Chapter 17: Modern Cosmology

Olbers’s paradox

The conflict between observation and theory as to why the night sky should or should not be dark.

cosmologists

An astronomer or physicist whose research focuses on the overall properties of the Universe and its origin.

observable universe

The part of the Universe that is visible from Earth’s location in space and time.

expanding universe

The idea, supported by observed redshifts of galaxies, that space is stretching, carrying galaxies and galaxy clusters away from each other.

Big Bang

The theory that the Universe began with a violent explosion from which the expanding Universe of galaxies eventually formed.

Hubble time

An upper limit on the age of the Universe derived from the Hubble constant.

cosmic microwave background radiation

Radiation from the hot clouds of the Big Bang explosion. Because of its large redshift, it appears to come from a body whose temperature is only 2.7 K.

antimatter

Matter composed of antiparticles, which on colliding with a matching particle of normal matter annihilate and convert the mass of both particles into energy. The antiproton is the antiparticle of the proton, and the positron is the antiparticle of the electron.

recombination

The stage within a million years of the Big Bang when the gas became transparent to radiation.

dark age

The period of a few hundred million years during which the Universe expanded in darkness. Extends from soon after the Big Bang glow faded into the infrared to the formation of the first stars.

re-ionization

The stage in the early history of the Universe when ultraviolet photons from the first stars ionized the gas filling space.

isotropic

The condition of being uniform in all directions. In cosmology, the characteristic of the Universe by which, in its general properties, it looks the same in every direction.

homogeneous

The property of being uniform. In cosmology, the characteristic of the Universe in which, on the large scale, matter is uniformly spread through the Universe.

cosmological principle

The assumption that any observer in any galaxy sees the same general features of the Universe.

critical density

The average density of the Universe needed to make its curvature flat.

nonbaryonic matter

In cosmology, a suspected component of the dark matter composed of matter that does not contain protons and neutrons.

WIMPs

Weakly interacting massive particle, a hypothetical type of subatomic particle of which dark matter could be composed.

large-scale structure

The distribution of galaxy clusters and superclusters in walls and filaments surrounding voids mostly empty of galaxies.

superclusters

A cluster of galaxy clusters.

voids

A region containing relatively few galaxies, part of the large-scale structure of the Universe.

hot dark matter

Invisible matter in the Universe composed of low-mass, high-velocity particles such as neutrinos.

cold dark matter

Invisible matter in the Universe composed of heavy, slow-moving particles such as WIMPs.

cosmological constant

Einstein’s constant that represents a repulsion in space to oppose gravity.

quintessence

The proposed energy of empty space that causes the acceleration of the expanding Universe.

dark energy

The energy of empty space that drives the acceleration of the expanding Universe.

big rip

The possible fate of the Universe if dark energy increases rapidly and the expansion of space-time pulls galaxies, stars, and ultimately atoms apart.

flatness problem

In cosmology, the circumstance that the early Universe must have contained almost exactly the right amount of matter to close space-time (to make space-time flat).

horizon problem

In cosmology, the circumstance that the primordial background radiation seems much more isotropic than could be explained by the standard Big Bang theory.

inflationary universe

A version of the Big Bang theory that includes a rapid expansion when the Universe was very young.

What does the darkness of the night sky tell you about the age and size of the observable universe?

It is not infinitely old.

During the time of re-ionization, which was dominant: dark energy, matter, or radiation?

matter

During the time of the "photon and particle soup," which was dominant: dark energy, matter, or radiation?

radiation

Today, which is dominant: dark energy, matter, or radiation?

dark energy

If you accept the cosmological principle, how can Earth be located at the center of the observable universe?

We cannot observe the entirety of the Universe due to the travel time of photons. Our observable universe is only a bubble embedded in the larger Universe.

Why can't an open universe have a center?

It is infinite.

How can a closed universe not have a center?

It has no edge.

What evidence shows that the Universe is expanding?

the redshifts of all distant galaxies

What evidence shows that the Universe began with a Big Bang?

• If the universe is expanding, the expansion must have had a beginning.

• The universe has a finite age.

• The cosmic microwave background radiation is proof of the big bang.

Why couldn't atomic nuclei exist when the Universe was less than about 2 minutes old?

The Universe was too hot.

Why are measurements of the current density of the Universe important?

The overall density of the Universe is the main clue that can allow us to choose between open, closed, and flat models of the Universe.

How does inflationary universe hypothesis resolve the flatness problem?

Such rapid inflation would have forced the curvature of the universe to be nearly zero.

How does inflationary universe hypothesis resolve the horizon problem?

The universe could have been small enough for all parts to be in thermal contact.

What is the evidence that the universe was homogeneous during its first 400,000 years?

The observed background radiation is very uniform.

What evidence can you cite that the universe's expansion is accelerating?

Type Ia supernovae are fainter than expected.

What is the difference between hot dark matter and cold dark matter?

Hot dark matter moves much faster than cold dark matter. Cold dark matter clumps easily into large structures, while hot dark matter does not.

How does this difference between hot dark matter and cold dark matter affect cosmology?

Cold dark matter is more likely to clump together into structures such as galaxies.

What evidence can you cite that the Universe is flat?

The tiny variations in the temperature found in the cosmic microwave radiation caused by density differences is strong evidence that the Universe is flat.

In the raisin bread analogy, each raisin represents which of the following?

a galaxy

Why do two raisins that start out close to each other move apart at a lower velocity than two that are far apart?

There is less dough between them.

Just as raisins are carried away from each other by the rising dough, which of the following is true for galaxies?

They are carried away from each other by the expanding universe.

In the raisin bread analogy, the raisins do not swim through the dough. How does this resemble galaxies?

• The galaxies are nearly at rest with respect to the space near them.

• Galaxies are carried along with the expansion.

• The expansion of the universe is not caused by the motion of galaxies through space.

Why does the raisin bread analogy break down?

• The universe cannot have an edge.

• The crust of the loaf implies there is a center.

• You can't measure inward from an edge to find the center of the universe.

The raisin bread analogy demonstrates which of the following?

It doesn't matter which galaxy an observer lives in. They all see the same expansion.

Photons can be deflected easily if they encounter which of the following?

electrons

Because the photons were coupled to the gas, which of the following had to be true?

The photons had to have the same temperature as the gas.

Why did the universe become transparent at the time of recombination?

protons combined with electrons

Which of the following became true after recombination?

• Photons were free to travel through the gas without interacting with it.

• Gas could cool without affecting the photons.

• Photons did not cool with the gas but expansion stretched their wavelengths.

Why do photons from the age of recombination arrive at Earth from all directions?

Earth is surrounded by the universe in all directions.

Which of the following is true of the photons now arriving from the age of recombination?

• They have a temperature of approximately 2.7 K.

• They are microwave radio photons.

• They are strong evidence that the big bang really happened.