Window to the Very Early Universe and some problems of “ μK Astrophysics”

Abstract

A few years ago it was demonstrated by several groups that we can see predicted by theory small scale CMB anisotropy at the recombination epoch, z = 1000. It means, that:

• 1. We can check the history of evolution of the primordial disturbancies up to the recombination time.

• 2. There was an extended epoch of “neutral” Universe when gravitation formed proto-objects from very small relative disturbancies, say, 10^•5, up to more than 10⁵ (”DARK AGE”, 10 < z < 1000).

• 3. The size of the “Luminous Universe” is limited by some redshift, say, z = 10.

There are many projects connected with item 1 (PLANCK mission - the most ambitious one), and with item 3, (NGST - the most ambitious in optics from Space, and 100m — 5000m in diameter - the most ambitious ground-based optical- IR telescopes, SKA - in Radio).

The situation with facilities for the ”DARK AGE” Universe is not so clear [1], and we shall discuss here this item. Neutral epoch of the Universe cannot be observed by optical tools because of strong absorption beyond Layman limit, and radio tools are the first and may be the only possibility. As in the early history of Radio (first half of the 20th century), when there where many researches connected with propagation effects in the Earth atmosphere before optimal band for the “people-to-people” communication was found, at the end of the last century hundreds of papers appeared in attempts to find the best widow to the Early Universe and “DARK AGE” of its history (“people-to-Big Bang” window). Here we present a recent data, collected by different groups, including new data from RATAN-600 multi-frequency focal field array [2]. New data on the “polarization noise” from the Galaxy will be discussed, and a large difference with predicted values from lower frequencies and greater scales will be explained by “Faraday cleaning” process. A new upper limit on the “spinning dust” emission, more stringent, than before, simplifies the situation in the low frequency part of the Galaxy window [3].

Most groups agree that in the frequency domain the best window is in the 1cm – 1mm region. In the angular scale domain (or on the “l-axis”) the lower l is limited by “recombination noise” by l=3000, above which “Silk Damping” process acts strongly. High l boundary for small telescopes is instrument-depending and was computed individually for all projects.

Appearance of the “Redshift cutoff” effect suggests that there are no instrumental boundary now, if the number of antenna beams on the sky is greater than the number of galaxies in the Universe. Even existing instruments are close to this situation (e.g. VLA at short wavelengths).

In this paper we estimate how deep the window on the frequency-scale plane is and how close we can be to the expected secondary anisotropy effects suggested in literature (e.g., [4, 5, 6]).

We show that there is small window to the “DARK AGE” epoch where experiments with sensitivity below 1 μK in brightness temperature are possible if adequate instrumentation will be invented. One way of the solution connected with very big receivers focal array in big reflectors will be discussed in much more detail. Present day status of this direction at RATAN-600, the biggest reflector type instrument, will be shown. A possible role of the next generation bolometers in Space [7] will be mentioned as well.