LONDON – Possibly the most daring piece of modern science is the attempt to predict the patterns that galaxies make in the sky. The bold starting point is a statement on what the universe was like at a time when the entire visible universe was compressed into something the size of a beach ball.
That idea takes some getting used to.
For starters, the notion that the entire visible universe could even fit into something as small as a beach ball is little short of mind-blowing. There are hundreds of billions of galaxies in the visible universe, our Milky Way being just one of them, and each one typically contains several hundred billion stars.
Squeezing all that into a beach ball is testament to the fact that the substantial matter of our everyday experience is, in fact, largely empty space.
Once we have a prediction for how the beach-ball universe looked, we can use the equations of physics to run the clock forward to the present day.
By comparing with the astronomical observations, we can test the model — and it works. The way that galaxies clump together into large-scale structures is precisely as anticipated.
By itself, that would be impressive enough, but there is another source of information that allows for a second test.
The Earth is bathed in radio waves that were produced when the visible universe was around 1,000 times smaller than it is today. At that time, the universe underwent a crucial change: It went from being opaque to transparent. Specifically, it had cooled sufficiently for the electrons and protons to pair up and make atoms of hydrogen.
Prior to this, particles of light could not travel very far before colliding with the electrically charged electrons and protons. After it, the electrically neutral hydrogen provided an environment through which the light could pass unhindered.
The evidence that this is what happened would be the observation that the Earth is bathed by light that has been streaming through a transparent universe ever since this critical epoch.
That light was first detected in 1964 by Arno Penzias and Robert Wilson in the form of radio waves impinging on a 6-meter-wide, horn-shaped antenna in New Jersey. Their observation was evidence in support of the idea that the universe was, at some time in the distant past, hot enough to prevent hydrogen from forming, which was a prediction of the “hot big bang” theory.
Since then, this “cosmic microwave background radiation” has been the subject of considerable investigation and, toward the end of last month, the most precise measurements to date were presented by the Planck collaboration of several hundred astronomers and cosmologists from around the world. Together, they have analyzed the latest tranche of data collected by the European Space Agency’s Planck satellite, which was launched on the Ariane 5 rocket in 2009.
The results are stunning and include precise measurements of the matter content of the universe and a tweak to the best estimate for its age.
For now, at least, the standard model of cosmology, which describes a universe that is made up of ordinary and dark matter evolving according to the equations of gravity formulated originally by Albert Einstein, including a mysterious cosmological constant term, is in excellent shape.
Buried in the Planck data is the all-important information on what the nascent beach-ball universe was like.
The light bathing the Earth is very nearly the same in all directions. This means that it can be characterized, to a good degree of accuracy, by a single temperature: a chilly 2.7 degrees above absolute zero. This temperature is determined by that of the electron-proton plasma at the time when the universe made its transition from opaque to transparent.
Crucially though, the plasma was not exactly the same everywhere. Some regions were slightly hotter and more dense than others. Through the action of gravity, matter was drawn into the more dense regions, which therefore grew in size, eventually forming the galaxies we see around us.
These initially tiny differences from a perfectly uniform universe also induced fluctuations in the characteristic temperature of the background radiation measured by Planck, which means that the light arriving from different directions does not have exactly the same temperature. The precise mapping of these deviations across the sky is what makes Planck such an important tool to explore the physics of the very early universe.
The challenge now is to understand what caused some regions to be slightly hotter or slightly cooler than others.
The best idea we have to explain these deviations from perfect uniformity, and hence the large-scale structure in the universe, involves contemplating what the universe looked like when it was even smaller than a beach ball.
First described in the 1980s, the idea is that the universe underwent a very rapid period of expansion, inflating a region very much smaller than that of an atom to the size of a beach ball in a time very much shorter than the blink of an eye.
This behavior is anticipated in many theories of particle physics, where it relies upon the existence of something called the “inflaton field.” We can think of this as something that pervades the universe, acting as a reservoir of energy that drives the rapid expansion. We do not know much about the inflaton field — or fields — yet, but we do know that the laws of quantum physics imply that it must fluctuate from place to place.
Crucially, these irrepressible fluctuations provide a natural explanation for the temperature fluctuations observed by Planck.
The Planck data are starting to have an impact in ruling out some variants of the inflationary theory. Future data will help us in our quest to uncover the precise mechanism behind early universe inflation.
It is thrilling that through observations of the universe at the largest distances, we are, at the same time, gaining new insights into the behavior of the universe at the very smallest distances.