ASTR 1210 (O'Connell) Optional Reading

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COSMIC HISTORY: A BRIEF NARRATIVE

A frame from a supercomputer simulation
of a forming cluster of galaxies several
billion years ago (B. Moore)

We have been discussing the impact astronomy has had on human society and civilization. The quest to understand humanity's place in the cosmos has been one of the prime motivations for astronomy in all cultures. One of astronomy's fundamental contributions has been to establish our basic perspective of space and time on the largest possible scales. It is the only science that attempts to understand the ultimate origin of the universe around us (in empirical, not religious or mythological, terms). The study of the origin and evolution of the universe is called cosmology.

Even though cosmology is not the subject of this course (it is covered more thoroughly in ASTR 1220, 1280, and 3480), it helps to set the stage to briefly describe what we have learned so far about the universe and how its contents have changed through time.

You may want to refer to Supplements 2 and 3 for background on some of the technical topics included in the following narrative.

A. The Big Bang

We think the Universe began in a Big Bang --- that is, a superdense and superhot state from which it has been expanding ever since. The best evidence for the existence of this hot state is the very faint, diffuse radiation called the cosmic microwave background, which is the cooled remnant of the heat of the Bang. This was discovered in 1964 by scientists at Bell Labs, although it had actually been predicted (on the basis of the Big Bang concept) 16 years earlier. It was recently studied in great detail by the "COBE", "WMAP", and "Planck" satellite missions, and its properties are in complete agreement with the Big Bang picture.

WHEN the Big Bang?

A very long time ago. As best we can determine, the Universe is almost 14 billion years old. This estimate is based on observations by WMAP coupled with measures of the expansion rate of the universe by the Hubble Space Telescope and other instruments. (The expansion rate of the universe implies an age, since by comparing the current velocity of expansion to the separation between galaxies, you can determine the time in the past when they would all have been on top of one another.)
One of the firm predictions of the Big Bang picture is that there can be nothing in the universe older than the time of the Bang. Astronomers can test that by determining the ages of stars, star clusters, and galaxies. So far, the oldest of these are about 12 billion years old---consistent with the predictions.
Recall the time-scale analogy we discussed in the first lecture. We compared the number of letters in the textbook to the age of the Sun and decided that if the whole text represented the age of the Sun (5 billion years), then a single letter represented 2000 years. So: in the textbook analogy, the universe would be almost 3 textbooks long.

WHERE the Big Bang?

When you think of an ordinary explosion, say of a firecracker, it is always localized in a small volume, and the effects expand outward. The Big Bang was not like this: in the case of the BB, the explosion happened everywhere simultaneously. It filled the whole spatial volume of the universe. The unimaginable pressure of this event caused not only the matter of the universe to expand with great violence but also caused space to expand as well.
This is because, according to the theory of General Relativity as formulated by Einstein (1915), space and time are not independent of mass and energy --- space and time are dragged with the expanding matter. The space between galaxies is actually expanding. So, if you could run the "movie" of the Big Bang backwards, you would see material around you getting ever more compressed and ever hotter, no matter where you are in the universe.
As for the volume of space in the universe at the time of the Bang, the best evidence is that it was infinite then and has remained infinite ever since (after all, space is expanding). The universe has no center, and we are certainly not at a center. [Yes, this is a difficult concept---so don't worry if you can't visualize it.]

WHY the Big Bang?

Now you've got us. In order to answer that question, we would have to be able to assess the state of things before the Big Bang. But, as just described, space and time are tied to matter and energy, and, according to our best understanding, space and time did not exist before the Bang itself. There is no way to probe what happened earlier because our basic conceptual equipment becomes meaningless at the instant of the Bang.

B. Early Evolution of Matter

Physical conditions near the time of the Bang were utterly alien to everyday intuition. Just after the Bang, the universe was filled with an unimaginably hot, disorganized mixture of subatomic particles. The temperature was too high for even ordinary protons and neutrons to exist, and certainly too high for atoms and molecules. But the expansion caused quick cooling, and more familiar kinds of matter rapidly began to "freeze out."

