
#19
Feb2013, 12:34 PM

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P: 9,183

How would you know the universe was expanding if things did not get further apart?




#20
Feb2013, 02:15 PM

PF Gold
P: 1,547

You keep thinking in terms of Inertia. Here is the problem with that thinking. In metric expansion. Only the non gravitationally bound areas are expanding. So the only thing that is changing is the distance between two large scale structures. This does not mean that the expansion is causing a force to move the large scale structure it merely increases the distance. Inertia requires a force. F=MA. In expansion the scale of space itself is increasing. For example if expansion suddenly stopped. By Newtonian laws the large scale structures should keep expanding until an opposing force stops it. We all know this isn't what will happen.
The acceleration of objects moving away from each other in an expanding universe is not the sort of acceleration which can be associated with a force as in Newton's Second Law because the expansion is an intrinsic property of the way space and time are measured rather than being due to dynamical interactions. A metric defines how a distance can be measured between two nearby points in space, in terms of the coordinates of those points. A coordinate system locates points in a space (of whatever number of dimensions) by assigning unique numbers known as coordinates, to each point. The metric is then a formula which converts coordinates of two points into distances. Now thinking of metric and the above. the coordinates are not changing. The only change is the distance between two coordinates. Hence as the coordinates are not changing we state that it is not moving. As their is no force acting upon the body stating that its accelerating is misleading. as acceleration requires constant force. Unfortunately thats one of the hickups with lanquage and its restrictions on usage. They would rather see a word poorly used than a new one created lol. Hubble's Law demonstrates that the Universe is expanding in a systematic way: The further away a galaxy is from us, the faster it appears to be moving away from us. Hubble Parameter: Rate of expansion of the Universe H0 is the value of the Hubble Parameter today. Hubble's Law v = recession velocity in km/sec d = distance in Mpc H0 = expansion rate today (Hubble Parameter) The more distant a galaxy, the faster its recession velocity what is recession velocity well simply put its NOT motion through space but rather its Expansion of spacetime: galaxies carried along Hope that helps as I cannot think of any other way to answer your confusion. Hopefully others can better. 



#21
Feb2013, 11:59 PM

P: 571





#22
Feb2113, 01:09 AM

P: 863

Now take two separated observers A and B, both inside this sphere at a constant relative distance, and assume the mass of this sphere is uniformly increased, or diameter uniformly decreased, such that the total gravitational depth is increased for both observers. If observer A sends a reference signal to observer B then, due to the relativity of simultaneity, B will receive this signal at a later time, such that the gravitational depth has increased from when A sent the signal. B will then say A's signal is redshifted by that amount, even though the relative distance between A and B never changes over time. The Hubble redshift is zh=(ωoωe)/ωe, where: ωo is the wavelength as measured by a future observer. ωe is wavelength as measured at the time of emission. If expansion is characterized as a time dependent change in gravitational depth, as described in the thought experiment, then the same effect applies expansion as in the thought experiment. 



#23
Feb2113, 02:01 AM

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P: 9,183

If you inject variance of the fundamental constants of nature into the equation, you are merely asserting the possibilty of a universe finely tuned for deception. That does not qualify as a theory of anything.




#24
Feb2113, 02:30 AM

P: 863





#25
Feb2113, 05:55 PM

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PF Gold
P: 9,183

Actually, there is effect similar to that you are describing in #22. It is called the integrated sachs wolfe effect. CMB photons passing through dense regions of the universe are slightly more blue shifted than those passing through less dense regions. This is because dense regions become gravitationally diluted due to expansion over the course of the millions of years it takes a photon to travel through a dense region. As a consequence, the photons retain some of the blue shift they acquired when approaching the over dense region.




#26
Feb2113, 10:25 PM

P: 863

Yes, the primary difference is that the integrated sachs wolfe effect is the product of nonuniform density variations. The difference is that the effect I described can occur in flat, albeit expanding, spacetime, and is only observable as a result of the relativity of simultaneity (RoS).
Going back to the two observers in the sphere, if valid, standard GR dictates each will perceive themselves to occupy a lower gravitational depth than themselves. Just as a signal sent to A from B will be blue shifted, so will a signal from B to A by the same amount. If expansion is uniform in an otherwise flat spacetime then its observational dependence on a finite light speed (RoS) dictates that not only does each observer frame perceive themselves to exist at a maximum gravitational depth but this depth will appear to decrease at a 1/r scale. 



#27
Feb2813, 06:24 PM

P: 503




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