Wednesday, May 26, 2021

Quantum Eras of the Very Early Universe





HERE YOU WILL DISCOVER THE MANY QUANTUM ERAS OF THE VERY EARLY UNIVERSE. HOWEVER, AS AN INTRODUCTION, I WOULD BE REMISS IF I DO NOT TO MENTION HERE THE FUNDAMENTAL "QUANTUM CONSTRUCTS" OF PROCESS PHILOSOPHY BY WHITEHEAD. YOU MAY SKIP THE TWO VIDEOS PROVIDED TO GET TO THE BULK OF THE MATERIAL BELOW BUT DO NOT FORGET THERE IS A LARGER METAPHYSICAL INTEGRAL OF THE COSMIC UNIVERSE IN PLAY HERE THAN MERELY THE QUANTUM PHYSICS ASPECT OF IT - AS MARVELOUSLY WONDERFUL AS I FIND QP TO BE. HENCE THE SHORT  (AND LONG) INTRODUCTORY PROCESS VIDS HERE. - RE SLATER

 


The Process Metaphysics of Alfred North Whitehead (*1861, †1947)
Alfred North Whitehead Project

A short introduction to the process philosophy & process theology of Alfred North Whitehead (*1861, †1947), containing several photos and 4 speakers, describing some core hypotheses of Whitehead's metaphysics. The speakers are: John B. Cobb, David Ray Griffin, Charles Hartshorne and Rupert Sheldrake.



TheoCon: Process Theology
Jun 24, 2020













Lectures in The Big Bang


















QUARK ERA


















Prof. Dale E. Gary | NJIT
Cosmology and the Beginning of Time

The Instant of Creation

Given Hubble's Law, and the fact that the universe is expanding, we can imagine running the clock backwards, allowing space to shrink, until all of the galaxies are on top of one another. If we did that, we would find that the universe would heat up until stars and galaxies would be vaporized into their constituent atoms, which would all collect into a single point of unimaginably hot, dense matter and energy. From this simple consideration, we imagine that the universe began in such a hot, dense fireball, which we call the Big Bang. Did the universe really begin this way? We can never really know for sure, but we can predict some things we should see today, if the Big Bang actually happened. Astronomers have found that a lot of the predictions do hold up to experimental tests, so the theory is widely accepted now, but there are a lot of unanswered questions that we are still searching for the answers to.

The amazing fact is that we can trace the Big Bang back to its earliest moments, at least as far back as 10-10 s, and possibly as far back as 10-43 s! This is an incredibly short time, and we can truthfully say that we can trace the evolution of the universe back to the first instant of creation. In so doing, we are probing not just the very earliest universe, but also the highest energy particle physics, so that particle physicists and astronomers are working on two aspects of the same puzzle.

Assuming that the Big Bang actually happened, what would the early moments of the universe be like? The figure below, from the text, shows an overview of all of time and space, which you can refer back to as we discuss the different eras of the past.

Eras of the Big Bang

The eras of the universe, from the time of the Big Bang, are listed below. We will discuss each in turn.

  • Planck Era (All four known forces are unified.)
  • GUT (Grand Unified Theory) Era (Gravity "freezes out" and becomes distinct.)
  • Electroweak Era (The nuclear strong force "freezes out" and becomes distinct).
  • Particle Era (particles begin to form)
  • Era of Nucleosynthesis (nuclear fusion creates Helium, and tiny amount of heavier elements)
  • Era of Nuclei (electrons are not yet bound to nuclei)
  • Era of Atoms (electrons recombine to form neutral atoms, and the first stars are born)
  • Era of Galaxies (Galaxies begin to form, leading up to the present)

The earliest eras were very short lasting, and very high energy. The first few eras are when the laws of physics were considerably different than they are know, but we can still predict some of the behavior.

Let's look at each era in more detail.

Planck Era






The Planck Era is prior to 10-43 s after the Big Bang, when we believe that the four basic forces of nature, 1) gravity, 2) nuclear strong force, 3) nuclear weak force, and 4) electromagnetic force were combined into a single "super" force. The idea is somewhat like the different phases of water (ice, liquid, and vapor), which are all aspects of the same thing. You can imagine that at certain pressure and temperature there might be conditions in which these three phases of water become a single phase, no longer distinct. Physicists believe that we will eventually find a theory that succeeds in combining all four of these fundamental forces, but at present there is no such theory. (We have names for such a theory, however: supersymmetry, superstrings, or supergravity.) So we really do not know what the universe was like in the Planck Era. Some superstring theories call for spacetime to have 11 dimensions during this time.

