“Its turtles all the way down!”
Glen Rees is a guide at Sydney Observatory, and is currently completing his PhD at Macquarie University and CSIRO. His PhD is supposed to focus on improving our understanding our universe as a whole, but he often gets distracted by interesting side projects involving black holes, galaxy evolution and star-formation.
I am writing to you from Sydney Observatory, 1003 Upper Fort St, Millers Point, Sydney, NSW, Australia. But just how much more of an address do we have? Perhaps in the next century we will have to add ‘Earth’ to our postal address, but what would our address be if we wanted to guarantee we received mail from EVERYWHERE?
Throughout history Astronomers seeking to understand our place in the universe have steadily added to our cosmic address. We identified other planets, suns, clusters and eventually the vast cosmos as we currently know it, filled with countless galaxies and an infinitude of stars. Our address is much larger than we might at first have thought. Currently there are countless scientists studying how all of these objects form, evolve and die but Cosmology, the study of this universe as a whole still seeks to answer those very oldest of questions: ‘what is our place in the universe, where did it come from and what will be its ultimate fate?’
Currently the constraints we have on our universe’s fundamental properties (how fast it’s growing, how much matter there is, how powerful gravity is etc.) are far better than could be conceived of even 50 years ago. Even better is the fact that these incredibly accurate observations match the predictions from our Standard Model of Cosmology very well. Unfortunately for us, the standard model actually requires the invocation of not one but two extra components that we have very little understanding of: dark matter and dark energy. Dark matter is what we need to exist in order to explain why gravity seems to be acting much stronger than it should be on large scales. Dark energy is included in our models to explain the recently discovered accelerating expansion of the universe. The down side of this is that while these two components allow us to accurately match our models to observations we have no real idea what they might be caused by, or made of. This is pretty embarrassing since between them they make up over 95% of our universe!
Theoreticians are constantly coming up with new theories of what these two unknowns are, but the only way to distinguish which model is correct is to test them against new observations.
Some of these new observations will come from the new Australia Square Kilometre Array Pathfinder (ASKAP). This mammoth telescope will consist of 36 radio telescopes spread across the vast Murchison Desert in Western Australia and will be used for a number of different science projects, from probing the formation of galaxies, to testing Einstein’s theory of general relativity on the largest scales.
In particular, the project I’m currently a part of (when not working here at Sydney Observatory) is ‘The Evolutionary Map of the Universe’ or “EMU” survey. EMU is going to focus on understanding how galaxies form and evolve across billions of years but it also turns out that the sheer scale of these observations (covering more than 75% of the sky at radio frequencies and producing images of over 70 million galaxies) is going to make it incredibly useful for testing Cosmology as well!
But how on earth can a set of images (no matter how awesome) tell us about things as fundamental as the shape and age of the universe?
The thing about our universe is that it’s actually pretty weird out there! Galaxies form groups or ‘clusters’ depending on the strength of gravity, they warp space-time into lenses allowing us to see even further away than normal and the universe itself is expanding with every passing second! This leads to some very odd observable effects in our night sky and by measuring these effects we can tell something about what causes them.
For example, imagine a universe where gravity was greatly increased. In such a universe galaxies would clump together much more strongly than we observe and hence you can imagine that by changing the strength of gravity you might eventually reproduce the same clustering of galaxies that we observe in reality!
Now instead imagine a universe where dark matter didn’t exist. Apart from causing galaxies to fly apart under their own spin (which would be bad), it would also result in far less ‘cosmic magnification’ which is where the mass of foreground galaxies warps space into a lens, magnifying distant, usually unseen, background sources. Hence you would see far fewer distant galaxies that we do in reality.
Lastly imagine a universe with 10 times more dark energy than we think is out there. Dark energy powers the accelerating expansion of the universe and this causes the light from distant sources (or even light from the initial Big Bang) to be slightly shifted in its colour as it passes through the gravity field of large clusters of galaxies. How much it shifts depends on how fast the universes is accelerating so too much dark energy will result in much stronger colour shifts than we usually see!
Hopefully you can kind of see where this is going. By creating a simulated universe and then simultaneously tweaking all the physical laws that govern it, we can eventually reproduce something that looks very much like reality. From this it’s as easy as reading off which physical laws and inputs best match the observed sky and voila, you have a new measurement of the universes laws!
By combining this with the most powerful of previous studies, we hope to increase the accuracy of our current cosmological constraints by a factor of 5 or even 10! This will allow scientists to put to the test even the most outlandish of currently acceptable modifications to the Standard Model, including the existence of multiple universes and higher dimensions!
Maybe someday soon we will have to expand our cosmic address even further!
Our full Cosmic Address: Sydney Observatory, 1003 Upper Fort St, Millers Point, Sydney, NSW, Australia, Earth, The Solar System, Orion Arm, The Milky Way, Local Group, Virgo Cluster, Virgo Super-Cluster, Universe … One?