Chapters 14 & 15 – The Cosmos
There is a theory which states that if anybody discovers exactly what the Universe is for
and why it is here, it will instantly disappear and be replaced by something even more bizarre
and inexplicable. There is another theory which states that this has already happened.
- Douglas Adams
The most energetic events we have discussed so far involve exploding stars:
Nova: white dwarf stealing mass from a binary companion and igniting a helium flash
Type 1a supernova: just like a nova, but the entire dwarf explodes in a “carbon flash”
Type II supernova: collapse of a supergiant into a neutron star
Hypernova: collapse of a supergiant into a black hole (we think)
More massive objects can be more energetic, but what single object is more massive than a
supergiant star? Remember Sgr A*, our 2,600,000 solar mass black hole. Let’s look at galactic
centers (aka “galactic nuclei”), and find the most massive, most energetic objects known.
Sgr A*, the supermassive black hole in the Milky Way’s nucleus, is a dormant black hole: it is no
longer eating material at a considerable rate. When the galaxy formed, matter that was moving
too slow for orbit fell into the black hole (that’s how it got so big, and we’ll return to
supermassive black hole formation later), and matter moving too fast sped off into intergalactic
space. So now Sgr A* is pretty stable, and is quiet.
Not all supermassive black holes are so quiet. Seyfert galaxies (discovered by Carl Seyfert in
1943) have nuclei that produce tremendous amounts of energy – the nucleus can outshine the
stars! By looking at relative red- and blue-shift around the center of a galaxy, we can tell the
rotational speed of the galaxy; the gas and dust around the core of a Seyfert galaxy moves ~30x
faster than a typical galaxy. Lastly, Seyfert galaxy nuclei can change luminosity in a matter of
minutes, so the source of the energy must be no more than a few light-minutes across. The source
of energy at the core of a Seyfert galaxy must be a supermassive black hole, actively eating
material. Active galactic nuclei are often found in galaxies that recently interacted with other
galaxies, either through collision or tidal interactions (galaxies produce tides on each other!).
This is likely why material is being thrown towards the central, supermassive black hole.
We are still figuring out how active galactic nuclei
work, but the unified model predicts some
features that match our observations. Just like
neutron stars and “small” black holes, the spin of
a black hole (or the orbit “spin” of material falling
into it), there is an equatorial accretion disc of
infalling material, and jets of energy screaming
out from the two poles. We also predict a donutshaped ring of dense gas around the accretion
disc. (See Fig 14-5 on page 309 for a diagram.)
According to the unified model theory, all active galactic nuclei are essentially the same, but the
difference we see is exactly that, our viewpoint. Seyfert galaxies are spiral galaxies with active
centers, and the different types of Seyferts simply reflect our viewing angle. BL Lacs are giant
ellipsoid galaxies where the active nuclear jet is pointed straight at us. All active galaxies are
different viewpoints of the same phenomenon: matter falling into a supermassive black hole.
In the 1960’s astronomers noticed that some “stars” had very bizarre
spectra. The unified model did not yet exist, and nobody knew what
these quasi-stellar objects were. In this Hubble photo, the left is a
quasar, the right is a star; at visible wavelengths, they look identical
even to Hubble! Quasars red-shifts suggest that they are billions of
light-years away (this will be explained later), which would make
them the most luminous objects in the universe. According to the
unified model, a quasar is a distant, active black hole oriented so that one of its two jets is
pointing straight at us. (A blazar is a kind of quasar.) Quasars were more common long ago,
when galaxies were forming, because more matter was falling into the central black holes. This is
why we see quasars far away, when we look into the past.
Wait a minute, red-shifts imply how fast an object is moving away from you, not how far away it
is, right? Well, according to Hubble (and lots of astronomers since), the very fabric of space-time
is expanding. Over the billions of years that a blue photon will travel through space towards your
telescope, space itself will expand, and the photon will be red by the time it gets here! The
Doppler effect can tell you relative speed, but for vast expanses, red-shift is really telling you
how long a photon has been flying through space. Most quasars have a red-shift between two
and 2.7, meaning there must have been an age of quasars. This suggests to us an age when
galactic nuclei were actively forming.
(Chapter 15) Because of Hubble’s observation of an expanding universe, we made the Big Bang
theory. Just after the Big Bang, the universe was very small and bursting with energy. At such
high energies, pure energy can convert into mass in a process called pair production, because
one particle and one antiparticle are created simultaneously. These pairs would then annihilate
each other, converting back into pure energy, and the cycle continued. Why we ended up with
more matter than antimatter, I don’t know. Within four seconds of the Big Bang, the universe
cooled enough that pair production stopped, and we were left with an excess of matter. (Of
course, if we were left with an excess of antimatter, we would’ve called it “matter”). The
protons, neutrons, and electrons that make you were all created in the first four seconds.