GUT Era


The GUT Era is when three of the four fundamental forces are combined, but gravity has become distinct. There are a class of theories called Grand Unified Theories (GUTs) that attempt to describe all forces except gravity in a single framework. The leading type are so-called string theories, and some are partially successful, but there are further details to be worked out. Theorists would say that in the GUT Era the gravity force "froze out" of the universe. The GUT Era lasted from 10-43 s to 10-38 s. Near the end of this era, grand unified theories predict that the universe cooled to the point that the nuclear strong force began to freeze out, leaving three fundamental forces: gravity, the strong force, and the still combined electroweak force. This "phase transition" released a huge amount of energy, causing space to undergo a rapid inflation. In a mere 10-36 s, pieces of our universe the size of an atomic nucleus might have grown to the size of our solar system. We will later discuss observations of the universe that seem to require such extreme inflation. Note that this inflation is very very large compared to the speed of light, but again, space itself is what is expanding, so it does not have to obey the speed limit of the speed of light.

Electroweak Era


During this era, only the electromagnetic and nuclear weak forces are still combined. The temperature of the universe at this stage is more than 1015 K, and there are no ordinary particles yet, just photons and pure energy. We do have a complete theory that can be used to understand the universe at the end of this era. By the time of 10-10 s, the temperature cools below 1015 K, and finally, the last of the fundamental forces, electromagnetic and nuclear weak forces, become distinct. We have also done particle physics experiments at energies corresponding to a temperature of 1015 K, so we can probe the Big Bang conditions experimentally from 10-10 s onward.

Particle Era


When the four fundamental forces were finally distinct, ordinary particles could start to form. However, both matter and anti-matter were formed in almost equal numbers, created out of the energetic photons present at that time. Once both types of matter were formed, a particle would not go very far before it met up with its anti-particle and annihilated to become pure energy again. During this era, particles continually were created and destroyed until, by 0.001 s (one millisecond), the universe had expanded and cooled far enough (to 1012 K) that creation and destruction of this kind ended. For some reason, the universe created slightly more matter particles than anti-matter particles. If the numbers had been exactly the same, the particles would eventually annihilate entirely and there would be only photons in the universe. This slight asymmetry for matter (1 billion and 1 protons for each 1 billion anti-protons) left us with all of the baryonic matter that we find today.

Era of Nucleosynthesis


When the universe was only 1 millisecond old, nuclei were hot enough and dense enough to fuse to create heavier elements, but it was so dense that the nuclei broke apart again as soon as they formed. This fusion and breakup continued until about 3 minutes after the Big Bang, when the universe cooled enough (109 K) that fusion ended. At this point, 75% of baryonic matter was in the form of hydrogen, 25% in the form of helium, and trace amounts were in the form of other atoms, mostly lithium. One of the great successes of the Big Bang theory is that it predicts just the right amount of these different forms of matter. At the end of the Era of Nucleosynthesis, the universe contained the "primordial" mix of hydrogen, helium, and lithium that went into making the first stars. All heavier elements have been created by fusion inside of stars and during supernova explosions.

Era of Nuclei


During the next 500,000 years, the universe was too hot to form neutral atoms, and all of the particles were in the form of atomic nuclei (hydrogen, helium and a few lithium nuclei) and free electrons. As long as the universe was made up of these fully ionized particles, it was a largely featureless ball of hot plasma that could not condense to form galaxies or stars. During this time, the particles and photons (light) were locked into an equilibrium in which the photons could not escape. Finally, after 500,000 years, the universe cooled to 3000 K, and hydrogen and helium nuclei began to capture the free electrons. At this stage, photons could not react with the electrons except in narrow energy ranges, so most of the gas became transparent and the photons were free at last to stream out of the plasma and cross the universe.

It is these photons that we see today as the cosmic microwave background, which we will discuss shortly. When we look out into the universe, we can never see back in time beyond 500,000 years, which is the time of last scattering of photons. Earlier than this, we could only see the hot surface of the universe.

Era of Atoms


After the photons decoupled from the matter, and the nuclei started combining with the electrons, we reach the era of atoms. The initially hot atoms slowly assembled and cooled into protogalactic clouds. The first galaxies formed by about 1 billion years, which marks the end of the Era of Atoms and the beginning of the Era of Galaxies.

