Launch Pad, Day Six, Mike Brotherton Leaves Us with a Lecture

To read the rest of my launch pad posts, click here.

We spent the morning playing with images in an astronomy program that you can download for free if you, also, would like to spend your morning playing with images — SAOImage DS9.


Is Anyone Out There? by Frank Drake and Dava Sobel
The Space Environment: Implications for Spaceship Design by Alan C. Tribble

Distance measurements to other galaxies:

a) Cepheid method: using period/luminosity relationship for classical Cepheids. This is what’s classically done, but it’s hard to do outside the local group of galaxies.
b) More recently, we’ve been able to categorize Type Ia supernovae (collapse of accreting white dwarves in binary systems) to get distances to much more distant galaxies, but these don’t let you calculate within galaxies well.

These are standard candle methods. If you know the brightness of a standard candle, then you can calculate the distance.

Once we have distances, we can calculate size and luminosity.

Cepheid distance measurement– repeated brightness measurements of a Cepheid allow the determination of the period and thus the absolute magnitude and distance.

We can use those kinds of arguments to get distances to nearby galaxies. Historically it’s the only usable technique. And back in the 1920s, Edwin Hubble started calculating things like the distances of galaxies and relating them to the redshift–their velocity moving away from us. He looked at spectra and saw they were redshifted.

From a measured redshift you assume objects are moving away from you. You can get a velocity. When he plotted velocity v. distance, he found a correlation–objects 1 megaparsec distant were redshifted at x, and at 2 megaparsecs at (function)x. This gives us the expansion rate of the universe, although this rate has not been stable throughout the history of the universe. Hubble got this correct, but miscalculated his original distances.

Knowing this relationship, if you just measure a galaxy’s redshift, you can use that to get a distance.

Ian: Is anything blueshifted?

Mike: Andromeda. It will merge with the Milky Way in 3.5 billion years.

Ian: Is anything else?

Mike: Some of the nearby galaxies are blue-shifted. It’s the general expansion of space that carries galaxies apart, but galaxies are also moving in relation to each other with peculiar motion. You have to go out a few megaparsecs before the Hubble reaction (space expansion) moves faster than the peculiar motion.

The extragalactic distance scale.

Many (most) galaxies are millions or billions of parsecs from our galaxy, expressed as mega- or giga-parsecs.

At distances like this, it’s interesting to note that we’re looking at light that left the galaxy we’re looking at millions or billions of years ago. We’re looking at how these objects looked when light left them, when the universe was much younger. This is red-shift time, with “look-back” time of millions or billions of years.

If we want to study the early universe, we just find the most distant objects possible.

So, galaxy sizes and luminosities. Galaxies come in vastly different sizes and luminosities. From small, low-luminosity, irregular galaxies (that are much smaller and less luminous than the Milky way) to giant ellipticals and spirals.

Rotation curves of galaxies–we use the rotation curve of the milky way to get the mass of the milky way. We can do the same with other galaxies. Spiral galaxies have a rotation, with one bit red-shifted and one bit blue-shifted, we can figure out the speed of the rotation. Based on the rotation, we can use Newtonian gravity to determine the galactic mass.

Supermassive black holes–almost all massive galaxies have supermassive black holes at their centers, some of them much more massive than ours (ours is millions, there are some with billions). The simple story is smaller galaxies have smaller ones, larger galaxies have larger ones. We only have dozens of black holes measured. We don’t know whether or not most galaxies have them, though some suspect so. It is also not clear that dwarf galaxies have black holes at their centers.

To explain the rotation curve of these far galaxies, mass does not follow the starlight. In order to understand these rotation curves, most of the mass of the galaxies has to be invisible. We don’t know what it is, but we have ideas about some components, and we know it probably *isn’t* brown dwarfs. Brown dwarfs are failed stars, which may be even fainter than M stars, which don’t shine or have fusion in their cores. They would be really faint. They’re out there and contribute a lot of mass, but calculations to discover how many of them there are indicate that there’s not enough mass from them to comprise all dark matter. The mass might also be small, primitive black holes from the beginning of the universe, but we don’t think that’s it either. It’s also possible that Newtonian physics doesn’t work as we think it should on the galactic scale, so there is no missing mass problem. However, we’re pretty sure most of the mass is exotic particles and anti-particles.

Clusters of galaxies–galaxies tend not to exist in isolation, but in clusters. Rich clusters have more than 1,000 galaxies of approximately three megaparsecs, condensed around a large, central galaxy.

Poor clusters have less than 1,000 galaxies, often just a few, with a diameter of a few megaparsecs, and are generally not condensed toward the center.

