Expanding universe is flying apart into itself

It is a real mind-bender, but everything is contained within the cosmos, which is still nonetheless expanding outwards from the Big Bang.

- Nicole Granucci

When you bake a loaf of bread or a batch of muffins, you put the dough into a pan. As the dough bakes in the oven, it expands into the baking pan. Any chocolate chips or blueberries in the muffin batter become farther away from each other as the muffin batter expands.

The expansion of the universe is, in some ways, similar. But this analogy gets one thing wrong — while the dough expands into the baking pan, the universe doesn’t have anything to expand into. It just expands into itself.

It can feel like a brain teaser, but the universe is considered everything within the universe. In the expanding universe, there is no pan. Just dough. Even if there were a pan, it would be part of the universe and therefore it would expand with the pan.

Even for me, a teaching professor in physics and astronomy who has studied the universe for years, these ideas are hard to grasp. You don’t experience anything like this in your daily life. It’s like asking what direction is farther north of the North Pole.

Another way to think about the universe’s expansion is by thinking about how other galaxies are moving away from our galaxy, the Milky Way.

Scientists know the universe is expanding because they can track other galaxies as they move away from ours. They define expansion using the rate that other galaxies move away from us. This definition allows them to imagine expansion without needing something to expand into.

The expanding universe