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I am investigating inertial forces and looking to identify both the most and least dense directions in space and determine by how much they differ.

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- Thread starter KingSnackMan
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I am investigating inertial forces and looking to identify both the most and least dense directions in space and determine by how much they differ.

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bapowell

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The universe is isotropic, meaning that its density is the same in all directions.

I am investigating inertial forces and looking to identify both the most and least dense directions in space and determine by how much they differ.

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Chronos

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bapowell

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phyzguy

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bapowell

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Sure, there will be tiny differences in the density as a function of direction since the universe isn't perfectly uniform. But you'd have to add up all the matter along each line of sight (each with some small cross section) between earth and the edge of the observable universe to determine the anisotropy. This is not possible with today's cosmological observatories.

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Chalnoth

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I'm not aware of any method to measure the density along an entire line of sight. The only observable I can think of that might reflect that would be the lensing of the CMB, but that's a pretty subtle effect to measure, and I think you could only get some very generic statistics of the mass distribution, not the specific density along a particular line of sight.

I am investigating inertial forces and looking to identify both the most and least dense directions in space and determine by how much they differ.

It's definitely possible to measure the masses of particular structures via gravitational lensing (both strong and weak), but that won't get you the entire density along a line of sight.

I think if you just want to get an idea of the statistical properties of the matter distribution in our universe, look up measurements of the matter power spectrum.

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mfb

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It is also unclear what that value would represent, as you integrate along an arbitrary line in both time and space. The integrated Sachs-Wolfe effect is something not too far away from that concept, and measurable.Sure, there will be tiny differences in the density as a function of direction since the universe isn't perfectly uniform. But you'd have to add up all the matter along each line of sight (each with some small cross section) between earth and the edge of the observable universe to determine the anisotropy. This is not possible with today's cosmological observatories.

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Chalnoth

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Yes. But the ISW effect doesn't measure the density. It measures the change in gravitational potentials over time. When there's dark energy around, gravitational potentials get shallower over time, which blueshifts CMB photons that pass through large overdense regions. The problem is that the redshift/blueshift which results is indistinguishable from the fluctuations of the CMB itself, meaning that it can only be detected through the fact that it changes the statistical properties of the CMB.It is also unclear what that value would represent, as you integrate along an arbitrary line in both time and space. The integrated Sachs-Wolfe effect is something not too far away from that concept, and measurable.

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