The Milky Way is in a poor cluster, the local group has three large spirals and a bunch of dwarf ellipticals.

Hot Gas in Clusters of Galaxies–space between galaxies is not empty, but filled with hot gas that is observable in X-rays. That this gas remains gravitationally bound provides further evidence for dark matter. There is more mass in the hot X-ray gas than there is in all the stars in these galaxies.

When we’re talking about galactic masses… the milky way is maybe 100billion in gas and stars, maybe 1trillion if we include halo and dark matter (that’s solar masses) which is a moderately large spiral galaxy. Some of these elliptical galaxies will be a hundred times more massive. A whole cluster might be as much as 10^15 solar masses.

Gravitational lensing–the huge mass of gas in a cluster of galaxies can bend the light from a more distant galaxy.The image of the galaxy i strongly distorted into arcs. This actually gives us another way to measure mass. You can measure the mass of a cluster in a similar way to how you measure the mass of a galaxy; they may rotate. You look at the velocities; you look at the size scale; you get a mass. Without the mass we calculate, these galactic clusters should fly apart. Even when you add in x-ray gas, they should still fly apart–arguing, again, for dark matter.

Our galactic cluster: the local group. Similar to the diagram we’re looking at. We’ve always thought Andromeda was more massive than our own galaxy, and in fact it probably is, but we recently figured out our estimate of our own galactic mass was off by a factor of 2, so it’s closer. A few of the galaxies in our groups are spirals, but most are dwarf ellipticals. This is a similar thing to the chart of stars–most stars are small, red stars, but if you just look at the brightest stars, then you see the rare massive stars. Likewise, if you just look around the sky, you tend to miss the dwarf ellipticals and see the bright, massive ellipticals and spirals. So probably dwarf ellipticals are the most common kind of galaxy, but we don’t know about many of them because we can only see the local ones and past that they’re too faint.

Ian: So, there are 13 dwarf ellipticals?

Mike: About, although there are edge cases.

Me: Any reason to think having few galaxies in our local group would be conducive to life? Would it reduce

Mike: The type of galaxy and galatic environment is probably correlated with life if we assume that life as we know it on earth happens and the conditions here are preferable because a lot of these rich galaxy clusters have almost no spirals, and the big elliptical galaxies are likely the result of mergers of spiral galaxies, and mergers of spiral galaxies do two things: they disrupt all the stars, they use up gas making mores tars, but passage through this hot medium has a tendency to strip gas from the galaxies themselves, quenching star formation, so that at some point, you stop making new stars, and at some point you’re stuck with older ,less massive stars, and you end up with low metallicity which is important in planet formation. When you crash two galaxies together, the space between stars is so large you essentially never get stellar collisions, but it may be easier to disrupt solar systems.

Me: It’s in those galaxies you see gamma ray bursts?

Mike: No… those are in galaxies with a lot of star formation, they seem to come from hypernovas.

Mike: Let’s talk more about these processes. When you have galaxies merging and colliding, you can set off star formation. Some of these excitements may not come from collisions, but just some kind of interaction.

Starburst galaxies are often very rich in gas and dust, and they’re bright in the infrared because dust forms from supernovas, and gets heated by the supernovas, so they shine very brightly.

Interacting galaxies–Particularly in rich clusters, galaxies can collide and interact. Galaxy collisions can produce ring galaxies like the cartwheel galaxy which we think passed through the center of another galaxy, and when it did, it produced a shockwave that set off star formation around the center. A movie showing the cartwheel galaxy formation.

Questions about movie: Bud: What’s the yellow lump?

Mike: The stars from the other galaxy.

Kelly: What happens to them?

Mike: They keep on flying.

Monte: When a collision does cause a star to end up outside a galaxy, what happens there? Does not being in a galaxy affect them in any way?

Mike: Probably not. They’d have a slightly better chance of not being killed by a supernova.

Monte: So they’d be a bit safer for life?

Mike: Maybe. It could be cool to have aliens on a planet outside a galaxy. They’d figure out galaxies faster than we did. Probably.

Bud: What happens to the momentum of the two galaxies?

Mike: It’s conserved. There is a kind of friction that slows galaxies down, though. As the big galaxy goes through the little galaxy, it pulls some of the little galaxy’s stars into it. As it passes through, the situation is different when it passes through to the other side. Having more stars can slow it down. So it’s slower than when it collided with the other galaxy.

Colliding galaxies can also produce tidal tails with all kinds of star formation going on.

There’s another galaxy called the mice with interesting features.

