Cosmology
The Observable Universe
The universe is 13.8 billion years old, yet we see 46 billion light-years in every direction, and some galaxies we see today we could never reach. Three horizons explain it: the Hubble sphere, the event horizon and the particle horizon. Watch them grow and shrink through cosmic time, then follow light down our past light cone, shaped like a teardrop.
particle horizon: the observable universeevent horizon: what we can still reachHubble sphere: v = cour past light conegalaxy and its light
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Radius of the observable universe—
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1 million yr100 billion yr
Distances
0.1CMB, 1100
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Numbers
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What to observe
- Today the particle horizon, the farthest light could have come from, is 46 billion light-years away, over three times 13.8 billion: the space it crossed kept growing behind it.
- The event horizon is only 16.6 billion light-years. Galaxies between the two horizons are ones we can still see, as they were, but light we send now will never reach them, nor theirs us.
- Run time forward: the particle horizon keeps growing, but the event horizon closes in as dark energy takes over. In 100 billion years every galaxy beyond our own group will have slipped out of reach.
- Our past light cone, drawn in proper distance, is a teardrop. The light now reaching us from a galaxy at z = 2 left it when it was only 5.8 billion light-years away, then was carried away from us at first before it gained ground.
- In comoving distance with conformal time, the same cone is straight at 45°, and the CMB is simply the oldest light on it.
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