On the 18th of November, 1989, NASA has launched a Cosmic Background Explorer mission, now generally known as COBE. It was aimed at experimental measurement of the earliest light in the universe. It actually set foundations for experimental cosmology as a whole and brought our theoretical models into an experimental light. It was a point in the scientific history where theories about the universe were bound to rise or fall.
Now we have a chance to engage in a dialogue with one of the masterminds behind COBE mission, Professor John C. Mather. He was the scientific leader of the COBE team and has been awarded a Nobel Prize as well as Gruber Cosmology Prize for his research in 2006.
“We’re explorers,” Professor Mather has said in the past, “We need to understand where we and our universe come from.” Since we truly share this cosmic aspiration there was no time to waste before embracing the really big picture. I wanted to know what COBE has already taught us and what is to expect in the future, whether the universe is infinite and what the limits of our knowledge are. Professor Mather was kind enough to answer my curiosity and now I invite you to join the exploration.
LAURYNAS ADOMAITIS: Up until the discovery of CMBR (Cosmic Microwave Background Radiation) our thinking about the edges of the universe (including ancient and modern philosophies) has been rather hypothetical. Now we have very precise measurements of CMBR including those of Planck observatory. Could you sum up what we have so far learned from the data – what previous hypotheses seem to be vindicated and what have been thrown into doubt?
JOHN C. MATHER: The main results include:
The CMBR has a blackbody spectrum (brightness versus wavelength), matching precisely the predictions of the expanding universe model, also known as the badly named Big Bang Theory. (It’s a bad name because people imagine a small bang like a firecracker, instead of an infinite universe expanding into itself.) No other idea (cold initial conditions, stead state theory, etc.) can match the observations.
The spectrum also shows that the early universe did not have any other exotic sources of energy that could heat the material after the first year of the expansion.
There are hot and cold spots in the material of the early universe, corresponding to regions of more or less density of the material.
The observed spots have many different sizes, and the brightness of the spots of different sizes can be matched to detailed models of the early universe. To make a good match, we need to include the ordinary matter we see around us, cosmic dark matter (which should be called transparent matter) that is much more abundant than ordinary matter, cosmic dark energy (which appears to make the expansion accelerate), and cosmic neutrinos (as predicted from the properties measured here on Earth). The calculated age of the universe from this method matches the age calculated from Hubble’s law of the receding galaxies when we include the measured acceleration.
The force of gravity, acting on the matter (ordinary and dark) in these primordial density variations, explains the ages and distributions of the galaxies we see with telescopes. Supercomputer simulations based on these measured variations show a wonderfully complex and beautiful structure that can now be compared with images taken with telescopes.
The measured hot and cold spots are consistent with predictions of the theory of inflation, which says that the first sub-microseconds of the expanding universe included a period of extraordinary acceleration. In this theory, the size of the universe doubled about 50 to 100 times in a very short time, so rapidly that the initial structure was frozen into the expanding material.
There is a prediction that the cosmic microwave background radiation may be polarized due to the effects of gravitational waves propagating in the primordial material. When this prediction is fully tested, we will know much more about the properties of that material. As far as we know today, this may be all we can measure about those earliest times.
LA: It seems that CMBR research has now developed into a more theoretical endeavor to match the data with theoretical models of the universe. But if we were to go back in time to the moment before COBE results – what were your expectations?
JM: When the COBE was first proposed, there were no serious analyses for the anisotropy (fluctuations), and there were many ingenious theoretical explanations for the (wrong) measurements of the CMBR spectrum. At the time, we could not rule out all sorts of exotic processes, such as cosmic strings, decay of elementary particles, explosive energy release as a seed for large-scale structure, etc. Inflation had not yet been proposed, so there was no explanation for the observed homogeneity and uniformity of the universe on large spatial scales. By the time that the COBE was launched there were some pretty good predictions for the anisotropy, based on matching the measured large-scale distribution of the galaxies.
I definitely did not anticipate the main features of the now-standard model of the universe. I was amazed that theorists were able to agree on the equations to be used and the solutions that they found. I was amazed at their ability to match the detailed predictions from the standard model with detailed measurements from COBE and other missions like WMAP and Planck. In retrospect, I would say I did not fully appreciate the fact that the whole standard model works because it deals entirely with small fluctuations so that “perturbation theory” is extremely accurate.
