Expansion of the Universe
The expansion of the universe
Definition
The expansion of the universe means that, on sufficiently large scales, distances between widely separated gravitationally unbound regions increase with cosmic time.
Important Idea
Hubble's observations showed that distant galaxies generally have redshifts increasing with distance. This led to the picture of an expanding universe. Cosmic expansion is not best understood as galaxies exploding from one point into empty surrounding space. Rather, the large-scale geometry itself evolves so that cosmic distances increase.
Bound Systems
Expansion does not simply stretch every object. Local forces can dominate within: atoms; planetary systems; many galaxies; other gravitationally or electromagnetically bound systems.
Formula / Key Relation
Conceptually: **d(t) ∝ a(t) and: H(t)=ȧ/a At the present epoch: H(t₀)=H₀**
Exam preparation
Exam Focus
Quick revision
Revision Zone
Expansion of the Universe
Sticky Notes · Quick Revision
Expansion:
- Cosmic distances ↑
- Scale factor `a(t)` ↑
- `H=ȧ/a`
- No ordinary center required
60-second revision
- Cosmic expansion is the large-scale increase of distances described by the scale factor.
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