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Michael Seeds Dana Backman

Chapter 14
Modern Cosmology

Look at your thumb.
The matter in it was present in the fiery beginning of the universe.

Cosmology, the study of the universe as a whole, can tell you where your matter came from and where it is going.

Cosmology is a mind-bendingly weird subject.
You can enjoy it for its strange ideas and theories.
It is fun to think about space stretching like a rubber sheet, invisible energy pushing the universe to expand faster and faster, and the origin of vast walls of galaxy clusters.

Notice that this is better than speculation—it is all supported by evidence.
Cosmology, however strange it may seem, is a serious and logical attempt to understand how the universe works.
It leads to wonderful insights into how you came to be a part of it.

Everyone has an impression of the universe as a vast depth filled with galaxies.
As you begin exploring the universe, you need to sort out your expectations so they do not mislead you.

Introduction to the Universe

The first step is to deal with an expectation so obvious that most people, for the sake of a quiet life, don’t even think about it.

Introduction to the Universe

In your daily life, you are accustomed to boundaries.
Rooms have walls, athletic fields have boundary lines, countries have borders, and oceans have shores.
It is natural to think of the universe having an edge.
That idea, however, can’t be right.

The Edge–Center Problem

If the universe had an edge, imagine going to that edge.
What would you find there?
A wall?
A great empty space?
Nothing?

The Edge–Center Problem

A true edge would have to be more than just an end of the distribution of matter.
It would have to be an end of space itself.
Then, what would happen if you tried to reach past or move past that edge?

The Edge–Center Problem

An edge to the universe seems to violate common sense.
Modern observations indicate that the universe could be infinite and have no edge.
Note that you find the centers of things—galaxies, globular clusters, oceans, and pizzas—by referring to their edges.
If the universe has no edge, then it cannot have a center.

The Edge–Center Problem

It is a common misconception to imagine that the universe has a center.
As you study cosmology, you must take care to avoid thinking that there is a center of the universe.

The Edge–Center Problem

Of course, you have noticed that the night sky is dark.
That is an important observation—because reasonable assumptions about the universe can lead to the conclusion that the night sky should glow blindingly bright.
This conflict between observation and theory is called Olbers’s paradox after Heinrich Olbers, an Austrian physician and astronomer, who publicized the problem in 1826.

The Necessity of a Beginning

Olbers was not the first to pose the question.
However, it is named after him because modern cosmologists were not aware of the earlier discussions.
You will be able to answer Olbers’s question and understand why the night sky is dark by revising your assumptions about the universe.

The Necessity of a Beginning

The point Olber made seems simple.
Suppose you assume that the universe is infinite and filled with stars. (The clumping of stars into galaxies can be shown mathematically to make no essential difference.)
If you look in any direction, your line of sight must eventually reach the surface of a star.

The Necessity of a Beginning

You can consider the analogy of lines of sight in a forest.
When you are deep in a forest, every line of sight terminates on a tree trunk, and you cannot see out of the forest.

The Necessity of a Beginning

By analogy, every line of sight from Earth out into space should eventually terminate on the surface of a star.
So, the entire sky should be as bright as the surface of an average star.

The Necessity of a Beginning

It is like suns crowded “shoulder to shoulder”—covering the sky from horizon to horizon.
It should not get dark at night.

The Necessity of a Beginning

Today, cosmologists believe they understand why the sky is dark.
Olbers’s paradox makes an incorrect prediction because it is based on an hidden assumption.
The universe may be infinite in size but it is not eternal—it is not infinitely old.

The Necessity of a Beginning

That answer to Olbers’s question was first suggested by Edgar Allan Poe in 1848.
He proposed that the night sky was dark because the universe was not infinitely old, but instead began at some time in the past.

The Necessity of a Beginning

The more distant stars are so far away that light from them has not reached Earth yet.
That is, if you look far enough, the look-back time is greater than the age of the universe.
The night sky is dark because the universe had a beginning.

The Necessity of a Beginning

This is a powerful idea—as it clearly illustrates the difference between the universe and the observable universe.
The universe is everything that exists.
The observable universe is the part that you can see.

The Necessity of a Beginning

The universe is about 14 billion years old.
In that case, the observable universe has a radius of about 14 billion light-years.
The observable universe is finite.
The universe as a whole could be infinite.

The Necessity of a Beginning

In 1929, Edwin P. Hubble published his discovery that the sizes of galaxy redshifts are proportional to galaxy distance.
Nearby galaxies have small redshifts, but more distant galaxies have larger redshifts.
These redshifts imply that the galaxies are receding from each other.

Cosmic Expansion

The figure shows spectra of galaxies in galaxy clusters at various distances.
The Virgo cluster is relatively
nearby.
Its redshift is small.

Cosmic Expansion

The Hydra cluster is very distant.
Its redshift is so large that the two dark lines formed by ionized calcium
are shifted from
near-ultraviolet wavelengths
well into the visible part of
the spectrum.

Cosmic Expansion

The expansion of the universe does imply that Earth is at the center.

