Dr. Neelima Sehgal, an assistant professor in the department of Physics and Astronomy, wants to understand the earliest moments of the universe.

Specifically, she wants to know what happened within one second after the Big Bang—the event believed to have taken place 13.8 billion years ago and took the universe from a tiny, dense, finite point to the infinite cosmos of today. But studying events that took place billions of years ago is no small feat.

Luckily, the universe has left a trail of breadcrumbs throughout its development that researchers like Sehgal can use to uncover its history.

After the Big Bang, the universe was incredibly hot and the mix of high-energy particles within the universe was such that photons, or particles of light, were destroyed. After approximately 370,000 years, the universe cooled and the particle makeup of the universe shifted to a state that allowed photons to escape. The light produced during this time is the earliest light that can be detected, as it is the first light that was able to survive.

Since light travels at a set speed, this light created over 13 billion years ago is still reaching life on Earth today and can be detected by powerful telescopes. Thus, observing this billions of-years-old light is like a window into the past and researchers can use it to study the universe when it was only 370,000 years old.

This light is known as the cosmic microwave background, or CMB. The “cosmic” part means that that it comes from outer space. The “microwave” refers to the type of light waves. Microwaves have wavelengths longer than the light that human eyes can detect. Finally, “background” means the light originates not from stars or galaxies, but from all parts of the sky.

Scientists use a few different telescopes to observe the CMB. Some are telescopes sent out into space, like the Planck telescope placed in orbit around Earth some years ago. Others are ground-based telescopes.

No matter the type of telescopes being used, they need to be in regions that are exceptionally dry because water vapor in the atmosphere will absorb the CMB waves.

Therefore, the two main places hosting CMB-detectinga telescopes are the South Pole and the Atacama Desert in Chile. Sehgal, studies the CMB through the Atacama Cosmology Telescope, or ACT. Whereas space-based telescopes like Planck can observe more of the sky, ground-based telescopes like ACT can have larger dishes that provide better resolution.

With ACT, Sehgal cannot only get information about the CMB, but also about everything in between the CMB and Earth. Throughout the CMB’s nearly 14 billion year journey, it interacts with numerous galaxies along the way, which leaves imprints on the light waves that can be studied.

One of the ways that the interactions between the CMB and galaxies can be studied is through polarization. Light, like that emitted from the lamp on a desk, typically travels in all directions. However, that light can be manipulated to move in a specific direction, or polarized.

For example, the light from the sun is traveling in all directions, but when it is reflected off of the hood of a car on a sunny day, it has been polarized and is now moving in a specific direction. Sunglasses often contain polarizing filters that block the light coming from reflecting surfaces, like car hoods, reducing the glare that can often impede vision.

As the CMB travels through space to Earth, the galaxies that it interacts with cause the light to be polarized. This type of polarization is called lensing and it leaves a lasting imprint on the CMB light waves. Scientists have also predicted a second way that CMB light can be polarized–by the events thought to have occurred within the tiniest fraction of a second after the Big Bang.

Soon after the Big Bang, many scientists believe that the universe underwent an incredibly rapid expansion, called inflation, that lasted less than a second and if true, would have left a lasting imprint on the CMB in the form of a type of polarization dubbed primordial B-modes. This idea has yet to be confirmed, but explains many of the major questions scientists have about the universe. One of which is called the Horizon problem.

The Horizon problem describes the phenomenon wherein any given region of the CMB, even regions millions of light years apart, have remarkably similar properties, including temperature.

For this to occur, scientists have theorized that at some point all of these regions must have interacted within very close proximity to each other, taking on similar properties and separating later. Inflation would take this into account.

If the predicted primordial B-modes can be observed, it would confirm inflation, allowing scientists to better understand the formation of the universe within a fraction of a second after the Big Bang.

The largest impediment to this is that B-mode signals are small and incredibly hard to detect. Currently, a race is underway to be the first to detect primordial B-modes. With ACT, Sehgal and colleagues are measuring vast portions of the sky in the hopes of catching these elusive signals and confirming an important theory of the universe’s formation.