“A cosmic mystery of immense proportions, once seemingly on the verge of solution, has deepened and left astronomers and astrophysicists more baffled than ever. The crux … is that the vast majority of the mass of the universe seems to be missing.” -William J. Broad
Despite the wondrous, luminous sights of the night sky, we’ve learned that normal matter — protons, neutrons, electrons and the like — make up only 4% of the total energy in the Universe.
The galaxies and clusters of galaxies lighting up the deepest recesses of space — shining with stars — are mostly composed of dark matter, a massive, clumpy type of matter that experiences gravity, but does not collide with normal matter, photons, or itself. They — along with the entire fabric of the Universe — also expand as though a mysterious, constant source of energy supplied a negative pressure to the Universe: that’s known as dark energy.
And finally, despite the nuclei, atoms and light in the Universe all having their origins in the Big Bang, the Big Bang itself is not the beginning of the Universe, at least according to modern theory. That’s reserved for cosmic inflation, the theory that tells how the Universe got to have the structure and temperature properties that it has today: by stretching quantum fluctuations across cosmological scales during a period of exponential expansion!
For many of you, this seems like a far-fetched story. After all, if you can’t observe what happened before the Big Bang, and (as of yet) you can’t directly detect this dark matter or this dark energy, you might be inclined to be skeptical of their existence. And I wouldn’t blame you. But, I hope, you would encourage scientists to ask the question,
If these things are real, what new things would they predict that we could look for?
After all, that’s what science is, and how science moves forward.
If we were to have a Universe that contained dark matter and dark energy in the proportions described above, and that had its origins consistent with the theory of cosmic inflation, we would expect there to be very, very specific patterns in the temperature fluctuations of the cosmic microwave background! This background is a left-over relic from the Big Bang, and comes to us (mostly) untouched from when the Universe was only 380,000 years old, when we formed neutral atoms for the first time!
Initially, we had matter and radiation uniformly distributed throughout the Universe, with fluctuations in temperature that didn’t care about scale, at least, for the most part. What this means is that if we looked at all the 10-degree patches of sky, or all the 1-degree patches, or all the 0.1-degree patches, we would see the same types of temperature fluctuations. Some parts would be a few parts in 100,000 cooler, some parts would be a few parts in 100,000 warmer, and those temperature distributions would be independent of the size of our patches.
But over time, matter tries to collapse, and radiation pressure bounces it apart again, much like ripples in a body of water. As a result, we end up with a pattern of fluctuations in the matter and radiation that only depend on parameters like the amount of dark matter and dark energy, and whether or not we had inflation.
We got our first really good glimpse of this patter from the WMAP satellite, which basically confirmed this picture, above. The left-most part of the graph, the largest angular scales, have just about no tilt to them, which is what inflation almost predicts. Technically, the initial spectrum of fluctuations could be perfectly flat, given by a parameter known as the scalar spectral index (ns), which would equal 1. But practically all models of inflation predict that ns will be slightly less than 1, or somewhere between 0.92 and 0.98, depending on the model. This means there’s a very, very slight slope to that line, something that we couldn’t hope to measure until we reach multipoles well out past 1,000. This would be, needless to say, very difficult to measure, and would require measuring much smaller angular scales to much better accuracy than WMAP could on its own.
Via WMAP, we were able to measure that the Universe was flat, that there was around 27% of the total energy in dark and normal matter combined, and that the other 73% was dark energy. There were some uncertainties, however, that required other observations to constrain them. The flatness part was easy; the differences between a flat Universe and non-flat cases were extreme.
But what goes on out past where WMAP can measure, at those very small angular scales? WMAP is no good at measuring that; we’d need a specialized mission for that.
Luckily for us, the South Pole Telescope was designed especially for just such a task.
75 feet tall and 280 tons, the South Pole Telescope can measure the night sky — weather permitting — for six months at a time out of the year. At small angular scales, the cosmic microwave background will start to notice that there are galaxies and clusters in the way; the (slightly) warm gas in those objects will distort the temperature on top of the initial fluctuations and the ripples, and so we need to be prepared to detect them.
But if we are, the Universe with a certain amount of dark matter and dark energy should make very specific predictions, and our brand new measurements had better line up. First off, here’s the region of the sky that the South Pole Telescope measured, to a much greater precision and accuracy (by about an order of magnitude) than WMAP could.
When they did their temperature analysis, they were able to measure a spectrum of temperature fluctuations, just like WMAP did. The only difference is, instead of measuring from multiple moment 2 through about 800, it measured from about 700 to about 3,000!
Now, what it expected to see was a very specific pattern of wiggles, dependent only on the amount of dark matter and dark energy present. Since the WMAP data already constrained those, the pattern was already pre-determined. Because we also know the physics behind how much the warm gas in galaxies and clusters should shift the temperature (the Sunyaev-Zel’dovich effect), we should see a very specific pattern of wiggles added to a very predictable, slow rise as we move farther out to larger and larger multipole moments.
So, enough with the suspense. Does the model work right? Did we see something consistent with what we expected?
Let’s go straight to the data, and see how well the data (in points, with error bars) fits the model that includes all contributions (solid line) and the model that doesn’t include the Sunyaev-Zel’dovich effect (dashed line).
First off, look at those tiny error bars! And second off, look at how well they match that prediction!
To a breathtaking degree, they confirm exactly what we expected to find: a Universe that was about 3/4 dark energy, about 1/4 dark + normal matter combined, and that was consistent with inflation.
But there were two huge improvements that came out of this experiment, that I want to highlight for the entire world.
1.) The Universe is really, truly, very, awesomely flat! There is practically no spatial curvature, as constrained by the microwave background alone. No other measurements necessary. The WMAP data, on its own, had a fairly large degeneracy, but the addition of the South Pole Telescope data wipes that out, and tightly constrains what fraction of the Universe’s energy is in the form of matter (x-axis, above) and what fraction is in the form of vacuum energy (y-axis).
The flatness of the Universe is now known to a better precision than ever before, and we can now say that the curvature is — with error bars — -0.3% ± 1.4%, which is eminently consistent with no curvature at all. (And is a 20% stronger constraint than WMAP alone!)
But this next one truly boggles the mind.
2.) The scalar spectral index, ns, is very well measured to be less than 1! When you add in the newly-measured South Pole Telescope data, we find that ns = 0.9623 ± 0.0097, which is the strongest evidence yet for inflation based on the scalar spectral index! This might not sound as exciting to you as it is to me, so let me phrase it in slightly different terms:
We just made a measurement that provides the strongest evidence ever that inflation came before the Big Bang, and what we measured is consistent with our best models for what inflation might be!
That’s why I’m excited, and you should be, too! Go and read the preprint for yourself, here, if you like, and have the happiest of all Halloweens with the newly-confirmed 96% dark, inflationary Universe!