1. The Growth of Structure

2. Large-Scale Structure The universe is far from‘smooth’ now! There are clusters of galaxies (like the Local Group), but also much larger systems (superclusters).

3. Clusters of Galaxies

4. Superclusters, Walls, Sheets – and Voids!

5. Millions of Galaxies!

6. Where Did That Come From?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.

7. 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,…

9. 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).

10. But First: Were There Any‘Ripples’? But 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.

11. Structure Seen in the CMBR

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

13. How Does 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 rdinary matter (H and He gas) accumulates around these clumps, eventually forming the stars and galaxies.

14. 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!).

15. The “Millennium Simulation” [the computer model tracked > 10 billion particles!] https://www.youtube.com/watch?v=FBkYIqtYb0I and a ‘fly-through’ of the end product (this would require travelling billions of times faster than the speed of light, of course) https://www.youtube.com/watch?v=JNIXAKkuShQ

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

17. The Theory Fits the Observations The curve is a prediction of the expected ‘power spectrum.’ The details don’t matter; just note that the fit to the data (the red points) is exquisite!