Era of Galaxies


Evidence for the Big Bang

Today we see several lines of evidence that the Big Bang really happened. One of the earliest discoveries was made right here at Bell Labs in New Jersey by Penzias and Wilson, who received the Nobel Prize for their work. Using a radio telescope in 1963 to track down some unwanted noise in their receiving system, they found that when looking at the blank sky, no matter in what direction, they were receiving radiation with a temperature of 3 K. After discussions with some astronomers at Princeton University, they realized that they were seeing the 3000 K photons from the end of the era of nuclei, which has cooled since that time 500,000 years after the Big Bang, to a cool 3 K. This is known as the Cosmic Microwave Background. Since then, the COBE (Cosmic Background Explorer) satellite has measured this background radiation and found it to precisely fit a perfect blackbody spectrum at a temperature of 2.73 K. This perfect blackbody radiation coming from everywhere is strong evidence that we understand the universe at least back to 500,000 years before the Big Bang.

To probe even further back, to only 1 millisecond after the Big Bang, we can look at the proportion of elements created in the Big Bang. In order to know how much helium the universe should have made, we need to know the precise temperature of the Big Bang. Luckily, we can get that directly from the cosmic microwave background temperature. Using this precisely known temperature, 2.73 K, we can deduce that the universe should have made 24% helium, exactly what we observe. We can also predict the ratios of other isotopes, which again agree to a remarkable degree.

Lumpiness of the Universe

When we look at the universe today, we are immediately struck by the fact that all of the matter is clumped into galaxies, with almost no matter between the galaxies. We learned last time that there is also structure on vastly larger scales in the form of knots of superclusters and huge voids of empty space between them. We can now study this lumpiness of the universe to find out more details about the earliest moments of the Big Bang. The key is to look at the lumpiness, or anisotropy, of the Big Bang radiation. If the universe were too smooth, there would be few or no galaxies. Rather, the matter would be spread smoothly across the universe. However, if the universe were too lumpy early on, all the matter would be concentrated in small clumps, perhaps in the form of black holes of immense mass.

There is now a spacecraft called MAP (Microwave Anisotropy Probe) that is studying the lumpiness of the cosmic microwave background, but many groundbased experiments and some spacecraft have also looked for fluctuations. It turns out that the fluctuations are very tiny -- for a long time they seemed too tiny. During the expansion of the universe, parts of the universe were no longer in contact and should have cooled separately. Yet we find that they are precisely the same temperature.

Also, in the early Big Bang there should have been quantum fluctuations that, after expanding for 14 billion years, should still be on much smaller scales than galaxies. Why do we see such large-scale structure in the universe, yet relatively smooth structure on smaller scales?

Both of these difficulties can be explained by the inflation theory. We already mentioned that inflation would have been driven by the decoupling of the strong nuclear force from the electroweak force. During this time, locally adjacent parts of the universe would have expanded far faster than the speed of light and ended up at opposite ends of the universe. Yet these far distant parts of the universe could have the same initial temperature. Also, the tiny quantum fluctuations would have grown in scale to larger than the solar system in a tiny fraction of a second, and hence the fluctuations would exist on the large scale we see today.

Finally, another feature of inflation is to moderate the density of the universe to make space appear very close to flat. Imagine a balloon, whose surface is curved, then blow up the balloon to an immense size. As the size increases, the surface gets locally flatter and flatter. This is related to the density of the universe. Recall that we discussed last time the critical density of the universe, and said that the universe appears to be almost flat. Inflation can help that by taking an initially open or closed space and making it so large that it appears nearly flat. What we still have to understand is where is all the matter needed to make the universe flat. We said that there is far too little ordinary matter (maybe only 1 to 10% of the critical density), while there is also dark matter to help out, but matter and dark matter together only accounts for about 30% of that needed to flatten or close the universe. Current theories suggest that perhaps a new form of energy, dark energy, can make up the difference, but it has not been shown yet.

Did the Big Bang Really Happen?

