By Joshua Winn, Princeton University
There is a profound feature in the cosmic microwave background (CMB) radiation. Even though the CMB is said to be the most perfect blackbody in existence, it’s not completely perfect. It has slight irregularities. An all-sky map of the CMB shows hot spots and cold spots. The differences are exaggerated in the map. In reality, the temperatures of the hot and cold spots differ only in the 4th decimal place.
Ripples in CMB
One can imagine how difficult it is to make a map with such high contrast. This one comes from a space telescope called Planck, which observed the microwave sky for 4 years, ending in 2013. There’s an abstract beauty to this map.
When the detection of these ripples was first announced, in 1992, one of the scientists involved, George Smoot, said “If you’re religious, it’s like seeing God.” What did he mean by that? What he meant is that without these ripples in the CMB, the universe would be “without form, and void”.
Hot and Cold Spots
The hot and cold spots represent parts of the universe that were ever-so-slightly denser, or less dense, than the average. Those density fluctuations are what eventually led to the formation of galaxies, stars, planets—everything we see—through the amplifying effect of gravity.
A region of the universe slightly denser than average exerts a stronger gravitational force on its surroundings, pulling more material toward it, and increasing its density even further. Given enough time, this amplifying process can convert a hot spot—higher in density by only one part in 100,000—into a supercluster of galaxies.
Ancestors of Galaxies
Computer simulations of this process show the formless void condensing into the web-like texture of galaxies that we observe today. So, in that sense, when we look at the hot and cold spots in the cosmic microwave background, we’re looking at the ancient ancestors of galaxies.
And who were the ancestors’ ancestors? What caused some parts of the universe to be denser than others in the first place? Nobody knows for sure. The current best guess is fascinating: the primordial fluctuations might have been a manifestation of quantum theory.
Cosmic Inflation
The regions with different densities used to be so small that quantum effects were important, and the density, like everything in quantum theory, was subject to uncertainty. Then the universe underwent a phase of very rapid expansion—a period called “cosmic inflation”—that stretched these quantum fluctuations out to astronomical scales.
Now, that’s hard to grasp, and the only evidence for that hypothesis right now is indirect. But the theory of cosmic inflation does make predictions for the characteristics of those fluctuations, in particular, the polarization of the radiation they produce, and investigators are seeking to observe with new experiments.
This article comes directly from content in the video series Introduction to Astrophysics. Watch it now, on Wondrium.
Angular Size of the Spots
In the meantime, a clear pattern has turned up in the CMB that’ll be important in the study of cosmology. The all-sky map looks random, but there’s one aspect that’s not random: the angular sizes of the spots. Most of the prominent spots are about one degree across. Again, it reminds one of pointillist art —the hot and cool spots are like dabs of paint made with a single brush. Why is that? Let’s focus on that.
Imagine those early days before recombination, when the universe was a nearly formless expanse of hot plasma, with slight disturbances of higher and lower density. Eventually the over-dense regions underwent gravitational collapse. But that could only really get going after recombination. Before then, the plasma was hot and dense enough to exert an appreciable pressure, which could push back against gravity.
Acoustic Waves
Because of the pressure forces, the initial disturbances didn’t lead to gravitational collapse: they produced waves. A dense region had a higher pressure than the surrounding regions, so the over-dense plasma pushed outward, launching a density wave—a traveling pattern of slightly higher pressure. It’s like what happens when you clap your hands, and launch a sound wave into the surrounding air. In fact, that analogy is so close that these cosmic density waves are called acoustic waves.
The irregularities in the CMB are patterns formed by acoustic waves. That special size of one degree represents the distance a wave could spread, between the Big Bang and the epoch of recombination. After that, the universe became neutral. The photons were released, and the pressure dropped to the point that gravity could act, unopposed, to amplify the density fluctuations and cause them to collapse into galaxies.
Now, here comes a crucial point. Before the epoch of recombination, the universe was simpler than it is now: it was a nearly featureless plasma. That simplicity makes it possible for cosmologists to calculate the time of recombination: it was about 380,000 years after the Big Bang. And they can calculate the speed of acoustic waves from first principles, and how far they could have travelled during that time.
It was about 150 megaparsecs, in co-moving coordinates. So, we know that each hot or cold spot in the microwave background represents a chunk of the universe about 150 megaparsecs across.
Common Questions about the Significance of Ripples in the Cosmic Microwave Background (CMB)
The hot and cold spots represent parts of the universe that were ever-so-slightly denser, or less dense, than the average. Those density fluctuations are what eventually led to the formation of galaxies, stars, planets—everything we see—through the amplifying effect of gravity.
The all-sky map of CMB looks random, but there’s one aspect that’s not random: the angular sizes of the spots. Most of the prominent spots are about one degree across.
The irregularities in the CMB are patterns formed by acoustic waves. The special size of one degree represents the distance a wave could spread, between the Big Bang and the epoch of recombination.