Galactic interaction simulations:

Models of Merging Galaxies.
Chris Mihos’s New Galaxy Collision Movies

Mergers of galaxies:
*NGC7252: Probably the result of the merger of two galaxies about a billion years ago.
*Small galactic remnant rotating backwards.
*Multiple nuclei in gigantic elliptical galaxies.

Active galaxies: this is Mike’s specialty. So this is an image of a quasar, quasi-stellar radio object. What we see is a bright core with jets spurting out at mega-parsec scales. Jets squirt out. We think we have accretion onto a super-massive black hole in the center of the galaxy and these are giant, intergalactic-scale jets issuing from it. AGN for active galactic nuclei. These things can be thousands of times more luminous that the entire Milky Way, as a result of energy released from within a region about the size of the solar system.

Line Spectra of Galaxies: Taking a spectrum of the light from a normal galaxy they should have all colors.

Seyfert galaxies are unusual spiral galaxies with very bright cores and emission line spectra. They vary up to 50% in a few months, and have as their power source, most likely, accretion into supermassive black holes at about 100solar masses, up to billions of solar masses. They have emission line spectra, not absorption, like new star forming regions. The time scale of the variability can give us an impression of the size of these objects.

We don’t see the black holes, we see the hot gas in their accretion disks.

We see some objects where the accretion disk of the black hole appears to be perpendicular to its galaxy, and is jetting material out into the interstellar medium. Hard to write about this–you could write about aliens in the way of this sort of thing, but either they can’t do anything about it, or they have lots of time to do something about it.

Radio galaxies–Jets visible in radio and x-rays show bright spots in similar locations. Infrared images reveal warm gas near the nucleus. They show evidence for the galaxy moving through the intergalactic material. There are circumstances where the jets hit other galaxies, shock gas clouds, and trigger star formation. We see galaxies with two nuclei, two supermassive black holes, two accretion disks, and two jets coming out.

Formation of radio jets–jet production in active galactic nuclei are not well understood, but we think it has to do with conservation of angular momentum and twisted magnetic fields, that the disk has hot plasma right at the center that gets caught up in these twisted magnetic fields that’s very hot, very high velocity, and gets sent out along these magnetic field lines. This is tough research, we don’t have a lot of good observational data; we need to have solar-system-level data in the centers of galaxies far away, which is very hard to do.

The Jets of M87–nearby giant elliptical with more radio jets, which have a velocity of half lightspeed.

Model for Seyfert galaxies (on scales up to quasars) where we have an accretion disk in the center around a supermassive black hole, and this produces optical UV light. The gas is hot, high velocity, viscous. Heat shines brightly. There are gas clouds outside being ionized, and we see emission lines coming from them. And we think we have this structure called a torus. There are two types of seyfert: I and II. Seyfert I looks like a little quasar, you can see galaxy around it, it produces emission lines. Imagine trying to look through the dense dust torus, though, it would be like looking through something opaque. Try to look through the side and you can’t see the disk, but you can still see jets and ionized radiation. You might see emisison lines, but they’d be narrow. The gas is moving at 10,000km/s around the black hole.

Radio galaxies are Seyfert II, we see jets and the galaxy.

But if we look down the “hole” of the “donut” we get to see quasar light.

We don’t know how these supermassive black holes are made and grown. We know they grow when they shine as quasars, but before they can shine as quasars they have to build up to 100mil or 1bil solar masses. How that happens we don’t know but these things already exist in the early universe.

Quasars are the most luminous types.

Captive nuclei in elliptical galaxies with even more powerful central sources than Seyfert galaxies. Also show strong variability over time scales of a few months. Also show very strong broad emission lines in their spectra.

Kelly: Quasars are in the center of the galaxy just like the black hole is?

Mike: Quasars are kinds of black holes.

The spectral lines of quasars show a large redshit. They’re very distant and indicate they’re from early in the universe. They seem to be a phenomenon from early in the universe.

Quasars have been detected at the highest redshifts.

Quasars are the most luminous, non-exploding objects in the universe. They’re visible across the whole universe. They reveal the large structure of the universe and help us look back to the early history of the universe. They show us things about galaxy evoltuion and dark matter. And observing quasars at high redshifts lets us look back many billions of years to when the universe was only a few billion years old.

Gravitational lensing– we’ll look up and see two quasars in the sky, exactly the same. That’s because there will be a galaxy in front of the quasar, with the quasar almost directly behind, so the light from the quasar passes in two directions, meaning that we see the same quasar twice. There are dozens of these kinds of situations now. We can see quasars quadrupled or quintupled. If you get the geometry lined up perfectly, gravitational lensing can cause you to see the same object in a ring.

