Cosmology - First observations

Andrew Conway

Black as night

Olbers popularised this question as a paradox: if the Universe is infinitely large and old, why is it dark at night?

Possible (overlapping) answers:

Constructing Hubble's law

Constructing a Hubble's law plot involves measuring the following for many galaxies

Cepheids (Type I)

Delta Cephei - light curve


Source: ThomasK Vbg CC-BY SA 3.0

Cepheid - Period-luminosity graph

Cepheid period luminosity graph_

Source: Marcia Rieke Copyright not clear.

Method to determine distance to a galaxy

Cepheids in practice

Redshift and the Doppler effect

Redshift example


Source: Georg Wiora CC-BY SA 2.5

Redshift formula

If v is the speed of the source, c is the speed of light then the redshift z is:

z = v / c

If λ0 is the stationary wavelength and λ is the observed (shifted) wavelength, then the redshift is given by

z = (λ - λ0) / λ0


Redshift calculations

Hubble's law - 1929

Hubble's law_1929

Source: Hubble (1929) Public Domain (probably)

Hubble's law - 2005

Hubble's law 2005

Source: Reindl et al (2005) Published in ApJ

The Hubble constant

Estimates of the Hubble constant

Hubble constant estimates

Source: Alexander.stohr CC-BY SA 3.0

The age of the Universe

An estimate of the age of the Universe can be worked out from the Hubble constant.

The age of the Universe T is given by this equation:

T = 70/H0 × 14 billion years

Taking the current accepted value we can calculate the age of the Universe as 14 billion years.

More refined models of the universe are used to adjust this estimate, but this is still a plausible estimate of the age of the Universe.

Note: Look at the units of the Hubble constant and notice that if you convert Mpc to km then the units become just s-1.

Other distance techniques

At galactic scales, the following distance measuring methods can be used:

In astronomy, the term 'standard candle' refers to an object of known luminosity that can be used to calculate a distance.

For a long list of methods see

Type 1a supernova