Space DOES NOT Expand Everywhere
Why Physics Stack Exchange recommends it
Physics Stack Exchange users link it when someone asks why cosmic expansion does not expand matter, rulers or bound systems, since it is seen as clearer than many written answers.
Recommended by 1 people across 3 different months, 2021 to 2025.
What it is
A PBS Space Time episode hosted by Matt O'Dowd that asks where new space comes from in an expanding universe and why expansion does not stretch objects like atoms, planets or galaxies. It explains that expansion acts only on large scales where gravity and other forces do not hold things together.
What people said
Recommends BlueRaja - Danny Pflughoeft · 2021-02 · Physics Stack Exchange · on “If spacetime is expanding, shouldn't lengths expand at an equal rate?”
I frequently hear that galaxies are moving apart (in some instances, faster than the speed of light) because the fabric of spacetime is expanding. The common analogy is two people standing on the surface of an expanding balloon. However, this makes no sense. Since we are made of matter that exists within spacetime, shouldn't we (and all of our unit lengths) be expanding at the same rate? In the balloon analogy, if the people are drawn onto the balloon, they will expand at the same rate as its surface. If you draw a coordinate grid onto the balloon, distances measured using those coordinates will stay constant as the balloon expands. So what's really going on? Note: Astrophysicist Nick Lucid
Recommends BlueRaja - Danny Pflughoeft · 2022-05 · Physics Stack Exchange · on “Why does space expansion not expand matter?”
PBS Spacetime just released a video about this exact question [this video] , which I think does a better job of answering the question than any of the answers below.
Recommends BlueRaja - Danny Pflughoeft · 2024-12 · Physics Stack Exchange · on “Question on the concept of the Big Bang Theory”
Another related: If spacetime is expanding, shouldn't lengths expand at an equal rate? [LINK] . I did not get a good answer here, but PBS Spacetime [this video] eventually did an episode on it.
Cites Léo Vacher · 2024-05 · Physics Stack Exchange · on “Does the Schwarszchild solution require the cosmological constant to stop it from expanding?”
Great answer! Let me also add that the cosmological constant is expected to have absolutely no impact on dense and small scales systems (as e.g. planetary systems, black holes etc) such that the Schwarzschild metric would correctly describe such system without a need for a $\Lambda$ addition. Possibly more informations in this outreach video: [this video]
Cites Alex · 2025-08 · Physics Stack Exchange · on “Is Spacetime Contraction/Expansion Really Different from Spacetime Curvature?”
Do you disagree with the contents of this PBS Spacetime video? It seems to go against what you are saying. [this video]