Cosmic Expansion

To understand why, consider an analogy in which you imagine baking raisin bread.
As the dough rises, it pushes the raisins away from each other uniformly at velocities that are proportional to their distances from each other.

Cosmic Expansion

Two raisins that were originally close to each other are pushed apart slowly.
Two raisins that were far apart, having more dough between them, are pushed apart faster.

Cosmic Expansion

If bacterial astronomers lived on a raisin in your raisin bread, they could observe the redshifts of the other raisins and derive a bacterial Hubble law.
They would conclude that their universe was expanding uniformly.

Cosmic Expansion

It does not matter which raisin the bacterial astronomers lived on.
They would get the same Hubble law.
No raisin has a special viewpoint.
Similarly, astronomers in any galaxy will see the same law of expansion.
No galaxy has a special viewpoint.

Cosmic Expansion

The raisin bread analogy to the expanding universe no longer works when you consider the crust, the edge, of the bread.
The universe cannot have an edge or a center.
So, there can be no center to the expansion.

Cosmic Expansion

The expansion of the universe led astronomers to conclude that the universe must have begun with an event of astounding cosmic intensity.

The Big Bang Theory

Necessity of the Big Bang

Imagine that you have a video of the expanding universe—and you run it backward.
You would see the galaxies moving toward each other.
There is no center to the expansion of the universe.
So, you would not see galaxies approaching a single spot.

Rather, you would see the space between galaxies disappearing.
The distances between all galaxies would decrease—without the galaxies themselves moving.
Eventually, the galaxies would begin to merge.

Necessity of the Big Bang

If you ran your video far enough back, you would see the matter and energy of the universe compressed into a high-density, high-temperature state.
You can conclude that the expanding universe began with expansion from that condition of extremely high density and temperature.
Modern astronomers call it the big bang.

Necessity of the Big Bang

How long ago did the universe begin?
You can estimate the age of the universe with a simple calculation.
If you must drive to a city 100 miles away and you can travel 50 miles per hour, you divide distance by rate of travel and learn the travel time—in this example, 2 hours.

Necessity of the Big Bang

To find the age of the universe, you divide the distance between galaxies by the speed with which they are separating.
Then, you find out how much time was required for them to have reached their present separation.

Necessity of the Big Bang

For now, you can conclude that basic observations of the recession of the galaxies require that the universe began with a big bang about 14 billion years ago.
That estimated span is called the Hubble time.

Necessity of the Big Bang

Your instinct is to think of the big bang as a historical event, like the Gettysburg Address—something that happened long ago and can no longer be observed.
However, the look-back time makes it possible to observe the big bang directly.

Necessity of the Big Bang

The look-back time to nearby galaxies is only a few million years.
The look-back time to more distant galaxies is a large fraction of the age of the universe.
If you look between and beyond the distant galaxies, back to the time of the big bang, you should be able to detect the hot gas that filled the universe long ago.

Necessity of the Big Bang

Again, do not think of an edge or a center when you think of the big bang.
It is a very common misconception that the big bang was an explosion and that the galaxies are flying away from a center.

Necessity of the Big Bang

Your imagination tries to visualize the big bang as a localized event.
You must, however, keep firmly in mind that the big bang did not occur at a single place but filled the entire volume of the universe.

Necessity of the Big Bang

You cannot point to any particular place and say, “The big bang occurred over there.”
At the time of the big bang, all the galaxies, stars, and atoms in the observable universe were confined to a very small volume.
That hot, dense state—the big bang—occurred everywhere, including right where you are now.

Necessity of the Big Bang

The matter of which you are made was part of the big bang.
So, you are inside the remains of that event.
The universe continues to expand around you.

Necessity of the Big Bang

In whatever direction astronomers look, at great distances, they can see back to the age when the universe was filled with dense, hot gas.

Necessity of the Big Bang

The radiation that comes from this great distance has a tremendous redshift.
The most distant visible objects are faint galaxies and quasars, with redshifts up to about 8, meaning the light from them arrives at Earth with wavelength 9 times longer than when it started the journey.
In contrast, the radiation from the hot gas of the big bang is calculated to have a redshift of about 1,100.

Evidence for the Big Bang:
The Cosmic Background Radiation

Thus, the light emitted by the big bang gases arrives at Earth as far-infrared radiation and short-wave-length radio waves.
You can’t see it with your eyes, but it should be detectable with infrared and radio telescopes.
Amazingly, the big bang can still be detected by the radiation it emitted.

Evidence for the Big Bang:
The Cosmic Background Radiation

In the mid-1960s, two Bell Laboratories physicists, Arno Penzias and Robert Wilson, were measuring the radio brightness of the sky when they discovered peculiar radio noise coming from all directions.

Evidence for the Big Bang:
The Cosmic Background Radiation

In the 1940s, physicists George Gamow and Ralph Alpher had predicted that the big bang would have emitted blackbody radiation that should now be in the far-infrared and radio parts of the spectrum.

Evidence for the Big Bang:
The Cosmic Background Radiation

In the early 1960s, physicist Robert Dicke and his team at Princeton began building a receiver to detect that radiation.

