Cocktail Party Physics

Cocktail Party Physics

Physics With a Twist

Walking the Planck: Our Lopsided Universe


We were not hardy enough to stay up until the wee hours for the big announcement of the latest results from the Planck satellite mission, but it is all over the science blogosphere this morning, so we didn't miss much. The Bad Astronomer has a particularly nice overview. For those keen on learning even more of the gory details, with graphs and pretty pictures, Ethan Siegel of Starts With a Bang has two posts: one detailing everything you should know about the universe, up to this morning's announcement, and the second on what that new data tells us about the composition of the universe.

For the most part, it's business as usual: the new data, which -- every news report will tell you -- "gives the most accurate and detailed map ever made of the oldest light in the universe," pretty much confirms the major predictions of the standard model of cosmology. But there's an asterisk, just to make things interesting.

Planck's "map" of the cosmic microwave background radiation. Image credit: ESA and the Planck Collaboration

It seems there are a few, er, "anomalies" in the data. The biggest is that we should see a random distribution of fluctuations in the data from the cosmic microwave background radiation.

Mostly, this is true. But prior surveys found hints of a not-so-random distribution in the amplitudes of those fluctuations (how bright they are), and the Planck data confirms it. Per Phil: "A simple model of the Universe says that shouldn’t happen. The Universe is lopsided on a vast scale!" That's right: we live in a lop-sided universe.

WHAT DOES IT ALL MEAN??? Well, the physicists are still sorting that out, via a very long list of papers, with plenty more to come, but as Matt Francis observed at Ars Technica, mostly it means that "the universe is still weird and interesting." And that makes physicists very happy.

So, what is this cosmic microwave background radiation, some of you may be asking? (Not everyone follows cosmology closely, after all.) It's basically the afterglow from the Big Bang. Arno Penzias and Robert Wilson discovered the CMB in the 1960s, quite by accident. At the time, the physics community was divided into two camps on the question of the universe. The dominant view was that the universe was unchanging and would remain in a steady state forever. A few mavericks argued in favor of the Big Bang model, based on Edwin Hubble's 1929 discovery that the galaxies are moving away from each other.

In this view, the universe was once infinitely dense, with all matter emerging in a single cataclysmic explosion. But in order for that model to be correct, there should be the equivalent of a cosmic afterglow of about 3 degrees K, per the prediction of Princeton physicist Robert Dicke. And no one had been able to detect it experimentally.

Penzias and Wilson with their horn-shaped antenna.

It's a classic example of scientific serendipity. Penzias and Wilson weren't looking for the CMB; they were using a 20-foot horn-shaped antenna (salvaged from an obsolete satellite transmission system) as a radio telescope to amplify and measure radio signals from the Milky Way and other galaxies.

They just couldn't get rid of all the interference in order to make precise measurements: there was an irritating hissing noise in the background, like static. It was a uniform signal, in the microwave frequency range, and it seemed to be coming from everywhere at once.

They tried everything, even installing a pigeon trap to oust roosting birds and removing the accumulated droppings, but they couldn't get rid of the hissing. So they consulted with Dicke, who confirmed the discovery: "We've been scooped," he told his Princeton colleagues. (The lowly pigeon trap is now part of the permanent collection at the Smithsonian Institute's National Air and Space Museum.)

The CMB's discovery made the Big Bang the dominant model for the early universe but scientists were still a bit fuzzy about why there would be stars and clusters of galaxies instead of an evenly distributed dust cloud. They figured there had to be minute temperature fluctuations in the CMB, variations in the density of matter in the early universe.

This is known as "anisotropy": small variations in different directions, in this case, variations of temperature in the CMB in different directions. Lots of things can be anisotropic, even something as basic as the polarizing lenses in sunglasses: if you hold the lens in one direction, polarized light streams through, but hold it in a different direction, and that light is blocked. Since the lens behaves differently depending on direction, it can be said to be anisotropic. Plasmas can have anisotropic properties, too: they may have a magnetic field oriented in a preferred direction.