Einstein proposed in the 30s that we might see lensed objects in the future, because relativity predicted it. People thought that was ridiculous, but a few decades later, we saw it.

We often see quasars in colliding or merging galaxies. There therefore seems to need to be a trigger to make the supermassive black holes in the centers of galaxies turn on and become active galactic nuclei.

As Mike prepares to move on to cosmology, he reminds us to check out his online resources.

Mike’s quasar research:

He shows us hubble space images of quasars. Many of these have companion galaxies, show evidence of recent collisions (such as tidal tails), or are currently colliding. A video about it.

Back to Hubble’s law and the cosmological implications. We interpret Hubble’s law as an expansion of space. Over time, space itself expands, and carries the galaxies further apart. (The galaxies themselves are not expanding.) The galaxies next to you move away from you some, but the galaxies further move away faster. Space itself increases.

Dave: At what speed?

Mike: We usually characterize the expansion in terms of Hubble’s constant, or H./H. I can go into more detail, but it’s not the most straightforward thing. We use a concept of comoving distance, comoving volume. We concentrate on a region and can imagine it moving forward or backward.

We can take any galaxy and measure the same hubble’s law.

We can do a simple estimate if we assume the rate of universal expansion has been steady over time. For instance, if we run everything backward, assuming that we use today’s hubble’s constant, how long would it be before everything was occupying the same space? We can see objects 13 billion light years away (not well); the universe was only about a billion years old when the light left that galaxy.

Looking back toward the early universe: the more distant the objects we observe, the further back into the past of the universe we’re looking.

The observable universe — We can see in a sphere around us, in any direction, light coming to us from any object that’s emitting light and that’s had enough time for that light to reach us. We can see about 14 billion years.

When you go back far enough, the universe was very compressed, and very dense, and very hot. And at some point, we see light coming to us from the microwave background radiation.

At some point, in the history of the universe, conditions everywhere were similar to conditions on the surface of a star. It doesn’t look like that when we look up.

It turns out when you redshift a black body, it’s still a black body, but much smaller. We see that in all directions. So nearby we see galaxies; further away we see forming galaxies; past that, there’s a void where we see little; we keep going to higher redshifts and we see background microwave radiation. This was predicted before it was seen.

The cosmic background radiation–the radiation from the very early phase of the universe should be detectable today, and is. It was discovered in the mid-1960s as the cosmic microwave background radiation. It is the most perfect black body known to science.

When you first look at the microwave radiation, what you see is the galaxy. Dust, cool dust in our galaxy emits at the same … as the background radiation. Filter that out. Then there’s the dipole, which is the blue and red shift caused by our galaxy’s movement, and you see that effect in the background radiation, so you have to remove it. Then you’re left with an image of a 2.73 kelvin black body that’s got these little tiny deviations, about 1 part in 100,000, that are a little hotter or a little colder. You have to up your contrast to see that. There are a lot of steps to go through before you can discover the temperature fluctuations in the background radiation.

We’ve been looking really hard to find these fluctuations because it’s these hot and cold patches that correspond to the clusters and voids of galaxies that exist today. These small fluctuations represent not just temperature but density. All the gas is distributed very uniformly, except for these tiny deviations. Those deviations grow. The dense bits get denser, the less dense bits become voids. So cosmologists explain how these deviations evolved to become the large scale structure of the universe we see today.

Temperature correlates with density; time correlates with redshift.

Today the universe is about 14bil years old. If we chart temperature/density and time/redshift from one second to 14bil, then we see the temperature moving from 10bil degrees kelvin down to about 2 degrees kelvin. On this logarithmic plot, we can only see a small portion. Most of the dramatic shift of the line characterizes the early universe. The energy of radiation has changed, with the wavelengths increasing as the universe expands, and the energy decreasing. The conditions back in the first few minutes of the universe were similar to that in the cores of stars. We believe we only had hydrogen to start with–protons, electrons, and free neutrons, some of which were able to fuse in the first few minutes into helium. Also some other elements we find in star cores.

And then things cool. This happens before heavier elements like nitrogen and carbon can form.

Bud: Where did the light emerge?

Mike: The very beginning.

The universe is expanding and cooling until at some point you get protons, neutrons, and electrons that are forming without immediate collisions that transform them back into energy. Then you have time to make helium, but before you can make anything heavier, things have expanded too much.

