An Astronomy Question

Re: An Astronomy Question


The universe expands constantly in all directions, not just from a central point. It doesn't get 'smaller and smaller' no matter which direction we look.

Cheers

Cycloptichorn

Quincy
Cycloptichorn speaks true.

However, it's difficut for me to visualize everything expanding equally from all points. It seems there should be one center somewhere. However, believe me, the earth is not that center.

The best way for me to understand this is the old "taping a coin to a balloon" thing.

If you taped several dozen coins to a balloon as it expands, being blown up with air, you would see that the space between the coins increases equally. There is no center from which they are all running from.

Also, is the universe a sphere? I don't think so.

There's another thread from a while back where this is discussed. I'll try to find it.

Meanwhile, read Michael Greene's "the elegant universe"

The size of the universe has been increasing since the big bang. However, let's say you consider a sphere with a radius of only twice that of the Earth. You are looking back some tiny fraction of a second in time. Next consider a sphere with a radius three times that of the Earth. You are now looking back a slightly larger, but still tiny fraction of a second. Next consider a sphere four times the radius of the Earth, etc. The point is that as long as you are dealing with distances much smaller than the size of the universe even billions of years ago, and times much less than the time which has passed since the big bang, you won't run into complications. If you start to think about spheres on the same scale as the size of the universe, things will become more complicated. I should also add that on that scale, even the definition of distance becomes more complicated, since you run into issues of the curvature of space, etc.

I think only brandon gets the idea of what im trying to say. I willl try to re-word it.

I know we are not at the centre of the universe, but it is merely a tool for thinking about what I was trying to say.

The greater the distance away from us that we look, the order of millions of billions of parsecs, then we also look very far back in tim, like in the order of magnitude of billions of years. But, if we are looking further out, the "surface" or area we are looking at is greater (the whole centre/sphere/radius thing I was going on about). But, if we are looking further back in time, we know the universe was smaller then, because it is expanding now.

So, the further away from us we look (from our solar sytem yo our galaxy, to our local cluster, to groups of clusters, super clusters etc.) the greater the area we can see, but the smaller the size of the universe at the time (billions of years ago).

I hop that makes it clearer, because I don't think anyone really got what I was trying to say, except Brandon, who didn't really answer me on the scale I was thinking of.

Most of the assumptions that you are using are erroneous. Some of which I have no doubt you have read somewhere. There are two problems with popular level science materials: one is that many articles on science are not written by scientists but rather by science writers who interpret what the scientists are trying to say. Most science writers these days, especially those writing for general consumption, have no particular science background. So more and more of the articles that I read have errors in them, some ridiculous errors. During Mars most recent close approach many news agencies were reporting the closest approach of Mars for x number of years. They had both how long the interval was wrong as well as being off by 6 months from when it actually happened. It would not surprise me if a survey showed that the average science writer had a background as a paranormal investigator rather than a physics degree. The second problem is that unless you want to get very bogged down in technical minutia you have to give approximate answers that help conceptually but are incorrect. If the reader then takes this information and tries to draw inferences they will almost always be wrong.

I'll try and help out with some of these but this type of forum is a poor place to try to explain complicated concepts. If you want to believe any of my explanation, fine, if not that is fine too. Some of the things I say will only be approximations but the detail would be very long and complicated.

First there is no "center" of the universe as you are describing it. The universe started form nothing and is expanding everywhere in all directions. To use your balloon analogy, the universe is the balloon itself. This is three dimensional space and the universe fills it. What you are trying to do is to point to the "center" of the balloon. The problem is how would you point to it? You can only point to one of the three spatial dimensions, which are all on the surface of the balloon. Quick...point in a direction that is at right angles to reality. Until you can figure that out I think that you can see the problem. It is actually even more complicated that that.

You want to point to the center of a "hollow sphere" that you describe the universe as. I am sure you have read that analogy. It is absolutely wrong. This does not take advanced math or blind faith in what I am saying to easily understand. The universe cannot be spherical because a sphere is a three dimensional object in Euclidean space. So by definition it cannot be spherical. Since the universe fills three dimensional space how can it be curved in that space? To try to visualize this: Take a cube of rubber. Define this as all of three dimensional space. You are standing outside this space as you look at the cube. Now curve it, within those three dimensions. You can't.

The second problem is that space is not Euclidean. So even if you could manage to do the above it wouldn't be relevant as you are working in the wrong geometry. Space is actually described by Minkowskian space on a Riemannian geometry. You need the Minkowskian space to correctly incorporate time. So you could say that the universe curves into a fourth dimension in a Remanian geometry and creates a hypersphere. But a hypersphere in a Riemannian space does not have a center in the way you are probably visualizing it. Just to define that center requires verrry advanced math concepts (differential forms). And the way you would calculate the radius is NOTHING like you are doing.

But it gets even more complicated...The expansion of the Universe was not uniform over time. In fact, taking inflation into account it is just the opposite, the rate of expansion varied with time to a degree that is unimaginable. So at times space was expanding at a rate of at least 1 x 10 exp 30 times!!! faster than it is now.

I know this is getting a bit long. So to leave you on a positive note I would say the following. You can do, sort of, what it is you are trying to do. But you would be talking about light cones on a Minkowski space in a Riemannian geometry. When you look back in time think of looking down an ice cream cone (the kind with the pointy bottom), from the top to the base. As you look you can see the area of space getting smaller and smaller until you reach the bottom tip. If you want you can then think of this point as a center from where the universe started and your "radius" calculations would be more relevant. Please not that it is actually even MORE complicated that this but I hope it at least gives you an idea of why your assumptions and calculations are in error.

