In addition to breakthrough observations, creative new ideas are also driving progress in cosmology. Not only do we know more about the universe, but our understanding is deeper, and the questions that we are asking are more profound. Still, our understanding of the origin and evolution of the universe has not yet caught up with what we know about it. But a vital ingredient in furthering our understanding and shaping our questions has been the recognition of the connections that exist between the elementary particles on the smallest scales, and the universe on the largest.
The abundance of structure that has been mapped out today - from
galaxies of mass as small as 106
M
to
superclusters of mass exceeding 1016
M -
speaks to a remarkable transformation
that occurred in the early universe as small primeval
inhomogeneities in the distribution
of matter were amplified by the attractive forces of gravity
(1 M
2 ×
1033 g refers to the mass of the sun).
During the earliest moments the primordial fluctuations did not grow because
the expansion was controlled by radiation, and radiation does not
clump. When the universe became matter-dominated, the inhomogeneity
then grew at precisely the same rate as the cosmic scale factor,
(
/
)
a(t). Because the size of the
universe grew by a factor of about 10,000 during the matter dominated
era, initial fluctuations of amplitude 0.01% or so are all that is
needed to seed the nonlinear structure seen today. Formation of
structure then ceased a few billion years ago when the accelerated
expansion began, pushing the existing structures apart.
The required primordial lumpiness should have left its signature on the CMB in the form of temperature fluctuations of order ten microKelvins. And this is precisely what has been measured, both in amplitude and variation with angular scale (cf. Figure 9). Being able to account for how the structure seen today arose from a nearly homogeneous beginning is a major success of the FLRW model.
Of course, a natural question arises: What is the origin of the primeval lumpiness? Cosmologists have a working model for this: the seeds of structure arose from the stretching and conversion of quantum noise originating on subatomic length scales to density inhomogeneity on astrophysical length scales, caused by a tremendous burst of expansion called inflation. Inflation, coupled with the idea of including nonbaryonic dark matter in the universe, has led to a predictive and descriptive theory of how the structure in the universe plausibly arose. This descriptive theory is known as Cold Dark Matter (CDM) (Blumenthal et al., 1984). From the simple starting point of cold dark matter and inflation-produced lumpiness follows a highly successful picture for the formation of structure in the universe.
The defining prediction of CDM is that structure forms from the "bottom up": first galaxies, then clusters of galaxies, and finally superclusters. This ordering, now confirmed by observations, follows from the fact that the degree of lumpiness is larger on smaller scales, so that the smaller objects become gravitationally bound, stop expanding, and collapse back on themselves first.
As computing power and numerical techniques have improved, simulations of the evolution of structure in the universe have become increasingly sophisticated, providing additional insight into the evolution of cosmic structure. The initial ingredients for these simulations are specified by the CDM paradigm, and the ensuing growth of structure via the force of gravity is now computed by following the motions of more than a billion particles. The properties of the simulated universe (correlation function, power spectra, and masses and abundances of galaxies, etc) are calculated for different recipes (dark matter and baryon densities, with and without dark energy, and different values of the cosmological parameters) and can all be compared with observations. Numerical simulations were instrumental in demonstrating the failure of structure formation in the presence of hot dark matter, and the success of flat CDM models with dark energy in matching the observed distribution of structure seen today. The successful predictions of CDM go beyond being merely descriptive, and now include many quantitative predictions.
That being said, the successes of CDM largely involve predictions that follow from the basic paradigm and the action of gravity alone. However, understanding the development of the structure we see with telescopes requires an understanding of how baryons form into stars in galaxies. Complicated gas dynamics come into play and there may even be feedback from the energetics of star formation on the dark-matter structures themselves. Crisp predictions become more difficult. Understanding how real galaxies form and evolve is a rich subject with many outstanding questions whose answers will require more observations as well as detailed astrophysical modelling that goes beyond the gravity of cold dark matter alone. Pinning down the basic cosmological framework has helped significantly by removing uncertainty associated with the evolution of the Universe.