Just as Charles Darwin once used the fossil record to tell the story of the evolution of life on Earth, astronomers are using the first light to shine in the universe to understand the events that shaped the cosmos.
This early light is called the “Cosmic Microwave Background (CMB),” leftover radiation that spreads almost uniformly across the universe. The CMB carries the signatures of the physical processes of the early universe and has unique characteristics that can be used to determine the composition of the universe.
Just as the study of biological evolution has evolved since Darwin, the ways cosmologists use this cosmic fossil have changed, and future missions are set to increase attention to the CMB and what it can teach us about how l evolved universe.
At the National Astronomy Meeting 2023 (NAM 2023) held at Cardiff University in the UK on Monday, July 2, astrophysicist Erminia Calabrese gave an insight into both where CMB science is currently and where it is headed in the near future .
“The reason this light has really been the driving force behind modern cosmology is that it’s been there throughout cosmic history,” Calabrese said. “He was there from the beginning, he went through everything the universe has experienced. He traveled through the formation of the first stars, the formation and evolution of the large-scale structure of the universe.
“On this journey to us, he basically captured the fingerprints of all this physics and carries them with him today.”
Let there be light: what is the cosmic microwave background?
If you could go back some 380,000 years in cosmic history to the point where the universe was filled with a thick, hot soup of electrons and protons, the first thing you’d notice is how dark the cosmos is.
The reason this first epoch in the universe’s 13.8 billion year history is literally a cosmic dark age is because the abundance of free electrons meant that photons, particles of light, were scattered infinitely, preventing them to travel. At the time, the universe was essentially opaque to light.
“So what we’re looking at is the very first light ever emitted in the universe, made up of photons emitted during the Big Bang,” Calabrese explained. “The photons were caught in interactions with everything else, meaning that whatever particle phenomena that were happening in this very hot and dense phase of the universe were interacting with these photons.”
This means that while trapped, the photons were creating a record of physics in the early universe, but they couldn’t stay trapped and in equilibrium with matter forever.
Eventually, undergoing rapid cosmic inflation as a result of the Big Bang, the universe expanded and cooled enough for electrons to bond with protons and form the first neutral atoms. This is known as the recombination period, even though electrons and protons had not previously been connected.
A diagram showing the evolution of the universe over 13.8 billion years with the era of recombination highlighted (Image credit: NASA) Initially, the light comprising the CMB was incredibly hot and energetic, but as the universe continued to expand it cooled and lost energy, which saw the frequency of this radiation reduced to the microwave region of the electromagnetic spectrum.
Calabrese explains that the CMB currently takes the form of a radiation field with a temperature of 2.7 Kelvin (-455 degrees Fahrenheit or -270.4 degrees Celsius).
How do scientists use the cosmic microwave background?
Since recombination occurred simultaneously throughout the universe, CMB radiation comes to us from all directions evenly. This means that this cosmic fossil looks the same in all areas of the sky, which scientists describe as isotropic.
This uniformity, even on opposite sides of the universe in currently non-contacting areas, is one of the key pieces of evidence that the universe once existed in a hot, dense state and then underwent a period of rapid inflation, which we now call the Big Bang. But it is in areas where small differences emerge that scientists find a useful cosmic fossil record.
Within the CMB there are small deviations from this uniformity called anisotropies. It is through these anisotropies that the CMB contains information about the evolution of the universe.
The small-scale anisotropies in the CMB represent tiny density fluctuations in the early universe that ultimately led to the creation of galaxies and galaxy clusters. Though they may be tiny, without these variations, the large-scale structure we see in the universe today could not have taken shape.
It is the larger anisotropies that reveal the contents of the universe and the abundance of these elements throughout cosmic history. This includes not only visible “everyday” matter composed of atoms and which make up stars, planets, cosmic gas clouds and us, but also invisible dark matter and dark energy, the forces that are driving the current accelerating expansion of the universe. ‘universe.
“In particular, there are three methods we work with to study the CMB: we can go into space and we have had three different generations of satellites dedicated to measuring the anisotropies of the CMB,” Calabrese explained. “You can stay on Earth but try to get higher into the atmosphere in stratospheric balloons, or you can just stay on the ground and then deal with the atmosphere. All of these methods have pros and cons; no single experiment can give you access to everything.”
In Calabrese’s NAM 2023 speech, the researcher highlighted the need for future CMB study missions that could answer fundamental questions such as what dark matter is made of and what is the large-scale distribution of mass in the universe.
An illustration shows CMB observation missions LiteBIRD orbiting the Earth as it prepares to observe the CMB. (Image credit: ISAS/JAXA) One such mission Calabrese mentions is the Japanese Aerospace Exploration Agency (JAXA) mission known as the Space Lite (Light) satellite to study B-mode polarization and inflation from cosmic background radiation detection (LiteBIRD).
LiteBIRD will observe the entire sky for three years from orbit, and JAXA says it will achieve unprecedented sensitivity, allowing it to accurately distinguish between the CMB and foreground radiation signals from sources such as cosmic dust. This means that LiteBIRD, which will be launched in 2028, could help fill in the gaps in cosmic evolution that current Big Bang models cannot explain.
“We really don’t have answers to the big fundamental questions that we aimed to answer with CMB temperature, and now we need to take the next step and continue to explore and leverage everything in the CMB to be able to answer them,” said Calabrese .
An illustration shows CMB observation missions LiteBIRD orbiting the Earth as it prepares to observe the CMB. (Image credit: ISAS/JAXA) One such mission Calabrese mentions is the Japanese Aerospace Exploration Agency (JAXA) mission known as the Space Lite (Light) satellite to study B-mode polarization and inflation from cosmic background radiation detection (LiteBIRD).
LiteBIRD will observe the entire sky for three years from orbit, and JAXA says it will achieve unprecedented sensitivity, allowing it to accurately distinguish between the CMB and foreground radiation signals from sources such as cosmic dust. This means that LiteBIRD, which will be launched in 2028, could help fill in the gaps in cosmic evolution that current Big Bang models cannot explain.
“We really don’t have answers to the big fundamental questions that we aimed to answer with CMB temperature, and now we need to take the next step and continue to explore and leverage everything in the CMB to be able to answer them,” said Calabrese .
