The Dark Side

· by · in Astrophysics, Cosmology, Physics. ·

Before we start i’d just like to apologise for our low activity on Rationalising the Universe as of late – given the Christmas period we’ve both been rather busy with family festivities but we’re about to jump back on the science wagon with full force. I hope everybody had a very merry Christmas!

Today we venture to the dark side in order to encounter the two most mysterious and shady characters in our universe: dark matter and dark energy.

Dark matter was first identified as the “missing mass” in galaxies and clusters required to explain their kinematic properties. The reason it is called ‘dark’ is because the matter does not emit or interact at all with electromagnetic radiation (such as light) it is therefore invisible to the entire electromagnetic spectrum. We cannot see it in visible, we cannot see it in infrared, we cannot see it period. Observations of the cosmic microwave background radiation and the large-scale structure of the universe suggest that approximately as much as 85% of the matter in the universe is dark! This makes its nature one of the biggest questions in cosmology and physics today.

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The first person thought to concretely theorise the existence of dark matter was astro-physicist Fritz Zwicky, of Caltech in 1933. Though the less famous and first to actually realise there was invisible matter in galaxies was Vera Rubin of Princeton, a maverick female astronomer of her day. Rubin worked on spiral galaxies and noticed that the stars at the edges of the galaxies spun at the same speed at those in the center. This analysis of star velocities in spiral galaxies is called looking at the ‘rotation curve’ of a galaxy. Rubin realised these stars at the edge were travelling far too fast as a result of purely the observable matter and there must be something else tugging on them that we cannot see. Zwicky then went on to estimate the total mass of the Coma Cluster of Galaxies and he then compared that estimate to one based on the number of galaxies in the cluster that could be observed. He found there was about 400 times more estimated mass than was observable. Zwicky inferred that there must be some non-visible form of matter, dark matter, which would provide enough mass, and therefore gravity, to hold the cluster together – just as Rubin had previously done. Vera Rubin died today at the age of 88, and though her contribution was overlooked in her day (many believed she should have been awarded a nobel prize and was not justly treated because she was a woman!) we shall acknowledge her contribution now and include a wonderful and selfless quote by the most excellent lady ‘Fame is fleeting. My numbers mean more to me than my name. If astronomers are still using my data years from now, that’s my greatest compliment.”

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On with the explanation. Dark matter is particularly elusive as it does not emit, absorb or reflect light, but makes itself apparent only through gravitational attraction. To locate this mysterious matter a technique called gravitational lensing is used. Gravitational lensing is a phenomenon discovered by Einstein whereby light bends in the presence of matter due to its gravitational warping effect on the surrounding spacetime. The degree of bending of light rays from distant galaxies as they pass through the dark matter’s gravitational field reveals the mass of the dark matter. The greater the bend the greater the mass. By carefully plotting the way that the distant images are distorted, it is possible to quite accurately map the dark matter and estimate its mass.

Dark matter still interacts gravitationally with visible baryonic matter but there is a question as to whether it itself is baryonic. Baryonic matter is the everyday stuff comprised of protons and neutrons – the familiar constituents of matter. Together with electrons, proton and neutrons make up all the different atoms which constitute the building blocks of all objects – you, me, the desk, the table, your cat. One of the primary theories for dark matter was that it is comprised of massive baryonic objects which do not emit light – example candidates were black holes (they do not emit light because light cannot escape)  or neutron stars and cool white dwarf stars (they do not emit light because they are too cold to give off any radiation). Together these objects are collectively known as ‘massive compact halo objects’ or MACHOs for short. There were a number of searches along these lines but MACHOs failed to be detected in significant enough numbers to be the responsible culprit. Therefore, a small fraction of the dark matter in the universe may be baryonic but we would have to look elsewhere to explain the majority of the unaccounted for mass in galaxies and clusters.

We must therefore look to more exotic forms of matter for the answer, away from the everyday world of the protons and neutrons: non-baryonic matter. Non-baryonic matter can be broadly classified as either hot or cold. A candidate for dark matter that is hot and non-baryonic has already been identified -the neutrino. Neutrinos come in three known flavours, electron, muon and tau and they interact with baryonic matter only though the gravitational and weak force – making them perfect dark matter candidates. But arguments and observations from cosmology have again ruled them out as the main culprit accounting for the majority of the dark matter mass in our universe.

