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Posts Tagged 'Planck'



Planck results: our weird and wonderful Universe

The cosmic pie, before and after Planck. The size of the slices changed a little bit, but it's barely noticeable. [Credit: ESA/Planck Collaboration]

The cosmic pie, before and after Planck. The size of the slices changed a little bit, but it’s barely noticeable. [Credit: ESA/Planck Collaboration]

The big news today is that our Universe is a little older than we thought, has a little more matter in it, and is every bit as strange as we’ve come to expect. Some numbers got shifted around a bit, but things are pretty much what we cosmology-watchers expected. It’s not a bad thing, in my opinion. After all, we still don’t know what dark matter is, we still don’t know what dark energy is, and we still don’t understand inflation completely. Adding weirdness to weirdness is probably more than our poor brains could take right now.

We learned all of this from the newly released data from the Planck mission, a telescope orbiting a stable gravitational point beyond the Moon. (That’s the L2 Lagrange point, for those keeping track at home.) Like WMAP before it, Planck was designed to study the cosmic microwave background (CMB), the radiation left over from recombination, when the Universe became transparent about 380,000 years after the Big Bang. I wrote a primer on the CMB a few weeks ago on this blog, and summarized the Planck findings for Ars Technica today. I won’t repeat what I wrote there, but I wanted to highlight a few of the big important things to take away from the data release, and give a few opinions too cranky for Ars Technica.

First up, here’s the summary of Planck’s findings:

  • The age of the Universe has been revised slightly, from 13.77 billion years to 13.81 billion years. It kind of looks like a bigger deal than it is: the difference may be about 40 million years (or more, depending on which estimate we use), but it’s about 0.2% of the total age of the cosmos. We’re fiddling with the radio dial to get the best reception at this point, not changing stations. (Does that metaphor even work anymore?)
  • Similarly (and for similar reasons), the relative amounts of stuff in the Universe have been shuffled a little bit. There’s a little less dark energy, according to the Planck data, while both ordinary (baryonic) and dark matter got a bit of a boost. The current balance is 68.3% for dark energy, 26.8% for dark matter, and 4.9% for ordinary matter. Again, not huge shifts, but interesting nonetheless.
  • Planck is higher resolution than WMAP, so it was able to probe to smaller scales of CMB fluctuations. As a result, it obtained a little better estimate for the spectral index, which is a measure of how fluctuations scale with size. If the spectral index is exactly equal to 1, then all fluctuations started off the same size when the Universe was young. However, if inflation is correct, then the spectral index should be a little less than 1, and that’s what Planck saw. That’s not enough to constitute direct evidence for inflation, but it’s a pretty nice find anyway. (I think I’ll cover the spectral index for the “S” entry in the Alphabet of Cosmology, since I don’t want to spend too long on it for today’s post.)
  • While earlier cosmology experiments, including WMAP, left tantalizing hints that there might be more than 3 species of neutrino, Planck seems to have closed that door. The neutrino data from CMB experiments aren’t definitive, but in combination with other results, it seems clear: the only neutrinos out there are the normal “flavors”.
  • Finally, the strange anomaly WMAP first observed in 2001 at the largest scales is still there. Some people held out hope that the WMAP results were a fluke, an error arising from the observatory’s basic construction. However, Planck is sufficiently different in design to make that hope futile. The effect is real. (Some sources made this sound like it’s something new, but maybe that’s because cosmologists tried not to call attention to it in the last decade.)

If you want more (and who doesn’t?), check out Jennifer Ouellette’s summary at Cocktail Party Physics, Phil Plait’s take over at Bad Astronomy, and Ethan Siegel’s coverage at Starts With a Bang.

The Planck CMB map, in its full glory. As with the equivalent WMAP image, the colors represent tiny fluctuations (measured in millionths of a degree) around the 2.7 Kelvin temperature of the cosmos. [Credit: ESA/Planck Collaboration/D. Ducros]

The Planck CMB map, in its full glory. As with the equivalent WMAP image, the colors represent tiny fluctuations (measured in millionths of a degree) around the 2.7 Kelvin temperature of the cosmos. [Credit: ESA/Planck Collaboration/D. Ducros]

Now, time for me to geek out over the power spectrum for a bit.

Power spectrum geekery

The temperature fluctuations in the image above are pretty interesting by themselves, but the real way to deal with them is to convert them to a power spectrum. In essence, that’s figuring out how big the fluctuations are on the sky relative to each other, without worrying about exactly where they’re located. To put it another way, it’s hard to see the important patterns in the all-sky CMB map, so the power spectrum is a way of compressing some of that data into another form.

