Graduate Understanding distances in Cosmology

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SUMMARY

This discussion focuses on the definitions of distance in cosmology, particularly the proper distance and comoving distance. The proper distance at redshift ##z'## is defined by the equation $$d(z{'}) = \frac{1}{1+z{'}}\int_0^{z{'}} \frac{dz{'}}{H(z{'})}$$, while the comoving distance is represented as $$d_1(z) = \frac{1}{1+z}\int_0^z \frac{dz}{H(z)}$$ and $$d_0(z) = \int_0^z \frac{dz}{H(z)}$$. Additionally, conformal time is introduced with the formula $$\eta(z) = \int_0^z \frac{dz}{H(z)}$$. The discussion also touches on lookback time and its relationship to conformal time, indicating a need for further clarification.

PREREQUISITES
  • Understanding of cosmological redshift and its implications
  • Familiarity with the Hubble parameter, denoted as H(z)
  • Knowledge of integral calculus for interpreting distance equations
  • Basic concepts of cosmological distance measures
NEXT STEPS
  • Research the implications of the Hubble parameter in cosmology
  • Study the relationship between lookback time and conformal time
  • Explore advanced distance measures in cosmology, including angular diameter distance
  • Read the recommended paper on distance measures: astro-ph/9905116
USEFUL FOR

Astronomers, cosmologists, and physics students seeking to deepen their understanding of distance measures in cosmology and their mathematical formulations.

Arman777
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I am trying to understand this graph but I am confused about the distance definitions. So there's an object located at a comoving distance ##r##.

The proper distance of the object at ##z'## can be written as
$$d(z{'}) = \frac{1}{1+z{'}}\int_0^{z{'}} \frac{dz{'}}{H(z{'})}$$

In this case,

$$d_1(z) = \frac{1}{1+z}\int_0^z \frac{dz}{H(z)}$$

and

$$d_0(z) = \int_0^z \frac{dz}{H(z)}$$

And we can also define the conformal time where it represents the distance taken by photon from the distant galaxy to us and can be written as

$$\eta(z) = \int_0^z \frac{dz}{H(z)}$$

There is also lookback time/distance but I am not sure it's same as the conformal time or not...

Are these definition that I have made are correct ?
 
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