Evidence for the Big Bang:
The Cosmic Background Radiation

When Penzias and Wilson learned about the preceding work, they realized the radio noise they had detected—now called the cosmic microwave background (CMB)—is actually radiation from the big bang.
They received the
1978 Nobel Prize
in physics for their
discovery.

Evidence for the Big Bang:
The Cosmic Background Radiation

The detection of the background radiation was tremendously exciting.
Astronomers, though, wanted confirmation.

Evidence for the Big Bang:
The Cosmic Background Radiation

Critical observations in the far-infrared checking whether the CMB really has a blackbody spectrum could not be made from the ground.

Evidence for the Big Bang:
The Cosmic Background Radiation

It was not until January 1990 that satellite measurements confirmed that the CMB is blackbody radiation, with an apparent temperature of 2.725 +/- 0.002 K.
This is in good agreement with theoretical predictions.

Evidence for the Big Bang:
The Cosmic Background Radiation

It may seem strange that the hot gas of the big bang seems to have a temperature of only 2.7 degrees above absolute zero.
However, it has a tremendous redshift.
Observers on Earth see light that has a redshift of about 1,100.
That is, the wavelengths of the photons are about 1,100 times longer than when they were emitted.

Evidence for the Big Bang:
The Cosmic Background Radiation

The gas clouds that emitted the photons had a temperature of about 3,000 K.
Also, they emitted black body radiation with a λmax of about 1,000 nm.

Evidence for the Big Bang:
The Cosmic Background Radiation

The expansion of the universe has redshifted the wavelengths about 1,100 times longer.
So, λmax is now about 1 million nm (1 mm).
That is why the hot gas of the big bang seems to be 1,100 times cooler now—about 2.7 K.

Evidence for the Big Bang:
The Cosmic Background Radiation

Simple observations of the darkness of the night sky and the redshifts of the galaxies show you that the universe must have had a beginning.
Also, you have seen that the CMB is clear evidence that conditions at the beginning were hot and dense.

Particles and Nucleosynthesis:
The First Seconds and Minutes

Theorists can combine these observations with knowledge from physics how atoms and subatomic particles behave—to work out the story of how the big bang occurred.

Particles and Nucleosynthesis:
The First Seconds and Minutes

Cosmologists cannot begin their history of the big bang at time zero.
No one understands the physics of matter and energy under such extreme conditions.
Nevertheless, they can come amazingly close.

Particles and Nucleosynthesis:
The First Seconds and Minutes

Suppose you could visit the universe when it was only 10-millionths of a second old.
You would find it filled with high-energy photons having a blackbody temperature well over 1 trillion (1012) K and a density greater than 5 x 1013 g/cm3.
This is nearly the density of an atomic nucleus.

Particles and Nucleosynthesis:
The First Seconds and Minutes

If photons have enough energy, two photons can combine and convert their energy into a pair of particles—a particle of normal matter and a particle of antimatter.
When an antimatter particle meets its matching particle of normal matter, the two particles annihilate each other and convert their mass back into energy—in the form of two gamma rays.

Particles and Nucleosynthesis:
The First Seconds and Minutes

In the early universe, photons had enough energy to produce proton–antiproton pairs or neutron–antineutron pairs.
When these particles collided with their antiparticles, they converted their mass back into photons.
Thus, the early universe was filled with a dynamic soup of energy flickering from photons into particles and back again.

Particles and Nucleosynthesis:
The First Seconds and Minutes

While all this went on, the expansion of the universe cooled the radiation.
By the time the universe was 0.0001 second old, its blackbody temperature had fallen to 1012 K.

Particles and Nucleosynthesis:
The First Seconds and Minutes

At that point, the average energy of the gamma-ray photons had fallen below the energy equivalent to the mass of a proton or a neutron.
So, the gamma rays could no longer produce such heavy particles, and the creation of protons and neutrons stopped.
These particles combined with their antiparticles and quickly converted most of the mass into photons.

Particles and Nucleosynthesis:
The First Seconds and Minutes

From this, you might guess that all the protons and neutrons would have been annihilated with their antiparticles.
However, for reasons that are poorly understood, a small excess of normal particles apparently existed.
For every billion protons annihilated by antiprotons, one survived with no antiparticle to destroy it.
Thus, you live in a world of normal matter—antimatter is very rare.

Particles and Nucleosynthesis:
The First Seconds and Minutes

The gamma rays did not have enough energy to produce protons and neutrons after the universe fell below 1012 K.
However, electron-positron pairs, having lower mass, could still be produced.

Particles and Nucleosynthesis:
The First Seconds and Minutes

That continued until the universe was about four seconds old.
At that point, the expansion had cooled the gamma rays to the point where they could no longer create electron–positron pairs.
Most of the electrons and positrons combined to form photons—and only one in a billion electrons survived.
The protons, neutrons, and electrons of which the universe is made were produced during the first four seconds of its history.

