Three leading researchers connected to the Planck mission spoke recently with The Kavli Foundation in a roundtable discussion about the latest results. The participants:
- George Efstathiou – Professor of Astrophysics at the University of Cambridge in the U.K., Director of the Kavli Institute for Cosmology at Cambridge (KICC), and one of the leaders of the Planck project.
- Anthony Lasenby – Professor of Astrophysics and Cosmology at the University of Cambridge and Deputy Director of KICC. Dr. Lasenby is a member of the Planck Core Team, a co-investigator for the spacecraft's High Frequency Instrument, and member of the Planck Editorial Board.
- Krzysztof Gorski – Senior Research Scientist at the Jet Propulsion Laboratory in Pasadena, CA, and faculty member at the Warsaw University Observatory in Poland. Dr. Gorski is a Planck Collaboration scientist, Core Team, and Editorial Board member, and one of the Co-Investigators of the Low Frequency Instrument on board Planck.
The following is an edited transcript of the discussion.
THE KAVLI FOUNDATION: Before we discuss the results, let me ask each of you: when you began studying the cosmic microwave background (CMB), did you ever expect to see the kind of amazing detail that the Planck spacecraft has offered?
GEORGE EFSTATHIOU: The new Planck data have given us more detail of the CMB than we ever could have predicted early in my career. I certainly didn't envisage that we would ever see this in my lifetime. When the Planck mission was being reviewed for funding in 1996, one of the questions asked was, “Why should we approve a satellite designed to measure fine-scale features in the CMB?” Despite the fact that the Cosmic Background Explorer (COBE) team had announced its discovery of CMB anisotropies in the early 1990s, some people had doubts that we could detect smaller-scale temperature fluctuations. The thinking then was that during the first billion years of the universe’s history when the first stars and galaxies formed, re-ionization could have erased much of that smaller-scale detail in the CMB.
KRZYSZTOF GORSKI: I came into the field in 1986 when I was a postdoc at Berkeley, and at that time George was already a giant in the field. Direct observations up until that time had shown that the only apparent differences in temperature in the CMB were actually an effect due to the motion of the Earth through space. People were hoping for a discovery of more actual detail, and it came with COBE in 1992, then with the Wilkinson Microwave Anisotropy Probe (WMAP) launched in 2001 and now it’s come with Planck – as well a number of suborbital instruments. We've been pretty lucky that this has happened over the span of our careers.
ANTHONY LASENBY: I started somewhat differently, because at the beginning I was on the experimental side rather than the theoretical side. I began by making observations of the microwave background back in 1978. It was pretty speculative then to be looking for anisotropies. We started looking, from a telescope at Tenerife, at really large angular scales, on the order of several degrees on the sky. At that time, my horizons were very much focused on mapping temperature variations at larger angular scales on the CMB sky. Only after that did I gradually realize that going to smaller and smaller angular scales would reveal more information. At Cambridge University we started a series of small ground-based experiments, which gave detailed coverage of small sky patches, and then in the 1990s I recognized that observing the CMB from space was the best way to move forward. I joined the Planck mission in 1993, and I think I knew from the beginning that this was going to be for us a definitive experiment for measuring these temperature anisotropies. And that is how it has turned out.
TKF: So let’s talk about some strange things that Planck found in the CMB. The new data reveal anomalies that suggest that the distribution of fluctuations in the CMB is not as uniform, or isotropic, as inflation theory predicts. One of these anomalies is a large cold spot in the CMB sky. Do these new results change our thinking about inflation theory?
EFSTATHIOU: It means we have new questions that need answering. Today’s universe could be 10100 times larger than the original patch of universe that inflated nearly 14 billion years ago during a fraction of a second after the Big Bang. As a result, the theory of inflation predicts that today’s universe should appear uniform at the largest scales in all directions. That uniformity should also characterize the distribution of fluctuations at the largest scales within the CMB. But these anomalies in the CMB that previous experiments had hinted at and which Planck confirmed, such as the cold spot you mentioned, suggest that this isn’t the case.
Planck has revealed fine-scale features in the CMB in exquisite detail; these are the fluctuations that seeded the formation of galaxies and galaxy clusters that we see today. But by confirming the larger-scale anomalies, Planck has also shown us that the universe may not be uniform at the largest scales. This is very strange. And I think that if there really is anything to this, you have to question how that fits in with inflation. You can modify the simplest inflation models to generate these features, but from the theoretical point of view these models are really ugly. They involve fine-tunings and so on, and it sort of undermines the motivation for thinking up inflation in the first place. It's really puzzling.
GORSKI: Still, the idea that the universe is so highly isotropic did not come about easily. It emerged in the 1960s and people wrestled with it for several decades before the inflationary ideas emerged. So, isotropy of the universe is not a theoretical idea; it’s based on observations. Planck is making a statement about some features that indicate deviations from isotropy, and we’re not certain what this means. Perhaps we may still eliminate these anomalies with more precise analysis; on the other hand, they may open the door to something much more grand – a re-investigation of how the whole structure of the universe should be.
