If we don't even know how large the universe is, how do we know that it is expanding? <br /><br />To determine that the entire universe is expanding, wouldn't that require observing the that the edges are exanding away from each other? If we don't know where the edges are or even if the universe has edges, then how are we so certain that the entire univers must be expanding? Thanks!
- Status
What we do know, is that the further galaxies are away from us, the faster they are receding from us. Hubble discovered that circa 1920s, that is STRONG evidence that the universe is continuing to expand everywhere. But that is not to say that at some point thousands of light years, that part of the universe could be contracting. But we currently have no theory that would predict such a contraction, and no way to test any such theory.
Welcome to SDC!<br /><br />Some misconceptions must be worked out to help you better understand the universe.<br /><br />First of all, to the best of our knowledge, there is no edge to the universe. You won't hit an imaginary barrier in your space ship if you travel for a long enough period of time. Einstein stated that although the universe is finite, it is unbounded. What he meant is that you will never hit an end to the universe, but if you travel far enough and long enough, you will eventually end up back where you started, having circumnavigated the entire universe. <br /><br />We can see the universe expanding due to the 'doppler shift'. Light that is emitted from a source that is moving away from the observer has longer wavelengths, meaning it has a higher redshift; light that is emitted from a source that is moving towards the observer has shorter wavelengths, meaning it has a higher blueshift. Here's a website to help illustrate doppler shift:<br /><br />http://www.astro.ucla.edu/~wright/doppler.htm<br /><br />As we look into space, we notice that all galaxies have an apparent redshift. And, the farther we look into space, the stronger this redshift becomes. We can conclude from this that the farther a galaxy is from Earth, the faster it is receding from us. <br /><br />I can use a simple diagram to show how this recession of the universe works:<br /><br />A--B--C--D<br /><br />Each letter represents a galaxy, now lets expand the space between them:<br /><br />A-------------B-------------C-------------D<br /><br />Imagine that A represents us in the Milky Way galaxy. The distance from galaxy A to galaxy D is much farther than it is from A to B. In addition, in the same amount of time, D appeared to recede to a greater distance than B. From this observance, we humans in galaxy A, can conclude that galaxy D covered more distance than galaxy B in the same amount of time; thus meaning it moved away from us a <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"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.</strong><strong>" -Albert Einstein </strong></font></p> </div>
You explained everything well... except for the diagram which doesn't visualize the further the galaxy, the faster it's recession.<br /><br />abcd<br /><br />Expansion from our perspective being 'a' :<br /><br />a--b----c--------d<br /><br />Expansion from the perspective of galaxy 'd' :<br /><br />d--c----b--------a <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
Both sets are helpful to understand the whole thing.<br /><br />ABCD<br /><br />Expansion from our perspective being 'A' :<br /><br />A--B----C--------D<br /><br />Expansion from the perspective of galaxy 'D' :<br /><br />D--C----B--------A<br /><br /><br />Actual metric expansion :<br /><br />ABCD -- /> A----B----C----D<br /><br />So over a given period of time, from the viewpoint of "A"<br /><br />B has moved from 1 to 5 spaces away, receding by 4 spaces.<br />C has moved from 2 to 10 spaces away, receding by 8 spaces.<br />D has moved from 3 to 15 spaces away, receding by 12 spaces.<br /><br />C is receding at 2 times the speed of B. <br />D is receding at 3 times the speed of B. <br /><br />And of course it works the same way from the viewpoint of "D". <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>
<font color="yellow">Einstein stated that although the universe is finite, it is unbounded. What he meant is that you will never hit an end to the universe, but if you travel far enough and long enough, you will eventually end up back where you started, having circumnavigated the entire universe. </font><br /><br />That is an accurate statement of Einstein's belief, but there is currently no evidence whether Einstein was correct, there is also, however, no evidence that he is incorrect.
