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Originally Posted by snowflakeuniverse
Hi Tobin Dax
You mentioned that “ The self-gravity of planets, stars, galaxies, and even clusters prevent them from expanding as space does. Only intercluster space (well, maybe intergalactic space, to a smaller degree) expands via the Hubble law.”
Why stop the expansion of space at the boundary of galaxies? Why assume that self-gravity prevents stars and galaxies from expanding? Why not have the expansion of space-time be a unifying property of the universe? There is a lot of space with in galaxies, why not expand them. There is a lot of space within an atom, why not expand the atom?
So long as I provide a model for expansion that preserves celestial and atomic stability, what is the problem?
Snowflake
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I'll tell you why. Because we don't observe that sort of expansion. What is observed is this:
v_rec (km/s) = H_o x d(Mpc), where distance is measured in units of megaparsecs (3.26 million light years).
H_o is measured to be 70 km/s/Mpc. On the scale of the galaxy (100,000 light years = 0.03 Mpc), the Hubble flow has a value of 2 km/s. Well, the virial (gravitationally induced) motions of stars around the galaxy are FAR larger (local gravity is far stronger than the tendancy of space-time to expand). On the scale of the solar system (d ~ 1 AU = 4.8e-12 Mpc), the Hubble flow has a value of a grand total of 3 x 10^-10 km/s. The local gravitational field of the solar system is FAR, FAR stronger than the tendancy of space-time to expand. Likewise, the electromagnetic forces (and quantum mechanical effects) that hold together and give structure to atoms and molecules are gargantuan in comparison.
Now IF the expansion of space-time is indeed accelerating due to some energy density with the property of negative pressure, and if this acceleration tends toward an exponential growth in Hubble's parameter, then it is in principle possible that at some future time the Hubble flow could exceed the local forces of gravity in a galaxy or perhaps even the forces holding together atoms. This scenario is called the "Big Rip", and is described in this speculative paper
Phantom Energy and Cosmic Doomsday
and contested in papers such as this one:
Phantom scalar fields result in inflation rather than Big Rip