There’s a point at which we go from being dominated by radiation to being dominated by matter, and we get recombination. Recombination is when the universe goes from being ionized to neutral (hydrogen and helium atoms where the electrons combine with ions to form neutrals). When this happens, the radiation decouples from the ions. Before that, the universe is opaque because the radiation is thermally linked to the matter. When we go neutral, and have hydrogen and helium ions, the gas becomes transparent, and there’s no longer thermal equilibrium. The gas becomes transparent. That’s when the background radiation comes to us from. That’s when the photons can fly across the whole universe, and be picked up as background radiation, or as static snow on TV.

Ian: Did the universe become transparent immediately?

Mike: No. It took tens of thousands of years.

There’s some reason why we have an excess of matter versus antimatter today, but this is a recent paper and Mike doesn’t remember. There was an excess initially on the order of one part per billion, and that’s where we are today.

Bud: I understand that the universe became transparent, but at that point there’s still no stuff. Why is there no stuff?

Mike: We think the first stars started forming in the first few hundred million years, but we don’t have observational evidence. We see galaxies from when the universe was less than a billion years old, some of the oldest galaxies.

Mike: It turns out that making the models without the metals is much harder than without the metals. It has to do with the fact that the opacities that govern energy transport in stars are strongly affected ins tars today by the presence of trace metals. Take those away and all the numbers change dramatically and it’s hard to understand how the processes work.

Bud: So we don’t understand how the first objects started to form?

Mike: No, it’s an active area of research. We don’t know, for instance, where these supermassive black holes come from and how they got so large. We see black holes from when the universe was less than a billion yrs old and we don’t understand how you can get that many solar masses so fast.

Mike: In the first few minutes, we get elements, but there are some gaps that make it hard for fusion to proceed. This is one of the strengths of the big bang theory which is what I’m describing-with general relativity, you run the model forward, and you get certain ratios of elements, and these predictions agree with what we observe. The big bang at its essence is correct observationally. There are details, especially in early areas of the big bang, that are active research and not understood. But if you define the big bang more generally, then from an observational point of view, the theory is unassailable. Details are still in question.

This big bang synthesis creates a baryonic mass (not dark matter) creates 25% helium, 75% hydrogen.

So we have this radiation dominated era following that which goes on for a few thousand years where we have conditions like the interiors of stars. The universe is hot. It ranges from millions to tens of thousands of degrees. It takes time to cool. It’s in thermal equilibrium.

Then we get to this redshift of about a thousand where we get recombination. Ions and free electrons make it difficult for photons to travel far before being scattered, like being inside a cloud. Even at relatively low densities, it’s still hard for optical light to travel through that. When things go neutral, suddenly it’s a lot easier. Photons can travel more or less freely through space. The universe becomes transparent. The light from this era can travel all the way across fourteen billion years of space.

Bud: So the universe was 14 bil light years across at that point?

Mike: No, it traveled 14 bil light years to get to us today. The size of the universe is complicated. Our current cosmological understanding points to a model of the universe in which the universe is infinite, which means it was always infinite when it was first born.

Ian: Even though it’s expanding?

Mike: Space gets bigger. Density was infinite at the beginning. Space wasn’t. But if the universe is infinite, it always was, and that’s what our theories indicate. Our observable universe is finite, but we know the universe expands past that; we don’t know whether it’s a lot past that or if it’s infinite.

A lot of times you’re better off thinking about density. What is the universe like in this region of space? How has that expanded over time? You’re safer talking about that rather than the universe which has an infinite space.

Bud: So basically this shell of microwave radiation is expanding outward into infinite space.

Mike: No.

Bud: But you said the universe was infinite.

Ian: Are you saying space is a chunk within the universe?

Mike: No, the universe is all space. The observable space is a chunk inside the space of the universe.

Ian: Okay…

Mike: My understanding of this is more complicated than yours, but there are probably ways in which I have trouble, too.

Ian: It would be easier if it were a closed universe.

Mike: It would, but it doesn’t appear to be.

Kelly: So how fast is the universe expanding?

Mike: It depends on variables like whether we’re dominated by radiation.

Kelly: What about right now?

Mike: We can’t say. We discovered this thing called dark energy, and we don’t know how it expands over time. I can give you two models, but we don’t know which is more likely.

Recombination–at 400,000 years, our universe goes neutral, becomes transparent, photons can fly freely toward us, but between the epoch of recombination when the temperature was 3000K and now, the universe expands by a factor of 1000. So our 3000K black body looks like a 3K black body.

We can’t see with EM any further back because the universe is opaque. In theory if we had neutrinos, we could look back until even earlier when the universe was opaque to neutrinos. So if you want to work on far future with astronomers, they could be working on the neutrino background.

