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granpa
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[According to Hawking] our universe can appear out of nothing because the positive energy (mass and motion) and negative energy (gravitational potential energy) cancel each other out on a large enough scale
http://en.wikipedia.org/wiki/Zero-energy_universe
Hawking is probably right, however
the energy in the gravitation field of an elliptical galaxy is approximately:
E = Gm2/r
where G = 6.67×10-11 N m2/kg2
and of course E = mc2
so using r = 1021 m (100,000 Light years) we get
Gm2/r = mc2
m = r * c2 / G
m = 1.347×1048 kg = 6.774×1017 solar masses
thats 6 * 104 times larger than the largest elliptical galaxies
Elliptical galaxies vary greatly in both size and mass, from as little as a tenth of a kiloparsec to over 100 kiloparsecs (300,000 Ly), and from 107 to nearly 1013 solar masses
So in order to break even you need 60,000 galaxies worth of mass in a region of space only 100,000 Ly across.
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