What’s the Cosmic Microwave Background?

*UPDATE: Researchers have since retracted the claim that they found the first direct evidence for cosmic inflation—namely, B-mode polarization in the cosmic microwave background, caused by primordial gravity waves (which cosmologists theorize occurred during inflation). “A joint analysis of data from ESA’s Planck satellite and the ground-based BICEP2 and Keck Array experiments has found no conclusive evidence of primordial gravitational waves,” according to a press release from the European Space Agency, released on January 30, 2015. “However, there is another contender in this game that can produce a similar effect: interstellar dust in our Galaxy, the Milky Way.” For more details, check out Ron Cowen’s article in Nature and Adrien Cho’s article in Science.

Today, researchers announced the first direct evidence* for cosmic inflation—the idea that, less than a second after the Big Bang, the universe blew up like a balloon in a process of exponential expansion. The evidence comes from patterns detected by a telescope called BICEP2 in the cosmic microwave background, or CMB.

The CMB is light that was released 300,000-400,000 years after the Big Bang, and it’s the oldest light that we can detect in the universe. “The fact that there is a CMB is one of the great pieces of evidence for the Big Bang,” says Duncan Hanson, a cosmologist at McGill University and the lead author of a study published last year describing patterns found in the CMB.

Here’s how cosmologists think the CMB came to be:

The early universe was a piping hot, opaque “soup” of particles, according to Clarence Chang, a scientist at Argonne National Laboratory who helped build the South Pole Telescope, another telescope that studies the CMB. As it cooled and further expanded over time, the universe coalesced into a plasma consisting of more familiar ingredients, including protons and electrons. These charged particles constantly scattered photons as polarized light, which was trapped in the plasma like sunbeams lost in fog.

As the universe continued cooling, the charged particles found each other, forming neutral atoms less prone to scattering photons. Liberated, that early light—what we now call the CMB—could travel freely through space, and as it did, it stretched to wavelengths associated with very cold temperatures that highly sensitive telescopes can detect. By studying patterns in the CMB polarization, researchers can learn more about the origin and makeup of the big black yonder.

For instance, as the CMB radiation travels toward us, it experiences gravitational tugs from bodies of mass that warp its path. That tugging, called gravitational lensing, creates a pattern in the CMB’s polarization called B-mode.

But B-mode polarization can also result from a different phenomenon—what’s called primordial gravity waves. Cosmologists theorize that these waves formed as a result of quantum mechanical fluctuations during inflation. Detecting the imprint of gravity waves on the CMB polarization amounts to finding “the first tremors” of the Big Bang, according to Hanson—and that’s exactly what researchers using BICEP2 observed.* For more on the discovery, tune in to this SciFri segment from March 21, 2014.