DimaMazin Posted December 21, 2014 Author Share Posted December 21, 2014 There are many reasons for red-shift in different contexts. Doppler effect, gravitational and expanding space. Which are you referring to? Well.I am wrongly referring. Why expanding space exists between galaxies, but doesn't exist inside galaxy? Link to comment Share on other sites More sharing options...
Strange Posted December 21, 2014 Share Posted December 21, 2014 Well.I am wrongly referring. Why expanding space exists between galaxies, but doesn't exist inside galaxy? Because within galaxies (and galaxy clusters) the gravity holds things together - they are all in orbit around the center of mass. As noted in post 107, the expansion of space is not a force pushing things apart. It is just that distance between things will increase over time if nothing else happens. In that post I gave the analogy of two people walking due south from the north pole: they will tend to move apart because they are following lines of longitude. But imagine they were holding hands - then they wouldn't move apart as they moved south. Link to comment Share on other sites More sharing options...
DimaMazin Posted December 21, 2014 Author Share Posted December 21, 2014 (edited) Because within galaxies (and galaxy clusters) the gravity holds things together - they are all in orbit around the center of mass. As noted in post 107, the expansion of space is not a force pushing things apart. It is just that distance between things will increase over time if nothing else happens. In that post I gave the analogy of two people walking due south from the north pole: they will tend to move apart because they are following lines of longitude. But imagine they were holding hands - then they wouldn't move apart as they moved south. You have confused me agen. To be holding hands is force of resistance. Now I have no doubt that expansion of space has a force. But then objects should have force which is connected with space. I am confused. Edited December 21, 2014 by DimaMazin Link to comment Share on other sites More sharing options...
Mordred Posted December 21, 2014 Share Posted December 21, 2014 The spacetime metrics has energy density distributions . Depending on what particles are involved at a given region. This energy/mass density has an equation of state.(,formula ) that correlates how much pressure the particles exert. This used by the FLRW metrics in combination with the ideal gas laws pressure exerts a force. See the universe geometry article under my signature for more details 1 Link to comment Share on other sites More sharing options...
Strange Posted December 21, 2014 Share Posted December 21, 2014 You have confused me agen. To be holding hands is force of resistance. Without the force of the people holding hands (or galaxies being gravitationally bound) they would follow straight lines (lines of longitude on the Earth, geodesics in space-time). This would cause them to move apart over time. In other words, in the absence of force distance will naturally increase. It requires a force to move objects off those geodesics and keep them the same distance apart. This might help: "Expanding Space: the Root of all Evil?" http://arxiv.org/abs/0707.0380 Link to comment Share on other sites More sharing options...
DimaMazin Posted January 11, 2015 Author Share Posted January 11, 2015 (edited) Mainly that per m^3 the energy density of the cosmological constant is extremely weak. Roughly [latex]6.0 *10^{-10}[/latex] joules per cubic meter. This weak energy density per cubic meter is easily overpowered by local gravity as well as the strong force How does the energy create applying force to an object?What a volume of space with the energy interacts with an object and how does distance reduce the interaction? Edited January 11, 2015 by DimaMazin Link to comment Share on other sites More sharing options...
Strange Posted January 11, 2015 Share Posted January 11, 2015 How does the energy create applying force to an object?What a volume of space with the energy interacts with an object and how does distance reduce the interaction? As far as I know, dark energy does not exert any force on objects. And, as it is thought to be present throughout all space, the distance question is irrelevant. 1 Link to comment Share on other sites More sharing options...
Mordred Posted January 11, 2015 Share Posted January 11, 2015 (edited) A joules is also a unit of measure for energy density per volume. This site has the figure in both energy density and mass density per volume. http://math.ucr.edu/home/baez/vacuum.html This isn't where I got the figure from However it is the same figure. Dark energy does exert positive pressure, pressure is energy density per volume with the appropriate equation of state pressure is a measure of force per given volume Your confusion seems to be on the joules unit of measure itself. ,"In physics, energy is a property of objects, transferable among them via fundamental interactions, which can be converted into different forms but not created or destroyed. The joule is the SI unit of energy, based on the amount transferred to an object by the mechanical work of moving it 1 metre against a force of 1 newton.[1]" http://en.m.wikipedia.org/wiki/Energy Please note the link is on the physics definition of energy. Edited January 11, 2015 by Mordred 1 Link to comment Share on other sites More sharing options...
DimaMazin Posted May 14, 2015 Author Share Posted May 14, 2015 I don't understand why when cosmological force increases distance between masses then it increases light way, but when another reasons increase the distance between emission and reception then it doesn't change light way. Link to comment Share on other sites More sharing options...
