The Generation of Large Scale Structure in the Universe

• The Universe is filled with structure on many different scales. What process formed this structure? What can it tell us about the Early Universe? If the Universe is smooth on large scale, why is it so inhomogeneous on small scale?

• Gravity is the dominant force on large scales. It must be responsible for the existence of all the observed structure.

• If Universe starts with perfect homogeneity --then by symmetry it will remain homogeneous as it expands

• But imagine that there are slight irregularities in the density of the matter in the Universe. The regions that are overdense will have stronger self- gravity, and expand slightly more slowly. Regions that are underdense will have less gravity and expand more rapidly.
• Over time, the density fluctuations become larger. Once the fluctuations become 100% in amplitude, they separate from the expansion of the Universe, they collapse and form structures that no longer expand. This is called Gravitational Instability
• Numerical simulations can help in understanding how the process works-- very active field of study.
• Gravitational instability is thought to be responsible for the formation of galaxies and clusters of galaxies.

• Details depend on the nature of the dark matter. The observed large scale structure is a strong test of alternative models of dark matter..

Tests a decade ago showed that neutrinos with mass (hot dark matter) failed to explain the observed structure. Cold dark matter models work better.

Connecting CMBR fluctuations to LSS

• Consider a stereo receiver with knobs for tuning amplitude of different frequencies, bass, midrange, treble, etc. Adjusting these knobs tunes the spectrum of sound waves.
• Similarly the spectrum of inhomogeneous matter waves in the Universe is tuned by different types of dark matter, different cosmological models.
• Observations of Large Scale structure and CMBR fluctuations seen by COBE are approximately consistent with cold dark matter models. Current situation is that models are close, but not perfect. Much debate at present.

• Test of gravitational instability is to study fluctuations in the local galaxy distribution. These fluctuations, if correlated with the mass, will cause gravitational perturbations.
• Overdense regions should attract matter, causing matter to move relative to the uniformly expanding frame, leading to deviations from smooth Hubble flow.
• For MW, this is measured as dipole anisotropy of CMBR
• For other galaxies, it is possible to use Tully-Fisher and other methods to estimate the deviation of a galaxy's redshift from perfect Hubble flow.

• Comparison of the two using IRAS galaxy distribution shows that this actually works!!
• Motion of MW is toward the overdensity of galaxies as mapped by IRAS
• Detailed velocity field of nearby galaxies looks very much like predicted inhomogeneous gravity field

• Comparison also allows estimate of mass density of Universe, with best estimate of order W » 0.4

3 Fundamental unanswered questions

• Although we understand how the fluctuations grow, we know that initial seed perturbations must be present in very early Universe.

• What is the origin of these seeds for large scale structure???

• The Universe has a density close to that required for W = 1, the critical value. Yet the solution of the expanding Universe is unstable, and it is surprising that the Universe could have expanded by such a large factor and still be close to W = 1.
• In other words, the curvature of the Universe is remarkably small. Why is the Universe so nearly flat?

• We know that the particle horizon in the expanding Universe grows with time and that it becomes arbitrarily small as we go back toward the big bang.
• There can be no communication outside the particle horizon.
• This horizon, at the time of matter recombination (Z=1300) subtended an angle on the sky of only 1 degree. Thus regions separated by more than 1 degree were not in causal contact at the time of last scattering of the CMBR
• If these regions were not in causal contact, then why is the CMBR so smooth on these scales? How is it possible that regions which have never communicated with each other could have the same temperature, density, etc???
• How was this information transmitted outside the particle horizon??

How can one resolve these 3 profound mysteries ??