Introduction

Well what exactly are COSMOSOMAS? It is a bon mot which was suggested, half-jokingly, within our group to more conveniently refer to the first unambiguous cosmological ``features'' we had found in the 3K relic radiation reported in Hancock et al. [1], rather than having to constantly use words like ``fluctuation'', ``hot spot'' or ``bump''. This slightly forced acronym of COSMOlogical Structures On Medium Angular Scales does convey the idea of the cosmological seed structure required by theories of galaxy formation by gravitational collapse. Here I shall extend this definition of COSMOSOMAS to cover not only the angles of the Tenerife experiment but also the expected region of enhanced signal in the CMB angular power spectrum at about $1^{\circ}$, which is usually referred to as the first ``Doppler'' peak. In this way the spectrum can be divided, rather arbitrarily, into large ( $> 5^{\circ}$) and small ( $< 0.5^{\circ}$) angular scales on either side corresponding to the ``Sachs-Wolfe plateau'' and the ``Doppler foot-hills'' respectively, which will be described in following sub-sections. An example power spectrum expanded in spherical harmonics showing these features is shown in figure  for a standard Cold Dark Matter (CDM) model.

Before the location of the first COSMOSOMAS the aim of CMB anisotropy measurements was just to find evidence of fluctuations and to establish by statistical means the level of their amplitude. This was the case with the announced results in April 1992 of Smoot et al. [2] from their first year of data from the Differential Microwave Radiation (DMR) experiment on-board the COsmic Background Explorer (COBE) satellite, where all sky coverage with moderate sensitivity, fixed the large angular scale amplitudes corresponding to the initial primordial power spectrum. Early the following year a balloon-borne experiment, the MIT Far Infra-Red Survey (FIRS) produced similar results in Ganga et al. [3], which when cross-correlated with DMR data confirmed the amplitude and cosmological origin of the excess signal, although still no single feature could be identified as real. This changed with our results in Hancock et al where smaller sky coverage with very high sensitivity localized individual features on scales of $5^\circ$. Now the main aim is to establish the existence of the ``Doppler peak'', which will show itself as the most characteristic angular scale size of CMB ``blobs'' or COSMOSOMAS.

  

Figure: Example CMB angular power spectrum showing the three different regimes of the ``Sachs-Wolfe plateau'', the ``Doppler peak'' and ``Doppler foot-hills''.