Luckily there were very tiny deviations - a bit more stuff here, a bit less there - that grew over time under the influence of gravity to form structures. These tiny fluctuations were precisely measured first by COBE and then WMAP, and today Planck has joined in with the most precise measurements yet.
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| The Planck microwave background sky |
I say luckily because if the universe were completely smooth, it would not have been able to form galaxies or planets and we would not exist! This evolution from smooth to structured is driven by the gravitational clumping of "dark matter", which makes up most of the mass in the universe but cannot be seen by our telescopes. Instead, we infer its existence through the effect it has on the movements of stars and gas via gravity.
The problem with trying to understand the universe on large scales is that there is only one. We can't move to a different place billions of light years away to view it from a different location, and we can't create a new universe to study how it evolves. Instead, cosmologists in my field (also known as "large scale structure") rely on computer simulations that solve the complex nonlinear equations of gravity for different cosmological models. These models can then be tested by putting galaxies in the dark matter halos and comparing the simulations to observations.
The results of one such simulation can be seen in an interactive browser that lets you zoom in and out on a slice through a very large simulation box. The yellow parts are the high density regions and the dark matter "halos" that in our real universe host galaxies and clusters of galaxies. Also visible are the filaments that connect the clusters and the low-density voids in between, all together beautifully representing what we call the "cosmic web" of large scale structures.
This simulation is called Millennium XXL because it is very large - almost the size of the entire observable universe! I say "observable" because light takes time to travel to us, so the farther away galaxies are the longer it takes for their light to reach us. The amount of the universe that we can ever possibly hope to observe is limited by the distance that light can travel over the age of the universe, starting from the smooth, "plasma" phase mentioned earlier until it reaches us today. The Planck satellite has measured that the universe is 13.8 billion years old, or 13,800,000,000 years!
In the interactive browser, the numbers in the bottom right box give you the astrophysical scale of the image in units of Mpc/h, or co-moving Megaparsecs, which divides out the effect of the universe's expansion. This unit is useful to express the distance between galaxies, whereas the size of our Milky Way galaxy is about one thousand times smaller (kpc) and the distance to the nearest stars is about a million times smaller (pc). You can zoom in all you want, but galaxies would still be too small to be resolved.
Have fun zooming in and out, scanning around for interesting structures, and be sure to make it full screen! If you find something cool, you can right click to save a snapshot image (as I did above).
