When we have observed distant galaxies we noticed something peculiar about the way they move. Instead of moving like we’d expect them to, with things further away from the centre moving slower than those closer, everything past a certain point moves at about the same speed. This is contrary to how other, non-galaxy sized systems like our solar system move and so we’ve long looked for an explanation as to how this could occur. The commonly accepted theory is that there’s extra matter present throughout the universe that we can’t see but interacts through gravity, called dark matter. Direct detection of dark matter has so far eluded us however due to its incredibly elusive nature. However there’s one experiment, called XENON1T, that could potentially shine some light on this elusive substance.
XENON1T is an evolution of previous experiments XENON10 and XENON100 which all shared the same goal: direct detection of a dark matter particle. Even though dark matter is theorized to be abundant everywhere in the universe (millions of dark matter particles would be passing through you every second) since they rarely interact detection is incredibly difficult. However just like the neutrinos before them there are ways of making detection possible and that’s what the XENON series of experiments aimed to do. At the heart of the experiment is a cylinder of liquid xenon, a chilly -95°C, bounded at each end by an array of photomultiplier tubes which are essentially extremely sensitive cameras. The thinking goes that, should a dark matter particle interact with the liquid xenon, a flash of light will occur which can then be analysed to see if it was a dark matter interaction.
The process by which this is determined is pretty interesting relying on 2 different types of interactions in order to determine what kind of particle interacted with the liquid xenon. The first signal, dubbed S1, is the flash of light that occurs at a very specific frequency of 178nm (ultraviolet). The photomultipliers are sensitive enough to be able to detect single photon events so even the smallest interaction will be captured. This photon is then brought upwards through the liquid xenon by an electric field that’s present in the gaseous section of the XENON1T device and when it leaves the liquid xenon it rockets upwards. The photon then leaves a trail of ionization in the gas behind it, dubbed the S2 signal, which allows for the exact position of the interaction to be determined. This is critical as the researchers only want events from the centre of the device as that has a greatly reduced rate of background noise.
The methodology was thoroughly validated by the two previous experiments even though both of them failed to directly detect any dark matter. They did put bounds on the properties of dark matter however, notably their size and potential electron spin. XENON1T is going to be around 100 times more sensitive than its previous brethren so even if it fails to see anything it will put even more stringent constraints on how a dark matter particle could be constructed. This is critical in validating or eliminating certain particle physics models and could give credence to other non-standard physics models like Modified Newtonian Dynamics (MOND).
Research such as this is incredibly important in developing an accurate understanding of how our universe operates. No matter the outcome of the experiment we’ll learn something valuable and it could potentially drive research towards a whole new world of physics. It won’t change our daily lives but ensuring our understanding of the world is as close to its knowable truth is the heart of science and that’s why this research is so important.
It was 3 years ago that particle physicists working with CERN at the Large Hadron Collider announced they had verified the existence of the Higgs-Boson. It was a pivotal moment in scientific history, demonstrating that the Standard Model of particle physics fundamental basis is solid. Prior to this announcement the LHC had been shut down for a planned upgrade, one that would see the energy of the resulting collisions doubled from 3.5TeV per beam to 7TeV. This upgrade was scheduled to take approximately 2 years and would open up new avenues for particle physics research. Just last week, almost 3 years to the day after the Higgs-Boson announcement, the LHC began collisions again. The question that’s on my mind, and I’m sure many others, is just what is LHC looking for now?
Whilst the verification of the Higgs-Boson adds a certain level of robustness to the Standard Model many researchers have theorized of physics beyond this model at the energies that the LHC is currently operating at. Of these models one that will be explored by the LHC in its current data collection run is Supersymmetry, a model which predicts that each particle which belongs to one of the two elementary classes (bosons or fermions) has a “superpartner” in the other. An example of this would be an electron, which is a fermion, would have a superpartner called a selectron which would be a boson. These particles share all the same properties with the exception of their spin and so should be easy to detect, theoretically. However no such particles have been detected, even in the same run where the Higgs-Boson was. The new, higher energy level of the LHC has the potential to create some of these particles and could provide evidence to support supersymmetry as a model.
