rrw4rusty Posted October 14, 2009 Share Posted October 14, 2009 (edited) Hi! Does this statement make any sense? New space is being created between the Milky Way and a galaxy in the Great Wall named X and that's why they are getting further apart. Regardless... (more questions...) = what exactly is this 'space' that is expanding? Just the distance between the MW and X and... The distance between the individual atoms between the MW and X? Just all the empty space available to hold all the matter and energy there is? I take it the matter and energy in the universe remains constant? = in this expansion, the MW and X are not getting larger or thinner, only the 'empty' space between them is being stretched out and it is not the atoms being stretched out but the 'empty' space between them yes? = Concerning this expansion, in the limited space between the Milky Way and X is the distance between them and a uniform decrease in the average density of matter and energy the only things we can detect? The only things that change? = In so far as the average density of matter/energy and galaxies, this is the same here as it is at the edge of the observable universe yes? = I've read that the universe is 97 billion lys across but there is no 'across'... out beyond the observable universe -- lets say 48.5 billion lys from here we'd hit what? Nothing but more universe. So I'm not sure how this measurement applies or what it means. Comments? = This 'space' that is expanding... it is nothing you can hold -- lol -- that is to say there is no way to take a tiny amount of it and do anything to it to prevent or stall this expansion? Put another way, there is no way to protect or shield even the smallest amount of 'space' from the expansion? = What about space at the edge of an event horizon? Does time distortion affect or slow the expansion? Thanks! R u s t y Edit: Related to this expansion, between the MW and X, is there 'anything' -- any particle or wave or ??? that is expanding? Edited October 14, 2009 by rrw4rusty Link to comment Share on other sites More sharing options...
ajb Posted October 14, 2009 Share Posted October 14, 2009 Does this statement make any sense? New space is being created between the Milky Way and a galaxy in the Great Wall named X and that's why they are getting further apart. You should not think of the expansion as making new space. It is better to think of how we measure distances as changing. You should really start thinking 4-dimensional. Space-time can be "cut" into 3+1 dimensions (well, consistently under some conditions that won't bother us). This means we can think of a 3-d metric, that is the thing we use to define distance on each 3-d "sheet". Now, as time goes forward, that is we move up onto the next sheet the the 3-d metric changes. In particular, distances measured are increased. A simple example to picture is a cone (minus the canonical singularity). It looks like a circle times a real interval. As you move up (or down depending on the orientation) the circle gets larger. So distances measured along the circle get larger. Notice, that nothing happens to the total space of the cone. Nothing is needed for it to expand into. Link to comment Share on other sites More sharing options...
rrw4rusty Posted October 14, 2009 Author Share Posted October 14, 2009 You should not think of the expansion as making new space. It is better to think of how we measure distances as changing. Yikes! You lost me here! Or I'm somehow confused on your meaning. Did you mean 'how the distances we measure are changing'? 'How we measure distances' is not changing. Or, if it is -- how?? Has the way we measure the distance to a distant galaxy changed? If so... from what to what?? Link to comment Share on other sites More sharing options...
ajb Posted October 14, 2009 Share Posted October 14, 2009 In the context of general relativity, the metric defines what we mean by distance. For cosmological solutions the metric need not be static, it depends on time. Thus, when we make a 3+1 dimensional splitting splitting the 3-d metric changes in time. (The 4-d metric need not respect this splitting, but that is irrelevant for now.) Think about my analogy of a cone cut it into circles (thin cylinders). Link to comment Share on other sites More sharing options...
