Friday, March 22, 2013

Exciting News from the Planck Satellite

We live in a weird universe. It had a beginning. It is enormous, probably infinite, in extent. And yet at least our observable part of it started out in a volume smaller than the head of a pin 13 to 14 billion years ago. In the vast unobservable part of the universe, there may well be an exact copy of you reading this blog. The universe is getting bigger, but the expansion rate started out fast, slowed down, and is now speeding up again. Is that weird enough for you? No? How about all that and lopsided? Read on.

All this would sound crazy if we didn't have a lot of evidence that tells us that this is the universe we inhabit. That evidence includes:
  • The observation that almost all galaxies in the sky are moving away from us, and that the ones that are further away are moving away from us faster. 
  • The fact that we cannot find stars or galaxies that look like they are much older than about 14 billion years old (plus or minus a few billion). 
  • The observation that relatively pristine gas in the universe consists of about 73% hydrogen and 25% helium. This ratio was set in place during the few minutes during cosmic expansion when the universe was cool enough for protons and neutrons to exist, but hot enough and dense enough for them to be continually bumping into each other.
And the evidence includes measurements of the Cosmic Microwave Background (CMB) radiation - radio waves that are all around us. The CMB accounts for a very small fraction of the static you can hear on an FM radio if you tune between channels. These radio waves originated when the universe was only 380,000 years old and was still permeated by a dense fog of electrons that were bumping into each other too often to be bound to atoms.

The big news this week is the release of the first cosmology results from the Planck satellite, which is the latest in series of increasingly sophisticated observatories designed to measure the statistical properties of this radio noise from the early universe. I'm going to comment on a few of interesting results in the 29 papers that were just submitted for publication. I am not an expert on the measurements and I have only just begun reading the papers. I'm aiming here for a set of highlights that are a bit more detailed than the press release, but easier to digest than the papers. There are some nice blog posts at Bad Astronomy and Starts with a Bang as well. If you are an aficionado, feel free to comment if you find that I've botched something, or missed something important!

The Data

The map of the Cosmic Microwave Background from the Planck satellite, 
after removing the foreground sources of radio emission. 
The Planck satellite is a radio telescope that scans the sky repeatedly. By comparing the radio power it receives when pointing in different directions, it can build up a map of the temperature fluctuations in the gas that permeated the universe when it was only 380,000 years old. These temperature fluctuations are tiny -- less than one part in 10,000. The engineering feat of creating devices to measure this is quite amazing. There are also other sources of radio emission in the sky, and it has taken a lot very careful work to measure these sources and account for their effect on the Planck measurements. Once these "foreground" sources are removed, the result is a map of the surface of last scattering. The places where you see red are just a tiny bit hotter than the places where you see blue. These hotter and cooler spots agree remarkably well with the previous best measurements from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite.

A blowup of a small portion of the CMB map. The new map on the right is from Planck. The previous best map from the WMAP satellite is on the left. The Planck measurements agree extremely well, but obviously provide more detail. From the European Space Agency.

You can see by eye that these maps are not uniform. There are fluctuations from pixel to pixel. But these aren't completely random. There are somewhat coherent patches that are bluer and other patches that are redder. The tool that astronomers use to quantify this is the angular power spectrum. The basic concept of measuring the angular power is something like the following. Draw a little circle on the map. Measure the average temperature in that circle. Do that in a bunch of other little circles that are the same size and compare the results. If they are all the same, write down the number zero. If they aren't,  you can quantify the dispersion of the measurements using the standard deviation or some other statistical quantity. Now vary the size of these circles. You will find that the dispersion in these mean temperatures depends on the size of the circle. Plot up these statistical results as a function of the angular separation and you have an angular power spectrum, more or less. (In practice, the fluctuations are measured using Fourier analysis and spherical harmonics). The figure below shows the angular power spectrum from Planck.