Which brings up a question that shrouds our ongoing studies of the universe: Who the hell is right? <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"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.</strong><strong>" -Albert Einstein </strong></font></p> </div>
<blockquote><font class="small">In reply to:</font><hr /><p>If we don't even know how large the universe is, how do we know that it is expanding?<p><hr /></p></p></blockquote><br /><br />Technically we could only profess to believe that our own visible "sliver" of the universe is expanding, and even that assumption presumes that the redshift phenomenon is *strictly* due to a doppler effect. In other words it excludes MECO type "intrinsically redshifted objects" and all "tired light" theories. That alone should make us cautious in our claim of "expansion". <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>To determine that the entire universe is expanding, wouldn't that require observing the that the edges are exanding away from each other?<p><hr /></p></p></blockquote><br /><br />You might visualize the process like a solar wind process. The solar wind particles all accelerate as they leave the surface of the photosphere and they continue to accelerate as they approach the heliosheath. As the particles accelerate, they move away from one another and become more diffuse as they travel further from the photosphere and closer to the heliosheath. Two objects in that expansion process will see a "doppler shift" as they recede from one another over time. Two ions that started further apart when they left the photosphere will recede away from one another even faster than two ions that started closer together at the photosphere. The acceleration process and the diffusion process is responsible for this doppler effect.<br /><br />Now keep in mind that we can only see a very small sliver of what is now a much larger process. We could never hope to know that every item in the the whole universe is expanding anymore than an ion in the solar wind could be sure that every object in the universe is expanding only based on the other ions it can still see around itself that seem to be receding. The bigger picture could be and probably is a lot more complicated than we imagine base <div class="Discussion_UserSignature"> It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. - Kristian Birkeland </div>
Actually the assumption that our visible "sliver" of the universe is expanding is based on the presumption of <i>cosmological redshift</i>, not a relativistic doppler effect. That and the <b>big clue</b> of increasing apparent angular diameter of galaxies with high redshifts.<br /><br />Only in the local supercluster are red and blue shifts assumed to be due to relativistic doppler effect, caused by the relative inertial motions of galaxies bound by gravity. Outside of that gravity bound cluster of galaxies, all the other clusters of galaxies are redshifted and this is assumed to be cosmological redshift, where the light we are seeing has been "stretched" by the expansion of the universe.<br /><br />Above redshifts of z=1.6 we see an apparent increase in a galaxies angular diameter and the dimmer and more redshifted the galaxy, the larger its apparent angular diameter is.<br /><br />The angular diameter of an observed object can be used as an indication of the distance to that object when the light was emitted, and some of the dimmest most highly redshifted galaxies we have seen have a angular size that puts them less than 4 billion light years away, 13 billion years ago! <br /><br />A brighter, less redshifted galaxy has an angular size that puts it over 5 billion light years away when it emitted the light we are seeing, but that light has only been travelling for around 10 billion years.<br /><br />Let me quantify those examples:<br /><br />Galaxy A<br />Redshift: z=7<br />Light travel time: 13 billion years<br />Angular diameter distance: 4 billion light years<br /><br />Galaxy B<br />Redshift: z=1.7<br />Light travel time: 10 billion years<br />Angular diameter distance: 5.8 billion light years.<br /><br />Are there other possible explanations, other than expansion, for these observations?<br /><br />Well, if dimness and redshift were <i>only</i> due to light travel time, we might live in a static universe, but that would mean that the further away in time a galaxy is, the larger it <i></i> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>
Great explanation Speedfreek. <br /><br />There are a couple things I am still a little unclear on. <br /><br />So, the angular diameter distance suggests that galaxies appear larger than they should through our telescopes because they were closer when they emitted their light? <br /><br />The way I understand the angular distance is that a galaxy at 3 BLY (when it emitted its light) now appears to be 14 BLY only because of the cosmological redshift?<br /><br />And if you could please elaborate on the <i> z </i> equations that you so often use in your posts. <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"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.</strong><strong>" -Albert Einstein </strong></font></p> </div>
Well, perhaps I phrased it badly. The angular diameter distance <i>alone</i> only tells us how far away an object was when it emitted the light we see. The angular diameter distance does not in itself imply the object is receding, or is further away now.<br /><br />Whether an object is moving relative to us or not, its angular diameter tells us how far away it was when the light we are seeing was emitted.<br /><br />It is the apparent magnitude and cosmological redshift that tells us how long the light has been travelling and how much the universe has expanded since the light was emitted.<br /><br />It is when you combine the angular diameter distance with both the light travel time distance and the comoving distance that you can see the whole picture.<br /><br />I use the graph at Distance Scales of the Universe for quick and dirty calculations (it's a little old and its figures are now considered a little high but I used it in my last post) and Ned Wrights Cosmological Calculator if I want more accuracy. I cannot elaborate on the z equations any more than that, except to point you to Ned Wrights explanations in his excellent cosmology FAQ.<br /><br />From Neds Calculator, using a flat universe, we find:<br /><br />Age of the universe: 13.665 billion years<br />Redshift of galaxy : z=9<br />Age of the universe at z=9: 556 million years<br />Light travel time: 13.109 billion years<br />Comoving distance: 30.728 billion light years<br />Angular size distance: 3.0728 billion light years<br /><br />If you look at a range of redshifts from z=0.1 upwards you will see that the relationship between angular diameter and light travel time changes direction after the point where the apparent recession velocity reaches the speed of light. For the nearest 9 billion light years or so (light travel time), the higher the redshift, the smaller the a <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>
Great Post. That is a really nice discussion of angular diameter, it is one of the clearest descriptions I have seen on that topic. Thx.
