The mass density of our universe is measured to be about 10-29 kg/m3. If an arbitrary point is chosen as the center, how large is the radius of a spherical surface centered at the point so that the mass enclosed in the surface will become a blackhole observed by someone outside the surface? A. 4.2 trillion light years B. 420 billion light years C. 42 billion light years D. 4.2 billion light years

1. Please help me answer A and B. Well explained please. (Not written on a paper please) A. Sets out the two postulates of Albert Einstein's theory of special relativity. B. What is simultaneity? Use a concrete example in your explanation.

1. What is escape velocity? 2. What does it mean to have a binding energy? 3. Under what conditions would you have to achieve in order for a satellite to escape the gravitational field of another body? 4. Why don't astronauts feel the effects of gravity while in orbit around Earth? 5. Why don't we feel the physical attraction from people around us, if there is a force of gravity between every obiect containing mass?

QUESTION 10 What curvature will the universe have if the matter and energy density of the universe is equal to the critical density? Open Closed Flat O None of the above QUESTION 11 Why can you not observe beyond the horizon distance? O Because of opaque gas O Because our telescopes are not powerful enough O Because light/information cannot travel faster than the speed of light O Because of the of black holes QUESTION 12 According to the Olber's Paradox, the night sky should be completely bright. However, when we observe at night, the sky is dark. Why does this happen? O Because the universe has a finite age O Because as the universe expands, objects at larger distances move faster, and the light from them shifts out of the visible part of the electromagnetic spectrum O Both a) and b) O Neither a) nor b)

Which axiom of set theory is used to define the constructible universe L? A. Axiom of Infinity B. Axiom of Power Set C. Axiom of Choice D. Axiom of Constructibility