Most scientists today would agree that the Big Bang is a successful theory, for which there are at least two bits of very clear observational evidence: 1) the cosmic microwave background radiation and 2) the relative amounts of hydrogen, helium, and other elements in the universe. However, from the above arguments you can see that several aspects of the Big Bang and related issues are not understood. From time to time you will see newspaper headlines that claim that the Big Bang is wrong, but those claims invariably turn out to be arguments over details, not fundamental disagreements in the theory as a whole. One observational fact stands out in favor of something like the Big Bang -- the sky is dark at night! If we imagine that the universe is infinite in all directions, then obviously there must be an infinite number of galaxies. If so, no matter which direction we look in the night sky, our line of sight must intersect a star in a galaxy somewhere. It is like being in a dense forest, in which no matter where you look you can only see tree-trunks, never the open sky. This argument is now called Olber's Paradox, after a German scientist in the 1800's. The universe is not like that -- the sky is dark at night -- so the observable universe cannot be infinite. The text casts this as showing that the universe must have a distinct beginning, since we can look back only to the cosmic horizon (14 billion years ago), but there is another possible horizon -- the one set by the expansion of the universe. If the universe is expanding such that the outer edges are moving away at greater than the speed of light, then we can only see a limited part of the universe up to a recession speed of the speed of light. So the universe may extend beyond this light-speed horizon.

In fact, if the expansion were to slow down, as we expect due to gravity slowing the expansion, then the light-speed horizon would move outward with time, and new galaxies would appear inside our observable universe. However, recall that we mentioned last time that the universe may be increasing its rate of expansion through the action of some kind of anti-gravity producing dark energy. If that is the case, then galaxies that we can see now, at the edge of the universe, would speed up and go outside the speed-limit horizon.

So the universe is very strange, and it seems to get stranger with each new discovery. But astronomers will continue to devise new observations to explore the universe, and new theories to explain it, and new particle experiments to verify the theories. But reflect for a moment on the following question from the first lecture.


Time out to think

Some people think that our tiny physical size in the vast universe makes us insignificant. Others think that our ability to learn about the wonders of the universe gives us as humans significance despite our small size. What do you think?

When we look at how much we know (or think we know) about the universe -- things that seemed completely unknowable only a few decades ago -- we have to marvel at the power of our intelligence to understand the universe in which we live. Perhaps you will help humanity solve some of the mysteries.


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THE VERY EARLY UNIVERSE

The Early Universe
  • Planck Era
  • GUT Era
  • Electroweak Era
  • Particle Era
  • Nucleo-Synthesis Era
  • Nuclei Era
  • Radiation vs Matter

More Subjects

Overview

The following is the current "theory" of the very early universe. Inflation, while not conclusively confirmed, is generally believed to be essentially correct in its overall implications if not in exact detail. Cosmologists believe that Inflation may be tweaked in the future but the essentials of it will survive. Several signs of Inflation have been observed, but not enough to "confirm" it. Below is a illustration that breaks down the early universe by "Era" of significant events.

Major Events Of The Early Universe


The Planck Era (Big Bang To 10^-43 Seconds)

The time from the exact moment of the Big Bang until 10^-38 of a second later is referred to as the Planck Era. While we have no way of knowing what this era was like from the equations of physics (as they break down in this era), it is "assumed" to be as follows. The universe was a tiny hot gaseous soup (a plasma) consisting of packets of "primal" particles at extremely high energies. The universe was smaller than the size of a proton. During this phase physicists believe matter and energy were not separated as they are currently. The primal particles were packets of radiation unlike anything we know today. Also, the four primary forces of the universe as we know them today were believed to be one united force. The temperature of the universe was 1 x 10^32 degrees Celsius. This hot thick soup was intense and everywhere. It also began to instantaneously expand and cool extremely fast.

The GUT Era (10^-43 To 10^-38 Seconds)


The Grand Unified Theory (GUT) is a model in particle physics in which at very high energy and temperature, the three fundamental forces (the Strong Nuclear Force, the Weak Nuclear Force, and Electro-magnetism) of the Standard Model are merged into one single force. With the exception of gravity, physics in this era are described by GUT Theory. The GUT "Era" begins at 10^-43 when gravity separates from the other three unified forces, which do remain unified. The GUT Era ends when Inflation begins at 10^-38 seconds.

Electroweak Era (10^-38 To 10^-10 Seconds)

The Electroweak Era began at 10^–38 seconds after the Big Bang, when the temperature of the universe cooled enough to separate the Strong Force from the Electroweak Force (the name for the two unified forces of Electro-magnetism and the Weak Nuclear Force). The Electroweak Era contains two sub-eras - Inflation and Reheating.


Inflation (10^-38 To 10^-35 Seconds)

Alan Guth, now a Professor at MIT, proposed an Inflation addition to the Big Bang Standard Model in 1981. As shown in the chart at the left produced by Guth himself, the universe went through an expansionary phase that was faster than the speed of light. (Space-time is not limited by the speed of light, only objects "within" space-time are.) In this brief interval of Inflation, the "observable" universe expanded by a factor of about 10^70 (1 followed by 70 zeros) from being unimaginably smaller than a subatomic particle to about the size of a grapefruit. That is the equivalent of going from about the size of a grape to the current size of the observable universe in the blink of an eye - awesome!

