The 1st light to flood the universe can help unravel the history of the cosmos. Here's how

The cosmos has been there for 13.8 billion years, and the Cosmic Microwave Background contains a history of those events.

Astronomers are using the first light to ever shine across the universe to comprehend the events that have created the cosmos, just like Charles Darwin once utilized the fossil record to explain the narrative of the development of life on Earth.

The residual radiation that makes up this first light, known as the "Cosmic Microwave Background (CMB)," is distributed very uniformly throughout the cosmos. The CMB has distinctive characteristics that may be utilized to ascertain the composition of the cosmos and carries the traces of the physical activities of the early universe.

The way cosmologists use this cosmic relic has changed through time, much like how the study of biological evolution has since Darwin, and next missions will put more emphasis on the CMB and what it may tell us about the development of the universe.

Astrophysicist Erminia Calabrese gave overviews of the state of the CMB science at the moment and where it is heading in the near future on Monday, July 2, at the National Astronomy Meeting 2023 (NAM 2023) hosted at Cardiff University in the U.K.

According to Calabrese, "this light has really been the driving force of modern cosmology because it has been there throughout the entire history of the cosmos." It was present at the beginning and experienced everything the cosmos did. It traversed the creation of the first stars, the emergence and evolution of the universe's large-scale structure.

It basically captured impressions from all of these physics along its trip towards us and carried them with it when it arrived at its current location.

What is the Cosmic Microwave Background, then?

The first thing you would notice is how black the cosmos is if you could travel back in cosmic time around 380,000 years to the time when the universe was filled with a dense boiling soup of electrons and protons.

This early period in the universe's 13.8 billion-year existence is considered to be a "cosmic dark age" because there were so many unbound electrons that photons, which are light particles, were constantly dispersed and couldn't move. The cosmos was basically opaque to light at this period.

Accordingly, Calabrese said, "What we are looking at is the very first light ever released in the universe, composed of photons that were released during the Big Bang." Any particle phenomena occurring in this extremely hot and dense phase of the cosmos was interacting with these photons because they were "trapped in interactions with everything else."

The photons were recording the physics of the early cosmos while they were confined, but they were unable to remain trapped and in an equilibrium with matter indefinitely.

The cosmos eventually expanded and cooled down as a result of the Big Bang, undergoing fast cosmic inflation, allowing electrons to combine with protons and create the first neutral atoms. Despite the fact that electrons and protons were not previously coupled, this is referred to as the recombination period.

As the cosmos continued to expand, the light that makes up the CMB was initially extremely hot and energetic, but as it cooled and lost energy, its frequency was decreased to the microwave area of the electromagnetic spectrum.

According to Calabrese, the CMB is now manifested as a radiation field with a temperature of 2.7 Kelvin (-455°F or -270.4°C).

How is the Cosmic Microwave Background used by scientists?

The CMB radiation comes to us uniformly from all directions because recombination took place simultaneously throughout the whole cosmos. This indicates that this cosmic fossil is isotropic, meaning that it appears the same everywhere in the sky.

One of the most important pieces of evidence that the universe formerly existed in a hot and dense state before undergoing a period of fast inflation, which we now refer to as the Big Bang, is this sameness, even at opposite ends of the cosmos in places not now in contact. However, it is in these regions where minute variations appear that researchers discover a helpful cosmic fossil record.

Small variations from this uniformity, known as anisotropies, exist within the CMB. The CMB includes knowledge about the development of the cosmos thanks to these anisotropies.

The microscopic density variations in the early cosmos that eventually led to the formation of galaxies and galaxy clusters are represented as small-scale anisotropies in the CMB. Despite the fact that they may be little, without these changes, the universe's current large-scale structure would not have developed.

The contents of the cosmos and the quantity of these elements over cosmic history are revealed through greater anisotropies. This includes not only the atom-based "everyday" matter that is visible to the naked eye and makes up the stars, planets, cosmic gas clouds, and even ourselves, but also the unseen dark matter and dark energy that are responsible for the universe's present, rapid expansion.

There are three specific approaches we use to research the CMB, according to Calabrese. "We can travel into space, and we've had three different generations of satellites that have been dedicated to measuring the CMB anisotropies," he said. "You can either stay on Earth and use stratospheric balloons to climb higher in the stratosphere, or you can stay on the ground and deal with the environment. Each of these approaches has advantages and disadvantages, and no single experiment can reveal everything.

Future CMB-studying missions that might address important issues like what dark matter is comprised of and how the universe's mass is distributed on a grand scale were mentioned in Calabrese's NAM 2023 presentation.

The Japanese Aerospace Exploration Agency (JAXA) mission known as the Space Lite (Light) satellite for the study of B-mode polarization and Inflation from cosmic background Radiation Detection (LiteBIRD) is one such mission that Calabrese mentions.

According to JAXA, LiteBIRD will reach remarkable sensitivity, enabling it to properly differentiate between the CMB and foreground radiation signals from sources like cosmic dust. It will monitor the whole sky for three years from orbit. That implies that LiteBIRD, which is scheduled to launch in 2028, may be able to fill in the cosmic evolution gaps left by the limitations of the Big Bang theory.

"We really don't have answers to the big key fundamental questions that we were aiming to answer with CMB temperature, and now we need to take the next step and continue exploring and exploiting everything that is in the CMB to be able to answer them," said Calabrese.