Another key concept is blackbody radiation. Emitted radiation by the early universe (for our purposes, the "body") should be distributed between the various wavelengths of the electromagnetic spectrum, and the shape of that spectrum depends entirely on temperature. So if we know the temperature of such a "blackbody" (technically, a perfect emitter and absorber of radiation, not literally "black"), we can precisely predict what the resulting spectrum should look like.

NASA launched the COsmic Background Explorer (COBE) satellite on November 18, 1989, and got the first results after a mere nine minutes of observations. The accumulated data points formed a perfect blackbody spectrum -- the universe is a perfect emitter and absorber of radiation. It was such an exact match with theory that, when the resulting curve was first shown at the 1990 American Astronomical Society meeting, there were audible gasps in the assembled scientists, followed by a standing ovation. Check it out:

Source: Wikimedia Commons. Public domain.


Isn't it a thing of beauty? It's a rare event indeed when experimental data matches the predictions of a theoretical model so perfectly. From this, the team was able to measure the minute temperature fluctuations in the CMB, and therefore where matter in the universe began to aggregate.

That "map" of the early universe produced by COBE was announced at the 1992 APS April Meeting in Washington, DC -- one of the first official physics conferences I attended as a young science writer. The press conference was standing room only, with TV cameras from all the major news networks, as well as radio and print reporters, and it was easy to get swept up in the excitement, especially since the gist of what they found could be easily grasped: a snapshot of the universe in its infancy, and an explanation for the origin of galaxies and stars.

COBE was the first experiment sensitive enough to detect those tiny fluctuations, even though the variations were at the level of parts per hundred thousand. COBE also provided the most precise average temperature of the universe to date: 2.726 degrees K. When COBE measured faint fluctuations in the CMB's temperature, it lent considerable support for the Big Bang model of the universe's birth. That's why Stephen Hawking called the COBE results "the greatest discovery of the century, if not of all time."

The story didn't end with COBE. The Boomerang and DASI detectors added even more detail to the microwave background, and most recently (around 2008) the WMAP project supplied the best values known thus far for such critical cosmological parameters as the actual age of the universe; the curvature of spacetime; and when the first atoms, stars, etc. began to form.

An artist's concept of the Planck spacecraft. Image credit: NASA/JPL-Caltech

And that brings us back to the Planck mission and today's big announcement. Planck is the successor to COBE and WMAP. I loved the press release quote by JPL scientist Krzystof Gorski describing Planck as "the Ferrari of cosmic microwave background missions. You fine-tune the technology to get more precise results," he said. "For a car, that can mean an increase in speed and winning races. For Planck, it results in giving astronomers a treasure trove of spectacular data, and bringing forth a deeper understanding of the properties and history of the universe."

Apart from the aforementioned anomalies, Planck gives us a new recipe for the makeup of the universe -- or rather, even more precise measurements of how much of each "ingredient" there is. To wit (per the folks at Symmetry):

"The map reveals that dark matter makes up about 26.8 percent of our universe, an increase from the previously measured 24 percent, while normal matter makes up 4.9 percent rather than 4.6 percent. The results also indicate that dark energy makes up 68.3 percent of the universe rather than the 71.4 percent previously estimated."

We also have a new estimate for how fast the universe is expanding (a.k.a. Hubble's constant): 67.15 plus or minus 1.2 kilometers/second/megaparsec. (One megaparsec = around 3 million light years). That is a slightly less expansion rate than those derived from previous data collected by NASA's Spitzer and Hubble space telescopes, which employ different measurement techniques. It means the universe is also a teensy bit older than previously measured: 13.82 billion years old, to be exact, compared to an even 13.8 billion years (cosmologists care about those extra decimal places, even as the rest of us prefer to round up or down).

So, you know, science marches on. We'll have the complete results from Planck next year, adding even more precision to these measurements. And after that? Who knows?

Portions adapted from an October 2006 post on the archived Cocktail Party Physics blog.

The views expressed are those of the author and are not necessarily those of Scientific American.

Share this Article:


You must sign in or register as a member to submit a comment.

The perfect movie companion to
Jurassic World

Add promo-code: Jurassic
to your cart and get this digital issue for just $7.99!

Hurry this sale ends soon >


Email this Article