Baryonic acoustic oscillations are sound waves that spread through the early universe. They were generated by the sudden clumping together of mass in the process that led to the formation of the first galaxies.
As luck would have it, we have a record of the universe as it was at that time in the form of the cosmic microwave background. A common misconception about the microwave background is that it is an echo of the Big Bang but this light first appeared in the universe some 400,000 years after the big event. Before then, the cosmos was filled with plasma that prevented light from travelling any reasonable distance. As soon as the universe cooled sufficiently, light began to leak into the cosmos. The cosmic microwave background is this light and gives us a snapshot of the universe as it was at that instant.
What it shows is that matter in the universe was smoothly distributed everywhere. There are some ten orders of magnitude difference between the variations in density then and the variations we see now. Clearly something has changed.
The thinking is that matter was gravitationally attracted to any small variations in density and that these grew rapidly. These original density variations were caused by quantum variations in the universe immediately after it formed and also by the sound waves that were travelling through the universe as it cooled.
We can see the variations in the cosmic microwave background that quantum variations should have caused. But the variations due to sound waves are much smaller and so more difficult spot.
Nevertheless, we know that galaxies ought to have formed in the regions of overpressure in these sound waves. So the distribution of galaxies around us should reflect that.
We know relatively little about the detailed distribution of the galaxies in our part of the universe (or any part of it for that matter). However, astronomers are gathering this data right now in a project called the Sloan Digital Sky Survey which is measuring the red shift of galaxies nearby.
This is where the physics becomes a little mirky. Various groups who have analysed this data say they can see in these galaxy maps the characteristic pattern that sound waves ought to have produced. That’s a difficult call–this isn’t a simple pattern by any means. Imagine the pattern of waves a handful of thrown stones would make on the surface of a pond and that’s a just a simple 2D version of a problem the size of the universe.
So it’s no surprise to find that others looking at the same data say they’ve found nothing.
Today, we get a new insight into this problem thanks to the work of Anna Cabre at the University of Pennsylvania and Enrique Gaztanaga at the Institute of Space Sciences in Barcelona, Spain.
These guys have done away with the universe entirely and used a simulation instead carried out by the Mare Nostrum supercomputer in Barcelona (not the world’s most powerful but surely the most beautiful).
This simulation shows how 8.5 billion particles behave under the force of gravity in starting conditions similar to those in the early universe. The result is a simulation of the evolution of a good fraction of the universe which ends up looking remarkably similar to the one we actually inhabit. And not satisfied with doing this once, the simulation has been done a couple of hundred times so that its statistical properties can be analysed.
Cabre and Gaztanaga use this model to ask an interesting question. They first imagine themselves inside their model and then ask whether the kind of measurements we can make on Earth would actually reveal the baryonic acoustic oscillations. Is it possible to spot baryonic acoustic oscillations in this model?
Their answer is a qualified yes. That’s reassuring but of course, it’s not proof that the measurements that have been made so far really do show the existence of these oscillations. There’s more work to be done on that score.
But why the fuss over a few sound waves? The reason astronomers are excited about baryonic acoustic oscillations is that they can calculate exactly how big they ought to be–about 500 million light years in today’s universe.
So these oscillations are a kind of yardstick with which to measure the properties of the universe on this scale, including things like the accelerated expansion of the cosmos. There’s no other way to do that so there’s a lot at stake for those who find them first.
Ref: arxiv.org/abs/1011.2729: Have Baryonic Acoustic Oscillations In The Galaxy Distribution Really Been Measured?