of planets around other stars (in the neighborhood of a hundred or so), and none of those would be capable of supporting life as we know it. There’s a catch, though: The only planets we’ve been able to detect so far are extremely large ones, extremely close to their parent star. For all we know, every star could have Earth-like planets around it, but we wouldn’t be able to detect them with our current technology.
So, then, what are the best bets for discovering nearby habitable planets? Well, it’s probably difficult to have habitable planets in a multiple star system, which rules out a lot of them. And you don’t want your parent star to be too hot or too cold, either: If it’s too cold, then a planet warm enough to be habitable would be so close that it’d be tidally locked, with the same side always facing the star, and if it’s too hot, the star won’t last long enough for life to develop. We know that G type stars can have life-bearing planets, since that’s the type that the Sun is, and probably F and K types as well, since those are the types closest to G.
In that case, our best bet close to us is probably the star [symbol]t[/symbol] Ceti, a single G, or the star [symbol]e[/symbol] Eridani, a single K, both of which are within a handful of parsecs from us. We haven’t discovered planets around either of those, but some might be there, and if so, one of those planets might be reasonably Earth-like.
Quite right! The interesting thing to note is, of course, that the closer (in time and space) an observed galaxy is to us, the less foggy the clue is. We’re talking about particular galaxies here that, over billions of years, end up in great-grandfather’s-axe-type problems. To predict where Andromeda, being a relatively young 100-million-year-old observation, is “now” is far less foggy a prediction than where the most distant observed galaxy is “now”. If you hadn’t talked to your mother in a few years you’d be less certain about her condition than you are since you just called her last night. We’re talking about “particular” galaxies at this point, so it is relevant to consider the amount of time that has passed. In general, however, we can look at these galaxies as laboratories and give an overall view of where “most” of the galaxies we see at a given epoch are projected to end up. That’s the beauty of the physics.
Well, yes. I would not make any guesses whatsoever about any individual galaxies that we observe (courtesy of Hubble and some serious gravitational lensing) via 14 billion year old light. I would not say things about what they are like now, where they are now, how much farther they have traveled by now, etc.
But I was making assertions about galaxies of that distance in a broad general sense, and saying that their nature by this point is most likely akin to the nature of the galaxies in our own neighborhood. I was also saying, inversely, that our neck of the universe was probably richly studded with quasars 14 billion years ago.
I don’t know these things for sure–my only data concerning what our chunk of universe looked like 14 billion years ago is the 14 billion year old light that is available to us. From every direction we look, it looks like a quasar-rich high-energy young-galaxy world out there (which is, as you said, also “back then”). So my surmise about how things were around here back then is based on the only “back then” data that I have, which comes from “out there”.
Similarly, my only data concerning what’s out there now (as opposed to 14 billion years ago) is the light that comes from much closer to home. The nearest 3 billion light years’ worth of universe is decently consistent and, since I know the light from farther out is older light, it is probably more accurate to assume that today’s universe is pretty consistent with what is nearby. So my surmise about how things are out yonder as of today is based on the only “now” data that I have, which comes from local sources.
I understand what you are saying about time and light and spacetime and “is”, but we are creatures to whom time occurs, and passes, and observations and models don’t make much sense to us without that reference.
If I were to adopt your habit of speaking only of the “observable universe”, deprived of the ability to speak of the entire universe in its entirety “then” or “now” (or in-between), I’d be hard put to craft a theory to explain the origins of the universe, no?
Not in the least. In fact, when we talk about the “origins of the universe” I would say we are basically talking about the origins of the observable universe.
This is part of the reason the question of “what is beyond the universe” is impossible to answer. The answer is, “we don’t know, we can’t look there!” This doesn’t stop the theorists from guessing, conjecturing, and making intelligent hypothesis. But in all honesty, it’s what we have observed that counts.
For example, if I talk about the origins of the universe, I may be forced to deal with concepts of inflation. This means that stuff that was initially in causal contact gets flung out of causal contact. When it gets flung out of causal contact, it leaves the universe (similar to how light that falls into a black hole “leaves” the universe). Just because that happens, does that mean the theorist forgets about it? Nah! The theorist still deals with it. To be sure, though, an inflationary origin of our “universe” is the origin of our observable universe. The short-of-hand notation observationalists use when talking about the “universe” in general is just that, short-hand.
Chronos rightly pointed out the danger in taking this line of thinking too far, that one can lapse into a kind of lazy observational solipsism of sorts saying that if one doesn’t observe it, it doesn’t exist. This is not what I’m advocating at all. I’m simply saying if it is impossible to observe it, it isn’t in the observable universe. To say anything else is tantamount to saying the future (which hasn’t happened yet) is in the observable universe. It’s not! It may be in my projected future light cone, but it is not in the observable universe. Yet.
Of course there are other considerations to make other than simply what’s observable (especially in theoretical cosmology and models of large scale structure evolution), but then such models are couched in terms of the assumptions and principles that are employed to reach their conclusions. For example, when we talk about “bubble universes” we aren’t talking about the observable universe, rather we are talking about things that are inherently UNobservable.
So, when we talk about the way the universe is “now” or the way the universe was “then” we have the advantage of being able to do it in two different ways. One way is to go out and LOOK. This is the observational side of astronomy. The other way is to conjecture. This is the theoretical side of astronomy. They both offer us views of the universe, but when we are talking about OBSERVED pheonomena (getting back to the actual words of the OP if not the actual intentions), then we are talking about the OBSERVABLE universe.
Similarly, if I talk about a universe populated with quasars, I’m talking about the part of stuff that “exists” that has a time-like separation from us, and as such, might just as well be like that stuff which has the same time-like separation in a different direction (say in a projected worldline back from right where I’m standing). This is an observed feature of the universe, and this way of describing the universe is separated from us in time-like ways. That is the stuff we really “know” about from an observational point of view. That is the universe. A theoretician makes makes predictions about stuff that is separated from us in both time- and space-like ways. We assume some level of homogeneity to science, so their predictions are supportable. The whole of existence to them is not simply that which we see, but also that which we can predict without seeing. So, perhaps what this REALLY gets down to is the age-old conflict between the observationalist and the theorist.
Point being that both groups NEED each other. While I may tend toward the observationalist camp when describing the universe, that doesn’t invalidate theorical considerations of the way some quasar will evolve since the light we see has left. It actually becomes more of a philosophical/epsitemological question more than anything else.
There is one problem… when you say, “my only data concerning what’s out there now (as opposed to 14 billion years ago)” what you are really saying is “my only data concerning what’s separated from us by the same magnitude, only a different sign for the metric measurement”. This may seem nitpicky, and certainly is a cumbersome way to talk, but it really does address the the continuity problem of what is and isn’t in the universe more precisely. This is why I usually put the “now” and “then” in quotation marks, because it is all relative.