Yes, great post! Thanks <img src="/images/icons/smile.gif" /><br /><br />The graph helps a lot for better understanding. I think I learned a lot more about the big bang just in the last 2 days. <img src="/images/icons/laugh.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"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.</strong><strong>" -Albert Einstein </strong></font></p> </div>
<blockquote><font class="small">In reply to:</font><hr /><p>What he meant is that you will never hit an end to the universe, but if you travel far enough and long enough, you will eventually end up back where you started, having circumnavigated the entire universe.<p><hr /></p></p></blockquote><br /><br />I can't get past this. Wouldn't this mean the universe was circular or a globe? I mean, if it is unbounded, wouldn't we travel through nothingness? (I know that's not possible)<br /><br />How would we circumnavigate the universe?
Einstein proved a static universe is not possible (ignoring his little error with the Cosmological constant) so it is either expanding or contracting.<br /><br />Assuming odds of either are 50/50, then if a preponderance of the evidence leans one way or the other, you can be sure.<br /><br />Cosmic background radiation, youth of the universe, etc., all support idea the universe is expanding. <br /><br />A contracting universe of the size of the one we see would have greatly different properties than observed.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
<i>How do we know that the universe is expanding?</i><br /><br />Have you looked at people's waists lately? <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis: </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
Vintersorg: <font color="yellow"> I can't get past this. Wouldn't this mean the universe was circular or a globe? I mean, if it is unbounded, wouldn't we travel through nothingness? (I know that's not possible) <br /><br />How would we circumnavigate the universe? </font><br /><br />It's as if the entire universe is like Pacman; you exit off one side of the screen, and appear on the other side! <img src="/images/icons/laugh.gif" /><br /><br />Einstein did state that the universe is curved in such a way that it would allow you to travel back to your starting point.<br /><br />The idea that it is unbounded does not imply that there is an infinite amount of empty space out there with our universe taking up only a tiny fraction. It means that our universe is neither infinite in age nor in size, but you can't travel to an imaginary "edge" of the universe. <br /><br />Remember that the expansion of space is like the surface of a balloon. One of the most common misconceptions of the big bang is that there was an explosion of matter in a pre-existing empty space. The big bang was the beginning of space. <br /><br />None of us can actually visualize what it would look like to take a trip that circumnavigates the universe. The way I see it, Einstein was trying to describe (although impossible to visualize) what would happen if you traveled forever in a universe that is finite in size, but has no boundries. <br /><br />The two seem paradoxical, but even with unmanned probes, we haven't even seen the edge of our own solar system, so what do we know!? <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"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.</strong><strong>" -Albert Einstein </strong></font></p> </div>
Indeed, we know so little about our solar system, yet we wonder about the "ends" of the universe. Maybe we should just take one step at a time for now <img src="/images/icons/smile.gif" /><br /><br />Your post was really helpful to try and visualize, but again, millions of new questions pop into mind, and I think we won't have the answers so soon.
