AT 9th Ed.
Discussion
Assigned Ch. 26 RD 1,2,3,5,8,11,14
1. What evidence do we have that there is no structure in the
universe on very large scales? How large is "very large"?
The Sloan Digital Sky Survey provides evidence that there
are no structures larger than about 300 Mpc. (So "very large"
is greater than 300 Mpc.) Other consistent observations are
pencil-beam surveys, Hubble Deep Fields,
and the distribution of radio galaxies (not in text).
2. What is the Cosmological Principle?
The cosmological principle is made up of two assumptions fundamental to
cosmology. They are homogeneity and isotropy. At a large enough scale, the
universe is homogeneous; one part is pretty much like any other part.
Isotropy means that it looks the same in all directions. The cosmological
principle tells us there can be no edge to the universe and there is
no center to it either.
3. What is Olber's paradox? How is it resolved?
Olber's paradox was that, if the universe is infinitely large,
old, homogeneous and isotropic, the sky should be as bright as the
surface of the Sun. It is resolved by accepting that the
universe is not infinitely large and old. Actually, you could
relax the restriction to just "not infinitely old". (You would
still have a dark night sky if the universe was infinitely large.)
[ 4. Explain how an accurate measure of Hubble's constant can lead to
an estimate of the age of the universe.
Hubbleâ€™s law is a relationship between velocity of recession of objects in the
universe and their distance, v = H0d. Since we know that velocity is distance
divided by time, the Hubble constant, H0, is a measure of one divided by time,
the time of the expansion of the universe to its present size. It turns out
that this time gives an age for the universe for the case
of zero acceleration (or deceleration).]
5. Why isn't it correct to say that the expansion of the universe
involves galaxies flying outward into empty space?
Since the Hubble flow is an expansion of space itself, galaxies are not
rushing outward into unoccupied parts of the universe. The universe is
evenly filled with matter but space is expanding, which gives rise to an
appearance of galaxies flying outward from us.
[ 6. Where did the Big Bang occur?
It occurred equally everwhere. ]
[ 7. How does the cosmological redshift relate to the expansion of the
universe?
A wave of electromagnetic radiation, as it moves through the universe,
will experience the same expansion of the space experienced by the universe.
As the wave travels farther and farther, it expands more and more. By the
time it is observed, it appears redshifted in proportion to the distance it
has traveled. ]
8. What properties of the universe determine whether it will
or will not expand forever?
One important factor is the total density of matter in
the universe. Matter exerts the attractive force of gravity
and so inhibits expansion. Another important factor is the
amount (density) of dark energy. Dark energy contributes to the
expansion of the universe.
[ 9. Is there enough luminous matter to halt the current cosmic
expansion?
No. The luminous matter density is only about 1-3% of what
is required to close the universe (assuming no acceleration). ]
11. (or 10.) Do we live in a flat universe?
Yes, we think the universe has a "flat" geometry on large scales.
The term comes from the 2D analogs of the universe: a "closed"
universe is like the surface of a sphere, an "open" universe
is like a saddle surface, and a "flat" universe is like a flat plane.
On a surface, one can draw parallel lines and they will stay parallel
on a flat plane, diverge on a saddle, and converge on a sphere.
In 3 dimensional space, we can "draw parallel lines" by firing two
lasers parallel to one another. In a curved universe the beams will
eventually converge together (and then go all the way around the
universe to return to the starting point). In a flat universe,
the beams stay parallel and never return. Actual measurements to
determine flatness are done by counting galaxies in different
redshift ranges. If the universe is flat, the counts will go
up as a function of distance/redshift such that you get 8 times
more galaxies with every doubling of distance. If the universe
is closed, you will get less than 8 times more per doubling.
Also, the angular diameter of galaxies can actually increase
with redshift instead of decrease if you have a closed universe
but it should decrease linearly with distance/redshift for a flat
universe.
Another way that we observe flatness is by imaging the small
fluctuations in the CMB. Planck, WMAP and COBE all found a
characteristic angular scale for the fluctuations. This scale
would be larger for a closed universe and smaller for an open
universe.
Finally, the observations of Type Ia supernovae are consistent with a
flat universe in which Omega=1. A plot of redshift versus distance
is mostly linear with a little dip at the end. This is consistent
with an Omega=1 universe which has about 68% dark energy. Omega
is the ratio of the actual density to the critical density and
it is related to the geometry as follows. If there is no dark
energy, Omega > 1 gives a closed universe, Omega < 1 gives an open
universe, and Omega=1 is a flat universe). If there is dark
energy, we can have the flat curvature of an Omega=1 universe
combined with the accelerating expansion of an "open" universe.
Inflation theory also suggests a flat universe.
[12. What do observations of distant supernovae tell us about
the expansion of the universe?
They tell us that the universes' expansion is accelerating.
The type Ia supernovae are standard candles - we can tell
their luminosity by their light curves and so we can figure
out their distances without using the Hubble Law. Thus, we
measure the Hubble constant at different epoch's of the universe.
It appears that the far away SN are expanding slower than
they are now.]
14. Why are measurements of globular cluster ages important
to cosmology?
Because they provide a high, lower limit for the age of
the universe. For example, if stellar evolution theory shows
globulars to be 12 Gyrs old, then the universe must be older
than 12 Gyrs.
[ 15. What is the significance of the cosmic microwave background?
It is the radiation from a time about 400,000 years after
the big bang when the protons and electrons were first combining
to form neutral H atoms. This was before stars or galaxies formed,
but small density variations existed which would later grow to
form structure. This is observed as slight variations in the temperature
of the CMB. It is the farthest "thing" we can observe. The radiation
is also greatly redshifted from travelling through expanding spacetime -
it began as a spectrum of a ~5000-10000 K blackbody and now appears
to have the spectrum of a 2.74 K blackbody.
Thus, the CMB is one of the pillars of evidence for the big bang. ]
[ 16. Do you think it constitutes good science to explain the
universe mainly in terms of dark matter and dark energy,
neither of which is known or understood?
In general, it isn't good science, but it is the
best scientists can do for the moment.
Dark matter and dark energy, at this time, are simply names to
things that appear to exist but for which there is no known explanation.
Actually, explanations exist but astronomers are uncertain whether any are
correct! Is it good science? Right now, astronomers seem to be given little
choice in the matter because observations have contradictions to classical
theories. Many new things have been discovered, about this there is no doubt.
Whether dark matter or energy are the correct explanations will be determined
through more good science and observation. ]