In geochemistry, geophysics and geonuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed.
 Primordial nuclides were present in the interstellar medium from which the solar system was formed, and were formed in the Big Bang, by nucleosynthesis in stars and supernovae followed by mass ejection, by cosmic ray spallation, and potentially from other processes.
 They are the stable nuclides plus the long-lived fraction of radionuclides surviving in the primordial solar nebula through planet accretion until the present.
 Only 286 such nuclides are known.
The particle horizon (also called the cosmological horizon, the comoving horizon (in Dodelson's text), or the cosmic light horizon) is the maximum distance from which particles could have traveled to the observer in the age of the universe.
 Much like the concept of a terrestrial horizon, it represents the boundary between the observable and the unobservable regions of the universe, so its distance at the present epoch defines the size of the observable universe.
 Due to the expansion of the universe it is not simply the age of the universe times the speed of light (approximately 13.8 billion light-years), but rather the speed of light times the conformal time.
 The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model.
The Big Bang theory is the prevailing cosmological model for the universe from the earliest known periods through its subsequent large-scale evolution.
 The model describes how the universe expanded from a very high density and high temperature state, and offers a comprehensive explanation for a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB), large scale structure and Hubble's law.
 If the known laws of physics are extrapolated to the highest density regime, the result is a singularity which is typically associated with the Big Bang.
 Physicists are undecided whether this means the universe began from a singularity, or that current knowledge is insufficient to describe the universe at that time.
 Detailed measurements of the expansion rate of the universe place the Big Bang at around 13.8 billion years ago, which is thus considered the age of the universe.
 After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms. Giant clouds of these primordial elements later coalesced through gravity in halos of dark matter, eventually forming the stars and galaxies visible today.
This more than 20-billion-year timeline of our universe shows the best estimates of major events from the universe's beginning to anticipated future events.
 Zero on the scale is the present day.
 A large step on the scale is one billion years; a small step, one hundred million years.
 The past is denoted by a minus sign: e.g., the oldest rock on Earth was formed about four billion years ago and this is marked at -4e+09 years, where 4e+09 represents 4 times 10 to the power of 9.
 The "Big Bang" event most likely happened 13.8 billion years ago; see age of the Universe.
UX Tauri, abbreviated as UX Tau, is a binary star system approximately 450 light-years away in the constellation of Taurus (the Bull).
 It is notable for the fact that, despite its recent (in stellar terms) creation, the Spitzer Space Telescope discovered that its protoplanetary disk contains a gap.
 The dust, which normally accumulates in an expanding ring starting right next to the star at such a young age, is either very thin or nonexistent at a range of 0.2 to 56 AU from the star.
 Typically, this means that the early ancestors of planets may be forming from the disk, though the star only ignited about 1 million years ago.
 In contrast, Earth was formed approximately 4.54 billion years ago, placing its formation about sixty million years after the Sun's ignition around 4.6 billion years ago.
Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O).
 Photosynthetic prokaryotic organisms that produced O as a waste product lived long before the first build-up of free oxygen in the atmosphere, perhaps as early as 3.5 billion years ago.
 The oxygen they produced would have been rapidly removed from the atmosphere by weathering of reducing minerals, most notably iron.
 This "mass rusting" led to the deposition of iron oxide on the ocean floor, forming banded iron formations.
 Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event.
 This mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen.
 At current rates of primary production, today's concentration of oxygen could be produced by photosynthetic organisms in 2,000 years.
 In the absence of plants, the rate of oxygen production by photosynthesis was slower in the Precambrian, and the concentrations of O attained were less than 10% of today's and probably fluctuated greatly; oxygen may even have disappeared from the atmosphere again around 1.9 billion years ago.
 These fluctuations in oxygen concentration had little direct effect on life, with mass extinctions not observed until the appearance of complex life around the start of the Cambrian period,   million years ago .
 The presence of O2 provided life with new opportunities.
 Aerobic metabolism is more efficient than anaerobic pathways, and the presence of oxygen undoubtedly created new possibilities for life to explore.
 Since the start of the Cambrian period, atmospheric oxygen concentrations have fluctuated between 15% and 35% of atmospheric volume.
 The maximum of 35% was reached towards the end of the Carboniferous period (about 300 million years ago), a peak which may have contributed to the large size of insects and amphibians at that time.
 Whilst human activities, such as the burning of fossil fuels, affect relative carbon dioxide concentrations, their effect on the much larger concentration of oxygen is less significant.
The Einstein–de Sitter universe is a model of the universe proposed by Albert Einstein and Willem de Sitter in 1932.
 On first learning of Edwin Hubble's discovery of a linear relation between the redshift of the galaxies and their distance, Einstein set the cosmological constant to zero in the Friedmann equations, resulting in a model of the expanding universe known as The Friedmann-Einstein universe.
 In 1932, Einstein and de Sitter proposed an even simpler cosmic model by assuming a vanishing spatial curvature as well as a vanishing cosmological constant.
 In modern parlance, the Einstein–de Sitter universe can be described as a cosmological model for a flat matter-only Friedmann–Lemaître–Robertson–Walker metric (FLRW) universe.
The Swazian is a poorly defined geological stage in South Africa extending from about four billion years ago to 3 billion years ago , encompassing some of the Hadean and much of the Archean on the Geologic time scale.
 Other scales  assign the Swazian to parts of the Paleoarchean and Mesoarchean, 3.5 to 2.8 billion years ago.
 The Swazian is not recognized by the International Commission on Stratigraphy nor by the Geological Society of America, the two most widely accepted developers of international geologic time scales.
z8_GND_5296 is a dwarf galaxy discovered in October 2013 which has the highest redshift that has been confirmed through the Lyman-alpha emission line of hydrogen, placing it among the oldest and most distant known galaxies at approximately 13.1 e9ly from Earth.
 It is "seen as it was at a time just 700 million years after the Big Bang [...] when the universe was only about 5 percent of its current age of 13.8 billion years".
 The galaxy is at a redshift of 7.51, and it is a neighbour to what was announced then as the second-most distant galaxy with a redshift of 7.2.
 The galaxy in its observable timeframe was producing stars at a phenomenal rate, equivalent in mass to about 330 Suns per year.
In physical cosmology, structure formation refers to the formation of galaxies, galaxy clusters and larger structures from small early density fluctuations.
 The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.8 billion years ago.
 However, looking in the sky today, we see structures on all scales, from stars and planets to galaxies and, on still larger scales still, galaxy clusters and sheet-like structures of galaxies separated by enormous voids containing few galaxies.
 Structure formation attempts to model how these structures formed by gravitational instability of small early density ripples.

Question: What cosmological model for the universe took place approximately 13.8 Billion years ago and is generally understood to be nucleosynthesis in stars followed by mass ejection?
Answer: Big Bang