State Hubble’s law.

Measured values of the Hubble constant can vary between 40 kms^–1 Mpc^–1 and 90 kms^–1 Mpc^–1. State the reason for this wide variation in values.

The blue line in the spectrum of atomic hydrogen as measured in the laboratory is 490 nm. The same line in the spectrum of light from a galaxy has a wavelength of 500 nm.

Determine the distance of the galaxy from Earth. You may assume that the Hubble constant=70 km s^–1 Mpc^–1.

the recessional speed of galaxies is proportional to their distance from Earth/us/each other / v = H₀d (with terms defined);

there is a large uncertainty in the measurement of galactic distances / it is difficult to accurately determine galactic distances;

$v = c\frac{{\Delta \lambda }}{\lambda }$;
$= \left( {3 \times 10^8 \times \frac{{10}}{{490}} = } \right)6.1 \times {10^3}{\rm{km}}{{\rm{s}}^{ - 1}}$;
$d = \frac{v}{H}\left( { = \frac{{6100}}{{70}}} \right) = 87{\rm{Mpc}}$;