Is Earth At The "Center" Of The Universe?

zeestephen · Feb 10, 2008

I do not understand why an observer on Earth can point a telescope in ANY direction and find radiation from the earliest galaxies and from the Big Bang. Unless the Earth is at the "center" of the universe, it seems like the view from Earth should be lopsided. I do not understand how a "flat balloon" or a "flat saddle" shaped universe solves this problem. For example, let's say the Andomeda Galaxy is 12.5 billion years old. All but 2.5 million years of its radiation would have passed by Earth and disappeared forever. Under this hypothetical, if we look "past" Andromeda, why isn't there a big blank nothingness "behind" it since no galaxies existed before 12.5 billion years ago? I have the same question about the CMB. How is it that ALL the CMB radiation is arriving from ALL directions at the same moment? Why are there not big blank spots in the sky where the CMB has already reached Earth, and passed it, and disappeared forever? And here's an easy question. The "thickness" of the Milky Way is about 1000 LY. What is Earth's relative position, in light years, to the "top" and "bottom" of the Milky Way? Thanks to all for helping!

nimbus · Feb 10, 2008

You're not seeing the full picture. The expansion of the universe was faster than light, and still is today when you account for the cumulative "co-moving" distance between far objects. The center of the universe was nowhere and everywhere. Center is relative to edge, and there's no edge. The last thread on this topic, you'll find more of these centrism questions earlier too if you search. <div class="Discussion_UserSignature"> </div>

gemini59 · Feb 10, 2008

It could be that the earth as well as Andromeda are expanding at a constant rate. Therefore, the distance the radiation has to travel would remain a constant? Do we really know, has light been measured from N,S,E,W?

gemini59 · Feb 10, 2008

I agree on the co-moving, I call it a constant, but I believe it has an edge. We just con't know where it is because it is constantly expanding, or I should say still expanding. I believe it will never stop. If time does truly exist, no matter that our lives are short, we are still part of the beginning.

weeman · Feb 10, 2008

Speedfreek....calling Speedfreek....where are you!? <img src="/images/icons/smile.gif" /> Earth is at the center of the observable universe, not the material universe. The observable universe means that every direction we look into space, we see everything moving away from us, and all light that we see gets progressively older the farther we look into space. The material universe is the exact nature of the universe as it exists today, it is not how we actually see the universe. The observable universe has a visible radius of about 13.5 billion lightyears, while the actual radius of the universe (from Earth) is more like 45 billion lightyears. This is because the speed of light is finite, the light from distant galaxies doesn't reach us instantly, it takes time for it to reach us, since the universe is so massive! In other words, light leaves an object that was 3 billion lightyears away when it was emitted. The light had to travel against the expansion of space to reach us. So, in that time, the object that emitted that light is now farther away from Earth than it was when it first emitted the light. This means that the object appears to be 3 billion lightyears away, when in reality, it may be more like 6-8 billion lightyears away now. This is the difference between the observable universe and the material universe. There is no center to the universe, because every point of space expands away from every other point. This is where the balloon analogy comes into play. If you imagine evenly spaced dots scattered across the surface of the balloon, you will notice that no dot is at the center. And, as you blow the balloon up, you will notice that every dot moves away from every other dot, and no dot can be concluded as the center of the balloon's surface. The thing to keep in mind is that the big bang was not an expansion IN space, the big bang WAS space. In other words, the big bang happened everywhere, hen <div class="Discussion_UserSignature"> Techies: We do it in the dark. "Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity." -Albert Einstein </div>

SpeedFreek · Feb 10, 2008

Apologies in advance for the long post. <img src="/images/icons/smile.gif" /> There are different ways of measuring things, due to the speed of light. If it takes light from a galaxy 1 billion years to reach us, we are seeing that galaxy as it was 1 billion years ago. How large it looks gives us an idea of how close it was to us when it emitted that light. What we see when we look out into space are galaxies in all directions. Some are bright and easy to see, others are very dim indeed. We find that as we look at the dimmer galaxies, the wavelength of their light seems to have been lengthened, shifting the spectrum of their light towards red - this is called redshift. Now we also see the opposite effect, blueshift, in galaxies that are relatively bright and thus pretty close to us, but all the more distant, dimmer galaxies are redshifted, and the dimmer a galaxy, the more redshifted we find it to be. Now when we look in all directions we see pretty much the same thing, past a certain distance, the dimmer the galaxy the higher it's redshift. Now we know that light travels at a certain speed and that light spreads out as it travels, so we think we can estimate how old the light from a galaxy is by looking at its magnitude or brightness, amongst other things. What this gives us is a picture of a universe that is around 13.7 billion years old, as Weeman said. The most dim, most distant galaxies we can possibly see are less than 13.7 billion years old. If we want to, we can say our observable universe is 13.7 billion light-years in radius, as we can see no light that is older than that. We are in the centre of an observable sphere of space, and light has only had time to travel for 13.7 billion years from it's edges to its centre. Now then we come to what the blue and redshifts represent. If you had a look at that redshift link you should have a <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

