Lookback time for a given redshift, tL(z), is...
* time between the given redshift and the beginning of the Universe.
* time between the infinite and the given redshift.
* equal to the age of the Universe at the given redshift.
* time between now and the given redshift.
To determine the correct option for the lookback time for a given redshift, let's break down the definitions of each option:
1. "Time between the given redshift and the beginning of the Universe": This option is incorrect because the beginning of the Universe corresponds to a redshift of infinity, so there is no redshift value corresponding to the beginning of the Universe.
2. "Time between the infinite and the given redshift": This option is also incorrect. While a redshift of infinity corresponds to the beginning of the Universe, the time between infinity and a given redshift is not the lookback time.
3. "Equal to the age of the Universe at the given redshift": This option is not correct either. The age of the Universe at a given redshift refers to the time since the Big Bang until that specific redshift, whereas the lookback time is the time between the redshift and the present.
4. "Time between now and the given redshift": This is the correct option. The lookback time for a given redshift, tL(z), represents the time that has elapsed between the redshift and the present day. It tells us how far back in time we are observing an astronomical object or event.
So, the correct answer is: "Time between now and the given redshift."