Particles and Nucleosynthesis:
The First Seconds and Minutes

This soup of hot gas and radiation continued to cool.
By the time the universe was about 2 minutes old, protons and neutrons could link to form deuterium—the nucleus of a heavy hydrogen atom—and not be broken apart again.
By the end of the next minute, further reactions began converting deuterium into helium.

Particles and Nucleosynthesis:
The First Seconds and Minutes

However, almost no heavier atoms could be built.
There are no stable nuclei with atomic weights of 5 or 8 (in units of the hydrogen atom).
Nuclei with these atomic weights fall apart as soon as they are created.

Particles and Nucleosynthesis:
The First Seconds and Minutes

Cosmic element building during the big bang had to proceed rapidly, step by step—like someone hopping up a flight of stairs.
The lack of stable nuclei at atomic weights of 5 and 8 meant there were missing steps.
The step-by-step reactions had great difficulty jumping over these gaps.
Astronomers can calculate that the big bang produced a tiny amount of lithium but no elements heavier than that.

Particles and Nucleosynthesis:
The First Seconds and Minutes

By the time the universe was 3 minutes old, it had become so cool that most nuclear reactions had stopped.
By the time it was 30 minutes old, the reactions had ended completely.
About 25 percent of the mass was in the form of helium nuclei.
The rest was in the form of hydrogen nuclei (protons).

Particles and Nucleosynthesis:
The First Seconds and Minutes

That is the abundance of hydrogen and helium observed today in the oldest stars.
The cosmic abundance of hydrogen and helium were essentially fixed during the first minutes of the universe.

Particles and Nucleosynthesis:
The First Seconds and Minutes

The hydrogen nuclei in water molecules in your body have survived unchanged since they formed during the first moments of the big bang.
Heavier elements were built by nucleosynthesis inside later generations of massive stars.

Particles and Nucleosynthesis:
The First Seconds and Minutes

At first, the universe was so hot that the gas was totally ionized.
The electrons were not attached to nuclei.

Recombination and Reionization:
The First Thousands and Millions of Years

Free electrons interact with photons so easily that a photon could not travel very far before it encountered an electron and was deflected.

Recombination and Reionization:
The First Thousands and Millions of Years

The radiation and matter interacted continuously with each other and cooled together as the universe expanded.

Recombination and Reionization:
The First Thousands and Millions of Years

As the young universe expanded, it went through three important changes.

Recombination and Reionization:
The First Thousands and Millions of Years

The first important change occurred when the universe reached an age of roughly 50,000 years.
The density of the energy present in the form of photons became less than the density of the gas.
Before this, matter could not clump together because the intense sea of photons smoothed the gas out.

Recombination and Reionization:
The First Thousands and Millions of Years

Once the density of the radiation fell below that of matter, the matter could begin to draw together under the influence of gravity and form the clouds that eventually became galaxies.
The expansion of the universe spread the particles of the ionized gas farther and farther apart.

Recombination and Reionization:
The First Thousands and Millions of Years

The second important change began as the universe reached the age of about 400,000 years.
As the density decreased and the falling temperature of the universe reached 3,000 K, protons were able to capture and hold free electrons to form neutral hydrogen.
This process is called recombination—although ‘combination’ (for the first time) would be more accurate.

Recombination and Reionization:
The First Thousands and Millions of Years

As the free electrons were gobbled up into atoms, they could no longer deflect photons.
The photons could travel easily through the gas.
So, the gas became transparent.
Also, the photons retained the blackbody temperature of 3,000 K that the gas and photons together had at the time of recombination.

Recombination and Reionization:
The First Thousands and Millions of Years

Those photons are what are observed today as the CMB—with a large redshift that makes their temperature now about 2.7 K.

Recombination and Reionization:
The First Thousands and Millions of Years

Recombination left the gas of the big bang neutral, hot, dense, and transparent.
At first, the universe was filled with the glow of the hot gas, which would have been partly at visible wavelengths.
As the universe expanded and cooled, however, the glow faded.

Recombination and Reionization:
The First Thousands and Millions of Years

The universe entered what cosmologists call the dark age.
This was a period lasting hundreds of millions of years until the formation of the first stars.
During the dark age, the universe expanded in darkness.

Recombination and Reionization:
The First Thousands and Millions of Years

The dark age ended as the first stars began to form.
The gas from which these first stars formed contained almost no metals and was, consequently, highly transparent.
Mathematical models show that the first stars formed from this metal-poor gas would have been very massive, very luminous, and very short-lived.

Recombination and Reionization:
The First Thousands and Millions of Years

That first burst of star formation produced enough ultraviolet light to begin ionizing the gas.
Today’s astronomers,
looking back to the
most distant visible
quasars and galaxies,
can see traces of
that reionization of
the universe.

Recombination and Reionization:
The First Thousands and Millions of Years

Reionization marks the end of the dark age and the beginning of the age of stars and galaxies that continues through today.

Recombination and Reionization:
The First Thousands and Millions of Years

The figure summarizes the story of the big bang—from the formation of helium in the first three minutes through energy–matter equality, recombination, and finally reionization
of the gas.

