Question 1

A galaxy with a lot of dark matter would have a high mass- to- light ratio compared to the Sun.

Accepted Answer

Dark matter does not emit light, so galaxies with a lot of dark matter will have a higher total mass than their visible (luminous) matter. This results in a higher mass-to-light ratio compared to the Sun, which is mostly made up of visible matter.

Question 2

One possible ingredient of dark matter is known as WIMPs, or weakly interacting massive particles. Our best guess is that WIMPs probably are made of protons and neutrons.

Accepted Answer

While WIMPs are hypothesized to be a potential ingredient of dark matter, there is currently no consensus on what they are made of. Protons and neutrons are only one possibility among many.

Question 3

Gravitational Lensing: Summarize the observational evidence that a gravitational field can deflect the path of light. In figures such as 18.7 on page 467, how would you prove to a skeptic that the distorted images indicated by the arrows are indeed multiple images of a single gravitationally- lensed galaxy? Muster as much observational evidence as you can.

Accepted Answer

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Question 4

The visible parts of galaxies contribute about 10 percent of the critical density of the universe.

Accepted Answer

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Question 5

Recent measurements of temperature fluctuations in the cosmic microwave background radiation indicate that about 70 percent of the mass- energy in the universe must be in the form of dark energy.

Accepted Answer

Recent measurements of temperature fluctuations in the cosmic microwave background radiation do indicate that about 70 percent of the mass-energy in the universe must be in the form of dark energy.

Question 6

If the universe is accelerating, it will expand forever.

Accepted Answer

This is the current understanding in cosmology, based on observations of the cosmic microwave background radiation and supernovae. The acceleration of the universe's expansion means that the rate of expansion is increasing over time, so it will continue to expand indefinitely.

Question 7

If we were to learn that the universe is a re- collapsing universe, then we must have misinterpreted Hubble's Law because the universe must be contracting, rather than expanding as generally believed.

Accepted Answer

Hubble's Law itself is an observation that galaxies are moving away from us at speeds proportional to their distances, indicating the universe is expanding. The concept of a re-collapsing universe doesn't necessarily contradict Hubble's Law but suggests that at some point in the future, the expansion could reverse into a contraction due to gravitational forces overcoming the expansion. This does not imply a current contraction or a misinterpretation of Hubble's Law.

Question 8

How do we know that there can only be insignificant amounts of dark matter in our solar system?

Accepted Answer

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Question 9

"Beauty" in Astronomy: In the absence of observational evidence, scientists often rely on other considerations to make decisions. One such consideration that we have met before (Chapter 3)is Occam's razor: the simplest explanation is to be preferred. Another consideration that is often used (and sometimes championed)by astronomers is the perceived "beauty" of the ideas themselves. Imagine you lived at a time before observational tests could be brought to bear on which of the ultimate fates (re- collapsing, critical, coasting, or accelerating)applies to our universe. Which fate would you support based on your own personal idea of "beauty"? Give your arguments, even if you think they are not scientific. Did you choose the observationally supported theory? What do you think of the role such considerations play in science? Are they sometimes necessary?

Accepted Answer

The answer of "Beauty" in Astronomy: In the absence of...

Question 10

Dark matter particles were definitively detected in 2013.

Accepted Answer

Dark matter particles have not yet been definitively detected. There have been several indirect observations that suggest the existence of dark matter, but direct detection of dark matter particles is still an ongoing area of research.

Question 11

Explain how observations of white- dwarf supernovae provide information on the expansion of the universe when it was younger.

Accepted Answer

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Question 12

GUT theories predict that protons will eventually decay, causing all solid objects in the universe to fall apart if the universe keeps expanding forever.

Accepted Answer

GUT theories suggest that protons are not stable and will eventually decay, leading to the eventual breakdown of all solid objects in the universe if it keeps expanding forever.

Question 13

Dark matter in galaxies and clusters of galaxies is purely hypothetical because we have no way of detecting its presence.

Accepted Answer

While we cannot directly observe or detect dark matter through traditional means such as telescopes, its presence can be inferred through its gravitational influence on visible matter and the overall structure of galaxies and clusters.

Question 14

Briefly describe the four possible expansion patterns (or fates)of the universe.

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Question 15

Changing minds: From the time the idea of dark matter was first proposed in the 1930s, it took nearly 50 years for astronomers to universally accept the existence of dark matter. Discuss the process by which that acceptance came about. What evidence was collected, and how was it checked?

Accepted Answer

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Question 16

Some galaxy clusters have not finished forming and are still attracting new members today.

Accepted Answer

Galaxy clusters are dynamic structures where galaxies are constantly moving and interacting. Some clusters are still in the process of forming and attracting new members through gravitational interactions.

Question 17

An Accelerating Universe? Current evidence that the expansion of the universe is accelerating is based entirely upon observations of distant white dwarf supernova. List all of the assumptions that you can think of that are made in using white dwarf supernova as distance indicators. Is it conceivable that any of these assumptions may be incorrect?

Accepted Answer

The answer of An Accelerating Universe? Current evidence that the...

Quiz 18: Dark Matter, Dark Energy, and the Fate of the Universe