Further to the supersymmetry model is every new particle physicist’s favourite theory: String Theory. Now I’ll have to be honest here I’m not exactly what you’d call String Theory’s biggest fan since, whilst it makes some amazing predictions, it has yet to be supported by any experimental evidence. At its core String Theory theorizes that all point like particles are made up of one-dimensional strings, often requiring the use of multi-dimensional physics (10 or 26 dimensions depending on which model you look at) in order to make them work. However since they’re almost purely mathematical in nature there has yet to be any links made between the model and the real world, precluding it from being tested. Whilst the LHC might provide insight into this I’m not exactly holding my breath but I’ll spin on a dime if they prove me wrong.
Lastly, and probably most excitingly for me, is the prospect of discovering the elusive dark matter particle. Due to its nature, I.E. only interacting with ordinary matter through gravity, we’re unlikely to be able to detect dark matter particles in the LHC directly. Instead, should the LHC generate a dark matter particle, we’ll be able to infer its existence by the energy it takes away from the collision. No such discrepancy was noted at the last run’s energy levels so it will be interesting to see if a doubling of the collision energy leads to the generation of a dark matter particle.
Suffice to say the LHC has a long life ahead of it with plenty of envelope pushing science to be done. This current upgrade is planned to last them for quite some time with the next one not scheduled to take place until 2022, more than enough time to generate mountains of data to either support or refute our current models for particle physics.
It’s hard to understate the significance of the science that has been done because of the Large Hadron Collider. Whilst it’s famously known for discovering the Higgs-Boson, the particle which gives all other particles mass, it has a long list of achievements outside of that singular event. What makes these discoveries even more interesting is that the LHC has been operating at something of a disadvantage since it was first turned on over 6 years ago, operating at around half the potential energy it was capable of. Shortly after the discovery of the Higgs Boson the scientists and engineers at CERN have been working to bring it up to full capacity and with it the potential for some even more radical discoveries.
The doubling of the collision energy increases the potential for the LHC to generate even more exotic particles than it has previously, ones which can give us insights into some of the most perplexing mysteries in particle physics to date. One such source of intrigue is how our universe, which is composed of nearly entirely matter, came to be that way. Another seeks to explain why the universe seems to be riddled with matter that’s not directly observable but is seen through its gravitational effects throughout the universe. These, and many other questions, have potential to find answers in the newly upgraded LHC which is slated to come online this year.
In the beginning, the beginning of everything according to scientific theory, there existed both equal quantities of matter and antimatter. Upon annihilation these two entities should have completely destroyed each other, leaving behind a wake of energy with no matter to speak of. However casual observation will show that our world, and the rest of the universe, is dominated by matter. This strange preference for matter (dubbed the CP Violation) has perplexed scientists for decades however the newly upgraded LHC has the potential to shed some light on where the Universe’s strange preference comes from. The LHCb detector focuses on the decay of the Beauty Quark, a fundamental particle that decays in all manner of strange ways when created in a collider. Studying these decays could grant us insight into where the CP violation comes from and why we live in a matter dominated universe.
However what’s far more interesting (for me at least) is that the LHC could have the potential to generate dark matter, the highly pervasive as-of-yet unobserved substance that binds galaxies together via its gravitational influence. There’s numerous theories that posit dark matter being made up of WIMPs (Weakly Interacting Massive Particles) which could potentially be generated in the LHC. It’s highly unlikely that we’ll be able to detect them directly, their very nature means that they’re far more likely to simply pass through the detectors, however should we generate them their signature will be left on the reactions. Essentially we’ll be looking for a reaction that’s missing energy and then seeing if that can be explained by a WIMP being generated. Should we find that we’ll have a solid basis to further investigate this elusive form of matter, furthering our understanding of just what makes up our universe.
It’ll likely be another few years before we hear any further news from the LHC as it’s going to take time to generate the data and even longer to sift through it to find the reactions we’re looking for. However I’m very confident that the results will forever change the scientific landscape as either confirmation of current theories or evidence against them will provide dozens of more avenues for further research. That, to me, is the beauty of science, the never ending search for answers that inevitably lead to more questions, starting the process of discovery all over again.
The spiral shape of a galaxy is an image that would be familiar to many of us but the story behind that shape is much less understood. We all know that gravity inexplicably pulls all matter together however if we were to weigh everything we could see it wouldn’t fully account for the resulting shapes and distributions we see throughout the universe. The missing mass is what is commonly referred to as Dark Matter, a theorized type of matter which is incredibly hard to detect directly yet must be pervasive throughout the universe due to its gravitational effects. However that might soon change if observations from our own parent star prove to be correct and we’ll be able to detect dark matter in our cosmic backyard.
