Discovered in 1965, the cosmic microwave background (CMB) is the most ancient light record in the history of the Universe. Despite being detected as a “noise” across the sky, it did not take long for scientists to realise that this radiation is an incredibly rich source of information about the history of the cosmos, setting them on a search for more and more details in this early cosmic signal.
The most perfect black body
Some issues started to emerge soon, as scientists tried to explain the physical meaning of this cosmic background. For starters, the discovery of Penzias and Wilson had been obtained at one single frequency, but to confirm the true, cosmological origin of the CMB as thermal radiation from the early Universe, observations at several wavelengths were needed. So astronomers set on a quest for the CMB and its black-body spectrum – the spectrum of radiation emitted by an object in thermal equilibrium, spanning many wavelengths and peaking at a specific one, with a shape that is determined exclusively by the body’s temperature.
This search required space-based observations, at least for certain wavelengths, and lasted around 25 years. Finally, in 1990, a team of astronomers led by John C. Mather and using NASA’s Cosmic Background Explorer (COBE) satellite detected the most precise black-body radiation ever observed, pinning down the temperature of the CMB as just above 2.7 K – less than three degrees above absolute zero.
The COBE satellite was the culmination of a couple of decades of increasingly precise experiments, last but not least the rocket-borne measurements conducted by Herb P. Gush from the University of British Columbia, Canada, which confirmed the black-body spectrum detected with COBE and might have even detected it earlier, had there not been delays in the launch of the rocket.
From extremely hot to incredibly cold
While its origin is deeply rooted in the very early Universe, the CMB did not set on its journey until about 380,000 years after the “big bang”. Before then, the cosmos was just too dense for photons – the particles of light – to propagate freely over significant distances.
Let’s go back to the first few moments of the Universe, which was pervaded by a dense fluid of particles and light tightly bound to one another by frequent collisions. While at the beginning there were only elementary particles, including quarks, electrons and neutrinos, the quarks would soon assemble and give rise to heavier particles: protons and neutrons. By the time the Universe was a few minutes old, all the neutrons were locked in the nuclei of light elements (this is in fact the idea by Gamow, Alpher and Herman that led to the prediction of the CMB) and the “cosmic soup” consisted mainly of protons, electrons, neutrinos and photons, continuously bumping into one another.
In the world we live in at present, protons and electrons are the main constituents of atoms, but in the young Universe, these particles were separated and remained so for quite a while. Indeed, if a proton and an electron did combine, attracted as they are to one other due to their opposite electric charge, they would form an atom but this would not last long, as a photon would soon break it apart and turn it back into its basic constituents.
As a result, the high density of the early cosmos prevented photons from travelling freely as well as neutral atoms from forming. Or, at least, it did so for a very long while. In the meantime, however, the Universe was steadily expanding, becoming cooler and less dense as time went by. So, slowly but surely, particles had more and more “room” to move around and their collisions became less and less frequent. As the temperature of the Universe, which had been as high as billions of degrees in its first few minutes, dropped to 3000 K about 380,000 years later, photons were finally set free and started their cosmic journey. Almost 14 billion years later, we can now detect them as the CMB, their wavelengths being stretched by cosmic expansion to the microwave domain, corresponding to a temperature of about 2.7 K.
Not so uniform after all
While the CMB was discovered as a uniform signal across the sky, scientists in the following years figured out that, given its origin, it must not be so uniform. As a snapshot of the early Universe, this ancient light must contain a record of how matter was distributed at the time: back then, stars and galaxies had not started forming yet, but their seeds must have been present already as clumps of matter slightly denser than their surroundings.
Theoretical calculations predicted that the slightly inhomogeneous distribution of matter in the early Universe would translate into tiny anisotropies that could be observed in the CMB. This was suggested, in particular, in two papers published in 1970, one by Rashid Sunyaev and Yakov Zel’dovich, the other by James Peebles and Jer Yu (1). The tiny fluctuations were expected to be of the order of 1/1000 K across the sky, but observational searches with increasingly sensitive detectors did not find any. Where were the seeds of the stars and galaxies that we see in the Universe today?
A possible solution to this conundrum, proposed by Peebles in 1982, invoked a different type of matter: dark matter (2). Unlike the ordinary matter that makes stars, planets, our bodies and many other things we are familiar with, dark matter is believed to manifest itself only via its gravitational attraction: for this reason, it emits no light. Astronomers had postulated the existence of dark matter ever since the 1930s, to explain the peculiar properties of certain galaxies and galaxy clusters.
Adding dark matter particles to the cocktail of the young Universe means that today’s structures could have their origin in tiny fluctuations of this dark component, which does not interact with photons, leaving no imprint on the CMB. Ordinary matter – basically protons and electrons – present in the early cosmos would still be inhomogeneous, but to an even lower degree: about one part in 100,000. After the release of the CMB, ordinary matter particles would feel the gravitational pull of denser clumps of dark matter, starting the process of structure formation that would eventually lead to the birth of stars and galaxies.
Several experiments kept looking for such anisotropies, including the Soviet satellite-borne Relikt-1, until the first detection was finally made in 1992 by a team of astronomers led by George F. Smoot using data from NASA’s COBE satellite. For their ground-breaking discoveries on the CMB, Mather and Smoot received the 2006 Nobel Prize in Physics (3).
While COBE demonstrated that the CMB contains tiny anisotropies of about one part in 100,000, it was up to its successors – NASA’s WMAP (4) and ESA’s Planck (5) satellites – to measure these small departures from uniformity in ever great detail. With much improved angular resolution (down to 15 arc minutes for WMAP and 5 arc minutes for Planck), these data enabled astrophysicists to quantify the amount of CMB fluctuations observed on different angles across the sky, using these information to delve into the past history of the Universe, constraining its geometry and the properties of its constituents.
Together with a number of other cosmological observations, these data led to what is currently acknowledged as the “standard model” for cosmology: a spatially flat Universe dominated not only by dark matter, but also by another mysterious component, dark energy, responsible for accelerating the present expansion of the cosmos. These extremely precise maps of the CMB also allowed astrophysicists to investigate the very beginning of the cosmos – an extremely brief phase of accelerated expansion known as inflation, when the Universe was only a tiny fraction of a second old.
And there is more, since the CMB conceals an additional source of information about inflation and the origin of the Universe. This treasure trove is “hidden” in the polarisation of the ancient cosmic light, and scientists have been seeking it in the past few years, with Planck and a variety of ground-based observatories, most notably the BICEP2 experiment, located at the South Pole. While this primordial signal has so far remained elusive, the search is still on, and we are all looking forward to the next exciting surprise in the coming future of CMB research.
1 – Sunyaev, R. A., and Y. B. Zeldovich, 1970, Astrophys. Space Sci. 7, 3; Peebles, P. J. E., and J. T. Yu, 1970, Astrophys. J. 162, 815.
2 – Peebles, P. J. E., 1982, Astrophys. J., Lett. Ed. 263, L1.
Claudia Mignone (@claudiascosmos) is an astrophysicist and science writer with a passion for science and telling stories about it. Originally from Salerno, in the south of Italy, she studied astronomy and cosmology in Bologna and Heidelberg (Germany). After having scrutinised the Universe’s expansion during her PhD, she became a full-time communicator of the wonders of astronomy, first as a science journalism intern at the European Southern Observatory and later as a science writer for the European Space Agency. She is based at ESA/ESTEC, in the Netherlands, where she has been writing space science stories for the past five years.