ABSTRACT
The set of problems under item 23 in the 'list' represents perhaps the most crucial points in astrophysics. It also includes cosmology (not everybody will agree with such a classification but this does not change the essence of the matter). The cosmological problem is undoubtedly a grand problem. It has always attracted attention, for Ptolemy's and Copernicus' systems are none other than cosmological theories. In the physics of the 20th century, theoretical cosmology was created in the works of Einstein (1917), Friedmann (1922 and 1924), Lemaitre (1927), and many other scientists. But before the late 1940s, all the observations significant from the point of view of cosmology had been made in the optical range. Therefore, only the redshift law had been discovered and, thus, the expansion of the Metagalaxy had been established (the works by Hubble are typically dated 1929, although the redshift had also been observed before and not only by Hubble). The cosmological redshift was justly associated with the relativistic model of the expanding Friedmann Universe but the rapid development of cosmology began only after relic thermal radio emission with a temperature Tr = 2.7 K was discovered in 1965. At the present time, it is measurements in the radio wavelength band that play the most prominent role among the observations of cosmological importance. It is impossible to dwell here on the achievements and the current situation in the field of cosmology, the more so as the picture is changing rapidly and can only be discussed by a specialist. I shall restrict myself to the remark that, in 1981, the Friedmann model was developed to the effect that at the earliest stages of evolution (near the singularity existing in the classical models, in particular, those based on GR), the Universe was expanding (inflating) much more rapidly than in the Friedmann models. The inflation proceeds only over the time interval t1t "' 10-35 s near the singularity (recall that the Planck time is tg "' 10-43 s and so the inflation stage can still be considered classically because the quantum effects are obviously strong only for t ~ tg)· However, the widespread inflation notions and models are criticized (178, 179; see also 204] and I have no clear opinion on this account. But the very existence of inflation is hard to deny and it is important that after the inflation, the Universe develops in agreement with Friedmann's scenario (at any rate, this is the most widespread opinion). The most important parameter of this isotropic and homogeneous model is the matter density p or, which is more convenient, the ratio of this density n = pf Pc, where Pc is the density corresponding to the limiting model (the Einsteinde Sitter model) in which the space metric is Euclidean. For this model, 0 = Oc = 1. The parameter
(1.3)
Some comments (astrophysics) 25 where the Hubble constant H appears in the Hubble law
v= Hr (1.4) which relates the velocity of cosmological expansion v (going away from us) with the distance r to a corresponding object, say, a Cepheid in some galaxy. The quantity H varies with time; in our epoch, H = H0 . This quantity Ho has been measured all the time since the Hubble law was established in 1929 (Hubble assumed that H0 ~ 500 km s-1 Mpc1 ). Now the value H 0 ~ 55-70 km s-1 Mpc-1 has been reached using various techniques (for example, the value Ho = 64 ± 13 km s-1 Mpc-1 has been reported [75], H 0 = 71 ± 8 km s-1 Mpc-1 has been obtained in [177]). For H0 = 64 km s-1 Mpc-1 = 2.07 x 10-18 c-1, the critical density is
(1.5)
Note that, from considerations of dimensionality, the Planck density is
(1.6)
Probably Pg is the maximum density near the singularity in which, according to classical theory, p -7 oo. Thus, the evolution of the Universe or, more precisely, of its region accessible for us has changed up to the present day (if we now have p'"'"' Pco) by 123 orders of magnitude (one should not, of course, attach any importance to the last figure).
