When it comes to the history of our universe, it's incredibly how influential the earliest moments were in terms of creating the conditions that would allow us to survive billions of years from now. We can say that the earliest phase of any meaningful one actually occurs before the start of the hot big **. The cosmic inflation occurs and then ends, sowing the seeds of quantum fluctuations in the universe, and then producing a hot big **. The universe cools and expands from its hottest, densest phases, producing more matter than antimatter, then stable protons and neutrons, then atomic nuclei, and finally even neutral atoms, all in the background ocean of radiation and neutrinos.
You might think that once a neutral atom is formed, the next step is driven by gravity: star formation. However, compared to everything that came before them, the timescales required to form them are enormous. When only half a million years passed, the universe was dominated by matter, the radiant sea was cool enough that atoms could not be ionized, and gravity began to work in earnest. Even with these components, even the first stars in the universe would take 50 million to 100 million years to form. In all the time in between, the universe went through the darkest part of this era, known as the Cosmic Dark Ages. This is what it looked like back then.
A universe where electrons and protons are free and collide with photons, transitioning to a neutral universe that is transparent to photons as the universe expands and cools. Shown here is the ionized plasma before CMB emission (left) and then the transition to a neutral universe that is transparent to photons (right). This is a spectacular two-photon transition in the hydrogen atom that allows the universe to become neutral as we observe. **amanda yoho for starts with a bang
The formation of neutral atoms is not only important for setting up the building blocks of all complex chemical structures, which may be produced by a combination of molecules, ions, and any combination of atoms that are held together. This is also very important for "releasing photons or light particles left behind" by the thermal**. When a neutral atom is first formed, this marks the time when the photon stops scattering from the free electrons because the free electrons are only present when your atoms are ionized in the form of plasma. Once all the neutral atoms are formed, the radiation simply travels in a straight line;There's nothing to scatter, it's just moving at the speed of light.
In terms of the number of photons present, the number of such lights far exceeds the number of atoms in the universe. Now, the light seems to be coming evenly from all directions and from all locations. At the beginning of the Cosmic Dark Ages, the temperature of this photon bath was from 2970Starting at 8 k, at that moment it will take on a yellow-orange color. Some areas are slightly hotter than others, with temperatures as high as 29710 k, while the rest of the world is slightly lower at around 29706 k。These tiny differences may not seem like much, but it is the most important factor in how our universe evolves and grows from now on.
The most comprehensive view of the cosmic microwave background is the oldest observable light in the universe, which shows us a snapshot of the universe just 380,000 years after the onset of the thermomegalophobia. While the blue area appears colder than average and the red area appears hotter than average, the reality is that photons from all of these regions distribute the same amount of energy between them. Only the difference in the depth of the gravitational potential they are in leads to the observed temperature difference. **ESA Planck cooperation.
Why are these tiny temperature differences important?Because in every region of space, all of these photons or light particles actually have the same amount of intrinsic energy, and that energy is distributed in the same way across all the photons. Essentially, radiation actually carries the same amount of energy in hot areas as it does in cold regions, but the environment in which the radiation is located varies from location to location. Some regions have an average density that is exactly the same as the average density of the entire universe, but others have slightly more (or less) more (or less) matter than average.
Areas with insufficient density, which have less gravity due to the less material in them. When a photon leaves the region, it has a smaller gravitational force, which means it loses less energy due to the gravitational redshift, resulting in a photon temperature that appears hotter than average.
On the other hand, overdense areas contain more matter, and as a result, they have more gravitational pull to fight against. When photons crawl out, they lose more energy than average, so the photons that emerge from these regions become colder or less energetic overall.
Areas of space that are slightly denser than average will produce larger gravitational potential wells, meaning that the light produced from these areas will appear colder when it reaches our eyes. And vice versa, areas with low density will look like hot spots, while areas with full average density will have perfectly average temperatures. **e. siegel/beyond the galaxy
If your universe contains the following areas:
Average density, density greater than average, density below average, you might think that all that's left to do is to bring these overly dense areas together and attract more and more material, as gravity indicates, until we form stars. It's part of the story, but it turns out it's not the only factor. Photons are part of the universe, and there is much more to add to the story before they simply disappear into the cosmic background.
Gravity works in a way similar to your intuition in your mind: all masses are attracted to each other, and as long as you have the greatest mass, it will prioritize all the other masses around it. Even in an expanding universe, these over-dense regions attract mass from any nearby areas with lower density, especially from areas that are less dense, which at best hold their matter weakly.