Therefore that leaves one other type – cold dark matter. Cold dark matter candidates are known as WIMPs – weakly interacting massive particles. Seeing as these particles would only interact via gravity and the weak force they are of course very difficult to detect, however a large number of experiments are on track in the hope to be able to provide a first direct detection. So sadly we’re still at quite a loss when it comes to understanding what the culprit for dark matter is. We are however a lot better off on our quest to understand dark matter than we are to understanding the even more mysterious character – dark energy.

Roughly 68% of the universe is thought to be dark energy making it whoppingly the largest constituent out there.  Dark energy is characterised in terms of its energy density and pressure, both negative. Its existence is thought to be cause for the accelerated expansion of the universe. We know how much is out there because we know how it effects the universe’s expansion but apart from that it is shrouded in mystery. There are few strands of thought at which we can grasp, the first is that dark energy is a property of space. As we’ve learn from the post on fields, empty space is not ‘nothing’, space is the gravitational field and can possess energy. If dark energy is a property of space itself and is the cause of expansion, then as the universe expands, more space would come into existence, more energy would appear and as a result this energy would continually cause the universe to expand faster and faster. In this case the dark energy also has a pseudonym – the cosmological constant, a constant energy density filling the space homogeneously which Einstein wrote into his equations for general relativity. (Although the energy increases as space expands, so of course does space, keeping the energy density (energy per unit volume) constant.

Instead, perhaps dark energy is new kind of ‘fluid’ or field that fills all of space but whose effect on the universe is opposite than that of normal energy and matter. Instead of being attractive due to gravity, this fluid or field is repulsive, causing the outward expansion of the universe. This new kind of fluid or field has been given the name ‘quintessence’ a greek name for a fifth element or fundamental force. Fields seem to be at the heart of everything.

So there we have an overview of our current knowledge regarding Dark Matter and Dark Energy which overwhelmingly make up the majority of our universe… not so enlightening I agree, I am sorry to disappoint you. Much ground is yet to be made in this area of physics and we hold hope for future advancements, for now however we largely remain in the dark. (excuse the all too easy puns)

47 responses to “The Dark Side”

  3. I hadn’t realized that ant-matter had a negative gravity so the spontaneous creation of positive and negative particles didn’t hit me in relation to dark matter. It makes me wonder if gravity lensing could be somehow mitigated by the presence of negative gravity particles. Also there has been some discussion as to the puzzle that the bulk of matter is not accompanied by an equal bulk of anti-matter and I wonder if this missing anti-matter is undetectable because it is somehow immune to electromagnetic radiation or rather has a setup of anti-electromagnetic radiation which exists in non-detectable form. something which might be labeled negative light. I have no idea as to whether my thoughts make sense. A good deal of speculation about anti-matter occurred in the science fiction in Astounding back in the 1950’s wherein the called the stuff contra-terrene.

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    • Hi Jiisand! So it’s not anti-matter that is speculated to have a negative gravity, anti-matter is a different beast to dark matter or dark energy. Anti-matter is composed of antiparticles which are same as their matter counter parts but have opposite charges and baryon numbers. (I’m going to write my next post on anti-matter!) The question of why there is much more matter than antimatter in our universe is still unknown but thanks to this discrepancy the universe as we know it exists, otherwise all particles would have annihilated with anti-particles straight after the Big Bang. Dark energy (who knows what it is!) is thought to perhaps be “repulsive” in it’s nature as this could explain why the universe is expanding, due to this dark energy filling the universe and creating an outward pushing pressure of sorts (sort of like a negative gravity as you say).

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  4. Reblogged this on Richard's Watch and commented:
    And there’s me, before getting up this morning, mulling over God’s use of basic science and mathematics in creating the universe. That’s not, of course, explicit in Genesis but implicit by nature in the opening verses. Moreover, there’s the original text’s alphabetical coding (maybe hinting at quantum coding?). So I searched New Testament for Jesus Christ’s part in this ‘mystery’: ‘through whom also He (God) made the worlds; who being the brightness of His glory and the express image of His person, and UPHOLDING ALL THINGS BY THE WORD OF HIS POWER, when He had by Himself purged our sins, sat down at the right hand of the Majesty on high…’ (Hebrews 1:2-3) So is today’s excellent overview by Mekhi merely coincidental in hinting my query’s solution? My own readers know me only too well on that score…

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  9. I apologize if I appear dense but your mention that anti-matter has negative mass leaves me mystified. The spontaneous random production of particles and anti-particles might produce an electron and a positron but I have not read mentioned that a positron has negative mass. The recent production of an atom of anti-hydrogen (https://home.cern/about/updates/2014/01/antimatter-experiment-produces-first-beam-antihydrogen)made no mention that the atom had negative mass. Wouldn’t that require something like an anti-Higgs boson – something I have never heard of?

