Radius of Universe 6 Gyrs Ago: What Was It?

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SUMMARY

The discussion centers on the size of the universe's radius approximately 6 billion years ago, particularly in relation to the cosmic scale factor (a) and the observable universe's radius at that time. Participants clarify the terminology, emphasizing the need to distinguish between the scale factor and the distance to the cosmological event horizon. The cosmic scale factor is dimensionless, while its current value is approximately 10^29 cm. Understanding the value of the scale factor at the time of accelerated expansion is crucial for accurate cosmological measurements.

PREREQUISITES
  • Understanding of cosmic scale factor (a)
  • Familiarity with observable universe concepts
  • Knowledge of cosmological event horizon
  • Basic principles of cosmology and universe expansion
NEXT STEPS
  • Research the calculation of the cosmic scale factor over time
  • Explore the implications of the observable universe's radius in cosmology
  • Study the concept of the cosmological event horizon in detail
  • Investigate the effects of dark energy on universe expansion
USEFUL FOR

Astronomers, cosmologists, and physics students interested in the dynamics of universe expansion and the historical context of cosmic measurements.

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What was the size of the 'radius' of the universe when it started to expand acceleratingly about 6 Gyrs ago?
 
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Not quite sure what are you asking. You probably asking either about scale factor (a), or about radius of observable universe at the time. Or maybe about distance to the cosmological event horizon ?

Can you be more specific about what you mean with 'radius of the universe' ?
 
Calimero said:
Not quite sure what are you asking. You probably asking either about scale factor (a), or about radius of observable universe at the time. Or maybe about distance to the cosmological event horizon ?

Can you be more specific about what you mean with 'radius of the universe' ?

Thank you for clarifying the question before answering it :smile:

If 'radius' of the universe is a, then cosmic scale factor is a(t), which is dimensionless. a has units of distance, which is presently ~10^29 cm. So I need to know how large was a when the universe started to accelerate ~6 Gyrs ago?
 

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