# Astro 3 Homework 8

In a photo like the Hubble Deep Field (Figure 16.1 in your textbook), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?
the galaxies that are farthest away

The most basic difference between elliptical galaxies and spiral galaxies is that __________.
elliptical galaxies lack anything resembling the disk of a spiral galaxy

Suppose we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What can we learn from this observation?
We can learn the distance to the galaxy.

In 1924, Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way, thus putting to rest the idea that it might have been a cloud within our own galaxy. How was he able to prove this?
By observing individual Cepheid variable stars in Andromeda and applying the period-luminosity relation

Assume that Hubble’s constant is 22 kilometers per second per million light-years. How fast would we expect a galaxy 100 million light-years away to be moving? (Assume the motion is due only to Hubble’s law.)
away from us at 2,200 km/s

Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?
White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.

Does Hubble’s law work well for galaxies in the Local Group? Why or why not?
No, because galaxies in the Local Group are gravitationally bound together.

What is the best way to determine a galaxy’s redshift?
Take a spectrum of the galaxy, and measure the difference in wavelength of spectral lines from the wavelengths of those same lines as measured in the laboratory.

Which statement below correctly describes the relationship between expansion rate and age for the universe?
The faster the rate of expansion, the younger the age of the universe.

What does cosmological redshift do to light?
stretches its wavelength

Why can’t we see past the cosmological horizon?
Beyond the cosmological horizon, we would be looking back to a time before the universe was born.

We can study how galaxies evolve because __________.
the farther away we look, the further back in time we see

Which of the following statements is not an assumption used in models of galaxy formation?
Gas contracted to form the disks of galaxies before any stars were born.

One possible explanation for a galaxy’s type invokes the angular momentum of the protogalactic cloud from which it formed. Suppose a galaxy forms from a protogalactic cloud with a lot of angular momentum. Assuming its type has not changed due to other interactions, we’d expect this galaxy to be __________.
a spiral galaxy

Which of the following phenomena are not thought to be results of collisions or other interactions between galaxies?
The fact that spiral galaxies have both disk and halo components

Interactions among galaxies also are thought to influence a galaxy’s type in at least some cases. Which of the following does not support the idea that interactions can shape galaxies?
The fact that more distant galaxies have larger redshifts.

Which characteristic is not generally true of a starburst galaxy?
The observed features of the starburst are thought to be caused by the presence of a supermassive black hole in the galaxy’s center.

Observations indicated that over billions of years, galaxies in general tend to change from __________.
smaller and bluer to larger and redder

A quasar’s spectrum is hugely redshifted. What does this large redshift tells us about the quasar?
the distance to the quasar

Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply?
Active galactic nuclei tend to become less active as they age.

Suppose we observe a source of X rays that varies substantially in brightness over a period of a few days. What can we conclude?
The X-ray source is no more than a few light-days in diameter.

All of the following observations are real. Which one does not support the model in which active galactic nuclei are powered by accretion disks around massive black holes?
The most luminous active galactic nuclei have huge redshifts.

According to the model in which active galactic nuclei are powered by supermassive black holes, the energy released as light comes from __________.
gravitational potential energy released by matter that is falling toward the black hole

The observed relationship between the masses of central black holes and the bulge masses of galaxies implies that:
Galaxy formation and supermassive black hole formation must be related somehow.

How do we determine the conditions that existed in the very early universe?
We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.

Why can’t current theories describe what happened during the Planck era?
We do not yet have a theory that links quantum mechanics and general relativity.

Which of the following statements best explains what we mean when we say that the electroweak and strong forces “froze out” at 10-38 second after the Big Bang?
These two forces first became distinct at this time.

According to the Big Bang theory, how many forces – and which ones – operated in the universe during the GUT era?
2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces

Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies _________.
the electromagnetic and weak forces into the electroweak force

What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?
The basic chemical composition of the universe had been determined.

According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?
During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.

Which of the following is not an observed characteristic of the cosmic microwave background?
It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.

In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times prior to about 380,000 years after the Big Bang. Why?
Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely.

If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature), then we would have no way to account for _________.
how galaxies came to exist

In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life). Why didn’t the same fusion processes produce carbon and heavier elements in the early universe?
By the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur.

How does the idea of inflation account for the existence of the “seeds” of density from which galaxies and other large structures formed?
Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure.

Suppose that inflation did not occur. In that case, the fact that the cosmic microwave background has the same temperature in opposite directions of the sky would be considered ___________.
surprising, because those locations would never have been close enough to have any light or matter exchanged between them

Which analogy best explains why inflation predicts that the overall geometry of the observable universe should appear to be flat?
Earth is so big that it the part explored by an ant appears flat.

Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be ___________.
uniformly bright

The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation below (these are not real observations) would call our current theory into question?
The discovery of a galaxy with a helium abundance of only 10% by mass.