LA: I, personally and philosophically, am most interested in one of the long-standing dilemma that is pertinent to CMBR research: the infinity of the universe. The question has certainly been debated by philosophers, theologians, and scientists for millennia. Now we might actually arrive at a feasible empirical argument. If I am not mistaken, one purpose that CMBR has is to investigate the density of the universe which in turn determines its overall geometry. As of now the density parameter is estimated at approx. Ω=1 which means the universe is probably flat and therefore infinite. Do you have a view on that? Is the universe actually infinite?
JM: We cannot measure anything that would prove that the universe is infinite in extent. We can only see a finite portion of it because light has only been traveling for 14 billion years now. However, everything we have observed is consistent with an infinite universe. In fact, it is consistent with a spatially flat universe; this statement means that if you can imagine a triangle of points in space-time at the same time after the beginning, that triangle will have the usual properties that we learned from Euclid – the sum of the angles will be 180 degrees. (But of course, as Einstein showed us, space-time in four dimensions is still curved by the action of gravity.) As it happens, the theory of inflation provides a natural explanation for this flatness: any initial curvatures would be stretched out by the expansion so that it would be unmeasurably small. Perhaps further advances in theoretical predictions will reveal that our universe need not be infinite, only extremely large.
LA: One of the most exciting aspects of CMBR is that it raises as many interesting questions as it answers. For example, the long-standing assumptions behind cosmology have been that the universe is isotropic and homogenous (the so-called “cosmological principle”). Some famous anomalies in the CMBR seem to threaten them – in the oldest and farthest observable edges of the universe we find asymmetrical cold spots et al. Do you think that these anomalies are conceptually significant, or should we see them as mere instrumental or statistical fluctuations?
JM: There is no simple answer to this question. We do not have an opportunity to measure from another location in the universe, so there is a limit to our knowledge. There is serious ongoing work investigating whether these measured anomalies can have physical explanations.
LA: So in a sense CMBR is the very limit of our observational knowledge. Since it is the most ancient and distant radiation, we can hardly imagine being able to access any information beyond it. Nevertheless, the universe might actually be infinitely bigger than that. It seems that no matter how much we know about our observable surroundings, we might still be infinitely ignorant about the universe itself. We might be conditioned to these limits alone. It feels that the more we are aware of our universe, the better we understand the limitations of our human knowledge. Do you agree? Do you see it as a pessimistic outcome?
JM: Yes, I agree that there is a limit on our knowledge of the CMBR, and we are close to reaching it. But there is still a possibility of great progress in the theoretical models of the universe. Right now the theoretical models that attempt to unify quantum mechanics and gravity seem not to work very well. But some time, maybe soon, maybe far in the future, I expect that there will be a satisfactory unification that matches everything we know. Then, perhaps we will be able to answer your questions.
LA: More generally, do you find humanities relevant to your work as a scientist? I mean, there are institutes around the world doing serious work in the history of science and in the philosophy of science dealing with epistemological and methodological issues. Does it have an impact on you or your field of research in general? And conversely, do you think your research has an impact on humanities or some interdisciplinary field? I know it made me rethink a lot of philosophical issues in a new light.
JM: I am not up to date about serious work on the history of science and philosophy of science. As far as I can tell the philosophers and epistemologists have not much affected the progress of physics. But it is certainly important to record history accurately at the time it is being made. It is very difficult for historians arriving decades later to know what was being thought and said. The best history I’ve come across about the CMBR is the one by three physicists, Peebles, Page, and Partridge, “Finding the Big Bang”. One of the beauties of this book is that it includes stories directly from the participants, combined with a very complete review of the early discoveries in the area.
I certainly see that the public is fascinated by the story of the universe, starting with the mysterious early moments, leading on to the formation of stars and galaxies and the Earth, and of course the possibility of life here and elsewhere. How did we get here, and are we alone? We are now making rapid progress on both questions. So in the long term, the cultural implications of science will be profound.
 Planck science mission (ESA) has provided the most precise measurements of CMBR so far. Planck was launched in 2009 and its results are continued to be presented. The latest results by the Planck Collaboration were prepared in 2015. More about Planck observatory can be found here.