Recombination and Reionization:
The First Thousands and Millions of Years

It may seem amazing that mere humans trapped on Earth can draw such a diagram.
However, it is
based on
evidence and
on the best
understanding
of how matter
and energy
interact.

Recombination and Reionization:
The First Thousands and Millions of Years

How can the big bang have happened everywhere?
To solve the puzzle, you must put what seem to be reasonable expectations on hold and look carefully at how space and time behave on cosmic scales.

Space and Time, Matter and Energy

The universe looks about the same whichever way you look.
That is called isotropy.

Looking at the Universe

Of course, there are local differences.
If you look toward a galaxy cluster, you see more galaxies.

Looking at the Universe

Furthermore, the background radiation is almost perfectly uniform across the sky.

Looking at the Universe

The universe also seems homogeneous.
Homogeneity is the property of being the same everywhere.

Looking at the Universe

Of course, there are local variations.
Some regions contain more galaxies and some less.
Also, if the universe evolves, at large look-back times, you see galaxies at an earlier stage.

Looking at the Universe

If you account for these well-understood variations, then the universe seems to be, on average, the same everywhere.
This is harder to check because you can’t actually go to the locations of distant galaxies and check in detail that things are about the same there as here.
However, all astronomical observations indicate that
is so.

Looking at the Universe

Isotropy and homogeneity lead to the cosmological principle.
This states that any observer in any galaxy sees that the universe has the same general properties—after accounting for relatively minor local and evolutionary variations.

Looking at the Universe

The cosmological principle implies that there are no special places in the universe.
What you see from Milky Way is typical of what all intelligent creatures see from their respective home galaxies.

Looking at the Universe

Furthermore, the principle is another way of saying that the universe has no center or edge.
Such locations would be special places.
The principle states there are no special places.

Looking at the Universe

Distance is the separation between two points in space.
Time is the separation between two events.

The Cosmic Redshift

Einstein’s theories of special relativity and general relativity (published respectively in 1905 and 1916) describe how space and time are related and can be considered together as the fabric of the universe.
This is called space-time.
You can think of space-time as the canvas on which the universe is painted.

The Cosmic Redshift

Einstein’s theories predict that the canvas of space-time can potentially expand (or contract).
Amazingly, that has been confirmed by observations.

The Cosmic Redshift

The stretching of space-time explains one of the most important observations in cosmology:
Cosmological redshifts

The Cosmic Redshift

Modern astronomers understand that, except for small local motions within clusters of galaxies, the galaxies are basically at rest.
They have kept approximately the same “address” in space since the big bang.
The distances between them increase as space-time expands.

The Cosmic Redshift

Furthermore, as space-time expands, it stretches any photon traveling through space to longer wavelengths.

The Cosmic Redshift

Photons from distant galaxies spend more time traveling through space and are stretched more than photons from nearby galaxies.
That is why redshift
depends on distance.

The Cosmic Redshift

Note that objects—such as the Milky Way, Earth, and you—that are held together by gravity or electromagnetic forces do not expand as the universe expands.

The Cosmic Redshift

Astronomers often express redshifts as if they were radial velocities.
However, the redshifts of the galaxies are not Doppler shifts.
That is why this book is careful to refer to a galaxy’s apparent velocity of recession.

The Cosmic Redshift

All a cosmological redshift shows you directly is how much the universe has expanded since the light began its journey to Earth.
The formula to calculate the distance a photon has traveled, given its redshift, is complicated.
Also, not all the parameters have been measured precisely.
Nevertheless, the Hubble law does apply, and redshifts can be used to estimate the distances to galaxies.

The Cosmic Redshift

Almost immediately after Einstein published his theory, theorists were able to solve the highly sophisticated mathematics to compute simplified descriptions of the behavior of space-time and matter.
Those “model universes” dominated cosmology throughout the 20th century.

Model Universes

The equations allowed three general possibilities.
Space-time might be curved in ways that are called an open universe or a closed universe.
Space-time might have no overall curvature at all—a situation that is called a flat universe.

Model Universes

Most people find these curved models difficult to imagine.
Fortunately, modern observations have shown that the flat universe model is almost certainly correct.
So, you don’t have to wrap your brain around the curved models.

Model Universes

Note that “flat” does not mean two-dimensional.
Rather, it means that the familiar rules of geometry you learned in elementary and middle school are true on the largest scales.
Different rules of geometry would apply on large scales if the universe were curved.

Model Universes

A main criterion separating the three models is the average density of the universe.
This, according to general relativity, determines the overall curvature of space-time.

Model Universes

If the average density matter and energy in the universe equals what is called the critical density, space-time will be flat.
The critical density is calculated to be about 9 x 10-30 g/cm3 (depending on the exact value of Hubble constant).

Model Universes

If the average density is more than the critical density, the universe must be closed.
If it is less, the universe must be open.

Model Universes

The expansions versus time of the three different models are compared in the figure.

Model Universes

The parameter R on the vertical axis is a measure of the extent to which the universe has expanded.
You could think
of it, essentially,
as the average
distance
between
galaxies.

Model Universes

Closed universes expand and then contract.
In contrast, flat and open universes expand forever.