Spyman Posted October 14, 2009 Share Posted October 14, 2009 (edited) That's an awful lot of complicated questions in one single thread, more than I am able to comfortable handle, but I am willing to make a try on this one: = I've read that the universe is 97 billion lys across but there is no 'across'... out beyond the observable universe -- lets say 48.5 billion lys from here we'd hit what? Nothing but more universe. So I'm not sure how this measurement applies or what it means. Comments? We don't know how large the universe is, but we have models based on the observations we are able to make. From our observational view we are in the middle of a sphere with a radius of ~46.5 billion lightyears, which thus could be said to have an diameter of ~93 billion lightyears across, we call this the Observable universe. Since Earth is not thought to be placed in such an important place of the universe, the universe itself is likely much larger than our observable part of it, but we can't know since it is beyond our horizon. If the space where not expanding then the edge of our observable universe would be ~13.7 billion lightyears away, but since space is expanding over time it gets a little more complicated to use a ruler marked with lightyears. With expanding space, light will have to pass through more space to reach us than the actual distance when the light was emitted and the object that emitted the light will be even further away when the light from it reaches us. One example of this is the Cosmic Microwave Background Radiation that reaches us today, it was emitted from an distance of ~30 million lightyears in the early universe and the material that emitted it is ~46.5 billion lightyears away now, but the CMBR has travelled ~13.7 billion lightyears through space to us. The observable universe is the theoretical volume from which it is possible in principle for photons from an object to reach us. In reality I don't think we are able to observe anything farther than around 5.9 to 6 billions lightyears distant, if the light has travelled through more space than that, then the object was closer when the light was emitted and even if it is more distant now, that light from the present distance has not reached us yet. Edited October 14, 2009 by Spyman Spelling Link to comment Share on other sites More sharing options...
Mr Skeptic Posted October 14, 2009 Share Posted October 14, 2009 Hubble's law says that the universe is expanding in such a way that distance increases by distance times Hubble's constant. Hubble's constant is H = 74.2 ± 3.6 (km/s)/Mpc. What this means is that, every second, what was 1 meter becomes 1 meter plus 2 (million million millionths) of a meter, or 1 meter + 2.4 X 10-18 meters if you prefer scientific notation. For reference, this is about 1000 times smaller than a proton, or about 10,000,000 times smaller than an atom. It would be really hard to notice that at small distances, even if the forces acting at those distances didn't act to negate the expansion. Really, the expansion of the universe only matters at very large scales. In fact, past a certain point, space is expanding between two points faster than the speed of light, which essentially means you can never get there nor communicate with someone there. Link to comment Share on other sites More sharing options...
rrw4rusty Posted October 14, 2009 Author Share Posted October 14, 2009 (edited) Sorry about all the questions -- I wasn't thinking! Thanks for the replies -- lots of fantastic info!! In fact, past a certain point, space is expanding between two points faster than the speed of light, which essentially means you can never get there nor communicate with someone there. All galaxies moving away from Earth faster than the speed of light would obviously be near the edge of the observable universe (assuming we can see them as they were 6-whatever billion years ago... now they would be further away and moving faster). Lets say galaxy Y is one of those traveling away from us faster then the speed of light... if you were an alien living in galaxy Y, galaxies nearer to you would look like the galaxies near us, yes? This is the question I'm most interested in though I fear I know the answer: Related to this expansion, in the space between the Milky Way and distant galaxy X, is there 'anything' -- any particle or wave or radiation -- that we can detect is growing... that is, more of 'it' is being added... sort of the structure that all other mater is spreading apart on? That may not make any sense or maybe dark energy is the closest answer. Maybe I'm trying to ask.. when space/time bends creating the gravity well around Earth... what exactly is bending? I think I'm looking for something that doesn't exist. Edit: Maybe what I'm looking for is the ether. Rusty Edited October 14, 2009 by rrw4rusty Add information Link to comment Share on other sites More sharing options...
Airbrush Posted October 15, 2009 Share Posted October 15, 2009 Yes, they cannot detect any such "thing" being added as space-time expands. Have you heard if the average density of matter in the observable universe is about one atom per cubic meter? Or is that the density of space in the middle of the great voids between supercluster? Maybe the more rarified space is, the more dark energy. Yes, you are looking for something that does not exist, because that space-time has not yet been activated as the expansion of space. Link to comment Share on other sites More sharing options...
Spyman Posted October 15, 2009 Share Posted October 15, 2009 ... that is, more of 'it' is being added... Maybe you should try to view the expansion of space as a rubber band being streached, there is no more parts of rubber or any new blocks of space added. Link to comment Share on other sites More sharing options...
Sorcerer Posted November 2, 2009 Share Posted November 2, 2009 Get a balloon, put dots on it, blow it up. The dots are stars/galaxys, anything u want really. You could even write MW and X. Space isnt created, its just "expanding". I'm not trying to be condescending, but maybe u could look up "expand" in the dictionary. Link to comment Share on other sites More sharing options...
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