It's sort of a dilemma. <br /><br />One-step-at-a-time is the more realistic aspect of space exploration. Yet, as humans, we want things done now! We want the answers sooner than later. <br /><br />I'd say we've learned a ton in the last century. Hubble has given us an incredible leap forward in the exploration of the cosmos. However, there are many things that Hubble can't do for us, things that are left to be answered only through equations! <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"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.</strong><strong>" -Albert Einstein </strong></font></p> </div>
<font color="yellow"><br />None of us can actually visualize what it would look like to take a trip that circumnavigates the universe. </font><br /><br />A photon wave knows. It isn't bounded by time. Therefore, I believe it can circumnavigate the universe in no time. Forget about the photon (particle) speed of light in our dimension. Think like a dual particle/wave photon! <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>
I just rejoined the message boards after a long absence. <br />I posted a question concerning the validity of the Red Shift method. My head hurts, now, a little, but I guess math and equations will have to suffice for now to explain what we think we see "out there". I was at first thrown off by the notion of farther away galaxies appearing to be larger than galaxies closer to us, but the math seems to explain the seeming non sequitor. I'm still a little cautious to swallow it "hook, line & sinker", but it seems to work. I guess wiser minds than my own have already verified these methods to be as reliable as one can currently hope for, at any rate.<br />Great posts!! Good to be back! <div class="Discussion_UserSignature"> We must progress and expand, or we will stagnate and die. </div>
I'm going to add a question that came to my mind last night to this thread considering its fairly relevant.<br /><br />Observations of distant stars and galaxies show that they present a red-shift indicating they are moving away from us. Also, the further away the object in question is the more of a red-shift is apparent leading us to believe its moving away at a faster rate. However, I was thinking about how because of the distance when we view these objects we are in essence looking at them as they were many many years ago. Wouldn't this then mean that we are viewing the red-shifts that were present at that time, and, considering those closer to us (and, hence, less in the past) present less of a red-shift, wouldn't that mean the expansion of the universe is actually slowing down?<br /><br />Sorry if its a stupid question, but it was kind of bothering me.
It's not a stupid question at all (there are no stupid questions, only stupid answers!). <img src="/images/icons/smile.gif" /><br /><br />It is the <i>light</i> we see from those distant galaxies that presents a redshift and we think that the redshifting of that light has been caused by that light being "stretched" as it travels through expanding space. When the light was emitted it wasn't redshifted at all.<br /><br />This <i>cosmological</i> redshift is cumulative in nature, and relative between objects at different distances. That galaxy <i>itself</i> emits ordinary light, it is what has happened to that light since it was emitted that tells us how much the universe has expanded, and so how far that object would be away now, and so how fast it has apparently receded from us.<br /><br />And just as we see those distant galaxies as they were 13 billion years ago, but redshifted by the expansion of space, so those distant galaxies would see what the Milky way (or what was here before it) looked like 13 billion years ago, and they would see this region of space redshifted by the same amount, or so we think! <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>
Thanks. Makes perfect sense now. Honestly, I was kind of hoping for the expansion to be slowing and eventually the 'big crunch'. There's a certain symmetry to it I find appealing. I guess its a cold dead universe in the future! 
<font color="yellow"> I guess its a cold dead universe in the future! </font><br /><br />Could be, none of us know for sure!<br /><br />There are three possible scenarios for the nature of our universe: <br /><br />1) We live in a closed universe, meaning the expansion will eventually stop and begin to collapse, leading to the 'big crunch'.<br /><br />2) We live in a flat universe, meaning the expansion will continue to slow down for an infinite amount of time. In other words, it will always be slowing down, but never come to a complete stop.<br /><br />3) We live in an open universe, meaning expansion will never cease, and the universe will expand forever. <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"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.</strong><strong>" -Albert Einstein </strong></font></p> </div>
- Status
Similar threads
TRENDING THREADS
-
China's Tiangong space station damaged by debris strike: report
- Started by Admin
- Replies: 1
-
NASA's Voyager 1 spacecraft finally phones home after 5 months of no contact
- Started by Admin
- Replies: 10
-
Space Force tests small satellite jammer to protect against 'space-enabled' attacks
- Started by Admin
- Replies: 0
-
-
-
A video made by me about space and the loneliness in the universe- Started by drakotol
- Replies: 54
-
Citizen scientists spot more than 1,000 new asteroids in old Hubble Telescope photos
- Started by Admin
- Replies: 3
Facebook
Twitter
Instagram