which part of the galaxy contains the coldest gas?
the disk
what part of the galaxy has gas with the hottest average temperature
the halo
What is the typical percentage (by mass) of elements other than hydrogen and helium in stars that are forming right now in the vicinity of the Sun?
2%
Where would you be most likely to find an ionization nebula?
the disk
The best measurements of the mass of the black hole at the galactic center come from
the orbits of stars in the galactic center
Where are most of the Milky Way's globular clusters found?
in the halo
How do we determine the Milky Way's mass outside the Sun's orbit?
from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun
Where would you least expect to find an ionization nebula?
in the halo
Which kind of star is most likely to be part of the spheroidal population?
an M star
During the history of the universe, what important event occurred about 0.001 seconds after the Big Bang?
Most matter in the early universe was annihilated by antimatter.
Which of the following important events occurred earliest in the history of the universe?
Spacetime rapidly expanded during a brief period of inflation.
During the history of the universe, what important event occurred about 380,000 years after the Big Bang?
Light began to travel freely through the universe.
Essentially all the hydrogen nuclei that will ever exist in our universe was created __________
by the time the universe was about 3 minutes old, because subsequent fusion in stars has consumed some of this hydrogen but has not made any more.
Compared to when the cosmic microwave background was first released, the radiation of the cosmic microwave background today is __________.
fainter and has most of its photons at longer wavelengths
Order of Eras
1. Planck
2. GUT
3. Electroweak
4. particle
5. era of nucleosynthesis
6. era of nuclei
7. era of atoms
8. era of galaxies
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
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.
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.
After this point, the density and temperature were too low to allow for any more fusion, so no new elements could be created (until stars began to do fusion much later on, but fusion in stars has changed the overall chemical composition only by about 2%).
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.
The excess was only about 1 extra matter particle for every billion particles of matter and antimatter, and the annihilation of the rest of the billion explains why there are so many photons in the universe.
Characteristics of the cosmic microwave background
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.
Until about 380,000 years into the history of the universe, hydrogen atoms were ionized and photons would scatter off electrons before they could travel very far; the universe was opaque.
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
Recall that models of galaxy formation presume that they formed from "seeds" of excess density present in the early universe. If the seeds really existed, they should have left their mark as slight variations in the cosmic microwave background, and we do indeed see such variations. Thus, if we did not see the variations, we would have been forced to reconsider everything we thought we knew about galaxy formation and the history of the universe.
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.
Temperatures at early times were far higher than this, but there was no helium around at those times.
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
In fact, with inflation, the quantum fluctuations lead to small differences in density and temperature. Without inflation, they would have no effect.
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.
diameter of earth
8000 miles
moons distance
60 radii(240,000 mi)
distance earth to sun
93 mil miles
1 pc
3.26 light years
milky way diameter
75000 light years