weeman · Feb 11, 2008

When our observable universe was 3 billion light years in radius there might have been galaxies 6 billion light years away, and their light would now just be reaching our most distant galaxies! Our most distant galaxies might see a similar picture to ourselves, with galaxies receding in all directions and our galaxy, as it was 13 billion years ago, on the edge of their observable universe. Very interesting, I never really thought of it like that! Although, I'm still not 100% clear on this statement. Are you saying that there could be galaxies that are even more distant than the most distant galaxies in our observable universe? <div class="Discussion_UserSignature"> Techies: We do it in the dark. "Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity." -Albert Einstein </div>

brigandier · Feb 11, 2008

From the surface of a perfect spherically inflated balloon, every point appears to be the "center" of the surface. It's the same concept

ctrlaltdel · Feb 11, 2008

I don't understand. If the universe has finite dimensions, then it shouldn't matter if it is expanding because you can always divide a finite number by 2 and thus find the centre. I understand the balloon analogy when used to describe how the universe is expanding, and how it's meaningless to discuss the centre of a surface that folds back on itself so every point is relative to the next, but the universe isn't shaped like a balloon: You can't travel back to your starting point if you go far enough, therefore the concept of a centre to the universe is valid surely?

SpeedFreek · Feb 11, 2008

Are you saying that there could be galaxies that are even more distant than the most distant galaxies in our observable universe? Yes, that is what I am saying. <img src="/images/icons/smile.gif" /> The reason we use the term "The Observable Universe" is that we do not know how much of the Universe is within our observable volume. We seem to be in the centre of an observable spherical volume of space whose radius is defined by the speed of light combined with the age of the Universe. We theorise that a fraction of a second after the Big-Bang, our observable universe was the size of a grapefruit! 380,000 years later, when recombination occurred, the universe became transparent and the CMBR was emitted throughout the universe, our observable universe is theorised to have been around 40 million light years in radius. The rate of expansion was so fast back then that the radius increased from a few centimetres to 40 million light years in only 380,000 years. If we move forward to the present, 13.7 billion years after the Big-Bang, our observable universe is theorised to be 46.5 billion light years in radius. But back when the observable universe was the size of a grapefruit, the whole universe could already have been any size larger. It might have been the size of 2 grapefruits, a mile across, a lightyear across, a billion light years across - any size (even infinite!). Let's take an example where the whole universe is 10 times the size of our observable universe. Back in that first fraction of a second, our observable universe was the size of a grapefruit - say 10cm in radius. Imagine if, at this point, the whole universe was 10 times larger - 1 meter in radius. Now move forward 380,000 years to when the universe became transparent and the CMBR was emitted. At that time, our observable universe, filled with the CMB radiation, was around 40 million lig <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

SpeedFreek · Feb 11, 2008

One of the reasons for the balloon model is that it is a simplified model of how the universe can expand but have no centre, but it is simplified by reducing the dimensions we are considering. The balloon model is modelling a 2 dimensional universe that expands through a 3rd unseen dimension. These models are referred to as manifolds. A manifold is simply an abstract mathematical space in which every point has a neighbourhood which resembles Euclidean space, but in which the global structure may be more complicated. A 1-manifold describes a structure where every point has a neighbourhood that looks 1 dimensional, but the structure is actually 2 dimensional. (your surroundings look like a line, but the overall shape is actually a circle. You cannot see that curve as you live in a 1 dimensional manifold and can only perceive 1 dimension.) A 2-manifold describes a structure where every point has a neighbourhood that looks 2 dimensional, but the structure is actually 3 dimensional (surroundings look like a flat disc, but shape is actually a sphere). This is the balloon model. A 3-manifold describes a structure where every point has a neighbourhood that looks 3 dimensional, but the structure is actually 4 dimensional! (your surroundings look like a sphere, but the shape is actually a 4 dimensional object). In all these cases, if you somehow mark your starting point, you can move in (what seems to you to be) a straight line, but you end up back where you started, so you might be able to work out the shape of the overall structure without being able to actually see that structure. Of course, when it comes to our universe, the furthest parts we can see are receding from us at speeds faster than light, so that theoretical straight line might loop round and meet itself, but we could never actually circumnavigate it. General Relativity models the universe using a 4 dimensional manifold where the dimensions of space <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

weeman · Feb 12, 2008

That's a very interesting way of looking at the overall scale of the universe. If only we could live on for another 5 billion years, we would find that there may be much more to the universe that we can't see yet! The idea that the universe could be much larger than 45 billion lightyears radius would increase the chances of intelligent life tenfold! If the material universe is so large that the majority of its light hasn't even reached us, then the number of total galaxies could easily be in the trillions or higher! Interesting stuff <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> Techies: We do it in the dark. "Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity." -Albert Einstein </div>

