ABSTRACT
If we can imagine a spherical surface that separates the observable from the unobservable, that surface constitutes a horizon. The Hubble spherical surface of radius https://www.w3.org/1998/Math/MathML"> R H https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003099581/7a0f57ba-a2aa-4ba0-9950-8db91dcea467/content/C002_equ_0001.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> is a horizon. Suppose we wish to know the recession speed of a galaxy at a distance https://www.w3.org/1998/Math/MathML"> R = α × 13.6 Gyr × c https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003099581/7a0f57ba-a2aa-4ba0-9950-8db91dcea467/content/C002_equ_0002.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> , where α is a dimensionless number. Using today’s value of the Hubble constant, https://www.w3.org/1998/Math/MathML"> v rec = H 0 R = 1 13.6 Gyr R = α × c https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003099581/7a0f57ba-a2aa-4ba0-9950-8db91dcea467/content/C002_equ_0004.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
