When the first major galaxy surveys started yielding meaningful results, we started to observe that the Universe was indistinguishable from scale-invariance, meaning that the Universe was not top-down and it wasn’t bottom-up; it was a combination of both. There are initial imperfections on small scales and large scales both, as well as the in-between scales. However, because gravitation only sends signals at the speed of light, the small scales begin to experience gravitational collapse before the larger scales can even begin to affect one another.
With the “seeds” of structure present on all scales, we fully anticipate the small scales to develop first, in tens or hundreds of millions of years, while the largest ones will take billions to fully form. Today, our best measurements of the Power Spectrum of the Universe and of the scalar spectral index, ns, tells us that ns = 0.965, with an uncertainty of less than 1%. The Universe is very close to scale-invariant, but it’s tilted to be just a little bit more top-down than bottom-up.
A century ago, we didn’t even know what our Universe looked like. We didn’t know where it came from, whether or when it began, how old it was, what it was made out of, whether it was expanding, what was present within it. Today, we have scientific answers to all of these questions to within about 1% accuracy, plus a whole lot more.
The Universe was born almost perfectly uniform, with 1-part-in-30,000 imperfections present on practically all scales. The largest cosmic scales have slightly larger imperfections than the smaller ones, but the smaller ones are also substantial and collapse first. We likely formed the first stars just 50-to-200 million years after the Big Bang Seed; the first galaxies arose 200-to-550 million years after the Big Bang Seed; the largest galaxy clusters took billions of years to get there.
The Universe is neither top-down nor bottom-up, but a combination of both that implies it was born with an almost scale-invariant spectrum. With future survey telescopes such as LSST, WFIRST, and the next-generation of 30-meter-class ground-based telescopes, we’re poised to measure galaxy clustering as never before. After a lifetime of uncertainty, we can finally give a scientific answer to understanding how our Universe’s large-scale structure came to be.
