How Big is The Universe?

The other day my daughter asked, "How big is the universe?"

Wow, a simple question to ask yet a really tough question to answer. I told her the universe is so enormously huge that it's beyond my comprehension to understand it even though I can describe it in gigantic measures of distance, like light years. It's strange to think of distance as light and its speed of travel, but there's no other way to do it. To try and define the size of the universe in miles would make the numbers so enormous it wouldn't make any more sense than saying it's a gazillion miles across in diameter.

I told my daughter, "Let's start simple." The distance of our solar system from our Sun at the center to the furthest planet (arguably Neptune or Pluto) is roughly 30 AU (astronomical units). Comparatively, Earth is just one AU or about 93 million miles from the sun. Light travels at 186,000 miles per second or 700 million miles per hour. Light takes eight minutes to go from the sun to the Earth and about five hours to reach Pluto.

Now, this is just our solar system which resides in the Milky Way Galaxy. How big is that? The Milky Way Galaxy contains an estimated 300 billion stars (like our sun) inside it and is about 100,000 light years across in diameter. Remember that a light year is not a time but a distance; it's how far light travels in an entire year. So if you made a large flash of light at one end of the Milky Way, that light would travel for 100,000 years before someone on the other side would see it. Even though we're just talking about the size of our Milky Way Galaxy, these kinds of numbers already perplex my brain. YouTube video on right from gohepcat.

But still, this is just the distance for our local galaxy. The universe has an estimated 300 billion galaxies, some similar to our Milky Way and some very different. Recent guesses by top astronomers say the universe is over 100 billion light years across. And yes, trying to understand how light could travel for 100 billion years is beyond the scope of my comprehension. Especially when the universe itself is estimated to be around 14 billion years old since the Big Bang theoretically occurred.

My daughter asked, "If the universe is 14 billion years old, then how can it be 100 billion light years across?"

My answer was, "You're right. That doesn't make any sense to me either. Have some more Cheerios." Smart kid.

What are your thoughts?

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