The Beginnings Of The Galaxy

## The Big Bang

In the beginning of our story, everything was dark. There was nothing in existence. This nothingness was called the void, and it contained no energy or matter. But then, a great explosion happened. This explosion was so powerful that it created all that we know and everything in existence. This is known as the Big Bang. From the Big Bang, the universe expanded outwards, becoming larger and brighter. Over time, the universe continued to expand and cool down, eventually forming stars and galaxies. Now, we come to the present day. The universe is still expanding, with new planets and stars being formed every second.

This is when life first began to exist in the universe. It started as simple single-called organisms, but over time these organisms became more complex and formed the basis for all life as we know it today. This process of complexification is called evolution and continues to occur in the universe today. It is believed that humans are the product of this evolutionary process, and that they continue to evolve and change into new and more complex forms. The story of the universe is the story of the evolution of life, and it is constantly changing and developing in infinite ways.

## The Start Of Everything

The first picosecond (10−12) of cosmic time. It includes the Planck epoch, during which currently established laws of physics may not have applied; the emergence in stages of the four known fundamental interactions or forces—first gravitation, and later the electromagnetic, weak and strong interactions; and the expansion of space itself and supercooling of the still immensely hot universe due to cosmic inflation.

Tiny ripples in the universe at this stage are believed to be the basis of large-scale structures that formed much later. Different stages of the very early universe are understood to different extents. The earlier parts are beyond the grasp of practical experiments in particle physics but can be explored through the extrapolation of known physical laws to extreme high temperatures.

This period lasted around 370,000 years. Initially, various kinds of subatomic particles are formed in stages. These particles include almost equal amounts of matter and antimatter, so most of it quickly annihilates, leaving a small excess of matter in the universe.

At about one second, neutrinos decouple; these neutrinos form the cosmic neutrino background (CνB). If primordial black holes exist, they are also formed at about one second of cosmic time. Composite subatomic particles emerge—including protons and neutrons—and from about 2 minutes, conditions are suitable for nucleosynthesis: around 25% of the protons and all the neutrons fuse into heavier elements, initially deuterium which itself quickly fuses into mainly helium-4.

By 20 minutes, the universe is no longer hot enough for nuclear fusion, but far too hot for neutral atoms to exist or photons to travel far. It is therefore an opaque plasma.

The recombination epoch begins at around 18,000 years, as electrons are combining with helium nuclei to form He+

. At around 47,000 years, as the universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule. (Much later, hydrogen and helium hydride react to form molecular hydrogen (H2) the fuel needed for the first stars.) At about 370,000 years, neutral hydrogen atoms finish forming ("recombination"), and as a result the universe also became transparent for the first time. The newly formed atoms—mainly hydrogen and helium with traces of lithium—quickly reach their lowest energy state (ground state) by releasing photons ("photon decoupling"), and these photons can still be detected today as the cosmic microwave background (CMB). This is the oldest direct observation we currently have of the universe.

This period measures from 370,000 years until about 1 billion years. After recombination and decoupling, the universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and galaxies, so there were no new sources of light. The only photons (electromagnetic radiation, or "light") in the universe were those released during decoupling (visible today as the cosmic microwave background) and 21 cm radio emissions occasionally emitted by hydrogen atoms. The decoupled photons would have filled the universe with a brilliant pale orange glow at first, gradually redshifting to non-visible wavelengths after about 3 million years, leaving it without visible light. This period is known as the cosmic Dark Ages.

At some point around 200 to 500 million years, the earliest generations of stars and galaxies form (exact timings are still being researched), and early large structures gradually emerge, drawn to the foam-like dark matter filaments which have already begun to draw together throughout the universe. The earliest generations of stars have not yet been observed astronomically. They may have been huge (100–300 solar masses) and non-metallic, with very short lifetimes compared to most stars we see today, so they commonly finish burning their hydrogen fuel and explode as highly energetic pair-instability supernovae after mere millions of years. Other theories suggest that they may have included small stars, some perhaps still burning today. In either case, these early generations of supernovae created most of the everyday elements we see around us today, and seeded the universe with them.

Galaxy clusters and superclusters emerge over time. At some point, high-energy photons from the earliest stars, dwarf galaxies and perhaps quasars leads to a period of reionization that commences gradually between about 250–500 million years and finishes by about 1 billion years (exact timings still being researched). The Dark Ages only fully came to an end at about 1 billion years as the universe gradually transitioned into the universe we see around us today, but denser, hotter, more intense in star formation, and more rich in smaller (particularly unbarred) spiral and irregular galaxies, as opposed to giant elliptical galaxies.

While early stars have not been observed, some galaxies have been observed from about 400 million years cosmic time (GN-z11 at redshift z≈11.1, just after the start of reionization); these are currently the early observations of stars and galaxies. The James Webb Space Telescope, launched in 2021, is intended to push this back to z≈20 (180 million years cosmic time), enough to see the first galaxies (≈270 Myr) and early stars (≈100 to 180 Myr).