Slide 1
Cosmic expansion increases large-scale unbound separations
Cosmic expansion describes the increasing separation of widely separated, unbound regions of the universe. It is a property of the large-scale cosmic geometry.
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This differs from an explosion of debris into surrounding empty space. In the standard model, the distances within space evolve; the account does not require an external room into which the universe expands.
Slide 2
More distant galaxies generally show greater recession
On large scales, more distant galaxies generally show greater recession associated with expansion. Measurements of distance and redshift reveal this relationship.
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Galaxies also have local motions produced by gravity. Those motions can dominate nearby, so an individual approaching galaxy does not overturn the large-scale expansion pattern.
Slide 3
Uniform expansion does not put Earth at its centre
A broadly uniform expansion can look similar from many locations. Seeing distant galaxies recede does not establish that Earth occupies a unique cosmic center.
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In a uniformly enlarging grid, separations increase from any chosen grid point. The analogy concerns relative distances; it does not imply that real galaxies sit on a physical lattice.
Slide 4
Bound galaxies, solar systems and atoms need not expand
Gravity and other interactions hold systems such as galaxies, solar systems and atoms together. Their sizes do not automatically grow with cosmic expansion.
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The cosmological description applies at large scales after averaging over local structure. Using it to predict that a ruler stretches each year confuses that scale with the forces maintaining ordinary objects.
Slide 5
The expansion rate changes through cosmic history
The rate of expansion can change over cosmic time. Observations at different distances help reconstruct that history within a cosmological model.
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The present expansion rate and whether expansion accelerates are different quantities. Keeping them separate is essential when interpreting measurements that address the universe’s evolution.
Sources
- A Model of the Universe
OpenStax, Rice University
- The Expanding Universe
OpenStax, Rice University