“Scientific discovery and scientific knowledge have been achieved only by those who have gone in pursuit of it without any practical purpose whatsoever in view.” –Max Planck
Tomorrow morning, at 8 AM my time, the press conference that cosmologists have spent the past decade waiting for will finally happen, and the Planck satellite — the most powerful satellite ever to measure the leftover radiation from the Big Bang — will finally unveil its results about the origin and composition of the Universe.
They’ve figured out how to subtract the galactic foreground in all of the seven wavelength-bands where Planck operates to unprecedented sensitivity, and the science is ready to be released! Let’s use this opportunity to take a look back on what we know right now, where-and-what the uncertainties are, and what Planck can (or, at the very least, might) teach us about the Universe!
1.) The Big Bang is safe.
The Big Bang is the idea that the Universe was once in a hot, dense, ionized state and expanded to become our star-and-galaxy-rich cosmos that we live in today. There are three separate cornerstones that lead to this picture: the observed Hubble expansion of the galaxies, whose recession rates increase with increasing distance from us, the observed primordial abundances of the light elements, which are predicted by Big Bang Nucleosynthesis to give us a Universe with about 75-76% hydrogen and 24-25% helium (by mass), and the leftover, nearly uniform blackbody (CMB) radiation at just a few degrees above absolute zero, coming from all directions in space, which marks the leftover glow from the Big Bang itself.
In the context of General Relativity, our tried-and-true description of gravity in this Universe, only an expanding, cooling Universe in the context of the Big Bang leads to these three predictions simultaneously, and nothing the Planck satellite observes will change that.
2.) The Universe is mostly dark energy, followed by dark matter, with normal (baryonic) matter making up just a small fraction.
There are three sets of large-scale observations that simultaneously lead to this picture, again in the context of General Relativity.
The observed patterns of large-scale galaxy clustering, combined with the data from ultra-distant distance indicators (like supernovae), and the already known patterns of fluctuations in the microwave background on both large (from WMAP) and small (from the South Pole Telescope and others) scales, all indicate a Universe that’s made up of about 71-74% dark energy, 20-24% not-too-hot dark matter, with the remaining 4.6% made up of normal, standard model particles. These standard model particles include everything we’ve ever observed directly, including protons, neutrons and electrons, photons and neutrinos, and all the exotic, unstable matter we’ve ever created.
So none of those things will change substantially, although the dark energy/dark matter numbers may shift around a small bit in that range. Although Planck will measure the large scales more accurately and in more wavelengths than WMAP before it, that science has already been done, and Planck will only refine it, not overthrow it. The way it will refine it is extraordinary; while WMAP was limited by the sensitivity of the instruments on it, that’s not the case for Planck, according to the ESA:
Planck will provide even more precise measurements with an accuracy set by fundamental astrophysical limits… In other words, it will be impossible to ever take better images of this radiation than those obtained from Planck.
But there are some things that Planck can shed some light on, which have the potential to be extremely exciting!
3.) The age, size and expansion rate of the Universe!
Yes, it’s true that we often quote that the age of the Universe is 13.7 billion years old, the diameter of the observable Universe is 93 billion light-years across, and the expansion rate — or the rate that all galaxies (on average) recede away from one another — is about 71 kilometers/second/Megaparsec. But these numbers are all related to one another, with the age-and-size numbers also dependent on the percentages of dark matter and dark energy.
But the expansion rate has a little bit of uncertainty attached to it. It probably couldn’t be as low as 60 or as high as 80, but no one would be shocked if it turned out to be 68 km/s/Mpc, or maybe as high as 74 km/s/Mpc. This could mean a Universe as old as maybe 14.2 billion years, or as young as 13.3 billion years, depending on how the dark matter and dark energy parameters adjusted. Half-a-billion years may not be a big deal to you, but when you consider that we’ve already got stars that push the 13-and-change billion year limit, it’s pretty important to astrophysicists that the Universe is at least as old as all the stars in it!
4.) There are three types of neutrino in the Universe.
We’re pretty sure of this one… aren’t we? I mean, we’ve got these huge particle colliders, we’ve been running them for decades, and we’ve seen how hordes of them decay. The decay of the Z-boson, for instance, tells us that there are 3.003 ± 0.006 neutrinos species whose mass is less than 4.5 × 1010 eV. Considering that the heaviest a neutrino is allowed to be is around 0.08 eV, it makes sense to conclude that there are three.
But the cosmic microwave background should also measure the number of neutrino species in an independent way, and would also be sensitive to a bizarre, hypothetical type of neutrino that particle physics wouldn’t find conventionally: the sterile neutrino! WMAP, with lousy sensitivity, has claimed to have found about 3.6 ± 0.5 neutrino species, and so while not conclusive, it’s suggestive that there could be a new particle (or maybe even 2?) out there! Planck should improve on the WMAP constraints, and this could be interesting.
5.) How did the Big Bang get started?
According to both the spectrum of density fluctuations imprinted in the CMB and the large-scale-structure of the Universe, and also the best theoretical solution to many open questions in cosmology, the answer to that is cosmological inflation, or a period where spacetime was expanding exponentially fast. At some point, inflation ended, setting up the Big Bang and creating all the matter and energy known to permeate our observable Universe.
Of course, we don’t quite understand how all of this happened. As in, there are many models of inflation that could have successfully done this, and we have no way to discriminate between many of them. But the two main classes of models — models of new inflation and models of chaotic inflation — have a major difference between them: chaotic models should produce large amounts of long-wavelength gravitational waves, while new inflation should produce almost none. In optimistic models of chaotic inflation, this would cause a polarization of some of the light from the CMB, something that Planck could — in principle — pick up. So Planck has the dual potential to either detect primordial gravitational waves and verify not only inflation but a particular model of it, or to disfavor the chaotic inflation scenario in favor of new inflation. (Full disclosure: new inflation has long been my preferred model.)
There are other, smaller refinements that could happen, such as a better pinning-down of the epoch of reionization or a more precise measurement of a few cosmological parameters, but these are the five big ones — confirmation of the first two and potential answers to the last three — that I’ll be waiting on. If you want to watch the NASA announcement live online, it’s at 8 AM pacific time on March 21st here, and you can check out the ESA’s page here or call in and listen to the teleconference. WMAP redefined the precision to which we understood the Universe when its first data release happened a decade ago, and now Planck has the potential to take us even further in our understanding of the greatest quest of all: the dream of understanding the entire Universe. I can’t wait to see what they found!