Fundamental cosmological observations

Question 1

What is Olbers’ Paradox, and what does it tell us about the universe?

Answer 1

Olbers’ Paradox asks why the night sky is dark if the universe is infinite and filled with stars. The resolution suggests that the universe is either finite in size or evolving (expanding), preventing us from seeing an infinitely bright sky.

Question 2

What is Hubble’s Law?

Answer 2

Hubble’s Law states that galaxies recede from us with a velocity proportional to their distance:

v=H0D

where H0H0​ is the Hubble constant, and DD is the galaxy’s distance.

Question 3

What is the physical meaning of redshift in cosmology?

Answer 3

Redshift (zz) measures the stretching of light waves due to the expansion of the universe. It is given by:

1+z=(λobs/λem)

where λobs/λobs​ is the observed wavelength, and λemλem​ is the emitted wavelength.

Question 4

What are the assumptions of the Cosmological Principle?

Answer 4

The universe is homogeneous (same in all locations) and isotropic (same in all directions) on large scales.

Question 5

What does the equation r(t) = a(t) * x represent?

Answer 5

Co-moving Coordinates. r(t) is the actual distance, a(t) is the scale factor, and x is the co-moving distance.

Question 6

What is the relative velocity equation? And how do you get the expansion rate?

Answer 6

v = H(a) r(t)

v will equal the expantion rate when H(a) = H_0 (Hubble Constant)