Me: So going back to the one second level is theoretical projection, not observation?

Mike: More or less, though we have observational tests, like seeing how the elements are what we predict. We think we know what happened in the first few minutes. We have observational evidence at 400,000 years.

It turns out the intergalactic medium today is ionized, not neutral. At recombination, it went neutral. Today it’s ionized. So there must be an epoch of reionization. About ten years ago we started discovering objects with redshifts of six and seeing signs of reionization. The first stars form less than a billion years ago, and they reionize the gas.

Even though the universe is ionized now, the photons can still travel unimpeded because the density is so low compared to the last time they were ionized that the universe is no longer opaque.

We need to use general relativity to explain the relationship between stuff in the universe and space. We have to make assumptions that are probably wrong if you look at them in detail. We have evidence they’re wrong. But they seem to be lcose enough to correct that they work together as a predictive model.

One assumption–homogeneity: on the largest scales, the local universe has the same physical properties throughout the universe. Every region has the same physical properties (mass density, expansion rate ,visible vs. dark matter, etc.) It’s like a sponge. If you look at the large view of a sponge, it looks similar, but if you look on the local level, you see it’s not uniform.

Isotropy: On the largest scales, the local universe looks the same in any direction one observes .You should see the same large scale structure in any direction .We have evidence that there are preferred axes, but we don’t know why.

Universality: The laws of physics are the same everywhere in the universe. That may not be true. Observation based on quasars indicates that the speed of light may have changed. There are other versions of the big bang that include things like varying the speed of light. Some of these constants may not be. People are trying to think of experiments to test those assumptions.

Ian: The physical laws may have changed over time, but at any given time, are they the same?

Mike: Maybe. We don’t have any evidence that’s not true.

Shape and geometry of the universe: back to our 2-dimensional reality: how can a 2-D creature investigate the geometry of the sphere? You have to measure the curvature of space. In principle we may have space with a closed surface and positive surface .Or it could have an open surface with negative curvature. Closed surface would be finite space with no edge. Or it could be a flat surface with no curvature.

We have to test the curvature of space to discover what kind of universe we live in.

People experiment like this, not using 2-D shapes, but they try to calibrate things like the radio jets from quasars, and think if they can calibrate cosmic rods and see how their lengths change over different distances, maybe we can discover which of these three options are the case?

But it’s hard to calibrate what the intrinsic size of a cosmic rod is.

Cecilia: If you don’t think of the limit of the universe as an edge… it’s just not an edge… I don’t have a word for what it would be then, but then the universe would be infinite even when it was small because it would be the same inside, but a different density, a different age.

Nick: Like in an egg.

Cecilia: But not in an egg. There’s no shell. The shell is a limit, but not an edge.

Kelly: That’s a good word. Limit> We think of limit as being the same as edge but it’s not.

Ian: The limit is how far you can run in the time available, but there may not be a wlal when you fniish.

Mike: The observable universe gets a little bigger as light comes from slightly further away.

Cecilia: But even if we had a neutrino detector and could see back to the densest…

Mike: We’d still see more of it.

Cecilia: It would be the same thing even if it were a different size.

Mike: Maybe.

Cecilia: It would still be infinite.

Mike: Yes. I like to talk about densities. The volume can stay the same and the density can change. There can be a denser or more rarefied infinity.

Ian: That background temperature is decreasing or staying the same?

Mike: Decreasing.

Einstein’s zombie, breaking into class:Space-time tells matter how to move; matter tells space-time how to curve.

Carrie: Time exists so everything doesn’t happen at once, and space exists so it doesn’t all happen to you.

Deceleration of the universe: for a long time we thought the universe stars off expanding, but gravity, we know there’s stuff in the universe, and there’s gravity associated with that. The gravity should slow the expansion, like breaks on the expansion. So we talked about the universe decelerating by virtue of gravity and the stuff in it. And this was thought to be critical because we thought the universe was initially expanding (like the balance between potential and kinetic energy; throw a ball up and it will come down; throw it with more energy and the gravity will pull it down; there’s a certain point where if you throw it with more energy, gravity will slow it to zero and hold it at infinity; but in each case, the gravity slows it) and the gravity would slow it down.

There’s a critical density where the universe will expand to infinity and stay at infinity.

So for a long time we wanted to measure the density to figure out whether it would continue to expand, or start contracting sometime. We thought the stuff int he universe should be decelerating it.

These models were important because they told us how to modify the hubble constant. In the no gravity model (universe keeps on expanding at the same rate), it would be 14billion years old. In the other two models (collapsing universes or flat universes where the universe expands but not as fast), the universe would be younger.

