# Tag Info

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Yes, in an expanding universe there is an increase in potential energy (with same caveats). The (Friedmann) equations of a homogeneous and isotropic universe with no spatial curvature or cosmological constant can, in fact, be derived very easily from nonrelativistic Newtonian gravity. (This derivation is shown, e.g., in Mukhanov's Physical Foundations of ...

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Total energy of a system is constant. In an open universe with cosmological constant $\Lambda = 0$, the potential grows (becomes less negative) while the kinetic energy decreases at the same rate. After a long time, the velocities tend to be constant (kinetic energy becomes constant) and the potential goes to 0. But, we now think that $\Lambda > 0$. ...

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Universe is picking up kinetic energy due to the expansion of space: A photon will gain energy (blueshift) when it heads into a supercluster on its way to the Earth. This is an effect of general relativity. And as it leaves the other side of the supercluster as it continues its journey, it will lose energy (redshift) as it climbs out of the ...

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It's not really clear what you're asking and most of these questions can be researched individually, but I'll give it a shot: What would Earthlike planets bigger than Earth with <10 m/s^2 gravity and complex life be like? Large is bigger than Earth. Assuming that such planets exist (though I guess if such planets don't exist that would ...

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Simple answer: the material is just too spread out. Forming a planetary-size object takes LOTS of collisions to build up enough matter to start gravitationally attracting the surrounding stuff. The Oort cloud is very far from the sun (starting at ~1000 AU, compared to a maximum of ~50 AU for Pluto), meaning that the icy fragments are moving very slow in ...

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What would really happen is that the 2 black holes would spiral around and into each other so fast that any theoretical safe point wouldn't be safe for long, like a few millionths of a second at most. But if you imagine a snapshot in time, where 2 black holes are near each other and your equidistant from both of them, that would be a gravitationally ...

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Your logic isn't unsound. By symmetry there would be a gravitational neutral point between them. Of course, the black holes could not then be described by the Schwarzschild metric, since that only applies to a spherically symmetric situation. As you brought the black holes closer together, the isopotentials would become distinctly aspherical, the event ...

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I'm not sure I agree with the double planet POV, but the calculation is pretty simple. The earth weighs 81 moons, so for the Barycenter to be outside the earth, the distance (center of Moon to surface of earth), = 81 earth radii. or about 515,000 KM. It's current farthest distance is 405,000 KM, average distance 384,000 KM and closest 363,000 KM ...

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Well, I wasn't going to answer but the other two answers are wrong, or at least incomplete. If you wish to make a black hole from a stellar-sized object, then there is no need to compress it to as small as the Schwarzschild radius (though that would certainly work and would certainly be the answer for smaller objects with negligible self-gravity). Instead, ...

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$$r_s = \frac{2GM}{c^2}$$ --Here: $r_s$ is the Schwarzschild radius; $G$ is the gravitational constant; $M$ is the mass of the object; $c$ is the speed of light in vacuum. The proportionality constant, $2G/c^2$, is approximately 1.48×1027 m/kg

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Well, it has to be crushed (compressed would be the better word to use here) below the Schwarzschild radius. The Schwarzschild radius is the radius of the object in which the escape velocity would be the speed of light from that object. When this radius becomes even smaller, even light cannot escape it and voilà, there's your black hole!

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The wikipedia page you linked to tells you that the solar system is gravitationally "chaotic", in part because the mass of the sun is not fixed over time. But even more simply than that, focusing just on the gravity (ignoring loss of stellar mass, etc.), the solar system is an N-body problem. We have 8 planets, a sun, and millions of asteroids, comets, and ...

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