Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. for this to be the case, which of the following would have to be true (based on current (a) dark energy is the dominant form of energy in the cosmos. (b) dark energy does not exist and there is much more dark matter than we are aware of to date. (c) neither dark energy nor dark matter really exist. (d) dark energy exists but dark matter does not.

Uranium has two naturally occurring isotopes. 238u has a natural abundance of 99.3% and 235u has an abundance of 0.7%. it is the rarer 235u that is needed for nuclear reactors. the isotopes are separated by forming uranium hexafluoride uf6, which is a gas, then allowing it to diffuse through a series of porous membranes. 235uf6 has a slightly larger rms speed than 238uf6 and diffuses slightly faster. many repetitions of this procedure gradually separate the two isotopes. what is the ratio of the rms speed of 235uf6 to that of 238uf6? express your answer to five significant figures.