Monday, April 28, 2014

BICEP2: Pinning Down Cosmic Inflation

Last month, the scientific collaboration working with the BICEP2 telescope in Antarctica (Background Imaging of Cosmic Extragalactic Polarization) published a surprising first result: after three years of deep microwave imaging of 6% of the sky, they may have found the smoking gun* of the theory of inflation. That smoking gun consists of a series of ripples or fluctuations of the microwave polarization in this part of the sky. These ripples signify that within the first trillionth of a trillionth of a trillionth of second, the universe underwent a rapid, exponential spatial inflation, making its temperature and density nearly homogenous, corresponding to the large-scale density-uniformity of our own observable universe today. Because the universe started out on such a small spatial scale before inflation, quantum alternations in the fabric of spacetime were present. As the universe rapidly inflated in this 10-32 second of time, those alterations were magnified and manifested themselves as large-scale gravitational waves. These gravitational waves made their imprint in the cosmic microwave background, that image of the hot early universe we see today, in that same background’s polarization information. (Just like ordinary polarized light you can see by turning your head 90 degrees while wearing polarized glasses, radio waves, including microwave radio emission from the early universe, can be polarized in perpendicular directions.) As a result of this imprint, there are patches of sky in which the CMB’s polarization goes one way, and other patches in which the polarization goes the other way:

Fluctuations in the polarization of the cosmic microwave background observed with the BICEP2 telescope. The high and low ripples (orange and blue) indicate a signature of gravitational waves from the era of inflation.

...the orange areas corresponding to one direction of polarization and the blue areas corresponding to another direction of polarization. The width and number of the orange and blue blobs is where the inflationary theory (conceived in the 1980s by Guth and Linde and others) comes in. That theory predicts that if inflation happened and if it produced large-scale gravitational waves, the number of widths of the blobs should be such-and-such. Now take a look at the next image:

Detailed analysis of the above fluctuation image.
The number and size of the blue and orange peaks and troughs
matches theoretical predictions closely., this image is fairly complicated and was of course given for the target audience of professional cosmologists. What it essentially means, though, is that the number and widths of blue and orange blobs (the black data points with error plusses overlaid) exactly corresponds to the predictions of the theory of inflation (the red curve which they follow).

This is really big news, and could potentially lead to a Nobel Prize. Of course, the result will be followed up with observations from other telescopes, and will probably take years to verify solidly. However, the results are so far looking incredibly good.

I’ve packed a lot into what I have said so far. Let me try to break it down a bit more, and to point out the relationship of this discovery to the research field of galaxy evolution, which we normally discuss in this blog.

First, what motivated the theory of inflation? A number of things, but most importantly it was the horizon problem. The CMB, even when first discovered in 1964 by Penzias and Wilson (later earning them the Nobel Prize), immediately showed itself as a strangely uniform microwave glow throughout the sky. Because of the CMB’s origin in the early universe, just 3% of the universe’s age after the Big Bang, such a strange uniformity was extremely surprising. 

I say “strange uniformity” for the following reason. The CMB’s overall uniformity (i.e., it is as bright when looking up from the north pole as it is when looking up from Antarctica) means that the early universe, 14 billion years ago, was the same temperature in one faraway volume of space as the other faraway volume of space. But the light-travel time from one side of the universe to the other is much longer than the age of the universe itself! (This conundrum is also called the "horizon problem".)

When I cook scrambled eggs, I have a passionate dislike for the brown, flaky parts that accumulate at the bottom of the frying pan. And so I am an obsessive stirrer. And why do we stir scrambled eggs? Because bringing the hotter parts in contact with the cooler parts equilibrates the temperature everywhere. The hot spots down by the hot frying pan won’t get brown. The fact that the early universe was the same temperature in vastly different parts of the universe means that something must have moved faster than light in order to bring the distant parts of the observable universe into contact with one another. Now we know from special relativity that no information can propagate faster than light. But that fact doesn’t prevent space itself from expanding faster than light. In other words, in very, very, very early times, space itself, i.e., the distance between things, would have stretched out at a rate faster than the speed of light. Thus, by a superluminal stretching, the entire observable universe would have been in causal contact with itself (just like stirred eggs) long before the hot-plasma era of the early universe we see in the CMB.

What would cause such a rapid inflation? An increase in spatial scale by 1026 in 10-32 seconds (as the theory states) is not reminiscent of any energy source known to us. There is a wealth of theoretical literature on the topic, but the short answer is: we don’t know, but it must have been something. (The best guess is that the mechanism would be something called scalar fields.) Although there are deep reasons in particle physics to look for such forces, doesn’t this sound strangely like magic at the end of the day?

But when the phenomenon known as dark energy was discovered in 1997 by CANDELS team member Adam Riess and collaborators, earning them the 2011 Nobel Prize in Physics, the theory of inflation sounded less like theoretical speculation and more like reality. The discovery, as can be read about in other CANDELS blog posts, showed that the universe in its more recent cosmological history (i.e., the space we see at around a redshift of 1) began accelerating in its expansion. It was certainly a far more gradual acceleration than the rapid inflation at 10-32 seconds, but nonetheless it was an inflation of sorts.
The BICEP2 facility, located at the South Pole.

The discovery of dark energy gave further impetus to search for the predicted polarization pattern in the CMB that would confirm inflationary theory. Now that a surprisingly strong polarization signal has been found (combining the BICEP2 data with data from the Planck mission), a path of independent confirmations lies ahead. The Planck space mission will confirm the result on larger spatial scales. Other ground-based telescopes such as the SPT and the Keck Array (the expanded BICEP2) will probe more deeply. They will verify that the polarization pattern is due to sources in the early universe and not something nearby – foreground galaxies, or even something in the vicinity of our own galaxy. Finally, the presence of gravitational waves in the early universe gives a bright green light for observations of gravitational waves passing the vicinity of the earth today: through pulsar timing (which, as discussed in previous posts, has significant implications for galaxy evolution models), LIGO, and the space-based eLISA mission.

Although I have not spoken much of galaxy evolution, suffice it to remember that had not the early universe inflated and laid the groundwork in which galaxy clusters, galaxies, stars, planets, and you and me would come to be, then I would not be able to write on this galaxy evolution blog about all that has happened in our universe.

*I don’t really like to use the term “smoking gun” in relation to scientific discoveries, because it usually takes a long time for the story to be over, if it ever is over. In this case, however, the results are rather dramatic, making me unable to resist.

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