Cosmic eras

Like the geological eras of Earth’s past, the history of the Universe is marked by critical periods in which our cosmos changed dramatically. One of these is the Recombination epoch, when charged electrons and protons combined to form neutral atoms, and light could finally travel freely.This era was followed by the Dark Ages and then Reionisation, when the first stars and galaxies formed; the energetic light they emitted changed the state of intergalactic gas from neutral to charged again. Cosmic Noon marks the period when the pace of star formation peaked.

Over the last century, scientists have used theory and observations to piece together the Big Bang model. This model gives a physical explanation of how our cosmos came to be and allows us to trace its history.

The Universe emerged from a state of extremely high temperature and density about 13.7 billion years ago. Initially, the Universe consisted of a uniform bath of fundamental particles, like quarks, electrons, neutrinos, and photons (the particles of light). Quarks then teamed up in trios, forming protons or neutrons – the constituents of atomic nuclei as we know them today. This all happened within the first second after the beginning: the so-called Big Bang.

About three minutes after, protons and neutrons combined to form the nuclei of hydrogen and helium atoms. At that time, collisions between electrons and photons were so frequent that light could not go far before hitting a particle again and being either absorbed or deviated from its path. Because of these frequent interactions photons could not travel freely, and the Universe was opaque.

As it expanded, the Universe grew cooler and sparser. When the temperature dropped to about 3000°C, electrons and protons combined to form (mainly) hydrogen atoms. This is the first moment in the history of the cosmos when matter was in an electrically neutral state. Particle collisions became so sporadic that photons started to travel freely across space.

Soon after the recombination era, there were no individual sources of light, such as stars. Normal matter existed in the form of clouds of hydrogen and, in a smaller fraction, helium. Because of this, the period is often referred to as the dark ages. Yet, within this dark arena tumultuous changes were taking place around slightly denser regions. The gravitational force exerted by these wells of gas and dark matter was relentlessly pulling in nearby material, and these regions were growing bigger and denser.

After a few hundred million years, some gas agglomerates had grown large and dense enough to ignite nuclear fusion and give birth to the first stars. At that point, galaxies also started to form and assemble on large scales.

As these first stars came to life, they filled their surroundings with powerful ultraviolet light. Far-ultraviolet rays are so energetic that they can strip electrons away from hydrogen atoms when hitting them. The gas in between the stars and galaxies became a mixture of charged particles, a process called cosmic reionisation. It took place slowly over a long stretch of time between around 200 million and one billion years after the Big Bang.

At that time, however, matter had already been diffused over large distances by the continued expansion of space. The interactions of photons and electrons were much less frequent than before electron-proton recombination. Thus, the Universe remained overall transparent to light, as it is today.

While reionisation was taking place, large clumps of neutral hydrogen were still present in intergalactic space – like clouds scattered in a clear blue sky. Their presence absorbed the far-UV light coming to us from the very early galaxies that were forming during this era.

That's why, when a galaxy is very far away, and very young, there is a prominent bump in the light spectrum of that galaxy. The bump appears in near-infrared and corresponds to the wavelength beyond which the far UV-light is absorbed by the hydrogen clouds. The huge distance travelled by that galaxy’s light through the expanding Universe causes its wavelength to stretch and become longer (cosmological redshift), so the UV bump appears in near-infrared light.

Scientists identify this sharp rise at near-infrared wavelengths and use it to accurately determine a galaxy’s redshift, and from this its distance in light-travel time and the Universe’s age at the time the light left the galaxy.

After reionisation, the pace with which stars form and galaxies grow entered a higher gear, reaching its peak at around two to three billion years after the Big Bang. Over this relatively short period, galaxies formed about half of their current stellar mass. Scientists refer to this era as cosmic noon.

Despite the great success of the Big Bang model in explaining many aspects of our cosmos, fundamental questions remain unanswered. One of the most profound mysteries is the composition of the Universe.

Based on the way galaxies rotate and orbit one another, and how the Universe is expanding, astronomers believe that two unseen components dominate the content of our cosmos: dark matter and dark energy. Yet, to date, we have not been able to detect either of them directly, only inferring their presence from the effects they have on the Universe at large.

ESA's Euclid mission will chart the large-scale structure of the Universe across space and time by observing billions of galaxies. It will help us shed light on the nature of dark energy and dark matter.