But the universe is trickier. We have gravity, but we also have other things.

One thing we have is dark matter.

We know that we have about 4% of the critical density to keep expanding from baryons. But we also know there’s about 6-7 times as much unknown dark matter as there is unknown dark matter. Put all this together and you get about 30% of the critical density. In that case, we should be in an open universe, with an expansion rate slowing, but not very much.

So we have to estimate the total amount of matter, including the dark matter. We estimate the mass of dark matter by means like thinking about gravitational lensing.

So this big bang model tells us what the early abundances of light metals should be, and puts a tight limit on what the density of baryon matter can be today, and was at the beginning of the universe. This is why we’ve spent a lot of time and effort trying to figure out the abundances of rare elements like lithium and deuterium; they’re not very abundant, but they tell us a lot about the conditions in the early universe where they were formed.

The nature of dark matter: can dark matter be composed of normal matter? if so then it’s mass would come mostly from protons and neutrons (baryons). The density of baryons right after the big bang leaves a unique imprint in the abundances of deuterium and lithium. The density of baryonic matter can only be about four percent of critical density.

We know that we have about 30% of the critical density in total, and that baryonic matter is only 4%, so therefore we know that most of the stuff out there is dark matter.

(Skipping over MACHOs, which are probably white dwarfs, possibly black holes, but basically we look at the vanishing line of clouds over time and occasionally we see a gravitational lensing event where a star gets 40 or 50 times brighter for a few days and can estimate how many dark things are floating… it’s not enough to make up for all the dark matter.)

(Notes on silly astronomical acronyms like MACHO or WIMP or FIRST.)

Problems with the classical decelerating universe:

1) The flatness problem. The universe seems to be nearly flat. It doesn’t look open. Even a tiny deviation from perfect flatness at the time to the big bang should have been amplified to a huge deviation today.

2) The isotropy of the cosmic background. Why is the microwave radiation the same when you look in different directions? Normally if you want to have things at the same temperature they need to be in thermal equilibrium, but if you do the calculations you find out that the part of the universe we see in the background radiation was not in communication with all the other parts we can see, making it confusing that they could be in thermal equilibrium.

21st century cosmology–the solution: inflation! Inflation is a period of sudden expansion during the very early evolution of the universe, about 10^25 seconds, at a very high temperature, and when that happened there was a time period when neither radiation or matter dominated, but some aspect of the field associated with the splitting of the forces dominated, and in this tiny fraction of a second, the universe expanded by 60 orders of magnitude. So the universe might not be exactly flat, but if it had expanded that fast, it would look flat because the curvature is so small. Start with a very small region of space in thermal equilibrium and blow it up exponentially (faster than the speed of light, without violating relativity because space itself is expanding), it would give us what we see. This is not settled. It seems to work, but there could be alternate solutions.

Inflation is part of the big bang theory that could change in the next decade or our lifetimes, whereas the other parts of the big bang seem very solid and well-tested. Inflation explains some strange properties of the universe, but could change.

Measuring the deceleration of the universe… we talked about standard candles before, like cepheid variables. We can see them far away, but not halfway across the universe. Type Ia supernova, however, we can see halfway across the universe. (Galaxies have about one supernova per century, and the supernovas are visible for about a year, so you can see these pretty easily.) So they looked at the standard candle supernovas according to redshift.

What fits the data we gather from the supernova redshifts is a universe with flat geometry and an accelerating expansion.

People are trying to think of other ways of explaining this.

One explanation could be cosmic dust obscuring the supernovas, but that was disproved.

Another team looked for supernovas again, and found the same results–expansion accelerates today, not slows. Why? And why is the universe flat? Why are we close to criticial density?

One solution that seems consistent with the data is the cosmological constant. The constant refers to a source of energy density through out space. Energy and mass are interchangeable in terms of relativity. So we think dark matter is about 30% of critical density; this constant give us the other 70%. It doesn’t act like gravity to decelerate. It acts to push us apart. This is dark energy.

People like to associate this with the energy from quantum foam, but when they try to make that come out the cosmological constant, it’s way off.

So we don’t understand something. And we don’t understand it a lot.

We don’t know if the dark energy is really a constant. If it is really a constant than we can predict from our current models that initially there was a period of radical acceleration, then a period of deceleration where gravity slowed things, and now we’re in a period where the constant is stronger than gravity again (because the objects are no longer close enough together to make gravity strong enough to overpower the constant), and this gives us exponential expansion. If we have constant dark energy, the universe will evolve so that what happens over time is that parts of the universe that were in causal contact will be disconnected. Our galaxy will hang together, but the other galaxies will fade from sight and we will no longer be causally connected to them, and the light from those galaxies will never reach us.