qso1 · Feb 12, 2008

zeestephen: Unless the Earth is at the "center" of the universe, it seems like the view from Earth should be lopsided. Me: Earth may well be at Universe center but we don't really know. One way to measure this would be to measure the most distant objects in all directions. Example being, the most distant object looking south, ditto looking north etc. The distribution of matter in the Universe is random at best so we do not actually see objects at the limits of our technical capability no matter what direction we look. zeestephan: And here's an easy question. The "thickness" of the Milky Way is about 1000 LY. What is Earth's relative position, in light years, to the "top" and "bottom" of the Milky Way? Me: Good question. The Milky Way is about 100,000 Ly in diameter and Earth is around 30,000 Ly from its center. If I had an image of a galaxy, I could estimate the distance you seek but roughly speaking...if were at the center of the spiral arm we reside in, we'd probably be around 200 light years from the top and bottom assuming your 1,000 Ly estimate is pretty close. Being 30,000 Ly from the center wher the top and bottoms of spiral arms thin out is the reason I estimated the way I did. <div class="Discussion_UserSignature"> My borrowed quote for the time being:There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened. </div>

alokmohan · Feb 12, 2008

Are we talking of visible universe only?

weeman · Feb 12, 2008

Visible and non-visible. The observable universe (what we can see) and the material universe (what's actually out there but hasn't become visible yet). <div class="Discussion_UserSignature"> Techies: We do it in the dark. "Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity." -Albert Einstein </div>

SpeedFreek · Feb 12, 2008

But if the expansion of the universe is accelerating, the amount we can see eventually decreases as time goes on. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

MeteorWayne · Feb 12, 2008

More correctly the amount increases, but the percentage decreases. At least I think so <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

derekmcd · Feb 12, 2008

The longer we stick around, the more time light has to reach us. So, in a way, we should be able to see more. However, with the expansion of the universe being the way it is, that light redshifted beyond anything we can detect. That's how I understand it... I could be wrong. <div class="Discussion_UserSignature"> <div> </div> <div>"If something's hard to do, then it's not worth doing." - Homer Simpson</div> </div>

SpeedFreek · Feb 12, 2008

Yes, I was being purposefully vague, and you rumbled me! <img src="/images/icons/tongue.gif" /> The distance in space that we can see will always increase with time, but in an accelerating universe there comes a point when new light emitted from distant objects cannot reach us - the expansion over that distance would mean the light would be receding from us! We will see more distant galaxies for a while yet (measured in billions of years) as light comes in, light that is being emitted right now up to a certain distance away, but in the end we will reach a distance where we can see no "new" light. We can illustrate this more clearly by looking at the early universe, when the expansion was initially very fast but was decelerating, and then reverse the picture. The oldest, dimmest, most distant (in light-travel time) objects we have seen are galaxies that are estimated to have been something under 3 billion light years away when they emitted the light we are now seeing, 13 billion years later. These galaxies look relatively large, as they were only 3 billion light years away when they emitted that light 13 billion years ago. The rate of expansion was very fast at this time, but was decelerating. The galaxies that look the smallest relatively, and were the furthest away when they emitted the light we are now seeing, are a lot brighter. They were nearly 6 billion light years away when they emitted that light, but that light was emitted only 9 billion years ago. The rate of expansion was a lot slower then, and still decelerating. So 9 billion years ago, the light from the oldest dimmest galaxies would have been as far away from us as those brighter galaxies were - their light reached us at the same time and light doesn't overtake light. What this means is that 13 billion years ago those oldest dimmest galaxies emitted their light at a distance of 3 billion light years away from us, and then for the n <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

weeman · Feb 13, 2008

Ahh....I see what you're saying! I had to read your post a few times, but I think I'm gettin' it. <img src="/images/icons/smile.gif" /> So, when we see galaxies that are the most distant in our observable universe, do they appear larger than they should because they are essentially pictures of galaxies who were only 3 billion lightyears away when they emitted their light? <div class="Discussion_UserSignature"> Techies: We do it in the dark. "Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity." -Albert Einstein </div>

robnissen · Feb 13, 2008

So, when we see galaxies that are the most distant in our observable universe, do they appear larger than they should because they are essentially pictures of galaxies who were only 3 billion lightyears away when they emitted their light? Yes.

Mee_n_Mac · Feb 13, 2008

Yes. Let me play Devil's Advocate for a second .... How do we know, or what makes us believe, that galaxies which are far away and thus formed in the early years of the Universe, weren't simply larger and thus have a larger angular diameter ? Is it a case of "Yes that cold be true but we have no reason to think so" ? <div class="Discussion_UserSignature"> -----------------------------------------------------Ask not what your Forum Software can do do on you,Ask it to, please for the love of all that's Holy, STOP ! </div>

Is Earth At The "Center" Of The Universe?

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