EFSTATHIOU: The challenge of making sense of these anomalies begins with the fact that we don’t have anything to compare our universe to. In other words, when you look at large-scale features in the CMB, we’re limited by the fact that we have only one realization of the universe. So we don’t have enough information to conclude that the anomalies we see are statistically significant.
Taken individually, I don’t think you can argue convincingly that any one of these anomalies is so unlikely that we can rule out inflation. But even the most die-hard inflation advocate would have to accept that the universe, on large scales, looks odd. The big question is whether new physics is associated with that oddness. I think there is very little doubt that the universe on large scales looks odd, compared with what we would expect from simple inflation models.
"The sense of this being a unique time in the history of this field is profound, because we've lived through a time of not being able to see what we can see now in the CMB sky." – Krzysztof Gorski
TKF: Why does it matter that inflation theory may not completely fit with what we see in the universe?
EFSTATHIOU: Inflation is a beautiful theory that tries to explain how the universe came to appear as it does today, from the presence of galaxies and galaxy clusters to how those large-scale structures are distributed throughout the universe. It’s fundamental to our understanding of how the universe began and evolved. If the Planck spacecraft is showing us features that inflation theory cannot easily explain, then we should be worried.
Perhaps our theory of inflation is not correct, despite its beauty and simplicity. We may have to either fix the theory, amend it in some way, or throw it out and look for another explanation for why we see the universe as it is today.
TKF: Apart from the large-scale anomalies that we’ve talked about, what do we think caused the fluctuations that we see at smaller scales – the variations that Planck has now mapped in such impressive detail?
EFSTATHIOU: The leading theory is that these began as quantum fluctuations, and they were amplified in scale as the universe inflated.
LASENBY: A major tenant of physics predicts there will always be fluctuations on the tiniest scales. So we expect that these fluctuations, present at the moment of the Big Bang, were magnified by inflation. And it’s these amplified fluctuations that led to the formation of galaxies and galaxy clusters.
TKF: Can we draw a direct connection from the smaller-scale fluctuations that we see in the CMB to the galaxies and galaxy clusters that we see today?
EFSTATHIOU: We cannot make a direct connection between what we see in the CMB and the galaxies and galaxy clusters that came after the CMB was generated. But, we can do large computer simulations where we start off with fluctuations that have the same statistical properties that we've observed in the CMB sky, and it works extremely well in describing the kind of large-scale structure – the cosmic web of galaxies and galaxy clusters – that we see today.
"The anomalies we found run contrary to the idea that isotropy at large scales points to how thorough inflation was. Inflation actually may have been more limited in scope than previously theorized." – Anthony Lasenby
TKF: Taken as a whole, what questions do the latest Planck data put to rest and what new ones do they raise?
LASENBY: Planck has shown, with much improved error bars, that the simplest inflation models are really doing fine. But there are still some mysteries, and Planck data is really putting pressure on some alternative inflation models. The anomalies we found run contrary to the idea that isotropy at large scales points to how thorough inflation was. Inflation actually may have been more limited in scope than previously theorized.
EFSTATHIOU: A more limited inflation period is possible, but it's just ugly. If Anthony could calculate why inflation may have been more limited than current theory predicts, then I would be more impressed.
GORSKI: It's a truly unique time that we've been going through over the past 25 years, and the sense of fulfillment is enormous. I've been privileged to be able to participate and work directly on the COBE mission and now Planck. The sense of this being a unique time in the history of this field is profound, because we've lived through a time of not being able to see what we can see now in the CMB sky. And now, we are moving beyond the time when it was measured exquisitely well and new horizons are being established for what to attack in the future.
Planck's legacy will be with us for a very long time, and I don’t expect there will be another mission like Planck for a long time. This will basically be it. Studying how CMB light is polarized will reveal more about the early universe, and this is the future of this field. There are now a lot of younger people participating in this CMB field, but they don't have that same connection to what happened in the 1980s and earlier. The three of us are in a group that does remember. We were direct witnesses to how everything changed dramatically.
EFSTATHIOU: I think there can be no doubt that with Planck we’re uncovering fundamental truths about the universe. On the one hand, the data shows us a strong confirmation that inflation occurred – and that suggests that our ideas about some of the earliest moments of the universe are correct. On the other hand, it points to something we don’t yet understand. As a scientist, if you have a theory that fits very well with certain aspects of the data, then you should look more critically at places where there might be discrepancies. There are aspects of the latest data from Planck – and we saw suggestions of it in data from COBE and WMAP – that don't fit well with our theoretical picture of the universe.
"I think there can be no doubt that with Planck we’re uncovering fundamental truths about the universe." – George Efstathiou
LASENBY: I echo what both Kris and George said. Speaking personally, I'm particularly interested in inflation, and the primordial power spectrum, which tells us about how the intensity of fluctuations in the CMB varies at different angular scales on the sky. I’m also interested in alternatives to standard inflation. It's intriguing to me that we are getting interesting hints that point toward some departures from the standard paradigm. For me, these latest results from Planck are intriguing, and there's a lot more I would like to see worked on over the next year or so.