According to the Cosmological principle, the Universe is isotropic, homogeneous and the laws of physics are universal. State what is meant by the term homogeneous.
Astronomers often use emission spectral lines to determine the relative velocity of distant galaxies. The wavelength of a specific hydrogen emission line measured in a laboratory on Earth is 650 nm650\text{ nm}650 nm.
A distant active galaxy, Markarian 501, is at a distance of 250 Mpc250\text{ Mpc}250 Mpc from the Earth and is known to host a supermassive black hole at its core.
Calculate, in nmnmnm, the wavelength λ\lambdaλ of this same spectral line when observed in the light coming from Markarian 501. Assume the Hubble constant is 72 km s−1 Mpc−172\text{ km s}^{-1}\text{ Mpc}^{-1}72 km s−1 Mpc−1 and the speed of light in a vacuum is 3.00×108 m s−13.00 \times 10^8\text{ m s}^{-1}3.00×108 m s−1.
State what is meant by the event horizon of a black hole.