The other possibility is called a big rip universe in which the dark energy increases in power in the future (and is not a constant) and in that case, it increases and increases and increases and atoms themselves are ripped apart.

Me: What’s the time scale to galactic or atomic separation?

Mike: I’ll have to check it out.

Large-scale structure: we see correlations between the fluctuations in the microwave background, and the kinds of large-scale structure we see today. Our projections from the microwave background don’t produce exactly the universe we see today, but they produce a similar universe with similar clustering and similar properties.

A large survey of distant galaxies shows the largest structures in the universe: filaments, walls of galaxy super-clusters, and voids, basically empty space.

Modern cosmology, the name of the game these days is primarily understanding what we can learn from the microwave background fluctuations. This is not a super-complicated topic, but it’s not conceptually simple either. These fluctuations have to do with acoustic waves, sound waves traveling early through the universe, and which ones get locked in so we can see. Analyzing the pattern of acoustic waves will give you the hubble constant, as well as lots of other important information. We can see structure in the cosmic background pattern. (Here’s the pattern. Thanks, Dave Williams!)

Here’s another way of looking at the microwave background fluctuations. It’s called a power transform. If we talk about looking for fluctuations in different angular sizes, basically if you took a caliper with a separation of one degree and put it down everywhere in that image, you’d be measuring how often you got the same temperature and how often you got a different temperature. We check our data against the model predicted by the flat universe, and they look very similar.

This is cutting edge cosmology, analysis at this level. It’s getting more abstract and harder for the public to follow. (Mike says it’s getting harder for him to follow, too, at the intermediate and advanced levels.)

(Mike says he needs to understand cosmology so he can turn the redshifts of his quasars into real distances.)

Cosmology great web resource–

Contains a cosmological calculator, a tutorial, and an FAQ list.

Also Wayne, with a webpage at

The angular size of these fluctuations lets us probe the geometry of space-time.


Fritz Zwicky discovered dark matter. He invented the term spherical bastard, because any way you look at ‘im, he’s a bastard.

In 1933, he realized that there should be something called dark matter holding structures together since otherwise they would fly apart.

We call him the father of dark matter, but Vera Rubin is called the mother of dark matter. A few decades after Zwicky, she started to notice flat rotation curves in spiral galaxies, noting the shape only worked if you thought about dark matter.

WIMPs are weakly interacting massive particles, such as neutrinos, which have mass but are too small to account for dark matter mass.

Are we sure dark matter is real? Or could we just have gotten gravity wrong on large scales? We are sure, and the smoking gun for the reality of dark matter is the bullet cluster. The bullet cluster is two galaxy clusters interacting, one moving through another. The reason this is important is we can get the total matter of these two galaxies, and also figure out how matter is distributed between the two galaxies .We know dark matter is not strongly interacting, so we know that two clouds of dark matter will pass without interacting. However, most galactic mass is in hot gas, and the two big clouds of hot gas will slow down as they pass through each other. So in principle this kind of cluster collision ought to separate the dark matter and baryonic matter, highlighted by the hot gas. That is a natural prediction based on dark matter. And what the image shows is the red light highlights the gas that dominates the mass of the clusters. The blue light is a model of where the dark matter is based on the gravitational lensing. So the center of mass of this cluster is in the blue, and the baryonic matter is between the blue and red. They’ve been separated.

There have been since then other interacting galaxy clusters that show the same thing. This is unlikely to be gravity being wrong on large scales. This is explained by dark matter theories.

We watched a youtube video simulating the bullet galaxy clusters collision, and then a 4 minute excerpt from Nova. I think this is the NOVA video. I think this is the simulation of the collisions, but I can’t get it to play on my computer, so good luck.

So! This is my last post on the lectures. We’re headed to WIRO (telescope the size of hubble) tonight since we weren’t able to last week. I’ll write a wrap-up in the next couple of days.

It’s been fun!


  1. Drax says

    SAOImage DS9: If I can figure out how to properly use this, I think I might love it! Thanks.

  2. says

    I’ve really enjoyed reading your notes on the lectures. Thank you so much for sharing them.

    I hope it’s okay, but I’ve put a kind of ‘quick reference’ to your notes on my blog so I can refind the bits that have inspired me ( When you put up your wrap-up post, I’ll add the link to my post :)

    Again, thank you for doing this for us.