Accelerating Universe & Gravity
Tusenfem tells us ...
Naturally, every first year physics student learns that only mass inside the sphere makes gravity. The notion of a huge universe outside of what we can observe pulling our little corner apart is preposterous.
And Mozina responds ...
http://www.space.com/scienceastronomy/080923-dark-flows.html
Unless of course you folks propose it and add the word "dark" in there somewhere, is that it?
The flows are already called "dark", despite the fact that they are actually "bright" structures made up of galaxies and galaxy clusters. These very large scale flows were first reported in
Kashlinsky, et al., 2008 &
Kashlinsky, et al., 2009. They see large scale flows out to about 300h
-1 Mpc.
Kashlinsky, et al., 2010 extend this result to larger catalogs with more galaxies, better data and improved error analysis, and extending out to about 800 Mpc, with the signal peaking at about 500h
-1 Mpc.
Fig. 2 displays the results obtained in this study compared to the expectations from the concordance LCDM cosmology for 95% of cosmic observers. These results cast doubt on the notion that gravitational instability from the observed mass distribution is the sole - or even dominant - cause of the detected motion. If the current picture is confirmed, it will have profound implications for our understanding of the global structure of space-time and our Universe's place in it.
Kashlinsky, et al., 2010, conclusions at end of paper.
Keisler, 2009 argues that
Kashlinsky, et al., 2008, overestimated the statistical significance of their results. However,
Atrio-Barandela, 2010 disputes that claim. Meanwhile,
Mersini-Houghton & Holman, 2009 argue in favor of "
superhorizon inhomogeneities induced by nonlocal entanglement of our Hubble volume with modes and domains beyond the horizon" as the cause for the large scale flows. This must be where the news report comment about "mass outside the visible universe" comes from.
In any case, we can say that these results are interesting, but the notion that they are somehow fatal to LCDM cosmology is wishful thinking and nothing more than that. At worst, it may mean that the concordance model needs to be modified to make it consistent with observations, which is hardly unprecedented in the annals of science.
Now let me say that these flows are totally irrelevant to the question. As usual, Mozina has no idea what he is talking about. The "dark flows" are extremely anisotropic, while the accelerated expansion of the universe is extremely isotropic, exactly opposite from the "dark flows". Clearly the "dark flows" are a separate phenomenon from the cosmological expansion.
Now, Mozina did say ...
Gravity can easily be empirically linked to "acceleration". It's therefore "possible" that mass outside of our visible universe "attracts" the mass inside out universe and "accelerates" that mass over time.
The obvious implication is that he thinks the accelerated expansion of the universe is caused by gravity pulling from the outside. But the accelerated expansion of the universe is
the same in all direction, very unlike the "dark flows" he linked to (once again choosing a news report rather than a science paper). One might naively assume that a spherically symmetrical distribution of matter around but outside the visible universe would cause an equally symmetrical outward acceleration. But one would be naively wrong, since, as tusenfem points out, it is physically impossible. Any freshman physics student will (or at least
should) learn that the gravitational acceleration due to a spherically symmetrical shell of mass is
exactly zero everywhere inside the shell. So that naive solution is obviously wrong, by virtue of being impossible.
So that leaves the question still not properly answered:
OK, since you think it is so easy, then do it.
Question: What is the "easy" link between the apparently repulsive, accelerated expansion of the universe and the apparently attractive force of gravity?
Like I said before ...
Who else could have realized that the universe is flying apart because of a fundamental force which only manifests itself by pulling things together. A truly amazing intellectual feat.
If Mozina thinks that the accelerated expansion of the universe is caused by gravity pulling from outside, he thinks wrong. The only explanation for accelerated cosmological expansion is a repulsion from within, and not an attraction from without.
Footnote: Cosmological distances are often presented in such form as I have done here, copying from the paper, e.g.,
500h-1 Mpc. A distance like "800 Mpc" depends on an assumed value for the
Hubble Constant (
H); change
H and the distance changes too. So cosmologists re-write the Hubble constant from
H km/sec/Mpc to
100h km/sec/Mpc. So if we assume
H = 70 km/sec/Mpc, then
h = 0.7. By expressing the distance in terms of
h the explicit dependence on
H is removed. The reader can translate the distance into a straight number of Mpc, in light of your favorite value for
H. And remember, 1
Mpc = 1
megaparsec = 3,260,000 light years.
And why is that? Well, in part because no one else (it seems) understands what you mean by the key terms you use, much less how to apply those criteria in an objective, independently verifiable way.
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