Note that Inflationary theory does not say anything about the "whole" universe, only the observable universe. The Inflationary model does not make any statement about the whole universe which could in fact be infinite. See paper on Inflation by Alan Guth. Inflation ends when the inflation field decays into ordinary particles in a process called "reheating", at which point the ordinary Big Bang expansion begins.

What caused Inflation to happen? Cosmologists are not sure of this and several theories exist. Guth proposed that the super cooling initiated a anti-gravitational field which then caused a violent growth spurt. There were some side issues with Guth's proposal and Andrei Linde of Stanford suggested "New Inflation" which could have various triggers, among them a random fluctuation of the initial field. There are several other theories floating about including one from Paul Steinhardt of Princeton. Steinhardt has championed an alternative to Inflation, in which the universe begins from a collision between a pair of universes known in string theory as "branes", short for membranes (see Branes in string theory sister page).

The important cosmological feature added by Guth is the rapid expansion of the very early universe which then blends in very smoothly with the standard Big Bang theory. Inflation solves the three issues with the initial theory (see theBig Bang Page.) and has now been incorporated as part of the standard Big Bang Theory. The recent WMAP satellite measurements are very consistent with the Guth type inflation models providing the first soft evidence for Inflation. (See the WMAP section under Early CMB Maps on the Microwave page.)

To Inflation purists, the Big Bang never really took place. Inflation was the start of the universe as we know it and all particles, stars, galaxies, etc. came about as a result of Inflation. There was no Big Bang explosion per se.

Reheating (10^-35 to About 10^-10 Of A Second)

Inflation was a period of super cooled expansion and the temperature dropped by a factor of 100,000 or so and continued to be cool during this phase. When Inflation ended the temperature returned to the pre-Inflationary temperature, back up by a factor of 100,000. This period is called "Reheating".

The huge potential energy of the inflation field suddenly decayed and filled the universe with elementary particles and radiation similar to water vapor in the atmosphere condensing into water droplets forming a cloud. Because the fundamental driving principles of Inflation are not known, this process is not well understood.

During Reheating, the elementary particles - photons, gluons, and quarks - were formed, but in a dense plasma state. Quarks and anti-quarks began to annihilate each other. However for reasons not yet understood, the mutual destruction of quarks and anti-quarks ended with a surplus of quarks. Because of this discrepancy, i.e. a surplus of quarks (matter), all galaxies, stars, planets, and human beings exist today.

Also, the three additional fundamental forces - the Electro-magnetic Force, the Strong Nuclear Force, and the Weak Nuclear Force - formed and then separated during this time according to the Grand Unified Theory. The universe, as we now know it, had evolved. The laws of physics and the four forces of nature began to apply. The fundamental particles had mass, but the temperature of the universe is still too high to allow quarks to bind together to form neutrons and protons. Reheating ended at about 10^-10th of a second.

The Particle Era (10^-10 To 10^-3 Seconds)

As the universe continued to cool, the particle era consisted of the formation of two particle families - hadrons and leptons. Hadrons are made up of quarks. Hadrons then formed two additional families - baryons and mesons. Baryons are "normal stable everyday matter" consisting of three quarks. Mesons are "short lived unstable particles" made of one quark and one anti-quark. All of the new mesons quickly annihilated each other leaving only protons and neutrons which would make up the nuclei of future atoms.

The formation of hadrons was followed by the formation of leptons. Leptons consist of electrons, neutrinos, muons and taus. Muons and taus are very unstable and quickly decay. During this period most of the matter in the universe consisted of leptons and their anti-particles. The lepton era drew to a close when the majority of leptons and anti-leptons annihilated one another, leaving a comparatively small surplus of leptons to populate the future universe. The lepton epoch ended at about 10^-3 seconds.

Big Bang Nucleosynthesis (10^-3 Seconds to 3 Minutes)


After Inflation, the universe slowed down to the normal "Hubble Rate" expansion and was filled with radiation and elementary particles, sometimes called "quark soup". As the universe continued to cool, new particles were formed out of pre-existing ones. This early formation phase is called the Big Bang Nucleosynthesis (BBN). With the temperature falling below 10 billion Kelvin, BBN took place from about 10^-3 seconds to about three minutes.

