The discussion revolves around the density of the universe along specific lines of sight, particularly in relation to right ascension and declination. Participants explore the implications of this density on inertial forces and the potential for identifying variations in density across different directions in space.
Participants express varying views on the isotropy of the universe and the feasibility of measuring density along specific lines of sight. While some agree on the general isotropy, others highlight the existence of small deviations and the challenges in measurement, indicating unresolved aspects of the discussion.
The discussion highlights limitations in current observational capabilities and the complexities involved in defining and measuring density along arbitrary lines in both time and space.
The universe is isotropic, meaning that its density is the same in all directions.KingSnackMan said:Does anybody know of a resource that I could use to determine the density of the universe along a specific line of sight? (right ascension and declination)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.
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.KingSnackMan said:Thank you bapowell and Chronos. I understand that on the universe is very close to isotropic, but because of the Large Scale Structure there must be some directions where there are just slightly more voids, and others where there I slightly more nodes, right? I am looking for very small deviations from the average density of the universe.
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.KingSnackMan said:Does anybody know of a resource that I could use to determine the density of the universe along a specific line of sight? (right ascension and declination)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 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.bapowell said: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.
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.mfb said: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.