Comoving distance facts for kids

Comoving distance is a special way scientists measure how far things are from each other in the Universe. It helps them understand the Universe's shape without getting confused by its constant expansion. Imagine the Universe as a giant, stretchy fabric. Comoving distance is like measuring on that fabric before it stretches.

What is Comoving Distance?

Scientists want to study the Universe's shape without worrying about it getting bigger all the time. So, they use a trick! They pretend the Universe isn't expanding for a moment. The distances they measure then are called comoving distances.

They know the Universe *is* expanding. They keep track of this expansion with something called the scale factor. This way, they can separate the Universe's true size from its shape. Thinking about the Universe with comoving distances makes some calculations much easier for them.

Why Do Scientists Use It?

When the part of the Universe we can see today first began, it was incredibly tiny. Everything was packed much closer together than it is now. It was also super hot, and tiny light particles called photons were zipping around everywhere.

Light travels very fast, but its speed has a limit. Also, the Universe was expanding incredibly quickly back then.

Imagine Ants on a Balloon!

Think of two ants on the surface of a balloon that is slowly being blown up. One ant starts crawling towards the other. But as it crawls, the balloon itself is also getting bigger! This means the distance between the ants is increasing because of the balloon's expansion, even as one ant tries to close the gap.

After a long time, the balloon might start expanding more slowly. Eventually, the crawling ant might finally reach the other one. This is a bit like how light travels in our Universe.

Some light from the stars you see at night has traveled for billions of years to reach your eye. It started very close to where you are now (or where the atoms that make up your body were). But it couldn't always keep up with the Universe's fast expansion. If something else started very close to you but wasn't traveling towards you, it would be very far away now because of the expansion. Comoving distance helps scientists measure these distances as if the balloon wasn't expanding.

Images for kids

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  This animation shows how the Universe evolves using comoving distances. The x-axis is distance in billions of light-years, and the y-axis is time in billions of years since the Big Bang. The right y-axis shows the scale factor, which represents the Universe's expansion. This model includes dark energy, which makes the expansion speed up later on, creating an event horizon beyond which we can never see.

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  Comoving coordinates help separate the Universe's expansion from its fixed spatial coordinates. This example shows the ΛCDM model, which is a common model for our Universe.

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  This animation shows the Universe's evolution using proper distances, which are the actual distances at any given time. It's the same model as the first image, showing how dark energy affects the expansion and creates an event horizon.

See also

In Spanish: Distancia comóvil para niños