Model Universes

You can’t find the actual age of the universe—the distance on the horizontal axis from the present to the beginning, when R was zero—from the expansion rate alone.
You also need to
know whether
the universe is
open, closed,
or flat.

Model Universes

The Hubble time is actually the age the universe would be if it were totally open.
This means it would have to contain almost no matter at all.

Model Universes

If the universe is flat, then its average density must equal the critical density.
Yet, when astronomers added up the matter they could detect, they found only a few percent of the critical density.
They wondered if the dark matter made up the rest.

Dark Matter in Cosmology

You have learned that our galaxy, other galaxies, and galaxy clusters have much stronger gravitational fields than you would expect based on the amount of visible matter.
Even when you add in the nonluminous gas and dust that you expect to find, their gravitational fields are stronger than expected.

Dark Matter in Cosmology

Galaxies must contain dark matter.
In fact, they must contain much more dark matter than normal matter.

Dark Matter in Cosmology

The protons and neutrons that make up normal matter—including Earth and you—belong to a family of subatomic particles called baryons.

Dark Matter in Cosmology

Modern evidence based on what can be determined about the products of nuclear reactions in the first few minutes after the big bang shows that the dark matter is not baryonic.

Dark Matter in Cosmology

If there were lots of baryons present during those early moments, they would have undergone two collision processes:
Colliding with and destroying deuterium nuclei
Colliding with some of the helium to make lithium

Dark Matter in Cosmology

The observed amount of deuterium sets a lower limit on the density of the universe.
The observed
abundance of
lithium-7 sets an
upper limit.

Dark Matter in Cosmology

Those limits indicate that the baryons you, Earth, and the stars are made of cannot add up to
more than four
percent of the
critical density.

Dark Matter in Cosmology

Yet, observations show that galaxies and galaxy clusters contain as much as 30 percent of the critical density in the form of dark matter.
Only a small amount of the matter in the universe can be baryonic.
So, the dark matter must be nonbaryonic matter.

Dark Matter in Cosmology

The true nature of dark matter remains one of the mysteries of astronomy.
The most successful models of galaxy formation require that the dark matter be made up of cold dark matter.
That is, the particles should move slowly and clump into structures with sizes that explain the galaxies and galaxy clusters you see today.

Dark Matter in Cosmology

Although the evidence is very strong that dark matter exists, it is not abundant enough by itself to make the universe flat.
Dark matter appears to constitute no more than about 30 percent of the critical density.

Dark Matter in Cosmology

There is more to the universe than meets the eye and more even than dark matter.

Dark Matter in Cosmology

A little dizzy from the weirdness of expanding space-time and dark matter?
Make sure you are sitting down before you read much further.

Modern Cosmology

As the 21st century began, cosmologists made a startling discovery.
To get a running start on these new discoveries, you’ll have to go back a couple of decades.

Modern Cosmology

By 1980, the big bang model was widely accepted.
However, it faced two problems that led to the development of an improved theory:
A big bang model with an important addition

Inflation

One of the problems is called the flatness problem.
The curvature of space-time seems to be near the transition between an open and a closed universe.
That is, the universe seems approximately flat.

Inflation

It seems peculiar that the actual density of the universe is anywhere near the critical density that would make it flat.
To be so near critical density now, the density of the universe during its first moments must have been very close—within 1 part in 1049 of the critical density.

Inflation

Why is the universe so close to exactly flat, with no space-time curvature, at the time of the big bang?

Inflation

The second problem with the original big bang theory is called the horizon problem.
When astronomers correct for the motion of Earth, they find that the CMB is very isotropic.
It is the same in all directions to a precision of better than 1 part in 1,000.

Inflation

Yet, background radiation coming from two points in the sky separated by more than an angle of 1° is from two parts of the big bang far enough apart that they should not have been connected at any previous time.

Inflation

That is, when the CMB photons were released, the universe was not old enough for energy to have traveled at the speed of light from one of those regions to the other.
The regions should always have been beyond each other’s “horizon.”
They could not have exchanged heat to make their temperatures equal.

Inflation

How did every part of the observable universe get to be so nearly the same temperature by the time of recombination?

Inflation

The key to these two problems—and to other problems with the simple big bang model—may lie with a modified model called the inflationary big bang.

Inflation

The theory predicts there was a sudden extra expansion when the universe was very young.
This expansion was even more extreme than that predicted by the original big bang model.

Inflation

According to the inflationary universe model, the universe expanded and cooled until about 10-35 second after the big bang.
Then, the universe became cool enough that the forces of nature—which at earlier extremely high temperatures would have behaved identically—began to differ from each other.

Inflation

Physicists calculate that this would have released tremendous amounts of energy.
It would have suddenly inflated the universe by a factor of 1050 or larger.

Inflation

As a result, the part of the universe that is now visible from Earth—the entire observable universe—expanded rapidly from no larger than the volume of an atom to the volume of a cherry pit.
Then, it continued a slower expansion to its present state.

