Suppose the Universe is currently 14 billion light-years in radius but only 8 billion years old, in another billion years how big will the observable universe be? Select one: a. 15 billion light-years in radius b. the observable universe is always infinite in size c. 14 billion light-years in radius d. 8 billion light-years in radius e. 9 billion light-years in radius
The correct answer and explanation is:
The correct answer is a. 15 billion light-years in radius.
Explanation:
The observable universe is defined as the region from which light has had time to reach us since the beginning of the universe. This means that the radius of the observable universe is roughly the age of the universe (in years) multiplied by the speed of light, because light travels at a constant speed and we can only see objects whose light has reached us so far.
- Currently, the universe is said to be 8 billion years old, and the observable radius is approximately 14 billion light-years. This discrepancy arises because the universe has been expanding during this time, causing the distances to stretch beyond just a simple multiplication of age by light speed.
- However, if we simplify the problem (assuming light speed is constant and ignoring complex cosmological expansion for this question), the radius of the observable universe roughly grows by 1 light-year for every 1 year that passes, because each additional year allows light from farther away to reach us.
So, if the universe is currently 8 billion years old and the observable radius is 14 billion light-years, then after one more billion years, the universe will be:
- Age: 9 billion years
- Observable radius: Approximately 15 billion light-years (14 + 1 billion light-years)
This is because with one more billion years passing, light from even farther away will have had enough time to reach us, increasing the observable radius by roughly 1 billion light-years.
Why not other options?
- b. the observable universe is always infinite in size
False. The observable universe is finite because light has only traveled a finite time since the Big Bang. - c. 14 billion light-years in radius
False. The radius increases as time passes, since light from farther regions reaches us. - d. 8 billion light-years in radius
False. The radius is larger than the age of the universe in years multiplied by light speed because of cosmic expansion, but it definitely grows over time. - e. 9 billion light-years in radius
False. This is just the age of the universe in years and would be true only in a static, non-expanding universe.
Summary
The observable universe grows as time goes on because light from more distant parts has time to reach us. Given the current radius is 14 billion light-years at 8 billion years old, in 1 billion years the radius will be about 15 billion light-years.