The Planck CMB power spectrum, which represents the temperature fluctuations (vertical axis) as a function of size on the sky (horizontal). The temperature fluctuations are squared because we don't care at the moment if they're hotter or colder than average - we're just after how much they deviate. The horizontal scale runs from 90 degrees - one quarter of the way around the sky - down to a tiny fraction of a degree, much smaller than the full Moon, which is half a degree on the sky. [Credit: ESA/Planck Collaboration]

The Planck CMB power spectrum, which represents the temperature fluctuations (vertical axis) as a function of size on the sky (horizontal). The temperature fluctuations are squared because we don’t care at the moment if they’re hotter or colder than average – we’re just after how much they deviate. The horizontal scale runs from 90 degrees – one quarter of the way around the sky – down to a tiny fraction of a degree, much smaller than the full Moon, which is half a degree on the sky. [Credit: ESA/Planck Collaboration]

The first thing to note is how much smaller the angles go in the Planck data compared with the equivalent WMAP power spectrum. The data points—the little red dots—are also more closely packed together, resulting in a lot more data to compare with theoretical predictions. The cosmology we know and love (?)—meaning the relative amounts of dark energy, dark matter, and ordinary matter—are determined by the size and position of the first three big peaks in the power spectrum.

I covered that in my previous post, so I won’t go over all the details again. However, if any of you doubt the existence of dark matter, I encourage you to go back to that post and then back to your man, then back to me look at the third peak. The Planck results are even stronger than WMAP’s for showing how much dark matter there must be in the cosmos. The ratio of dark matter to ordinary matter didn’t change—it’s still about 5 to 1—but there’s more of both in the Planck data, and the errors (represented by the vertical lines attached to the dots) on the estimates are a lot smaller.

A few highlights in the CMB power spectrum. a. The third peak indicates the total matter content of the Universe, both baryonic (ordinary) and dark. Planck has much better data for this peak than WMAP, strengthening the case for dark matter's existence and how much of it there is in the Universe. b. The long tail of smaller peaks is the small-scale fluctuations that gave birth to galaxies at later times. Awesome, right? c. The anomalous temperature fluctuations at the largest scales, first seen by WMAP in 2001, are probably the things getting the most attention today. [Credit: ESA/Planck Collaboration/moi]

A few highlights in the CMB power spectrum. a. The third peak indicates the total matter content of the Universe, both baryonic (ordinary) and dark. Planck has much better data for this peak than WMAP, strengthening the case for dark matter’s existence and how much of it there is in the Universe. b. The long tail of smaller peaks is the small-scale fluctuations that gave birth to galaxies at later times. Awesome, right? c. The anomalous temperature fluctuations at the largest scales, first seen by WMAP in 2001, are probably the things getting the most attention today. [Credit: ESA/Planck Collaboration/moi]

WMAP had to stop at the third peak: it didn’t have the resolution to go any smaller. However, Planck could probe down to very small angles on the sky (the bit of the image marked “b” above), which are the fluctuations that gave rise to galaxies—and ultimately to us. To put it another way, these are the direct evidence for the baryonic acoustic oscillations (BAO) I wrote about in an earlier post, and they correspond perfectly. Life is good.

Now for the yucky bit: at the left side of the power spectrum, the error bars get big, and the points don’t fit neatly to the theory—any theory. These are anomalous temperature fluctuations at very large scales on the sky. As Jennifer Ouellette and Sean Carroll described it, the Universe seems to be lopsided: the temperature on one side of the whole sky is just a little warmer than on the opposite side.

It’s not a huge difference, as you can see from the power spectrum: the highlighted bit in the image marked “c” is nowhere near the height of the peaks, or even the long tail. However, it’s enough to keep many cosmologists awake at night, and give writers like me heartburn as we try to combat the more sensationalist accounts of what those anomalies might mean. (Ethan Siegel in particular hates sensationalism, so I’ll turn the podium over to him for that rant.) One possible explanation, which seems kind of boring but which I can’t get out of my head, comes from a quirky aspect: the anomalies line up pretty well with the plane of the Solar System. That’s probably a freaky coincidence, but if it isn’t, then it’s probably some new kind of foreground signal that mimics the CMB, due to some undiscovered source associated with the Solar System.

I have no particular evidence to back up that guess, so we’ll see if I’m right as people continue to study the problem. Sean Carroll proposed an interesting idea a few years ago, based on a modified version of inflation. Others (including Planck scientist George Efstathiou) postulated that the anomalies could be due to an earlier Universe that preceded the Big Bang. My colleague Amanda Yoho pointed me to two papers that discussed the problem (on one of which she was lead author), so good brains have been pondering the problem for some time.

I’m confident that, however it works out, we’ll continue to be impressed with our beautiful, weird, wonderful Universe.

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