
Concept explainers
The definition of horizon and flatness problem and the steps adopted to resolve them by the idea of inflation.

Explanation of Solution
Introduction:
Horizon problem originates due to the observation of constant temperature of (CMBR) cosmic microwave background
Flatness problem is the cosmological problem, which describes that the density of the universe is close to the critical value of density, which is impossible and leads to the flatness problem.
Horizon problem originates due to the observation of the nearly constant temperature of the cosmic microwave background radiation. After the big bang, the radiation emitted is continuously traveling in the observable universe. Two points in the universe which are at very distant locations to each other, that is nearly a million light-years, cannot have the temperature of cosmic microwave background radiation similar to each other. This is because the radiation from the big bang cannot reach such a distant point in the space, so it leads to the horizon problem.
On the other hand, flatness problem arises due to the observation of density of the universe to be nearly equal to the critical density, which is considered impossible. After the billion light-years from the big bang, still the geometry of the universe resembles that of flat surface which is considered a problem in cosmology.
The solution to the horizon and the flatness problem is cosmic inflation which states that the universe is expanding after the big bang at an exponential rate in the initial inflationary period resulting in the isotropic nature of the universe observed today. This theory also states the fact that during the inflationary period the energy and mass were equally distributed resulting in constant temperature observed in the universe.
Conclusion:
Therefore, the cosmic inflation theory gives the solution to Horizon and Flatness problems.
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Chapter 26 Solutions
Universe: Stars And Galaxies
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