The angular power spectrum of temperature fluctuations measured by Planck. Small angular separations are on the right, large angles are on the left. The red points show the Planck measurements. The green curve is the prediction from the standard model of cosmology -- that weird model described at the opening of this blog post. (If you want to understand what causes the wiggles, I highly recommend reading  Wayne Hu's web site and looking at some of his animations.) A couple of amazing things to note. (1) Every point has an error bar. Most of them are tiny. This is  a great measurement. (2) The predictions from the cosmological model go right through the data. The shaded green region at large angular scales shows the uncertainty due to the fact that we can only observe from one vantage point whereas the models predict the fluctuations that would be observed for an ensemble vantage points placed randomly throughout the universe. From the European Space Agency.

What Does It Mean?

The standard (weird) model of cosmology is expressed as a series of mathematical equations, which include physical constants that we know quite well (for example the speed of light), and include other quantities, like the density of matter in the universe, that we don't know as well. These quantities are known as cosmological parameters. There are seventeen parameters involved in the model represented by the green curve in the angular power spectrum plot. Having adjusted the parameters to fit the data, it's then possible to infer other quantities -- like the current rate of expansion of the universe, its age, and the mean density of matter and energy. 

So here are a few highlights of the Planck findings.

The universe is expanding more slowly than we thought.

The present-day expansion rate of the universe is known as the Hubble Constant. The Planck estimate is only about 90% of the expansion rate that we measured a couple years ago with the Hubble Space Telescope. This is a bit surprising given the estimates of the uncertainties from both measurements. We usually learn something when two sets of precision measurements don't quite agree. So it's going to be interesting to try to understand what the source of the discrepancy is. 

The universe has only three flavors of neutrinos.

One of the interesting inferences from the previous, higher, expansion rate was that the model that provided the best fit to all the data required more than three types of neutrinos. Neutrinos are very light subatomic particles that carry no charge and rarely interact with atoms. There about 100 million neutrinos passing through you right this second. The standard model of particle physics has three types of neutrinos. The previous cosmological observations suggested there were four. Because this would have a profound impact of our understanding of the forces of nature, the measurements created a lot of interest in a hypothetical particle called a sterile neutrino, which interacts with normal matter only through gravity. If the sterile neutrino had mass, it could be part of the dark matter that pervades the universe. It would be warm dark matter, and its existence might help resolve some of the problems that we encounter when trying to explain galaxy properties using just cold dark matter. If Planck is right, we will need to look elsewhere for solutions.

The universe is flat.

It's that time of year, so imagine yourself on the surface of a giant basketball, but one that is poorly inflated. If you start walking one direction, you will eventually end up where you started. If you count your steps, you can estimate the size of the basketball. Now imagine someone inflates the basketball. It's still more or less spherical, but it will take more steps to arrive back at your starting point. The number of steps gives you the size. The fact that you ended up where you started, tells you that the basketball is a closed surface. Another thing that tells you this is that two parallel lines will eventually intersect -- like those two lines that meet in the middle of the picture. 

The universe is harder to picture because the "surface" has three dimensions, not just two. If the universe is closed, a beam of light sent out in one direction will eventually (billions of year later) return to you. In the CMB measurements, changing the curvature of the universe changes the position of the peaks in angle on the sky. The Planck observations confirm what previous measurements were telling us, that the universe is spatially flat. Two lines that start out parallel won't intersect, and they won't drift apart. Flatness is one of the consequences of the early, rapid expansion of the universe called inflation. So it is good to see this holding up. 

The universe has a bit less Dark Energy and a bit more Dark Matter than we thought.

We don't know what Dark Matter and Dark Energy are. Dark Matter is probably some kind of subatomic particle that interacts with normal matter primarily through gravity. We think it is there because we can measure its gravitational influence on large scales in the universe. There are many, many observations that are best explained by invoking dark matter. But we still don't know what it is. Dark Energy is even stranger. It has been invoked to explain the observation that the universe has started to expand faster. It is probably has some link to Vacuum Energy in particle physics, but there is no consensus among physicists on how it is related. The best fit to the wiggles in the Planck power spectrum suggests that the universe consists of 68% Dark Energy, 27% Dark Matter and 5% normal matter (what all of the stars and galaxies and people in the universe are made of). The previous best estimate was 72% Dark Energy.