The picture above is what’s known as the Bullet Cluster, the collision of 2 galaxy clusters with each other which is thought to provide some of the best evidence for dark matter. It’s theorized that in a collision of this nature the dark matter would avoid interaction with all the normal matter and would essentially race ahead. This theory is supported by the gravitational lensing observed between the two galaxies as without some form of dark matter the lensing would follow the matter consistently whereas here it appears to be ahead of its observable matter brethren. There’s still not enough evidence here to call it a direct detection of it, especially when other modified standard models can accommodate the effect readily enough.
However physicists at the University of Leicester in the UK have detected what could be the decay of dark matter particles coming from our sun which, if proven to be correct, would be the first direct detection of dark matter. The theory goes that axions, particles which were theorized to solve one of the more puzzling problems in quantum chromodynamics and are theorized to be a component of cold dark matter, are created in the sun and make their way to earth. Much like neutrinos they don’t interact with matter very often and thus race from the core at light speed, unimpeded by the sun’s great mass. When they reach our magnetic field however they decay into a x-ray photon which means that the level of background x-rays should be higher within the earth’s magnetic field. This is what the researchers have found and data from other orbital observatories seems to corroborate this.
Of course the theory is not without its problems, notably that the properties of the axion that they’re theorizing would be different to the one that’s predicted by the current model. There’s evidence to suggest this from other observations so in order to prove the theory one way or the other further analysis with that additional data will have to be taken into consideration. It’ll likely be years before we’ll have a definitive answer on this particular theory thanks to the large data sets that they’re working with but either result will provide some insight into where dark matter might be hiding.
For hundreds of years we humans have been staring off into the vastness of space for varying reasons. Initially man looked at the stars for mythical and spiritual purposes, hoping to derive meaning from what they saw up in the sky that they could then apply to their lives. As time went on we began to discover that the sky could be used as an extraordinarily accurate navigation tool that was used for hundreds of years. Even today celestial navigation is still used by modern technology to guide craft that venture beyond Earth’s atmosphere, a tribute to how useful gazing towards the heavens is.
Perhaps the most interesting part about our constant star gazing is that the more we discover the more we find out we don’t know. The following pie chart shows just how what we can see from Earth makes up a small part of the universe:
In essence the visible universe accounts for a mere 4% of what exists with the remaining 96% being made up primarily of dark matter and dark energy. This idea that the visible universe is so small, even when you consider something like VY Canis Majoris, is something that still amazes me even today.
Consider dark matter, something that we’ve never directly observed. If you take a look around the universe you would begin to notice some strange behaviour that you couldn’t explain if the universe was exactly as we see it. Some of the best examples are effects like gravitational lensing where light coming to us appears to be bent around another object. In the case of a dark matter object we can’t actually see the object doing the bending. Whilst this would traditionally lead to a review of classical physics models (and indeed it has) observational evidence like the bullet cluster are giving strength to the dark matter model of the universe.
Even more curious is the concept of dark energy. As far as we can tell the universe is expanding at an accelerating rate. For this to occur there has to be some form of energy fueling the process and so far it is best explained using dark energy. By its definition dark energy exerts an outward pressure on the universe which is arguably weak, but it is constant throughout the universe. Current models show that previously gravity was overcoming the outward pressure that dark energy was applying to the universe. However as the force that gravity can exert does not increase as the volume of space increases eventually the force of dark energy took over, and caused the acceleration of expansion.
And herein lies the fun of science. We’re constantly finding out how our view of the universe is incomplete and needs to be updated and changed. We’re only really just finding solid data for dark matter (the bullet cluster analysis is barely 3 years old) and dark energy was coined just over a decade ago. One of my most favourite sayings from all my science teachings was :
The most interesting discoveries aren’t usually eureka moments, they’re more along the lines of “That’s not supposed to happen…”
So after millena of gazing up at the stars the biggest mystery about them turns out to be something that we can’t see. Isn’t that just so awesome? It’s like the universe watching us and waiting until we think we have everything figured out and then throwing us a giant curveball.
Maybe I just like it when secrets play hard to get 😉