The Expanding Universe

“Distant galaxies separate not from a visible center but because the scale of space changes.”

01 / PLAINLY

What it means, plainly

Cosmic expansion is the growth over time of distances among unbound regions of the universe on large scales.

02 / CONTEXT

A little more

Light from distant galaxies arrives redshifted, and the relationship between distance and recession reveals expansion. It is not an explosion of galaxies from one point into empty space. Bound systems such as atoms, people, or the Milky Way do not expand with the cosmic flow.

03 / WHY IT MATTERS

Why it matters

It connects the cosmos's thermal history, the age of observed light, and evidence that today's expansion is accelerating.

04 / EXAMPLE

A familiar example

The farther a galaxy lies in the general flow, the greater its redshift tends to be, although local motions add deviations.

05 / LIMIT

What it does not mean

There is no observed center from which everything was launched, and expansion does not appreciably stretch your body or the solar system.

06 / NOTICE

Notice it in your day

Mark dots on an elastic band and stretch it to illustrate growing distances. Then list three limits of this material analogy.

FINAL NOTE

The idea worth keeping

Separate metric expansion, local motion, and structures held together by gravity.

QUESTIONS / 02

Questions people still have

Where is the center of cosmic expansion?

In the homogeneous model, no privileged spatial center exists within the observed universe.

Does a bound galaxy expand as well?

Not generally; its gravity and other forces dominate over expansion at that scale.