As this is a web forum I fully expect many will come along and say this is all wrong and many will probably say that I am even incorrect about the existence of ice cream cones, but what can you do?

Thanks for your answer, but I still think everyone is missing the point of what i'm saying. I never said imagine the centre of the universe, but rather we are the centre of our viewing point. And when i said a sphere i merely meant looking outwards in all directions.

In fact TheCorrectResponse I have read most of what you are saying in science books written by physicists.

I think I'll just drop the question, because no-one seems to understand what I'm saying. I knew I wouldn't be able to effectively communicate what I was thinking about

I agree that this type of forum is not a good place for this type of question. Since you say you are familiar with the concepts I described I'll leave you with a clue in case you want to think about this on your own. You are describing a Euclidean problem on a non Euclidean space. Also, the increased area you are talking about is only correct if you don't take into account that these are not simultaneous areas they are space-like separated. Both of these facts make your increased area, at best, an illusion. Face-to-face with a blackboard is the only way I know to really get into these topics.
Sorry I couldn't help. But as I always tell students the motto I live by is: There are no stupid questions, only stupid people! Some seem to take this as an insult, the ignorant prigs!!

I reread your question more carefully. Your assumption is:




But then you say:



Simply, the assumption that you are somehow able to look along this radius as it expands is wrong. This would not be looking back in time it would be going back in time. It is the equivalent of looking at how a past time was by actually looking into the past. At the telescope I am only looking into the present at photons that have reached me from the past. It is the coincidence of the physics that light affords me this pleasure.

Ah, well thanks for your answer, now I know im wrong, but the reason why im wrong is probably to complicated for me to understand. I guess I will probably never get real insight into this. But thank you everyone for your answers. Appreciated.

Re: An Astronomy Question

Quincy, I have wondered the same thing myself.

Light currently reaching earth was emitted billions of years ago from galaxies on the surface of a sphere 40 million light-years in radius and centered on earth, but intervening space has expanded (ant on a rubber band analogy) so it is now a much larger sphere. Galaxies are gravitationally bound so the expansion only takes place in the voids, but I would expect some magnification in the apparent size of galaxies. I don't know if this is actually observed. The galaxies should also appear to be much farther apart since the number if galaxies remains the same but spherical surface of their apparent position is much larger than the one they occupied when the light was emitted. Perhaps someone more versed in astronomy can clarify this.

Don't forget that what you see was the position of the stars/galaxies those billions of years ago. They might not be there now but farther away.

Didn't read everyone's reply so here is mine:

Be very clear that the light you are seeing when you look deep into space is not looking "back in time" as it has been referenced a few times on this thread.

You are seeing old light from stars that exsisted a long time ago and it just took very long for it to get to us so we could see it, and to see it we have to use very powerful telescopes that can gather the light because we are so far from where the light originated the radius is also very large so we receive a very small percentage of that light from the expanding surface area (thought of as a sphere).


ok lets see if i answered the question...

"the longer the radius, the greater the area we must be able to see"

here is your problem right here. From this, you observe that if the radius (distance from us) increases, then the "surface area" of the sphere is going to expand which is true, the deeper you look into space the more you are going to see and more things are going to appear inbetween the things you can already see.
Like when you look at the stars at night, stand in darkness for 15 minutes so your pupils dialate and your eyes will gather more light and you will see more stars.

"But, the universe gets smaller and smaller the farther back in time because the universe is expanding, but the area we can see gets larger! How can we reconcile these two thoughts?"

If the radius is the distance we can see into space, things that are farther away will appear smaller and more numerous (cause we will see more, more surface area)

Ill say more if this doesn't make it clear.

Now I will ask,

I'm sorry your explanation does not really clear things up for me ScienceLawyer.

Re: An Astronomy Question
Quincy,

Your question IS worded badly, but I get you. Let me rephrase it:

If you look far away you see the microwave background radiation (CMB) that we attribute to the big bang. You see it uniformly in alll directions, meaning that it fills a huge space. Yet, you heard that the big bang started from a small point...so why, when you look back in time, does it appear to be everywhere?

The answer:

Well, you aren't looking back through a portal in time to see CMB. Think about it like this. Take a balloon, fill it with gas -- thats CMB. Now expand the balloon (thats space expanding). Then looking out from the center, you see CMB in all directions even though it started in a small point. Whenever you look at ANYTHING you are looking back in time because it takes time for light to travel to your eyes, but you aren't looking back to the time of the big bang itself, you're just seeing residual radiation in this case.

Quincy's question
What we observe when looking through a telescope, is an image of what the object that emitted the light, looked like at the time the light was emitted,not really looking back in time.To look back in time, one would have to travel back in time.This is thought to be impossible by many, but is still debated.

Also, the expansion of the universe is not like that of an explosion. The term "Big Bang" gives a false impression and should never have been used to begin with. The expansion of the universe is more like a loaf of raisinbread cooking in an oven. As the loaf cooks the raisins move away from each other. Consider the raisins as galaxies. The loaf is infinite. An observer on one raisin sees the expansion from their reference only. Another observer on a distant raisin sees a different "sphere" of observation. This is an inacurrate representation of the universe because it is limited to 3 dimensions and the "real" universe is expanding in at least 4 dimensions. For a better representation one has to use math that is beyond me.

I hope this helps as it is my understanding only, and I am just an enthuiast.