Introduction to Cosmology

Cosmology is the study of the universe as a whole. Other astronomers study specific objects in the universe, asking questions like:

Cosmologists, on the other hand, are concerned with the big picture of how all the different pieces came to be arranged the way they are. For example, cosmologists ask questions like:

My specific research interests within cosmology are:

Lyman alpha Emission and Cosmic Structure

My PhD project, supervised by Martin Haehnelt at the Institute of Astronomy, has looked at the light emitted by galaxies as they form. We believe that galaxies grow by gravitational instability: lumps of matter grow by attracting nearby matter. The more mass in a lump, the stronger it attracts the surrounding matter, and the larger it grows - the big get bigger.

I am specifically studying the emission of Lyman alpha (Lyα) radiation, which is light at a certain frequency that is emitted by neutral hydrogen. There are three ways that we think that galaxies produce Lyman alpha radiation as they form:

More details on my work can be found in my thesis.

The Expansion of Space

My masters project at the University of Sydney looked at geodesic motion in expanding spacetime, and its connection with the expansion of space. I was supervised by Geraint Lewis, and collaborated with Berian James and Matt Francis.

The ideas of modern cosmology have always been prone to misinterpretation. Tamara Davis and Charles Lineweaver wrote an article for Scientific American entitled Misconceptions about the Big Bang, which is a excellent introduction to the topic.

There has been debate among cosmologists recently about the interpretation of cosmological models, namely the Friedmann-Robertson-Walker (FRW) models. In particular, are we justified in saying that the expansion of the universe occurs because space itself is expanding? What exactly does the expansion of space mean? How can empty space expand? Are some galaxies really moving away from us faster than the speed of light? Does the expansion of the universe affect the way things move locally?

Our work focussed on the last question - what does the expansion of space mean for how things move through the universe? Our attention was drawn to the topic by the following comments by John Peacock in his textbook Cosmological Physics:

The idea of an expanding universe can easily lead to confusion … The worst of these is the 'expanding space' fallacy. A common interpretation … is to say that the galaxies separate 'because the space between them expands', or some such phrase. From a global point of view, this is a sensible statement: the total volume of a closed universe is a well-defined quantity that increases with time, so of course space is expanding. But does this correct global idea have a meaningful local counterpart? Is the space in my bedroom expanding, and what would this mean?

… there is no local effect on particle dynamics from the global expansion of the universe: the tendency to separate is a kinematic initial condition, and once this is removed, all memory of the expansion is lost. [We] have proved that 'expanding space' is in general a dangerously flawed way of thinking about an expanding universe.

It seemed to us that Peacock was mistaken to claim that "all memory of the expansion is lost" when a particle is removed from the expansion of the universe (the "Hubble flow"). Peacock admits that the particle rejoins the Hubble flow eventually, but because it can initially move toward the origin, this is not an effect of expanding space. Alan Whiting made even stronger claims, saying that particles don't really rejoin the Hubble flow.

Our first paper looked at "joining the Hubble flow": exactly what it means for a particle to join the Hubble flow and what conditions need be fulfilled in order for this to happen. The results: it depends! There are a number of different ways to define what it means to join the Hubble flow. For some of them, the expansion of the universe is enough to ensure their fulfillment; for others, they will fail if the expansion of the universe decelerates too much. The question was: what does this mean for the expansion of space?

Our second paper attempted to defend the concept of expanding space against the objections leveled against it. Our conclusion was that, while the concept is not without its subtleties, if formulated correctly it can serve as a useful conceptualisation of FRW models. Finally, our third paper replied to some new objections that had appeared in the literature.

More details can be found in the papers, and in my masters thesis:

Dark Energy

My honours project at the University of Sydney looked at the effect of dark energy on cosmological models and observables. I was supervised by Geraint Lewis and collaborated with Matt Francis and Eric Linder, from the University of California, Berkeley.

We published a review paper in PASA, summarising the effects of some generic dark energy models. More details can be found in my honours thesis:

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Black Holes

Kerr Black Hole Accretion Disks

My masters project at the University of Sydney also looked at how light moves in the vicinity of a black hole. Einstein's General Theory of Relativity predicts that the path of a light ray will can be bent by the force of gravity. In the case of weak gravitational fields, like those on the surface of Earth, this would cause the light to slightly deviate from a straight line path. However, in the very strong gravitational fields around black holes, gravity can make light follow funky paths like this:

A funky path of a light ray passing close to a black hole.

The grey section of the path passes behind the black hole, shown as a grey sphere.

I considered the case of an accretion disk around a spinning (Kerr) black hole. As matter falls toward the black hole (i.e. as it accretes), it will often settle into a disk of matter orbiting the black hole around its equator. (It's a bit like Saturn's rings). We can't see the black hole (hence the name), but we can see the accretion disk. In fact, black hole accretion disks are thought to be one of the brightest things in the universe. If the black hole didn't bend the paths of light rays, then the disk would look like an ordinary disk, viewed at an angle:

An accretion disk around a black hole, with no light bending.

The top of the picture shows the back of the disk - it's as if you're looking down on a dinner plate, with a hole in the middle. The black circle indicates where the black hole would be. The disk is coloured according to the distance from the centre of the black hole. (Black hole accretion disks aren't really rainbow coloured, I'm afraid).

If we now turn on light bending, the same disk looks like this:

An accretion disk around a black hole, with light bending.

One of the main effects is that the back of the disk (at the top of the picture) looks as if it has been bent upwards. this is because light rays from the disk are being bent toward the black hole, and so appear to be coming from a point above the disk. This is shown in the following diagram, where we view the accretion disk edge on:

The accretion disk, as viewed from the side.

The solid red line shows the actual path of the light ray, while the red dashed line shows where the ray appears to be coming from.

The other thing that you can see in the image of the disk is the bit at the bottom. The top of the picture shows the far side of the disk; the middle of the picture shows the near side of the disk; the bottom of the picture shows the underside of the far side of the disk. Light rays are passing under the front of the disk, then being bent by the black hole up onto the underside of the far side of the disk. This is shown in the black dotted line in the diagram above.

Unfortunately, telescopes aren't powerful enough yet to actually see images like the ones I've simulated.

More details can be found in my masters thesis:

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