In questioning the fundamental nature of the universe, cosmology regularly grabs the public’s attention.
But in an era in which we are observing deeper and more widely than ever before, our knowledge of the make-up of the universe is both unsatisfying and unsettling.
It all started with the realisation that we can describe the dynamics of the universe within a mathematical framework – namely Einstein’s General Theory of Relativity – and that these dynamics are implicitly dependent upon what the universe is “made of”.
By the close of the 20th century, after decades of observations with the largest telescopes on Earth and in orbit, after checks, calibrations and extensive analysis, we knew what the universe was made of.
And it was not what we expected.
Dark matter and dark energy
We had known for some time that the motion of stars is not dominated by the gravitational pull of the matter we can see, but apparently by a pervasive “dark matter” which envelops galaxies, including our own Milky Way.
In fact, it’s this dark matter which guides the sun in its orbit, not the 200 billion stars we can see.
On cosmic scales, not even dark matter dominates, and it was established that the universe is expanding at an increasing rate.
It’s been suggested that this is due to the presence of a ubiquitous energy unlike anything encountered previously.
This enigmatic substance was given the name “dark energy”.
Knowing what we don’t know
We appear to have learnt a lot, but we are left with a rather stark reality: while we live in a dark-dominated universe, we don’t actually know what this dark stuff (be it matter or energy) actually is.
Again, after significant observational effort, we now know a lot about what they are not. For example, we know that dark matter is not a sea of massive black holes.
But until we discover the source of the universe’s dark sector – be it a particular particle or fundamental property of space itself – we are faced with this ongoing mystery (or, as some would say, embarrassment).
Testing the dark cosmological model continues with more intricate and detailed experiments, with a recent focus upon the imprint of dark matter and energy on the very distribution of galaxies through the universe.
Again, the dark cosmos accurately accounts for what we see, with media headlines telling us that observations tell us that dark energy is “real”.
But such a proclamation makes many uneasy, because this “real” comes with some important caveats, caveats that are typically missed by the media.
Context is everything
The important issue is that these observations are interpreted within the framework originally laid down by Einstein. If this framework is the correct description of the universe, then we cannot do without the dark sectors.
But what if this basic framework is not correct?
Speculation is rife that this is the case, with myriad proposed modifications to both Newton’s laws of motion and Einstein’s theory of relativity.
These range from straight-forward extensions, to well tested ideas right through to outright speculation and guesswork.
Frustratingly, the difference between these theories is rarely discussed in the mainstream media.
This is important because any alternative theories are competing with the exceedingly well tested laws of Newton and Einstein.
The future is dark
Cosmologists such as myself are likely to continue to use the established laws to explain our observations, even though they contain the unsatisfying concepts of dark matter and dark energy.
Of course, the search for the dark matter particle is now the key goal for a range of experiments (including work being done at the Centre of Excellence for All-Sky Astrophysics) involving many researchers from around the world.
While we may favour our current framework in explaining the universe, virtually all scientists involved must have a niggling doubt that those closer to the fringe may be right.
But this should be the normal state of science, with no theory conclusively proved.
Any good scientist should be aware that the rug could be pulled out at any time, with cherished ideas assigned to the rubbish-bin.
So, is dark matter a currently undiscovered particle rather than a modification of the laws of physics? My gut tells me that this would be a good bet.
But is dark energy really a weird energy that pervades the cosmos? While I might be willing to risk a $1,000 flutter, I am not prepared to bet my house on this.
I am sure many other cosmologists would agree.