Cosmology (Chapter 26)

Based on deep field images taken by the Hubble and James Webb Space Telescopes, about how many galaxies do we believe exist in the observable universe?

About 100 Billion to 2 trillion.

What is cosmology?

The study of the structure and evolution of the universe as a whole.

What does it mean to say that the universe is homogeneous on large scales?

It means the universe has a uniform composition and structure when averaged over large distances.

What does it mean to say that the universe is isotropic?

It means the universe looks the same in all directions.

What does the cosmological principle state?

The universe is both homogeneous and isotropic on large scales.

What two things does the cosmological principle imply?

There are no special places or directions in the universe.

What is Olber’s paradox?

The question of why the night sky is dark if the universe is infinite and filled with stars.

What did Olber’s paradox make clear to us regarding the size and ability of the universe to change?

That the universe is finite in age and is evolving.

What evidence do we have from Hubble that the universe has been changing in time?

Galaxies are moving away from us, with speed proportional to their distance.

How long has it taken for all galaxies to get to where they are today?

About 14 billion years, based on Hubble time calculations.

What is the value of the Hubble time? What is this value essentially equal to?

About 14 billion years; it's approximately the age of the universe.

What is the Big Bang known as?

The beginning or origin of the universe.

How does the Big Bang solve Olber’s paradox?

It implies the universe had a beginning, so not enough time has passed for light from all stars to reach us.

The universe is still expanding as a result of the according to __, which is the same for any observer anywhere in the universe.

initial explosion; Hubble’s law

No matter what galaxy the observer is in, Hubble’s constant is always calculated to be what value based on the observed recessional speeds and corresponding distances?

About 70 km/s/Mpc.

If you asked an observer in Galaxy C to describe how Galaxy B appears to move, what would this person say?

C. 'Galaxy B is moving away from Galaxy C.'

Where did the Big Bang occur, and why?

Everywhere at once; space itself expanded.

What is a good analogy to describe the expanding universe? (Hint: It involves baking!)

Raisin bread rising in the oven.

State three specific reasons why blowing up a balloon with coins taped to it is another great way to visualize the expansion of the universe.

1. Galaxies (coins) move apart as space (balloon) expands. 2. Expansion is uniform. 3. No center of expansion.

Galaxies expanding in the universe have cosmological redshifts, which are what?

Stretches in the light waves due to the expansion of space.

Objects observed farther back in time have higher/lower cosmological redshifts.

Higher

What are the two options for the fate of the galaxies in the universe?

1. Expand forever. 2. Eventually collapse.

What determines which of the above fates will be?

The density of the universe.

Describe the expansion of a low-density universe.

Expansion continues forever and slows gradually.

Describe the expansion of a high-density universe.

Expansion slows, halts, and reverses.

What is the critical density? Describe what it physically is.

The dividing point between eternal expansion and eventual collapse; it's the density needed for a flat universe.

Describe the “Big Crunch” or “Heat Death” possible fate of the universe.

Big Crunch: collapse back to a point. Heat Death: everything spreads out and cools.

Describe the “Big Freeze” or “Cold Death” possible fate of the universe.

Universe continues expanding and cools down indefinitely.

If the density of the universe is greater than the critical density, what does this mean?

B. The universe is expanding at a rate less than the escape speed of the universe.

The presence of matter and energy __ space.

Curves

What do we need to apply to the universe as a whole, instead of Newtonian mechanics?

General relativity

The degree of curvature will depend on the total __ of the cosmos.

Density

The total density of the cosmos takes into account what two things?

Matter and energy

What is the cosmic density parameter (Ω₀) and what does it indicate about the universe's type and geometry?

Ω₀ < 1: Open, negative curvature; Ω₀ = 1: Flat; Ω₀ > 1: Closed, positive curvature

In a closed universe, you can travel in a “ __” and possibly end up right back where you started!

Circular path

Why is it important to consider the curvature of space when mapping out the universe?

It affects how we interpret distances and the structure of the cosmos.

What does the expansion rate of the universe depend on?

Total density of matter and energy.

When we measure the amount of luminous matter in a large volume of space, what percentage of the critical density is it? Is it accurate?

About 1–2%; no, it's incomplete.

When we include dark matter, what is the percentage of the critical density? Is this enough to halt expansion?

About 30%; not enough to halt expansion.

Does measuring the amount of matter in a large volume of space give a local or global density measurement?

Local

What objects have been used to obtain a global measurement of the universe’s density?

Supernovae and galaxy clusters.

What measurements from those objects help determine the global density?

Luminosity and redshift.

If the expansion of the universe was decelerating, would distant objects move more quickly or slowly?

More quickly.

What do we actually see, and why?

They move more slowly, indicating acceleration.

What does this imply about the expansion rate of the universe?

It’s increasing.

What is dark energy?

A mysterious force causing the acceleration of the universe.

What happens to dark energy’s effects as the universe expands?

Its influence increases.

What is the cosmological constant?

A constant energy density filling space, proposed by Einstein.

What is a second candidate for dark energy?

Quintessence.

The expansion rate of the universe is:

A. Increasing.

What type of universe do cosmologists believe we live in?

Flat

What are the constituents of our universe and their corresponding percentages?

Dark energy: 70%, Dark matter: 25%, Ordinary matter: 5%

What astronomical objects help determine the age of the universe and rule out models?

White dwarfs, globular clusters, and distant supernovae.

Since we live in an accelerating universe at the critical density, what is the estimated age of the universe?

13.8 billion years

What is the cosmic microwave background (CMB)?

Radiation left over from the Big Bang.

What type of radiation did the CMB start as, and what is it today?

Started as visible/infrared, now microwave.

What temperature has the CMB cooled to?

2.7 K

Why was the discovery of the CMB important?

It confirmed predictions of the Big Bang theory