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wolram
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how do we know that the expansion of space is universal ,rather than local to our observable portion of it?
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Originally posted by wolram
how do we know that the expansion of space is universal ,rather than local to our observable portion of it?
We don't.Originally posted by wolram
how do we know that the expansion of space is universal ,rather than local to our observable portion of it?
Originally posted by wolram
how do we know that the expansion of space is universal ,rather than local to our observable portion of it?
Originally posted by marcus
you can plot it out on graph paper and see---it is the old
raisinbread dough picture: when the dough is rising and expanding it looks the same to every raisin. each raisin sees
the others getting farther away
Originally posted by wolram
it warms my heart to know that you all do your best to impart your knowledge to the ignorant, thanks esp MARCUS
Originally posted by marcus
1. the "Copernican axiom" or "we ain't special"...
Originally posted by marcus
They assume there is no preferred direction and no preferred location.
Originally posted by marcus
2. There is another more pragmatic way that we can kind of tell.
The Hubble law is linear at the present moment. This actually means that expansion would look the same from a neighbor's viewpoint as long as everybody is at rest w/rt CMB.
Originally posted by marcus
People who are "comoving" or have a common idea of what it means to be at rest also have a common idea of the present moment.
Originally posted by marcus
One can define a "comoving" distance in the present
---also called the "Hubble law distance" because it is the idea of distance that works in the v = H0D law. this law is linear.
Originally posted by steinitz
This elegant way of finessing the issue of comoving coordinates does make sense, but ony because of the isotropy in this important but special case. In general (and yes I'm sure you know this marcus, these remarks aren't aimed at you), just as in SR, simultaneity is an observer-dependent concept, and there are an infinite number of ways general spacetimes can be foliated by spacelike hypersurfaces. In this more general context, the analogy between comoving coordinates and "chronologically like minded" observers breaks down.
Originally posted by steinitz
That's just a misleading artifact of working in comoving coordinates which hides the physical significance of hubble's law. Firstly, comoving coordinate distances unlike physical distances between points carried from slice to slice remain constant. Also, as I explained in another thread, the use of recessional velocities is in practice problematic due to the difficulty of separating out the gravitational component of redshift. It's really the basic linearity of the relation between physical distance - as opposed to comoving coordinate distance - and redshift implicit in hubble's famous relation that in terms of comparing with redshift data is unambiguous and for that reason should be used.
The expansion of space is a theory in cosmology that suggests the universe is constantly expanding, and the distance between galaxies, stars, and other celestial bodies is increasing over time.
Scientists have observed the redshift of light from distant galaxies, which indicates that they are moving away from us. This phenomenon is known as the Hubble expansion, and it is evidence of the universal expansion of space.
No, the expansion of space is not happening at a constant rate. The rate of expansion, also known as the Hubble constant, has been found to be increasing over time. This means that the expansion of space is accelerating.
The exact cause of the expansion of space is still unknown. However, scientists believe that it is due to a mysterious force called dark energy, which makes up about 70% of the universe.
It is currently believed that the expansion of space will continue indefinitely, and the distance between galaxies will keep increasing. However, as our understanding of dark energy and the universe evolves, this theory may change.