@ 3 Minutes After the Big Bang: Atomic Nuclei Form

At this point, the universe is "cool" enough (about 10-20 million degrees) for ordinary protons and neutrons to form---the constituents of atomic nuclei. Conditions are similar to those in the cores of stars like the Sun, and so the nuclei interact the same way they do inside of stars (or a hydrogen bomb) --- in nuclear fusion reactions. These reactions turn hydrogen nuclei (one proton) into heavier nuclei, like helium (2 protons, 2 neutrons). If the universe lingered at this density and temperature, all of the heavier elements of the Periodic Table could have been made. But it was still expanding and cooling quickly, so only helium and a few traces of other light elements like lithium were made this way.

@ About 1 Million Years After the Big Bang: Atoms Form

By this time, the universe has cooled to about 3000 degrees, at which temperature atoms can form. The nuclei acquire electrons by electromagnetic attraction from the hot surrounding gas, forming stable atoms. But still, because only a few types of nuclei exist, the gas is mainly only H and He. This stage is called "recombination" (electrons "combine" with nuclei) --- something of a misnomer since they had never previously been combined.
It is at this point that the cosmic microwave background radiation is released. Until this time, electromagnetic radiation had been strongly trapped by interactions with matter (as daylight is strongly scattered in fog, for instance). With the formation of ordinary atoms, most of this radiation is now free to stream away, even to cross the universe. That is, in fact, how we we are able to measure it directly today. We see that radiation coming to us from enormous distances (corresponding to a distance of almost 14 billion light-years). The CMB originates in the most distant regions we can directly observe in EM radiation; beyond this shell, the universe is opaque. Because the radiation has continued to "cool down" as the universe expands with time, it is detected today as being at an equivalent temperature of only about 3 degrees above absolute zero. We have now detected the CMB coming to us from all directions in the sky. Click here for a high-resolution view of the latest all-sky map of the CMB (from the European Planck mission).

C. Formation of Galaxies, Stars, and the Chemical Elements

Gravity Takes Over

What we've described so far is based on the fundamental physics of elementary particles, which can mostly be very well tested in the laboratory. Conditions like those just after the Big Bang are simulated in the microscopic fireballs produced when protons or electrons collide with one another at near the speed of light in particle accelerators. Experiments like the Large Hadron Collider (in Europe) will further test the kinds of physics prevailing only microseconds after the Big Bang. The interactions we've described are based on the so-called nuclear short-range forces and the longer-range electromagnetic forces.
What happens over the subsequent 12 billion years depends on how the basic constituents of the universe interact with each other gravitationally. Even though gravity is the weakest of the fundamental forces, it dominates the growth of organization in the universe from now on.
Physical structure in the present-day universe originated in tiny irregularities in the distribution of matter during the Big Bang which have been "amplified" over the intervening time by the weak, but inexorable, force of gravity and blown up to cosmic size by the expansion.
The next phase of evolution depends very strongly on how clumpy the matter in the universe is, how fast the clumps move, and so forth. We are just now starting to understand this, and the details are not well in hand. We do know that following recombination the universe will be "dark" for a period of some hundreds of millions of years, because the blaze of the Bang will have faded but stars will not yet have formed.

Star Formation and Nucleosynthesis

Gas begins to concentrate under gravity slowly at first but then faster. The numerous smaller clumps merge into fewer larger clumps in a hierarchical fashion. Eventually, the density of the clumps become high enough for gas clouds within them to cool, compress, and begin to collapse under their own self-gravity and hence to form the first stars. This could have happened anywhere from 50 to several 100 million years after the Bang.
The earliest generations of stars will consist only of H and He. But because they burn these nuclei in their centers through nuclear fusion reactions, stars create heavier elements, like carbon, nitrogen, and oxygen.
The newly forged elements wouldn't do much good if they stayed trapped in stars. Fortunately, they don't. Stars like the Sun, in their old age, tend to shed their outer envelopes, and this carries some of the heavier elements back into space (into the so-called "interstellar gas"). Even more importantly, some kinds of stars can explode violently in their old age. These supernovae not only spew most of the star's processed material back into space, but the explosions themselves generate some heavier elements like iron through uranium. Astronomers have recently found that direct collisions between "neutron stars" -- very dense stellar remnants -- also contribute importantly to heavy element generation.
So, the stars themselves create most chemical elements --- a process called nucleosynthesis. These get incorporated into later generations of stars, which can enrich the mixture further.
At the end of the chain, elements like C, N, O in turn become the basis of the organic molecules we find in our bodies and other life on Earth. Stars are an essential part of the ultimate human cosmic heritage. They are not merely incidental celestial decoration, as they were often considered in pre-scientific philosophy.
The ultimate manifestation of the inexorable attractive force of gravity is the black hole, a region containing so much mass that nothing -- no particle or electromagnetic wave -- can escape it. Black holes can in principle form in a wide range of sizes, but the ones we have detected so far (from about 6 solar masses to several billion solar masses) have probably all formed from stellar explosions or the coalescence of stars and subsequent accretion of gas.