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    • Apologies jiisand, I think I made a grave error, as I wrote this post far too quickly. Thank you for pointing it out, antiparticles do have the same mass as their particle counterparts, not sure what I was thinking, I must have meant charge. I have now omitted that section as I need to re-understand the argument that comes from particles and antiparticles myself with regards to how they could theoretically link to dark energy. Please forgive me and i’ll get back to you soon when i’ve sorted my brain out!

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      • Sorry if I dumped my confusion too liberally. The Higgs boson confuses me totally. I still retain much of the old concept of the atom as a kind of planetary configuration and if the Higgs particle is embedded somehow in every particle to give it mass aside from photons I have no way to give myself a graphic image of how this might appear. I am totally lost in this area.

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      • The Higgs field is the most important concept – this is a quantum field which permeates all of space and time. Physicists like to stress we are NOT talking about a universal ether – been there, done that and it was certainly not worth having a t-shirt. If we were to take the ocean, the water is the Higgs field and the fish are the particles in the universe. You can imagine a fish with a larger surface area may, in fish terms, feel heavier as it experiences more drag from the water (note in practice we are often looking at point particles so you need to drop the analogy of “size” but it helps to visualize). Now the Higgs boson is a manifestation of the higgs field – it is a guauge boson. Think of it in the same way as the electromagnetic field being comprised of photons. The Higgs Field is there – and if we can get a Higgs boson to pop out (which we believe we have) it is validation. They are not however embedded within particles.

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      • Let me plead total inability to enter further into this area. I frantically retreated to Wikipedia to explore the nature of field theory and rapidly collided with some of what probably are the elementary formulae and equations involved in advanced mathematics of both early and more current field theories from Faraday through latter explanations of the nature of the relationships of particles to fields. It confronted me directly with how distant I am from grasping what must be the fundamentals of the essential concepts of modern physics.
        It has led me to conclude that I have no more chance than the average horsefly of getting any sensible grasp of what must be going on. I very much appreciate your efforts to make this huge puzzle even somewhat transparent but I must now start rescribing pentagrams on the floor to protect me from the evil demons that threaten to confine me to that box with that cat which is neither dead nor alive. I am unsure at the moment which category would be more reassuring.

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    • Hi jiisand,
      I enjoy your spirit of questioning, your style of writing and your brand of wit as much as one can gleefully entertain the plausibility, ambiguity and ambivalence of (understanding, being or imagining) Schrodinger’s cat or the “elusive rabbit that . . . refuses to leap out of that magical top hat”.

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      • I very much appreciate the effort in telling me what the Higgs field is not But my machinery of conception is grinding its gears and emitting loud squeaks and groans to reach out to this ocean which is not an ocean, It is like whispering to me that a tiger is not a cockroach and however I transform my familiarity with cockroaches in the most extraordinary ways I have no idea what to do with those two superfluous legs and the antennae or what mental manipulations I must perform to mandibles to give them teeth and whiskers. That this encompassing Higgs fluid which is everywhere can be hammered into an elusive particle somewhat like an electromagnetic wave accepted as a flood of photons is an elusive rabbit that, in my pitiful mind, refuses to leap out of that magical top hat.

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  11. That was so interesting! I must say I have to read it again to understand everything. Thank you for taking your time and writing about this subject that fascinates a lot of us, specially me! I will reblog. Have a happy new year!

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  13. All I can say is the truth is much stranger than fiction. It’s not an easy subject to put into words and you have made a good interesting job.
    Of course there are always questions and often some facts seem to contradict others; consistency is far in the future. I suppose as humans we will always equate light with understanding and dark with impenetrable mystery.

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  20. Thanks for the article and your work, and all the contributions. Dark Energy/Dark Matter is a great subject and promotes excellent discussions, as noted here. I’ve written on the subject often. Good job!

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  22. One aspect of dark holes that has not been discussed (or so it seems to me) is the nature of the dark hole itself. It has been proposed that this is a passage to another part of the universe or maybe to a different universe but if the matter has been captured and is no longer in the center of the black hole then how can the mass of the hole be increased by the addition of more matter? But, assuming all the matter is still there, in what state must it be? If light itself cannot escape how are the normal forces of the matter which maintain standard atomic forces affected? Are all normal particles such as electrons, protons, neutrons or whatever else is present squeezed together in a new form of matter or are they all so distorted that they no longer exist as separate particles?

