Surprisingly or not we still don’t know what most of the universe is made of! But then again it’s a big place with lots of stuff (technical word) in it and we’ve only be observing it with helpful technical assistance for four hundred years or so. Decades ago we thought we had solved this puzzle but in recent times the plot has thickened with the discovery of the aptly named dark matter and dark energy which have become holy grails in cosmology, and without understanding these we can never form any firm conclusion about the nature or future of the universe.
Unaccounted-for gravity, what next! Decades ago astronomers realised that the gravitational force in galaxies couldn’t be sufficient to bind them together. Instead they should be falling apart. So a typical spiral galaxy like our own Milky Way galaxy is rotating around its center so fast that the laws of physics mean the stars should fly off into intergalactic space. But physics has a problem because they don’t! So there had to be something else out there, accounting for this extra gravity to hold the stars in orbit, something holding them together that couldn’t be seen or detected. A sort of ‘cosmic glue’ which became known as ‘dark matter’. In fact it was shown in calculations that the majority of the mass of the universe, up to 85%, is made up of this invisible dark matter. It seems to be everywhere and yet it couldn’t be detected as it neither emits, absorbs or reflects radiation and it doesn’t react with the standard model of atomic matter we are familiar with. The only evidence of its existence so far comes from gravity as its presence is betrayed by its gravitational influence on regular matter.
Could Einstein’s general relativity be wrong in which case we may not need this dark matter to explain the rotation of galaxies? Basically either the laws of gravity are wrong or the estimated mass of the galaxies is wrong.
Briefly, the early history of dark matter started with the astronomer Fritz Zwicky who was the first to notice the unexpected rotation of galaxies when he observed the Coma galaxy cluster in the early 1930s and he concluded that there must be missing, invisible or unseen matter, and lots of it in these galaxies to account for his observations. The galaxies weren’t behaving as they should. However the world of cosmology seemed to largely ignore Zwicky’s conclusions and it was left to the American astronomer Vera Rubin decades later in the 1970s to re-affirm the existence of dark matter with her work on the rotational rates of galaxies. The stars she observed moved much too fast and this movement could not be attributed to the observed stars and interstellar gas alone. She concluded there had to be more unseen mass influencing this movement.
The galaxies we see strewn across the universe today wouldn’t exist if it were not for the gravitational effects of this dark matter as gravitational effects of observed matter would not be powerful enough to clump matter together to form galaxy structures. It’s the biggest building block in the universe and is responsible for our very existence. The universe is full of it, and it plays a critical role in determining the fate of the cosmos in the battle between expansion and contraction, and yet we know very little about it. We know what it’s not; diffuse clouds of gas since the absorption and emission of electromagnetic radiation would be a tell tale sign which we don’t observe. So what might it be? Chief candidates are (in no particular order);
1. MOND and MOG: Perhaps gravity is stronger on intergalactic scales than predicted by Newton and Einstein. How credible is MOND (Modified Newtonian Dynamics)? According to Stacy McGaugh, one of the MOND founders it may eliminate the need for dark matter altogether as a few modifications to Newton’s Laws on gravity (tweaking the inverse square law) can explain the observed discrepancies in galaxy rotation. And MOG (modified gravity) is another proposal for explaining the discrepancy in observations without invoking dark matter, see this link.
Does the Bullet Cluster collision spell the end of modified gravity theory. When the galaxies in the bullet cluster collided the stars continued on their paths unhindered due to the enormous distances between stars. However as the hot gas in the merging galaxies collided the friction caused a drag and a slowing down of the hot gas. So the stars now had separated from the clouds and allowed astronomers to measure their total mass (without the trailing hot gas). They discovered that the mass was far higher than expected indicating that something unseen had also travelled through the galaxy collision, namely dark matter! However Mordehai Milgrom, the founding father of MOND, thinks that this Bullet Cluster evidence can still be explained by MOND, see this link.
2. Is it Baryonic matter, as in the standard model particles (protons, neutrons) that don’t emit or absorb enough radiation to be detected such as MACHOs (MAssive Compact Halo Objects) in the outer reaches of galaxies, dead or failed stars like black holes, brown dwarfs or white dwarfs? Many of these stellar remnants are too faint to observe directly but many have been detected through gravitational micro-lensing, but still only accounts for a fraction of the known dark matter.
3. Or is it non-baryonic matter like theoretical WIMPS (Weakly Interacting Massive Particles) or Axions (low mass)? While neutrinos are weakly interacting (not massive), and populate the galaxies in great numbers their mass is not nearly large enough to account for the observed rotational effects. However it’s conceivable that there could be as yet undiscovered WIMPS, such as the hypothetical neutralinos, populating the galaxies, so the search goes on through experiments in trying to create them in particle accelerators or trying to detect them coming from the cosmos through highly sensitive detectors.
4. Or is Dark Matter or rather the effects of dark matter just the gravitational attraction of ordinary matter, from another parallel universe, on our universe. Can gravity transcend universes? Perhaps we can see its effects in our universe but can never see the cause. If there are parallel universes maybe gravitational energy is not bound to any one universe but ‘leaks’ across many universes.
Are there scientists in a parallel universe to ours scratching their heads and grappling with the same conundrum where the stars and galaxies that we can see when we look up to the sky are having an effect in their parallel universe? Of course if that is the case why do we not observe this ‘leaking’ gravity effect on smaller scale galaxy structures like our solar system?
Dark matter – the hunt goes on. It’s hiding in the dark (sorry!) but it can’t hide forever, and we have found a chink in its armour – gravitational lensing. We can see the distortion of light as photons pass through dark matter since light doesn’t discriminate between ordinary matter and dark matter. In fact now astronomers are able to produce dark matter maps in individual galaxies, galaxy clusters and superclusters. The Dark Energy Survey (DES) has produced detailed maps showing large amounts of dark matter concentrated where visible galaxies and galaxy clusters are positioned and large areas empty of dark matter where very few galaxies reside see here.
Dark matter appears to be evenly spread in halos around galaxies. But why does it not clump together over time to form large clumps of dark matter. Does it not obey the laws of gravity like normal matter while at the same time can cast gravitational affects? Why do stars and planets not contain dark matter, why is their mass not split roughly 80%/20% rule between dark matter and observed matter, or planets for that matter – why is it that it’s the space between stars that seems to contain dark matter and not stellar objects or Earth itself ?
Dark matter has collapsed (due to gravity) to form the cosmic web structure and galactic halos we see today, but why not collapse further and be incorporated into galactic objects like stars and planets? How does dark matter oppose this fundamental law of gravity?