Question

1. Choose the correct statements from the following list. (Give ALL correct answers, i.e., B, AC, BCD...)
   
   A)
   B) The horizon problem in cosmology is that regions of the universe that should not have ever had thermal contact with one another have the same temperature.
   C) Assuming no dark energy, if the matter density of the universe is greater than critical, the universe is open.
   D) The inflationary model of the universe solves both the flatness and the horizon problems.
   E) Observations show us that the geometry of our universe must be very close to flat.
   F) Assuming no dark energy, if the matter density of the universe is greater than critical, the universe is will expand forever.
   G) The critical density is the density needed to cause the Big Bang.

Answer 1

B is true .since,theHorizon Problem in cosmology says, essentially, that regions on opposite sides of the observable Universe are in thermal equilibrium even though they are so far apart that due to the limited speed of light, they could not have ever been in contact.

C is false.As,Critical density is the value at which theUniverse is at balance, and expansion is stopped. If the ratio is less than one, then the actual density is less than the critical density, and the Universe is open and expands forever in every direction.

D is also correct statement.The small universeinflated by a large amount and the part of the universe you can observe appears to be nearly flat. That solves the flatness problem. The horizon problem issolved by inflation because regions that appear to be isolated from each other were in contact with each other before the inflation period.

E is also correct statement . Since,The distance between every observer and horizon of events (radius of the Universe a) is very great, and the body of every observer is verysmall. We compare themselves with a ,therefore the space of the Universe seems as a flat.

F is false as the universe is open and expanded only when the matter density is lesser than criycrit density.

G is wrong.

The 'critical density' is the average density of matter required for the Universe to just halt its expansion, but only after an infinite time.It is the value at which universe is at balance and expansion stops.

Therefore ,B D and F are correct statements.