Answer to Question 1
By 1980, the big bang model was widely accepted, but it faced two problems that led to the development of an improved theory-a big bang model with an important addition.One of the problems is called the flatness problem. The curvature of space-time seems to be near the transition between an open and a closed Universe. That is, the Universe seems approximately flat. It seems peculiar that the actual density of the Universe is anywhere near the critical density that would make it flat. To be so near critical density now, the density of the Universe during its first moments must have been very close, within 1 part in 1049, of the critical density. So, the flatness problem is: Why was the Universe so close to exactly flat, with no space-time curvature, at the time of the big bang?The second problem with the original big bang theory is called the horizon problem. When astronomers correct for the motion of Earth, they find that the cosmic microwave background (CMB) is very isotropic, the same in all directions to a precision of better than 1 part in 1000. Yet, background radiation coming from two points in the sky separated by more than an angle of one degree is from two parts of the big bang far enough apart that they should not have been connected at any previous time. That is, when the CMB photons were released, the Universe was not old enough for energy to have traveled at the speed of light from one of those regions to the other-the regions should always have been beyond each other's horizon and could not have exchanged heat to make their temperatures equal. So, the horizon problem is: How did every part of the observable Universe get to be so nearly the same temperature by the time of recombination?
Answer to Question 2
A
Bronchi Model
Three-dimensional model of DNA.
A model of the 70S ribosome based upon early structural data
Three-Box Model
Hepatic Acinus Model of Liver Function
Computing Limits Problems