Dusty galaxies can be detected by their mass.

This result is interesting to those of us who study distant galaxies. There is a fluctuating background at far-infrared wavelengths that has nothing to do with the CMB. It probably arises from dusty galaxies, but we don't know much about them, other than that they are very numerous and probably very distant. The dust absorbs the optical and ultraviolet radiation emitted from stars and re-emits it in the infrared. Planck can measure this fluctuating background in its short wavelength channels, and by correlating with the radio maps, can see the statistical effect of gravitational lensing of the CMB by the mass associated with the dusty galaxies. The average dark-matter mass inferred from the measurement is only about 3% of the mass of the Milky Way. I find this a bit surprising because we tend to find that massive galaxies are very dusty and low-mass galaxies are not. On the other hand, the uncertainties of the estimate are quite large. The constraints might get tighter with more analysis.
This is a cool measurement. In addition to making radio maps, Planck can measure far-infrared radiation that comes from dusty galaxies. The maps in the right panel show the fluctuations in the infrared brightness of the sky at wavelengths of 350 and 545 microns due to these dusty galaxies. You can't see the individual galaxies, just their combined effect. The mass of each of these individual galaxies can imprint a gravitational lensing effect on the CMB. It's possible to detect that effect (the diagrams on the left and center) by stacking the CMB maps at the positions of the peaks (left) and troughs (center) of the far-infrared map. By measuring the effect, it's possible to infer the mass and distance of the galaxies responsible for the far-infrared radiation. Paper XVIII.

The Universe is lopsided?

This is the result that might engender the most discussion, because it suggests the universe might be even weirder than we thought. A fundamental assumption of cosmology is that this vast universe looks statistically the same in all directions. There is no preferred direction. There was a hint in the earlier data from the WMAP satellite that there might be a preferred direction. That hint has gotten much stronger with the Planck data, which confirms both the effect and the direction. One interpretation is that our entire galaxy is moving in one direction faster than we think it should if we add up the pull of gravity from all the other galaxies in that direction. But that interpretation doesn't fit all the data, and there are weirder possibilities that involve changing some of our assumptions about the cosmological model.

And one other tidbit that's going to cause problems.

The Planck papers have a lot of co-authors. The the authors are listed alphabetically after the words "Planck Collaboration." The first author is almost always Peter Ade of Cardiff University. It's rare (but not unprecedented) in astronomy to construct the author lists this way, because the tradition has been to recognize the people who have contributed most to a particular paper (even if it is from a giant collaboration like CANDELS) by listing their names near the top of the author list. By bucking tradition, the Planck collaboration is acknowledging  that it often impossible to judge who made the most significant contributions to a particular paper, particularly if it depends on the technical efforts of many, many other people. On the other hand, it is going to be interesting to see who gets invited to the conferences!

1 comment:

  1. A few refinements, after hearing a very nice summary of the results by Marc Kamionkowski at Johns Hopkins.

    First, it's probably wrong to say "the universe is expanding more slowly than we thought." Instead I should have said that the expansion rate estimated from the CMB disagrees with the more direct measurements in the nearby universe. One possible solution is that Dark Energy makes the change in the expansion rate different than the simplest model (in technical terms, the equation of state parameter w might be less than the simplest assumption w=-1). If that interpretation were confirmed it would be a profound discovery.

    Second, I was confused about the lopsidedness. The measurement of lopsidedness is in the "total power" of the fluctuations, rather than in the "mean temperature." In other words, the difference between the troughs and valleys on the right-hand side of the map is slightly bigger than on the left side of the map. So the explanation that our galaxy might be moving faster in one direction than previously thought is not viable. I expect there are theorists hard at work already trying to come up with simple explanations.

    Third, Marc was very excited about the implications for inflationary models (used to explain the first tiny fraction of a second of the expansion of the universe). The relative power of fluctuations on different scales was set in place as the universe emerged from its "exponential expansion" phase, so by getting this snapshot of the relative power you can (a) get a lot more confidence that inflation really happened, and (b) actually say something about this transition phase.