How far do we have to see, in order to see the milky way in its youth?
So far the most distant object ever recorded by human instruments is this galaxy we spied using gravitational lensing from another galaxy. Here is a link to the info on this: http://www.seds.org/hst/97-25.html
The light coming from this galaxy is roughly 13 Billion years old.
The universe is estimated to be 14-17 Billion years old by most cosmologists…so we must be approaching the point where we should be able to see “all the way around” the universe and see the light that left our own Milky Way so many billion years ago.
Anyone have any insights or comments on this?
Anyone have any links to astronomers who discuss this?
Well, there are two scenarios by which this might be plausible. In the first, the Universe is positively curved, which means that the shape is something like the surface of the Earth, but in three dimensions rather than two. In this case, in any direction you looked, your line of sight would eventually end on your starting point. It seems, though, that the Universe is not positively curved, or if it is, then the radius of curvature is far greater than we could hope to see. Even if it were positively curved, most models of the expansion have the Universe expanding fast enough that we still wouldn’t be able to see all the way around. So, we can discard that idea.
There is another possibility, though. It may be that the Universe is similar to a videogame screen, made up of a single “cell” which repeats indefinitely. Go off one edge of the cell, and you come on on the opposite edge. If this is the case, then you would be able to see your starting point by looking in some directions, but not in others. This also works for any choice of curvature, although the cell would be different shapes for different curvatures. Nobody considers it likely, but it’s possible that this cell would even be smaller than the observable Universe, so we could be seeing the birth of the Milky Way right now, and not recognize it. The MAP satellite, currently in operation, should be able to resolve this question with observations of the cosmic background radiation.
So let’s assume that the Universe is positively curved as you say. And if you look long enough in any single direction eventually you should be able to see yourself.
If you are right that we cannot see the Milky Way because the universe has always been expanding so fast that the light from the Milky way has not reached “all the way around” yet…then this has some startling consequences. (I think!)
If you are right, then the very first photons that escaped from big bang have not made their way all the way around the universe to us yet? That is, whatever the Milky Way was at the very birth of the Universe…the photons from it have not yet made it around the Universe and hit Earth?
If this is true, then when we create a Telescope capable of seeing far enough…say 17 Billion light years+…we will be able to “see” the first moments of the Big Bang, basically the birth of the Universe.
So either:
the photons from the moments after the big bang for the milky way haven’t reached us yet-meaning we should be able to “see” the birth of the Universe for at least some Galaxies that are closer to us then the circumference of the Universe
or the photons from the birth of the Milky Way have circumscribed the Universe and we should be able to see the Milky Way in some point in its youth.
We can’t see all the way around the universe - yet.
In the simplest positively curved models, we can see all the way around precisely at the moment of greatest expansion - just before the Universe starts re-collapsing. Depending on your assumptions, the moment at which this occurs may be different.
However, we do see photons from shortly after the Big Bang. It’s called “microwave background radiation”. It shows us a time about 100,000 years (I think) after the BB. We can’t see further (“earlier”) than that because the Universe ws opaque before tht time due to all the free charged particles lying about.
This radiation is starting to tell us quite a lot about the early Universe and how galaxies were formed, thanks to recent advances in satellite measurements.
Even if the Universe were positively curved, you could never actually see the back of your head, or the Milky Way in its youth, because you’d have to look “past” the big bang in order to do so. However, the Universe didn’t uncouple (become transparent) until it was about 100,000 years old or so. In other words, if you try to look “back” beyond a certain point, all you see is a glowing haze.
If the universe were small enough, the haze would eventually clear. However, the best bet is that the universe is much, much larger that what we can observe. The limit of our observable universe isn’t the size of the universe. Rather, our observable universe is just a bubble defined by the distance that light has been able to travel since the big bang. What we see is a horizon with lots and lots of stuff (probably) beyond it.
Under this scenario, the haze will never clear. It will just red shift, or if the universe is positively curved and does collapse eventually, blue shift. . . . I think.
I bet The Bad Astronomer has something about this in his new book, Bad Astronomy by Philip Plait that just hit the bookstores last week.
I’m not sure about seeing “all the way around” the universe. The furtherest we will ever be able to see is about 15 billion light years (at the current best estimate of Hubble’s Constant). This might be though of as a horizon. Like standing in a small boat on the surface of the sea the furtherest you can see is in the neighborhood of 8 miles, even though we know the earth extends much further than that.
Actually, the “horizon” keeps moving further and further away. We can see things 15 B ly away because the Universe is about 15 B years old. When the U. is 20 B years old, we will see things 20 B ly away. Roughly speaking, because the expansion rate is not constant.
To connect with the Earth analogy, imagine the Earth is a big balloon that is inflating. As the radius of the Earth increases, you can see farther and farther since the Earth is getting flatter.
You are quite right and thanks for pointing that out. However, as the analogous balloon gets bigger you will have to see even further to see all the way around it. Wouldn’t it be likewise with the universe? Assuming, of course, that “around it” makes any sense at all.
I might have been a little too hasty in agreeing. This is a concept about which I get easily confused. If you are an astronomer, or close to it, maybe you can straighten me out if what I say below is wrong. Or maybe the Bad Astronomer will drop by.
In any case, cosmological distances are measured by the use of the red shift of the light. The red shift is used to determine the recession velocity of the object in question. This velocity is then divided by Hubble’s Constant to get the distance in light-years, parsecs or whatever.
Now the greatest velocity of recession that can be measured is the velocity of light (I think). The velocity of light divided by the current estimates of Hubble’s constant yields and answer of about 15 billion years. If Hubble’s Constant is a constant then we will never be able to measure a greater distance than that.
If dividing the velocity of recession by Hubble’s Constant is a legitimate method of finding the age of the universe, and if the velocity of light is the fastest we can measure then when the universe gets to be 20 billion years old, Hubble’s Constant will have to have decreased during the intervening 5 billion years.
Actually, you measure Hubble’s Constant by independently measuring the distances and redshifts of objects, not the other way around. And Hubble’s Constant almost certainly is not a constant: The only way it can be is if the Universe is expanding exponentially, which it doesn’t seem to be doing. If the current acceleration keeps up, though, then exponential expansion will end up being a very good approximation.
Well, this is a sort of misleading statement, isn’t it? It is true that Hubble’s Constant is determined by using certain so called “standard candles” that are within direct measurement distance. Their distance away from us is measured along with their red shift. Assuming Hubble’ Law; i.e. increasing distance means increasing velocity of recession, you can get a number for Hubble’s Constant.
Having once gotten that number you then use that and the recession velocity (obtained from the red shift) to compute the distance to other objects whose distance can’t be measured directly.
Well, you can do that (and often do, because redshift is so easy to measure), but that doesn’t give you any new information. If you’re using the distances to objects to get the parameters of your cosmological model, then you might as well just choose your objects to be the same standard candles that you used to find H[sub]0[/sub].