A. write 2 -3 sentences describing the big bang theory and explaining why it is considered a theory, as opposed to a hypothesis.

b. write 2 -3 sentences explaining cosmic background radiation and what it led scientists to conclude about the universe.

c. scientists have proposed three models of the universe: flat, open, and closed. each of these describes a different future for the universe. write 2 -3 sentences identifying the model that is most widely accepted and describing the future of the universe according to that model.

a. Big Bang Theory explains the beginning of the universe. The entire matter of the universe is believed to be concentrated in a single point. This point had infinite density. At time, t = 0, at the big bang, universe started expanding and cooling down. This theory is most widely accepted theory describing the beginning of the universe. It is not considered as hypothesis because of the evidences received by the scientists- detection of cosmic microwave background, red shift of the light received from the galaxies indicating that universe is expanding.

b. Cosmic Microwave background radiation is a faint radiation having average temperature of 3 K detected every where around from the sky. This is a residual radiation from 400,000 years after the beginning of the universe. The universe was very hot and dense at the beginning and thus, opaque. Hence, detection of the CMB has provided the evidence of Big bang theory.

c. Scientists have proposed three models of the universe:

1. closed universe- The universe has finite volume and curves around itself. The space-time has positive curvature.

2. open universe - The universe has infinite volume and has a negative curvature.

3. flat universe - The universe has zero curvature. It has infinite volume. The actual density of the universe is equal to the critical density of the universe. The expansion rate slows down over time. This is the most widely accepted model.

it is a negative acceleration