Strange Posted May 14, 2015 Share Posted May 14, 2015 (edited) I don't understand why when cosmological force increases distance between masses then it increases light way, but when another reasons increase the distance between emission and reception then it doesn't change light way. What do you mean by "light way"? The distance light travels? Or wavelength? Or ...? (And there is no such thing as "cosmological force"; I assume you mean the metric expansion of space) Edited May 14, 2015 by Strange Link to comment Share on other sites More sharing options...
DimaMazin Posted May 14, 2015 Author Share Posted May 14, 2015 What do you mean by "light way"? The distance light travels? Or wavelength? Or ...? (And there is no such thing as "cosmological force"; I assume you mean the metric expansion of space) Time of light travel. Why when gravity don't let cosmological expansion to work then time of light travel doesn't increase? Link to comment Share on other sites More sharing options...
Strange Posted May 14, 2015 Share Posted May 14, 2015 Time of light travel. I'm not quite sure what you are asking. But if two objects are moving apart then the time it takes light to travel between them will increase (because the distance is increasing). Why when gravity don't let cosmological expansion to work then time of light travel doesn't increase? Because there is no change in distance. So why would the time increase? Link to comment Share on other sites More sharing options...
DimaMazin Posted May 14, 2015 Author Share Posted May 14, 2015 (edited) I'm not quite sure what you are asking. But if two objects are moving apart then the time it takes light to travel between them will increase (because the distance is increasing). Because there is no change in distance. So why would the time increase? Usually speed of object doesn't change length of light way during between emission and reception.Only not in case of cosmological expansion. Edited May 14, 2015 by DimaMazin Link to comment Share on other sites More sharing options...
Strange Posted May 14, 2015 Share Posted May 14, 2015 Usually speed of object doesn't change length of light way during between emission and reception. I'm still not sure what you are trying to say ... OK. Is this it: 1. We are on Earth watching a spacecraft fly away from us.. When it is 1 light year away, the spacecraft emits a flash of light. That light will take 1 year to get to Earth, even though the spacecraft is moving away. This is because the speed of light is the same for all observers and independent of the speed of the source. (But the light will be red-shifted because of the Doppler effect.) 2. In cosmological expansion, the light leaves the distant galaxy and starts travelling towards the Earth. But the space between the galaxy and the Earth is still expanding and so the light has an ever increasing distance to travel. So, for example, after the light has travelled half way to Earth, expansion means that it now has more than half the original distance still to travel. This is why it is wrong to think of distant galaxies as moving away from us: it is more accurate to say that the space in between is increasing. This is also why trying to calculate cosmological red-shift using the Doppler effect gives the wrong answers. Link to comment Share on other sites More sharing options...
DimaMazin Posted May 15, 2015 Author Share Posted May 15, 2015 2. In cosmological expansion, the light leaves the distant galaxy and starts travelling towards the Earth. But the space between the galaxy and the Earth is still expanding and so the light has an ever increasing distance to travel. So, for example, after the light has travelled half way to Earth, expansion means that it now has more than half the original distance still to travel. This is why it is wrong to think of distant galaxies as moving away from us: it is more accurate to say that the space in between is increasing. This is also why trying to calculate cosmological red-shift using the Doppler effect gives the wrong answers. Good answer for half of question. Why cosmological increase of space doesn't work when gravitational forces are stronger than cosmological force of division? Link to comment Share on other sites More sharing options...
Strange Posted May 15, 2015 Share Posted May 15, 2015 Good answer for half of question. Why cosmological increase of space doesn't work when gravitational forces are stronger than cosmological force of division? There is no cosmological "force". It requires no force for expansion to happen. It requires a force to stop it. Expansion only takes place where there is a homogeneous distribution of mass (which is true on very large scales). On small scales, galaxies are held together by gravity. Or, rather, the curvature of space-time caused b the galaxies prevents expansion. 1 Link to comment Share on other sites More sharing options...
DimaMazin Posted May 15, 2015 Author Share Posted May 15, 2015 There is no cosmological "force". It requires no force for expansion to happen. It requires a force to stop it. Expansion only takes place where there is a homogeneous distribution of mass (which is true on very large scales). On small scales, galaxies are held together by gravity. Or, rather, the curvature of space-time caused b the galaxies prevents expansion. Thanks. And is that in the complex equations? Link to comment Share on other sites More sharing options...
Strange Posted May 15, 2015 Share Posted May 15, 2015 Thanks. And is that in the complex equations? Do you mean the FLRW metric? Yes. I have tried to explain what the math says in a very basic way. 1 Link to comment Share on other sites More sharing options...
Mordred Posted May 15, 2015 Share Posted May 15, 2015 Expansion and the energy density to pressure relations that determines how the universe expands is covered under the acceleration equation. The wiki equations of state page has a decent coverage. http://en.m.wikipedia.org/wiki/Equation_of_state_(cosmology) 1 Link to comment Share on other sites More sharing options...
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