The density fluctuations of the cosmic microwave background (CMB) provided the seeds for the formation of modern cosmic structures, including stars, galaxies, galaxy clusters, filaments, and large-scale cosmic voids. However, the CMB itself is invisible, which will take hundreds of thousands of years, until the universe forms neutral atoms from its ions and electrons, and stars will not form even longer: 50 to 100 million years. **e.m.Huff, SDSS-III Antarctic Telescope, Zosia Rostomian
What this tells us is that gravity is an out-of-control force in this sense. There will be "winners" and "losers" in this universe in terms of attracting mass to a region, and the region with the greatest lead is the region that ends up having the most matter. As more and more matter is attracted to a region, the more successful the gravitational pull is in bringing more additional matter into it.
However, that's not all. Although this is true, matter and gravity are not the only major beings at work in the universe at work at the moment. There is also radiation, which appears in the form of the remaining background of photons. While matter – including dark matter and atomic matter – attracts massive particles under gravitational pull, it attracts all forms of energy, including radiation, to the region with the highest density.
When this happens, we must remember that radiation, unlike matter, has a considerable inherent pressure. For example, our Sun is 300,000 times more massive than Earth, but less dense than our own planet, due to the enormous pressure exerted outward by photons inside the Sun. The same type of pressure that keeps stars like the Sun resisting gravitational collapse can also stop these collapsing gas clouds before any star can form, slowing down their growth. Even in a universe dominated by matter, the excess density of matter can only grow very slowly as long as radiation remains important.
Over time, gravitational interactions will turn a universe of essentially homogeneous density into a universe with a high concentration of matter and huge voids. As long as radiation remains important, even if the universe becomes matter-dominant, it will exert pressure outward, and the growth of material defects is very small. **volker springel/mpe
For millions of years, all of these types of cosmic structures have been severely limited in their growth rate, driven by gravity. Radiative pressure simply does not allow the density of matter to grow at a rate that exceeds a certain rate. Because millions of years need to pass and matter becomes denser and denser, even in the densest regions, another process begins to take place: something inherent in the most common type of atom in the universe: hydrogen. Ever since we formed a neutral atom with (mostly) protons and electrons, a new form of light has emerged: light produced by spin flipping transitions within hydrogen atoms.
During these dark ages, 92% of the atoms in the universe were pure hydrogen. The hydrogen atom is made up of a proton and an electron, and both protons and electrons have intrinsic spins: +1 2 or -1 2. There is a slight difference in total energy between systems with protons and electrons with the same spin (+1 2, +1 2 or -1 2, -1 2), making their energies slightly higher than those of those with opposite spins (+1 2, -1 2 or -1 2, +1 2). On a timescale of about 10 million years, their configurations with the same spin will spontaneously flip, and when this happens, a photon with a specific wavelength of 21 centimeters will be emitted.
When a hydrogen atom is formed, the probability of spin alignment and anti-alignment of electrons and protons is equal. If they are anti-aligned, no further transitions will occur, but if they are aligned, they can quantum tunnel into a low-energy state, emitting photons of a very specific wavelength (21 cm) on a very specific and fairly long time scale. The precision of this shift has been measured to more than one part per trillion and has not changed for decades known. It was the first light emitted in the universe after the formation of neutral atoms: even before the formation of the first stars. **tiltec/wikimedia commons
Although the radiation present in the universe is mainly driven by the large ** remaining photons, cosmic microwave background radiation can be observed today, but now another signal is added to this signal: this faint, tiny signal from a 21-centimeter emission line. If 92% of the atoms in the universe are these simple hydrogen atoms, and protons and electrons have a 50 50 chance of initially aligning or realigning spins when making these atoms, then this means that about 46% of the universe (by number) should have a 21 cm emission signal.
Each hydrogen atom that is spontaneously formed in a state of proton and electron arrangement emits light in this way, and each time a new hydrogen atom is produced, including from a previously neutral hydrogen atom, it is ionized again. While we haven't detected the 21 centimeter emission signal from the first atoms formed in the universe, it's a signal that we know how to do and a radio telescope advanced enough should be able to detect if there is enough time to observe it.
