# Thread: Are we certain that we are measuring speed of acceleration w

1. Are we absolutely certain that we are measuring speed of acceleration with the Red Shift? If the Cosmological Constant (Dark Energy) is everywhere, what keeps it from pushing back on the other side of a galaxy? What lets it accelerate faster at a greater distance? Some of these things beg the question with me. Thanks for comments. Joe L. Ogan

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3. The red shift is due to the expansion of space. To illustrate why things are moving away from us at faster speeds the further they are away from us, think about this scenario. Let's say you have four points in a line at equal distance from each other. Now let the space between them expand. What you will notice from the first point in the row, is that the second one is moving away from you at speed , but when you look at point 3, it will appear to be moving away at and similarly point 4 will appear to be moving away at . This due to the fact that twice the amount of space is being doubled between points 1 and 3 as is being doubled between points 1 and 2 in the same amount of time. We are pretty certain that expansion is happening because of this relationship between distance and speed of recession.
What recent WMAP data suggests is that the rate of expansion is accelerating. This acceleration needs extra energy, but we don't know where it is coming from yet, so it is called dark energy. It is wrong to think about space pushing on matter, as the very space matter exists in is also expanding. If you made a black dot on a rubber sheet and stretched the sheet out, nothing would be pushing on the dot. The reason matter don't drift apart is because the force of gravity overwhelms the expansion.

What happens with the red shift is that the light waves are being stretched out on their way to us due to isotropic expansion, similarly to how a sine wave drawn on a rubber sheet expand when you stretch out the rubber. One other possibility is that light stretches out on its own over long distances, but this has never been shown and no mechanism for something like this has ever been shown either.

Hope that helps.

4. Originally Posted by Joe L. Ogan
Are we absolutely certain that we are measuring speed of acceleration with the Red Shift? If the Cosmological Constant (Dark Energy) is everywhere, what keeps it from pushing back on the other side of a galaxy? What lets it accelerate faster at a greater distance? Some of these things beg the question with me. Thanks for comments. Joe L. Ogan
"speed of acceleration" is either a misnomer on your part or it is called "jerk" and iis not relevant.

What seems to be increasing is the rate at which distant galaxies are receeding from us. This is determined using red shift to determine speed and other methods, particularly "standard candles" to measure distance.

Galaxies are not being pushed apart through space. Space itself is expanding.

The effect is modeled in general relativity as a positive cosmological constant in the Einstein field equations. This causes what is called "metric expansion" of space. It is just as if (in 2 dimensions) space were a rubbler sheet, the galaxies were dots and the sheet were being stretched.

Note that in this analogy the speed of recession from any fixed poin is proportional to distance. This is precisely the Hubble relationship.

No one knows what is fundamentally causing this expansion or positive cosmological constant. One possibility is the zero point energy of the vacuum as predicted by quantum electrodynamics. However, the usual calculation overestimates the effect by a huge margin, indicating that we don't really understand what is going on.

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