July 26, 2013
Planck Observations Support Prevailing Big Bang Theory, But They Also Raise New Questions
April Flowers for redOrbit.com - Your Universe Online
This spring, scientists from the Max Planck Institute showed humanity the most detailed map ever created of the early universe. The map was generated from data gathered by the Planck spacecraft. It shows fluctuations in temperature in the relic radiation left over from the Big Bang - the moment when space and time came into existence nearly 14 billion years ago. This relic radiation is sort of like an afterglow of the Big Bang. It is called cosmic microwave background, or CMB. Streaming towards Earth from everywhere in the sky, the CMB provides a snapshot of how the early universe appeared when it was generated 380,000 years after the Big Bang.
The Kavli Foundation recently interviewed three leading researchers on the Planck team.
Professor George Efstathiou is an astrophysicist from the University of Cambridge, Director of the Kavli Institute for Cosmology at Cambridge (KICC) and one of the leaders of the Planck project. He was joined by Anthony Lasenby, Professor of Astrophysics and Cosmology at the University of Cambridge and Deputy Director of KICC, as well as a member of the Planck Core Team, a co-investigator for the spacecraft's High Frequency Instrument, and member of the Planck Editorial Board. The final member was Krzysztof Gorski, Senior Research Scientist at the Jet Propulsion Laboratory (JPL), and faculty member at the Warsaw University Observatory in Poland. Dr. Gorski is also a Planck Collaboration scientist and one of the Co-Investigators of the Low Frequency Instrument on board Planck.
The following is an excerpt from that interview, provided by Kavli.
A PLANCK SPACECRAFT ROUNDTABLE
TKV: 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?
(The theory of Inflation is one of the major tenants of the Big Bang model of the evolution of the universe. Inflation suggests 1036 seconds after the Big Bang, the universe expanded exponentially, very quickly. The initial size was something that was billions of times smaller than a proton, which expanded to something that was about the size of a fist.)
EFSTATHIOU: It means we have new questions that need answering. Today's universe could be 10,100 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.
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.
KF: 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.
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.
LIVE ON GOOGLE
All three scientists will be participating in a live Google hangout webcast on July 31, 2013, from Noon to 12:30 PDT. The public is encouraged to drop by and ask questions about the mission and the CMB findings.