On the left, infrared light from the end of the Cosmic Dark Ages is shown, minus the (foreground) stars. 21 centimeters of astronomy will be able to detect epochs in the history of the universe, even older than the formation of the first stars. **nasa/jpl-caltech/gsfc
However, there are other processes happening at the same time, and these will be even more important for the story of the universe that is just beginning to unfold. It is true that, from an anthropocentric perspective, the normal matter in our universe – the matter that makes up all the stars, the planets, and all the chemical reactions that will take place – is arguably the most important component of our reality, but there is more to the universe as we know it than just this normal matter. The first step in the creation of these entities will be the formation of the first stars, but we still have a long way to go from the moment we form neutral atoms.
To achieve this, we need help: the help of a photon redshift, the gravitational pull of matter into an overly dense clump, and the help of time to allow these two effects to accumulate and have a substantial impact. In the first 3 million years after the formation of neutral atoms, the temperature cooled from 3000 K to 800 K, bringing radiation from yellow-orange to orange to red, after which it finally cooled enough to become invisible to the human eye. The reduced radiation pressure allows the material to grow in clumps, but only about four times as much as when the CMB is emitted.
Over time, the overdense areas continue to grow, but their growth is restricted due to the initial small size of the overdense areas and the presence of radiation that still has energy, which prevents the structure from growing faster. **aaron smith/tacc/ut-austin
When the universe is between 15 million and 20 million years old, it has cooled to the temperature we experience on Earth: the empty space is about room temperature. The density of the mass of matter, which is only slightly higher than average (perhaps 1/30,000th) of amplitude, has increased significantly, and is now about 10-15th denser than the average of the universe. The densest clumps have begun to grow at a faster rate and may be 30 to 90 fractions higher than the average density: about 02% or 03%。
It is important to track the density of these over-dense areas, as matter can reach a critical density where the simplest explanation of how these clumps grow no longer applies. Until then, in the so-called linear state of structure formation, the overdense regions grow as if they follow a simple law: when the temperature of the universe reaches half, the mass of matter grows to twice the original excess density. Once you pass a certain critical threshold, the clumps start to grow faster in what we call non-linear.
This critical shift occurs once an area is about 68% denser than average. After that, an out-of-control collapse became inevitable.
The first stars and galaxies in the universe will be surrounded by neutral atoms of (mostly) hydrogen, which absorbs starlight. If there are no metals to cool them or radiate energy, only massive clumps in the heaviest regions can form stars. The first stars were likely to form between 50 million and 100 million years ago, based on our best theories of structure formation. **nicole rager fuller / nsf
So when did this start happening?At some point, when the universe reached an age of about 50 million years (perhaps a little longer), the densest clumps had now transitioned to this postcritical stage and began to shrink and attract more matter at a breakneck rate. This will quickly lead to the formation of the first stars in the absolute densest region of space, but much of the rest of the universe will continue to grow slowly, taking more time for the clumps of matter to grow to the density where stars could form.
On a cosmic scale, the first wave of star formation takes about 200 million to 200 million to 2It won't start until 500 million years. However, in the densest regions, the matter in it collapses into large densities in as little as 50 million to 100 million years. At some point, due to cooling, the first star – defined by the first hydrogen-helium chain reaction of proton-proton fusion – will take place. In a universe full of dark matter and normal matter, the universe would have to cool down to about 100k before it could actually form its first true star.
Schematic diagram of the history of the universe, highlighting reionization. Before stars or galaxies formed, the universe was filled with neutral atoms that shaded the light. Although most of the universe until 5It was reionized after 500 million years, but the first time occurred in about 2In 500 million years, a few lucky stars may form 50 to 100 million years after the big **. **s. g. djorgovski et al.， caltech;Caltech Digital ** Center.
Before the formation of the first stars, conditions were very different from what they are today. Space is not transparent to light, but is filled with neutral, light-blocking atoms. The residual glow of the large **, today's CMB, was about 30 to 50 times warmer than the current temperature. And these conditions, in the expanding universe, occur too long ago that even with powerful infrared capabilities, even JWST cannot observe them. Although many hope that JWST will allow us to see directly the first waves of star formation, the first stars of all should remain obscure, even to its never-before-seen powerful eye.
It would take less than half a million years to take all the normal matter in the universe and complete the formation of neutral atoms, but it would take 100 to 200 times longer for neutral matter to collapse enough to form the first star in the universe. Until then, the only light that will be visible will be the remaining light of the Great **, whose energy drops low enough that the human eye will not be able to see it in 3 million years. For the next 47 million to 97 million years, the entire universe will be truly dark.
But once the first star is lit, "Let the Light" will eventually become a part of our cosmic history again.