Inflation

That sudden inflation can solve the flatness problem and the horizon problem.
The inflation of the universe would have forced whatever curvature it had toward zero—just as inflating a balloon makes a small spot on its surface flatter.
Thus, you now live in a universe that is almost perfectly flat space-time geometry because of that sudden inflation long ago.

Inflation

In addition, because the observable part of the universe was no larger in volume than an atom before inflation, it was small enough to have equalized its temperature by then.
Now, you live in a universe with the same CMB temperature in all directions.

Inflation

The inflationary theory predicts that the universe is almost perfectly flat.
Observations, however, seem to show that the masses scientists know about—baryonic matter plus dark matter—add up only to about 30 percent of the amount needed to make space-time flat.

Inflation

Can there be more to the universe than baryonic matter and dark matter?
What could be weirder than dark matter?

Inflation

The Acceleration of the Universe

Both common sense and mathematical models suggest that, as the galaxies recede from each other, the expansion should be slowed by gravity trying to pull the galaxies toward each other.

How much the expansion is slowed should depend on the amount of matter in the universe.
If the density of matter is less than the critical density, the expansion should be slowed only slightly.
Thus, the universe should expand forever.
If the density of matter is greater than the critical density, the expansion should be slowing down dramatically.
Thus, the universe should eventually begin contracting.

The Acceleration of the Universe

For decades, astronomers struggled to measure the distance to very distant galaxies directly, compare distances with redshifts, and thereby detect the slowing of the expansion.
The rate of slowing would, in turn, reveal the true curvature of the universe.

The Acceleration of the Universe

This was one of the key projects for the Hubble Space Telescope.
Two teams of astronomers spent years in competition making the measurements using the same technique.

The Acceleration of the Universe

They calibrated type Ia supernovae as standard candles.
They did this by locating such supernovae occurring in nearby galaxies whose distances were known from Cepheid variables and other reliable distance indicator.

The Acceleration of the Universe

Once the peak luminosity of type Ia supernovae had been determined, they could be used to find the distances of much more distant galaxies.

The Acceleration of the Universe

Both teams announced their results in 1998.
They agreed that the expansion of the universe is not slowing down.
It is speeding up!

The Acceleration of the Universe

That is, the expansion of the universe is
accelerating.

The Acceleration of the Universe

The announcement that the expansion of the universe is accelerating was totally unexpected.
Astronomers immediately began testing it.

The Acceleration of the Universe

The most likely problem was thought to be that the calibration of the supernovae by the original teams might be incorrect.
However, that has been ruled out by measurements of more recently discovered supernovae at very great distances.

The Acceleration of the Universe

Other possible problems have been checked and seem to have been eliminated.
The universe really does seem to be expanding faster and faster.

The Acceleration of the Universe

If the expansion of the universe is accelerating, then there must be a force of repulsion in the universe.
Astronomers are struggling to understand what it could be.
One possibility leads back to Albert Einstein.

Dark Energy and Acceleration

When Einstein published his theory of general relativity in 1916, he noticed that his equations describing space-time implied that the universe should contract because of the gravitational attraction of galaxies for each other.

Dark Energy and Acceleration

In 1916, astronomers did not yet know that the universe was expanding.
So, Einstein thought he needed to balance the attractive force of gravity.

Dark Energy and Acceleration

He did this by adding a constant to his equations—called the cosmological constant.
This represents a force of repulsion that would make the universe hold still.

Dark Energy and Acceleration

13 years later, in 1929, Edwin Hubble announced that the universe was expanding.
Einstein then said that introducing the cosmological constant was his biggest blunder.
Modern astronomers, though, aren’t so sure.

Dark Energy and Acceleration

One explanation for the acceleration of the universe is that there is a cosmological constant after all.
It represents a real force that drives a continuing acceleration in the expansion of the universe.
The cosmological constant, as its name implies, would be constant in strength over time.

Dark Energy and Acceleration

Another possibility is a type of energy that is not constant in strength over time.
Astronomers have begun referring to this type of energy as quintessence.

Dark Energy and Acceleration

In either case, the observed acceleration is evidence that some form of energy—either a cosmological constant or quintessence—is spread throughout space.

Dark Energy and Acceleration

Astronomers refer to this as dark energy.
This energy drives the acceleration of the universe.
It, however, does not contribute to the formation of starlight or the CMB.

Dark Energy and Acceleration

You have learned that acceleration and dark energy were first discovered when astronomers found that supernovae a few billion light-years away were slightly fainter than expected.

Dark Energy and Acceleration

Since then, even more distant supernovae have been determined to be a bit brighter than expected.
So, they are not as far away as the redshifts of their galaxies would seemingly indicate.

Dark Energy and Acceleration

This means that, sometime about 6 billion years ago, the universe shifted gears from deceleration to acceleration.
The careful calibration of type Ia supernovae allows astronomers to observe this change from deceleration to acceleration.
The careful calibration of type Ia supernovae allow astronomers to observe this change from deceleration to acceleration.
This discovery has the important consequence of increasing previous estimates of the age of the universe by several billion years.

