The Growth of Structure

Large-Scale Structure The universe is far from‘smooth’ now! We see mid-sized clusters of galaxies (like the Local Group), but also much larger systems (superclusters) and other features that we need to characterize and explain.

Clusters of Galaxies

Superclusters, Walls, Sheets – and Huge Voids Superclusters, Walls, Sheets – and Huge Voids! [overall, a ‘sponge-like’ structure]

Millions of Galaxies!

How Did That Arise? The Growth of Structure The universe can’t have been completely uniform when it was very young. There must have been some very small irregularities (‘ripples’) which grew into galaxies, clusters, and superclusters. Why this growth? Because material condensed here and there around the regions of slightly higher density under the effects of gravity.

Analogy Glance at an ancient map (say, of Roman Britain) and you can see how small settlements were the ‘seeds’ that eventually became the great cities of today. There is some logic to which ones will grow: for example, a major port, a good centre for trade, an easily-defended location,…

In the Universe The ‘logic’ for creating growth is gravity! The small ‘overdense’ regions will grow as time passes, drawing in more matter. We can model this behaviour on computers. We have to remember the influence of dark matter – it is very abundant, but obeys slightly different laws in some respects (e.g. it does not interact much with ordinary matter, except via gravity).

First: Were There Any‘Ripples’? In the Cosmic Background Radiation, we see small irregularities, showing up as slightly hotter and cooler spots. (Remember we are seeing the universe as it was!) These differences are very tiny, and difficult to measure – but it can be done! It’s regions like that will grow over time to form clusters and superclusters of galaxies.

Structure Seen in the CMBR Remember that this is literally a picture of the remote parts of the universe as it was billions of years ago!

COBE Those tell-tale ‘ripples’ were first detected with the COBE satellite (Cosmic Background Explorer) [and much better later on with experiments known as WMAP and Planck] The COBE work won the 2006 Nobel Prize (for G. Smoot and J. Mather).

How Did the Growth Proceed? Remember: we must include the effects of dark matter! (not just the gas and stars). In fact, the dark matter is gravitationally dominant! ‘Clumps’ of dark matter grow first; then the ordinary matter (mostly H and He gas) accumulates around these clumps, eventually condensing to form the stars and galaxies.

Is This Observable? Of course we can’t watch it as it happens, but we can model the effects in computers. We will then ask if the end product resembles our current (local) surroundings – the local clusters of galaxies and so on. Indeed, we can also ask if the intermediate stages resemble what we see at intermediate distances (not looking quite so far back in time!).

The “Illustris Simulation” [a computer model tracked with many billion particles!] https://www.youtube.com/watch?v=vOnWKkeLoyE

A ‘Snapshot’ of the Final Structure - very much like what we actually see

The Theory Fits the Observations The curve is a theoretical prediction of the expected ‘power spectrum’ (It is a purely mathematical representation, not what we see ‘on the sky.’) The details don’t matter; just note that the solid-line fit to the data (the red points) is exquisite!

One Problem The early universe was quite uniform (just very tiny ripples and differences were seen from place to place). But why so very smooth? We might have expected bigger variations from place to place.

Why Was Everythng so Nearly the Same Temperature?

Did Heat Flow ‘Back and Forth’ Did Heat Flow ‘Back and Forth’? [as when a drink eventually reaches uniform temperature] In the early universe, the two patches in the previous panel (‘A’ and ‘B’) were admittedly very close together – the universe was smaller then. But the universe was very young then, and there was no time for heat to flow back and forth between them, to reach ‘thermal equilibrium.’

An Unappealing Answer Maybe the universe came into existence with the unexpected and special circumstance of being essentially the same temperature everywhere, even in widely separated regions. This is a ‘special condition’ of its formation that we would philosophically prefer to avoid having to assume.

Cosmological Inflation! (Alan Guth)

A Remarkable New Understanding