The Universe is filled with photons at 2.7 K (2.7° C above absolute zero), left over from when the Universe was 380,000 years old. These photons reveal a lot about the early era of the cosmos: its contents, temperature, and incredible uniformity. Because these photons fall in the microwave portion of the spectrum, they are collectively known as the cosmic microwave background (CMB). Before the CMB formed, the Universe was an opaque soup of protons, electrons, and helium nuclei mixed up with photons energetic enough they could prevent stable atoms from forming. However, as the Universe expanded, it eventually cooled enough the photons could be freed, the first atoms could form, and the cosmos became transparent. The CMB provides some of the best data we have about our Universe.
The CMB is the glory and the despair of cosmology.
Well, that’s not quite true. It’s more that the CMB, for all its value, marks a boundary beyond which we cannot presently go. We usually think of “opaque” in terms of objects seen from the outside, but in the case of the early Universe, the opacity means we can’t see light coming from earlier than 380,00 years after the Big Bang. The light is prevented from reaching us, even though if we could somehow transport ourselves back to that era, everything would be blindingly bright (and very hot, so we’d also need to survive extreme temperatures).
That situation is analogous to the Sun in some ways: bright as stars are, their interiors are not accessible directly. If we could tolerate diving into the Sun’s insides, we’d find it very bright and hot, but we couldn’t see anything. Our information about the Sun’s core and the like comes from helioseismology (sound waves traveling through the Sun, analogous to earthquakes), magnetic fields, and neutrinos. Apart from the neutrinos, these phenomena leave their marks on the Sun’s surface and its atmosphere.
Just as the Sun’s surface is a barrier in space, the CMB is a barrier in time, and while it provides us with a lot of data about earlier times, we are limited in what we can see before it formed. Light (in all its forms, from radio waves to gamma rays) is the best source of information we have about the cosmos, so when we can’t use it, it makes us grumpy. Since the era before recombination—when electrons combined with nuclei to make the first stable atoms—contains the fluctuations that eventually led to the formation of galaxies, we could learn a lot if we could observe it directly. And that’s not to mention a lot of exciting things like inflation, when the Universe expanded very rapidly, smoothing out the chaos of the Big Bang, and the transitions that differentiated the fundamental forces that shape the cosmos. We can’t observe the quark-gluon plasma that preceded the first nuclei.
Seeing without light
However, we can get information about those early eras, and the CMB (like the Sun’s surface) is one of the best. Even more information could be obtained if we could only figure out how to extract it, from neutrinos and from gravitational waves.
Neutrinos interact very weakly with ordinary matter, so they pass through pretty much anything without interference. They travel from the Sun’s core to the surface in a tiny fraction of the time it takes a photon to make the same journey, since the light bounces off electrons like a pinball in an arcade. Neutrinos made in reactions in the first two seconds after the Big Bang permeate the Universe, but they are low energy compared to the ones made in the Sun and other cosmic sources. Even those neutrinos are difficult to detect—we have to build elaborate laboratories to find any out of the vast number passing through Earth all the time. If we could figure out how to detect the cosmic neutrino background (CνB, where the “ν” is the Greek letter “nu”), we would learn a lot about an era inaccessible to other means of observation. Like many cosmologists, I get giddy thinking about it.
Gravitational waves are even more difficult. Just as electromagnetic fields propagate as light waves, gravity propagates as waves, also moving at the speed of light. However, gravity is the weakest of the four fundamental forces. While we know gravitational waves exist, our evidence is all indirect: the inspiral of pulsars orbiting each other. Part of the problem is that gravitational wave detectors must be comparable in size to the large wavelength of the waves, yet sensitive to really tiny fluctuations due to the weakness of the force. Finding the gravitational radiation background (GRB) from the early Universe would require detectors larger than planets. That isn’t as crazy as it sounds—the detector could consist of a number of small spacecraft spread out over a huge volume—but it’s not easy to do, and there’s no guarantee of success, thanks to all the possible sources of interference. If we could somehow solve all those problems, however, gravitational waves might provide the best information possible about inflation.
The CMB will remain the best source of data about the first 380,000 years of the Universe’s life, at least for the foreseeable future. However, as we think about observational challenges for the next era of cosmology, I can’t help but think of what we could do…if we are creative enough.