Dark Energy and Acceleration

Furthermore, dark energy can help you understand the lack of curvature of space-time.
The theory of inflation makes the specific prediction that the universe is flat.
Dark energy seems to confirm that prediction.

Dark Energy and Acceleration

You have learned that energy and matter are equivalent.
So, dark energy is equivalent to mass spread through space.

Dark Energy and Acceleration

Baryonic matter plus dark matter make up a third of the critical density.
Dark energy appears to make up two-thirds.
That is, when you include dark energy, the total mass-plus-energy density of the universe seems to equal the critical density.
That means that the universe is flat.

Dark Energy and Acceleration

For many years, cosmologists have enjoyed saying, “Geometry is destiny.”
Thinking about models of open, closed, and flat universes, they concluded that the density of a model universe determines it geometry—and its geometry determines its fate.
In other words, they were sure that an open universe must expand forever and a closed universe must eventually begin contracting.

The Fate of the Universe

That, however, is true only if the universe is ruled by gravity.
If the power of dark energy dominates gravity, then geometry is not destiny.
Even a closed universe might expand forever.

The Fate of the Universe

The ultimate fate of the universe depends on the nature of dark energy.

The Fate of the Universe

If dark energy is described by the cosmological constant, then the force driving acceleration does not change with time—and our flat universe will expand forever.
The galaxies will get farther and farther apart.
They will use up their gas and dust making stars.
Ultimately, the stars will all die.
Each galaxy will be isolated, burnt out, dark, and alone.

The Fate of the Universe

If dark energy is described by quintessence, then its strength could increase with time—and the universe expansion may accelerate faster and faster.
Space will pull the galaxies away from each other and, eventually, pull them apart.
Then, it will rip the stars apart.
Finally, it will rip individual atoms apart.

The Age and Fate of the Universe

That has been called the big rip.
Don’t worry, though.
Even if a big rip is in the future, nothing will happen for at least 30 billion years.
There will probably be no big rip.

The Age and Fate of the Universe

Critically important observations made by the Chandra X-Ray Observatory have been used to measure the amount of hot gas and dark matter in 25 galaxy clusters.
These observations are critical for two reasons.

The Age and Fate of the Universe

First, the redshifts and distances of these galaxies confirm that the universe expansion initially slowed down—but shifted gears about 6 billion years ago and is now accelerating.

The Age and Fate of the Universe

Second, the results are almost good enough to rule out quintessence.
If dark energy is
described by
the cosmological
constant and not
by quintessence,
then there will be
no big rip.

The Age and Fate of the Universe

Notice that this method does not depend on type Ia supernovae at all.
It is independent confirmation that acceleration is real.
When a theory is confirmed by observations of many different types, scientists have much more confidence that it is a true description of nature.

The Age and Fate of the Universe

On the largest scales, the universe is isotropic.
On smaller scales, though, there are irregularities.

The Origin of Structure

Galaxies are grouped in clusters ranging from a few galaxies to thousands.
Those clusters appear to be grouped into superclusters.
The Local Supercluster, in which we live, is a roughly disk-shaped swarm of galaxy clusters 50 to 75 pc in diameter.

The Origin of Structure

Astronomers have measured the redshifts and positions of over thousands of galaxies in great slices across the sky.
Thus, they have been able to create maps revealing that the superclusters are not scattered at random.

The Origin of Structure

Superclusters are distributed in long, narrow filaments and thin walls that outline great voids nearly empty of galaxies.

The Origin of Structure

These aggregations and gaps are referred to as large-scale structure.

The Origin of Structure

This large-scale structure is a problem.
The CMB radiation is very uniform.
That means the gas of the big bang must have been extremely uniform at the time of recombination.
Yet, the look-back time to the farthest known galaxy clusters, galaxies, and quasars is about 95 percent of the way back to the big bang.

The Origin of Structure

How did the uniform gas at the time of recombination coagulate so quickly to form galaxy clusters, galaxies, and supermassive black holes in the centers of galaxies so early in the history of the universe?
The answer appears to lie in the characteristics of dark matter.

The Origin of Structure

Baryonic matter is so rare in the universe that it does not have enough gravity to pull itself together quickly after the big bang.
As you have learned, astronomers propose that dark matter is nonbaryonic.
Thus, it is immune to the smoothing effect of the intense radiation that prevented normal matter from contracting.

The Origin of Structure

Dark matter was able to collapse into clouds.
Then, it pulled in the normal matter—to begin the formation of galaxies, clusters, and superclusters.

The Origin of Structure

Mathematical models have attempted to describe this process.
Cold dark matter does seem capable of jumpstarting the formation of structure.

The Origin of Structure

What started the clumping of the dark matter?
Theorists say that space is filled with tiny, random quantum mechanical fluctuations smaller than the smallest atomic particles.
At the moment of inflation, those tiny fluctuations would have been stretched to very large, but very subtle, variations in gravitational fields.
These could have later led to the formation of clusters, filaments, and walls.

The Origin of Structure

This structure may be the ghostly traces of quantum fluctuations in the infant universe.

The Origin of Structure

Observations of tiny irregularities in the background radiation can also reveal details about inflation and acceleration.