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  23. Nanites are turning my mind is turning into a virtual particle so I’ll read more of your blog when I’m in this universe. Now following you. Of course, your subject matter is fascinating…

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  27. This item from the Slashdot site might interest you. Although today is April 1 2017 I have a hunch the item is genuine.

    boley1 quotes a report from New Atlas:
    According to the Lambda Cold Dark Matter (Lambda-CDM) model, which is the current accepted standard for how the universe began and evolved, the ordinary matter we encounter every day only makes up around five percent of the universe’s density, with dark matter comprising 27 percent, and the remaining 68 percent made up of dark energy, a so-far theoretical force driving the expansion of the universe. A new study has questioned whether dark energy exists at all, citing computer simulations that found that by accounting for the changing structure of the cosmos, the gap in the theory, which dark energy was proposed to fill, vanishes. According to the new study from Eotvos Lorand University in Hungary and the University of Hawaii, the discrepancy that dark energy was “invented” to fill might have arisen from the parts of the theory that were glossed over for the sake of simplicity. The researchers set up a computer simulation of how the universe formed, based on its large-scale structure. That structure apparently takes the form of “foam,” where galaxies are found on the thin walls of each bubble, but large pockets in the middle are mostly devoid of both normal and dark matter. The team simulated how gravity would affect matter in this structure and found that, rather than the universe expanding in a smooth, uniform manner, different parts of it would expand at different rates. Importantly, though, the overall average rate of expansion is still consistent with observations, and points to accelerated expansion. The end result is what the team calls the Avera model. If the research stands up to scrutiny, it could change the direction of the study of physics away from chasing the ghost of dark energy.
    “The theory of general relativity is fundamental in understanding the way the universe evolves,” says Dr Laszlo Dobos, co-author of the new paper. “We do not question its validity; we question the validity of the approximate solutions. Our findings rely on a mathematical conjecture which permits the differential expansion of space, consistent with general relativity, and they show how the formation of complex structures of matter affects the expansion. These issues were previously swept under the rug but taking them into account can explain the acceleration without the need for dark energy.” The study has been published in the Monthly Notices of the Royal Astronomical Society. You can view an animation that compares the different models here.

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  28. This item today from the Science Daily site should interest you.

    Dark matter filaments bridge the space between galaxies in this false colour map. The locations of bright galaxies are shown by the white regions and the presence of a dark matter filament bridging the galaxies is shown in red.
    Credit: S. Epps & M. Hudson / University of Waterloo

    Researchers at the University of Waterloo have been able to capture the first composite image of a dark matter bridge that connects galaxies together. The scientists publish their work in a new paper in Monthly Notices of the Royal Astronomical Society.

    The composite image, which combines a number of individual images, confirms predictions that galaxies across the universe are tied together through a cosmic web connected by dark matter that has until now remained unobservable.

    Dark matter, a mysterious substance that comprises around 25 per cent of the universe, doesn’t shine, absorb or reflect light, which has traditionally made it largely undetectable, except through gravity.

    “For decades, researchers have been predicting the existence of dark-matter filaments between galaxies that act like a web-like superstructure connecting galaxies together,” said Mike Hudson, a professor of astronomy at the University of Waterloo. “This image moves us beyond predictions to something we can see and measure.”

    As part of their research, Hudson and co-author Seth Epps, a master’s student at the University of Waterloo at the time, used a technique called weak gravitational lensing, an effect that causes the images of distant galaxies to warp slightly under the influence of an unseen mass such as a planet, a black hole, or in this case, dark matter. The effect was measured in images from a multi-year sky survey at the Canada-France-Hawaii Telescope.

    They combined lensing images from more than 23,000 galaxy pairs located 4.5 billion light-years away to create a composite image or map that shows the presence of dark matter between the two galaxies. Results show the dark matter filament bridge is strongest between systems less than 40 million light years apart.

    “By using this technique, we’re not only able to see that these dark matter filaments in the universe exist, we’re able to see the extent to which these filaments connect galaxies together,” said Epps.

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  32. This is a very interesting article and it provides me with a little more understanding of dark matter. It is interesting to see how much scientists are baffled by the many components of creation. Perhaps if they humbly ask the Creator to reveal His secrets, He will. I am looking forward to your article on antimatter

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