Galaxy Formation

The earliest stars were probably unusually massive, but as they seed their surroundings with heavy elements, ordinary star formation proliferates. In the larger collapsing protogalaxies or in collisions between big galaxies, billions of stars form in intense starbursts lasting only about 50 million years.
Galaxy build-up continues rapidly for the next 5 billion years or so, often involving turbulent and violent gravitational interactions between galaxies that result in mergers between them. This era, sometimes called "cosmic noon" because it features the most intense period of star formation, is being probed in ever-increasing detail by observations of deep survey fields like the Hubble Deep Field or the long exposure images taken with the James Webb Space Telescope. Star formation and galaxy build-up is still continuing today, though at a much reduced pace.
Supercomputers allow astronomers to make fairly realistic numerical simulations of this phase of evolution.
Seeing the Beginning of Time is a nice 2019 documentary video from the National Center for Supercomputer Applications that illustrates the beautiful work that is now being done in modeling the evolution of the universe from its earliest moments.

D. Formation of Planets and Life

Our Sun formed in the outer parts of an ordinary galaxy about 7 billion years after the oldest stars in the galaxy. Several generations of chemical enrichment had occurred, so about 2% of its mass is in the form of heavy elements.

The planets formed in a disky debris layer which accompanied the formation of the Sun. The terrestrial planets like the Earth accumulated from solid grains and chunks of materials in the inner part of the debris disk. These were predominantly rocky because the temperature of the inner disk was high. The outer planets accumulated from more icy materials in the cooler part of the disk.

By about 100 million years after the Sun settled down to burning hydrogen steadily in its center, the planets had assumed their present structures. The Earth's atmosphere and surface were very different from now, and both were subject to violent change by occasional collisions with asteroids, comets, and other proto-planets, which thickly populated the debris disk.

Within another billion years or so, conditions were sufficiently stable and favorable (in terms of water content, temperature, pressure, etc) that primitive living organisms had begun to thrive. Whether these originated on Earth or arrived from elsewhere we do not yet know. But the process of biological evolution through natural selection, which has continued to the point of producing human beings, had begun. Lifeforms on the planet underwent a series of dramatic transformations until, about 50 million years ago, mammals and flowering plants began to spread rapidly across Earth's surface. The human species appeared about 200,000 years ago.

And that brings us (on a cosmic scale) to the present time.

One of the most important lessons learned from astronomy to date is that there is nothing special about the Sun or the solar system. In fact, we already know of thousands of planets in orbit around other stars. Planet-based lifeforms are therefore probably widespread in the universe, and life could even exist in interstellar environments. The universe probably teems with life.

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Web Links:

Supplements 2 & 3 (Study Guide 10) provide background information on elementary particles, interparticle forces, and atomic structure. [This won't be formally assigned reading in the course until after the first midterm exam.]
To keep this narrative focussed on things we understand fairly well, I've deliberately omitted discussion of topics like cosmic inflation, dark matter, dark energy, and multiple universes. These ideas have helped, or have the potential to help, explain some mysterious aspects of the universe but have added new puzzles of their own. For more information, look them up in Wikipedia and follow the links there. If you're very curious about such topics, you might also consider taking ASTR 3480, which is taught every spring semester.

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Guide Index

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Last modified February 2023 by rwo

Text copyright © 1998-2023 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.