When the universe was 2 minutes old, it had cooled enough that particles were colliding with
each other and sticking together, forming helium and a trace of lithium. There’s no stable nucleus
with an atomic weight of 5, so jumping from 4 (helium) to 6 was difficult, and there was very
little lithium formed. This cosmic nucleosynthesis went on for one furious minute, until, at 3
minutes, the universe was too cool for nuclear reactions, and we were left with mostly
hydrogen, some helium, and a touch of lithium. The universe was still too hot for electrons to
stay in orbit around a nucleus, so we’re really talking about hydrogen, helium, and lithium nuclei.
Within a few hundred thousand years, the universe cooled to about 3000K, and nuclei were now
able to combine with electrons to form neutral atoms: an event called recombination. Aha!
Now that electrons are nicely packed away with atoms, photons can travel a bit without
constantly interacting with electrons. The universe became transparent! Photons from the
3000K recombination roamed free, and as the universe expanded, they became red-shifted, and
red-shifted, and red-shifted.
Our models predict that today, these photons have a huge redshift of 1100, and so it should look
like the entire universe has a weak, cold glow, as if the whole thing were 2.7K. This cosmic
microwave background radiation was first discovered (by accident!) in the mid-1960’s, and
was beautifully mapped by the COBE satellite in the 1990’s. Both these projects won physics
Nobel prizes, in 1978 and 2006. The Big Bang theory has withstood scrutiny!
Just after recombination, as the free photons became more and more red-shifted, there were no
stars yet, so there was no real source of energy – the universe just slowly cooled. This dark age
ended in ~400 million years, when the first stars began to shine (an event called reionization).
Today, as far as we can see, stars  galaxies  galaxy clusters  galactic superclusters.
(For us: Sol  Milky Way  Local Group  Virgo Supercluster.) Superclusters are sometimes
threaded together into filaments with voids between them. On scales larger than ~500 million
light years, the observable universe appears homogenous. This is called the End of Greatness.
The End of Greatness, of course, is limited to the observable universe. Since light travels at a
finite speed, and the universe’s age is finite, we can only see out so far. This is the solution to
Olber’s paradox, which wonders why any area of the night sky appears black: if the universe is
infinite and filled with infinite stars, every line of sight should end in a bright star! Whether the
universe is infinite or not, we don’t know, because we are limited to the observable universe.
One more cosmological question we’ll tackle: the curvature of the Universe. The observable
universe appears flat to us, like a sheet of paper, but there are other options. The universe could
be closed, like the surface of a sphere, which is finite but has no edges; or it could be open, like a
saddle that goes on forever. The universe’s curvature depends on gravity, and therefore mass.
If the mass is below the critical density, the universe will be open. (This is the same as saying
that, if there’s not enough mass in the universe, gravity will never pull everything back together
in a Big Crunch.) If above the critical density, we get a closed universe. Using methods I don’t
fully understand involving cosmic background radiation and the sound made by the Big Bang, the
universe appears flat even on the grandest scales, which means we’re at (or near) the critical
density. We do not see nearly enough matter to be at critical density, which further supports the
idea of dark matter. (We also see galaxy clusters form very quickly after the Big Bang, which is
theoretically possible only with cold dark matter to “seed” areas which will form galaxy clusters.)
One way to measure the fate of the universe is to measure how quickly the Hubble expansion is
slowing down. (Remember, telescopes are time machines, so we can measure speeds in the past!)
In 1998, two teams found a shocking discovery: the expansion of the universe is not slowing
down at all – it’s speeding up! There must be some energy we don’t know about, called dark
energy, which is pushing the universe apart. If the universe keeps expanding at an accelerating
rate, eventually everything will be torn apart in an event called the Big Rip.
Two “asides.” 1 – In the 1980’s the Big Bang theory was revised to include a massive inflation
right at the beginning. 2 – Scientists are working on unifying the electroweak and strong forces,
which would team up to make a “Grand Unified Theory,” or GUT. I’ll go over these in class if
we have time.
We just went over the entire history of everything, from the beginning of time to the eventual fate
of the universe! Next class, we’ll look back at our own home in this vast cosmos.
Homework
Read chapter 14
Read chapter 15
review questions on page 318: 3, 4, 9, 11
review questions on page 344: 1, 3, 4, 5