The diagram at the left illustrates two of the common nuclear reactions which occurred during the BBN. It shows single proton ions and neutron ions combining to form deuterium nuclei, D, (containing one proton and one neutron) plus the emission of high energy photons, γ. Subsequently it shows two deuterium nuclei fusing to produce one nucleus of helium-3 (with two protons and one neutron) and one free neutron. Note that "atoms" were not yet forming, the above reaction shows just the "nuclei" of future atoms forming from ions. Experiments can be done in labs today demonstrating these early BBN nuclei reactions.

The BBN theoretical calculations result in a nuclei abundance of about 75% hydrogen (1 proton nucleus), about 25% helium (2 protons and 2 neutrons in the nucleus), and about 0.01% of deuterium (1 proton and 1 neutron nucleus). Without this abundance of hydrogen nuclei, there would be no water and therefore no life as we now know it. That the observed hydrogen and helium abundances in early distant galaxies are very consistent with the above theoretical calculations is considered "strong evidence" for the Big Bang Theory. Big Bang Nucleosynthesis is one of the three pillars of support for the Big Bang Theory. Measurements from the WMAP satellite over nine years of collecting data (2001 to 2010) has confirmed the above ratios with much greater precision.

One nice feature of the BBN is that the physical laws that govern the behavior of matter at these energy levels are very well understood. Hence the BBN lacks some of the speculative uncertainties that characterize earlier periods in the "theoretical" life of the universe. During all this time, electrons and photons interacted with each other so intensely (instantaneous collisions) that the universe was dark (opaque), no light (photons) could escape the darkness.

The Nuclei Era (20 Minutes to 380 Thousand Years)

The Dark Ages

After the first three minutes, the universe obeyed the Standard Model and settled down to a much longer period of expansion and cooling in which change was much less dramatic. High energy radiation (photons) dominated the cosmos. As the universe continued to cool, more and more matter was created. Expansion caused radiation to lose more energy than matter so that after a while, matter (nuclei) particles exceeded massless particles (photons). About 70,000 years after the Big Bang, radiation and matter were about equal in density, shortly thereafter matter began to dominate.

Recombination (380,000 Years)

For the next 310,000 years the universe continued to expand and cool, but was still fiery hot and dark. Any visible light was immediately scattered by collisions with the ubiquitous electrons and protons. It contained only the simplest elements, mostly hydrogen and helium ions. As the universe cooled further, the electrons (with a negative charge) begin to get captured by the ions (with a positive charge) forming atoms (electrically neutral). This process happened relatively fast and is known as "recombination". The first bits of structure began to form. These small clumps of matter grew in size as their gravity attracted other nearby matter. At about 380,000 years of cooling, light (photons) began to travel through the spaces between the atoms which now "bond" the electrons in their orbits. The universe had become transparent.


Cosmic Microwave Background (CMB)

The first early radiation that could freely travel was the CMB, the remnants of which we can detect in the current universe 13.75 billion years later. 380,000 years is the earliest point in time we can ever look back and "see" because everything before that was part of the dark ages.

Note the CMB picture to the left taken by the European Space Agency (ESA) Planck satellite. The Planck satellite launched in 2009 is the third satellite exclusively designed to map the early CMB. NASA has previously launched the COBE in 1989 and the WMAP in 2001. Check out the Cosmic Microwave Background web page.

Currently, the earliest galaxy we can "see" is MACS0647-JD which formed about 420 million years after the Big Bang.

Early Radiation vs. Matter


On the very largest scales, we can regard the universe as a roughly homogeneous mixture of matter and radiation. As the universe expanded, the number of both matter particles and photons per unit volume decreased. But radiation is also reduced in energy by the acceleration rate, so its density falls faster than that of matter as the universe grows. Currently, matter dominates radiation in the universe. Matter consists of the familiar building blocks of atoms - protons, neutrons, and electrons, as well as mysterious dark matter.

Most of the radiation in the universe is in the form of the cosmic microwave background. Surprisingly, although the microwave background radiation is very weak, it still contains more energy than has been emitted by all the stars and galaxies that have ever existed. The reason for this is that stars and galaxies, though very intense sources of radiation, occupy only a tiny fraction of space. When their energy is averaged over the volume of the whole universe, it falls short of the energy of the microwave background by at least a factor of ten.

Even though today the radiation density is much less than the matter density, there was a time in the past when they were equal at the crossover point. Before that time, radiation was the dominant constituent of the cosmos.


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