Lecture - Motivation and Background

Radio telescopes play a tremendous role in modern astrophysics due to the manifold possibilities of observation. With radio astronomy, we are able to observe a broad range of thermal (e.g. H21-line at  a wavelength of $\lambda = 21\,\text{cm}$) and non thermal (e.g. synchrotron-) radiation. Radio astronomy also plays a unique role when investigating the most powerful objects in the universe, such as active galactic nuclei (AGN) or even the cosmic microwave background (CMB).

For information about the history of radio astronomy beyond the scope of this course, we recommend the book "The Invisible Universe" by Gerrit Verschuur (2015, Springer).

In the following a short summary of the history of radio astronomy will be given.

• 1931: The radio engineer Karl Guthe Jansky was assigned to study the electromagnetic radiation coming from thunder storms by the Bell Labs in Hondel, USA. The scientific focus was the polarization and the direction of arrival of the radiation. With a big, rotating, vertically polarized antenna at a wavelength of $\lambda = 14.6\,\text{m}$ $(\nu = 20.5 \,\text{MHz})$ he was the first person to discover electromagnetic waves from space. In subsequent studies, he inferred that this radiation came from the center of the Milky Way.

• 1937: The first radio telescope devoted solely to astronomical research was built (and paid for) by Grote Reber in his backyard. The size and broad receiving band of $160\,\text{MHz}$ to $3.3\,\text{GHz}$ lead to a series of new discoveries:

• The radiation spectrum of the Milky Way cannot be described by Planck's formula for black body radiation. Different, non thermal processes cause this radiation.
• The first radio astronomical sky survey could be performed. Several strong radio sources in constellations like Sagittarius, Cygnus, Cassiopeia, Canis Major, Puppis and Perseus were discovered.

• 1944: Hendrik van de Hulst proposed that interstellar, neutral hydrogen $HI$ may cause radiation in the radio regime.

• 1951: The $HI$ emission at $\lambda = 21 \,\text{cm}$ (H21-line) was discovered by Ewen and Purcell.

• 1965: Arno Penzias and Robert Wilson tried to verify that the sky temperature at the zenith is approximately $0 \, \text{K}$. However, they detected a $3 \, \text{K}$ background radiation at $\lambda = 7.4 \,\text{cm}$. For this first detection of the CMB, which is the remnant of the Big Bang, Penzias  and Wilson shared a Nobel prize with Pyotr Kapiza in 1978.

• 1967: Jocelyn Bell discovered the first Pulsar.

Fig. 1.1 Arno Penzias and Robert Wilson standing at the Holmdel Horn Antenna with which they discovered the CMB. Credit: NASA