Astronomy Quiz #2

Why is the night sky dark?

Our Universe has a finite age.

Suppose it were discovered that the Hubble constant is significantly larger than 68 km/s/mpc. What effect would this have on the calculated age of the universe?

It would decrease the age of the universe.

If gas is compressed, its pressure _____ and temp ______.

Pressure and Temp increases.

If the expansion of the Universe is faster (a higher Hubble constant), then the critical density required to halt the expansion is


The presence of matter (gravity) will cause the Hubble constant to change over time. In the distant past, how would the Hubble constant (expansion rate) compare to today?

It would be faster in the past.

How does the orbital speed of a merry-go-round change with distance from the center?

It increases, since children on the outside have to travel faster to cover the large distance.

How does the orbital speed of the planets in our solar system change with distance from the Sun?

It decreases since the the force of gravity decreases.

How old are the oldest globular clusters?

The oldest globular clusters are around 13 billion years old, giving them 1 billion years to form which is consistent with the age of our universe.

What is the best estimate for the age of the universe?

13.8 billion years old.

What is the Big Bang's answer to the Flatness problem (why is the universe so close to the critical density?) ?

the universe went through cosmic inflation in a short period of time.

What is the Big Bang's answer to the Horizon Problem (Why is the Universe the same everywhere?) ?

Inflation occured uniformly.

What is the Big Bang's answer to the Structure Problem (Where did all the structure in the Universe come from?) ?

Everything formed in the beginning, when the universe was very hot and dense.

What are the four best observational facts that support the Big Bang?

1. The Universe is homogeneous and isotropic.
2. The Universe is expanding.
3. The matter in the Universe is about 75% hydrogen and 25% helium.
4. The Universe is filled with electromagnetic radiation with a blackbody temperature of about 2.7 K.

How is matter created?

Particles can be produced from energy in a process called pair production. Gamma ray photons come together to produce matter. When the particle and its antiparticle collide, they will convert their mass back to energy.

What is the Nucleosysnthesis Epoch?

Protons and neutrons can fuse to form deuteurium, an isotope of hydrogen, through the reaction. As time goes on, more nuclear reactions occur and form other nuclei.

Explain the origin of elements.

The big bang can only produce elements no heavier than Li. All the heavier elements are produced in the cores of stars via nucleosynthesis.

What happened at the end of the Radiation Era?

After nucleosynthesis, the temperature of the universe is high enough that the atoms are ionized. (the electrons before were too energetic to attach to atoms).

Explain a closed universe.

A closed universe is one in which there is enough matter and energy that their combined gravitational pull will be sufficient to stop the expansion of the universe and cause it to contract again. Results in spherical geometry.

Explain an open universe.

An open universe is one in which the combined gravitational pull of all the matter and energy in the universe is insufficient to stop the expansion. Results in saddle-shaped geometry.

Explain a critical universe.

A critical universe has exactly the critical density needed to stop the expansion after an infinite amount of time. Results in flat geometry.

What are the five constituents of the universe?

Atomic matter, dark matter, light (photons), neutrinos, and dark energy.

What is the current best fit model of our universe?

The universe is flat. That is, the universe is a critical universe. This is because when you add up all the constituents of matter, you get nearly the critical density needed to halt the expansion.

What is the ultimate fate of the universe?

As time goes on, galaxies will recede from one another with ever increasing speed.
Nearby galaxies will recede greater than the speed of light eventually.
The Universe will grow cold and dark.

How big is the bulge of the Milky Way?

20,000 LY in the plane and 15,000 LY perpendicular to the plane.

What is our best guess at what Dark Matter is?

WIMPS (Weakly Interacting Massive Particles)