Briefly describe one of the three questions left unanswered by the standard Big Bang theory which are solved by inflation. That is, describe either the structure problem, the uniformity (or smoothness) problem, or the density (or flatness) problem

Question

What will be an ideal response?

Answer 1

Structure problem: The fact that there are galaxies means that the density of the early universe differed slightly from place to place. The temperature differences in the cosmic background radiation show that regions of enhanced density did exist at the end of the era of nuclei, when the universe was 300,000 years old. However, the standard Big Bang theory cannot explain where these density enhancements originally came from.

Smoothness problem: Observations of the cosmic background radiation show that the density of the universe at the end of the era of nuclei varied from place to place by no more than about 1 part in 100,000 percent. This means that widely separated regions of the universe that did not have time to be in thermal contact nevertheless ended up with exactly the same temperature. The standard Big Bang has no explanation for this "cosmic coincidence."

Flatness problem: Historically, measurements of the matter density of the universe typically found values from, perhaps, 0.1 to 100 percent of the critical density (today we know that the matter density plus dark energy adds up to exactly the critical density). These early measurements were quite puzzling: why would the density of the universe be so close to the critical value, when, according to the standard Big Bang theory, the matter density could be any value. Another way to state this problem is to say that the universe is very flat. If the universe had been 10 percent denser at the end of the era of nuclei, it would have re-collapsed long ago. If it had been 10 percent less dense at this time, galaxies would never have formed before expansion spread the matter too thin. Why this fine-tuning to the critical density? The standard Big Bang has no answer.