Astronomy Without A Telescope – Dark Matter Science

Article written: 29 Oct , 2011
Updated: 24 Dec , 2015
by

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Dark matter – there’s a growing feeling that we are getting closer to finding out the true nature of this elusive stuff. At least we are running a number of experiments that seem (on theoretical grounds) to have the capacity to identify it – and if they don’t… well, maybe it’s time for a rethink of the whole ball game.

There are two arguably quite separate requirements for dark matter to make sense of our current dataset and our theoretical schema for the universe. Firstly, the Standard Model of cosmology (Lambda-Cold Dark Matter) requires that 96% of the universe is composed of stuff of an unknown nature that cannot be directly observed.

About two thirds of this unknown stuff can’t possibly be matter since it apparently grows as the universe grows – so we call it dark energy. The remaining component we call dark matter since it represents a component of the dark side that is capable of generating gravity. But that’s about it. In this context, dark matter is invoked to balance the math – within a set of formulae which are already straining credibility by telling us that 96% of the universe is invisible and undetectable. So, if that was all there was to argue the case for dark matter, you would be justified if feeling a little skeptical.

But the second requirement for dark matter is much more grounded in sound observation and conventional physics. Galaxies – and the way in which galaxies cluster and dynamically interact – don’t make sense if they are composed of only the visible and other known types of matter that lie within them. The Milky Way itself is rotating in a manner that would result in much of it flying apart, if there wasn’t additional invisible matter generating additional gravitational attraction. So there are sound reasons to think that there really could be something else out there.

There’s been a recent kerfuffle about dark matter in dwarf galaxies – although this is largely about whether dark matter particles clump together at the centre or whether they are energetic particles whizzing about throughout the galaxy. Apparently the data better fit the latter scenario, which challenges the prevalent view that dark matter is ‘cold’ and prone to clumping.

Similar to the Bullet Cluster, MACS J0025.4-1222 represents the aftermath of the collision of two galaxy clusters. Most of the mass of each cluster remnant is in the cool blue regions - each having already moved beyond the collision point due to being only weakly interacting mass. The pink region represents strongly radiating and strongly interacting mass has been slowed up within the initial collision. Credit NASA.

A recent literature review on Arxiv provides a comprehensive coverage of the current status of dark matter science. Initial data from the PAMELA spacecraft, showing an anomalous cosmic ray flux, encouraged speculation that this might result from dark matter annihilating or decaying. This theory did not receive wide support, but such speculation was more recently revived with FERMI-LAT finding unexpected flows of positrons (i.e. antimatter) – followed by an announcement that FERMI-LAT and other telescopes will undertake a dedicated search for gamma ray lines arising from dark matter annihilation or decay. Here it is presumed – or at least hypothesised – that dark matter can be destroyed within the hot, dense and dynamic centres of galaxies, including our galaxy.

So space science could provide at least circumstantial evidence for one of the biggest mysteries in space science – although all findings to date are inconclusive at best.

Earth-based experiments are looking for more direct evidence of the particle nature of dark matter. For example, the Large Hadron Collider is looking for signs of supersymmetry particle signatures. The hypothesised neutralino would nicely fit the hypothesised characteristics of a dark matter particle (a particle that weakly interacts with other matter, has neutral charge, is stable over cosmic timescales and has no color charge), but there are no signs of the neutralino, or anything else clearly supersymmetrical, so far.

There are also experiments, like DAMA/LIBRA, deep down coal mines and the like, which are designed to directly identify weakly interactive massive particles – although again findings to date are all a bit inconclusive.

And ‘all a bit inconclusive’ is a statement that aptly represents the current state of dark matter science – we remain confident that there is something out there, but (obligatory play on words coming) we remain as much in the dark as ever about what exactly it is.

Further reading: Capoziello et al The missing matter problem: From the dark matter search to alternative hypotheses.



13 Responses

  1. Anonymous says

    Despite not getting my holidays on Mars I am glad to be born into this time because so much is up for grabs, An apparent increasingly expanding universe filled with undetected mass made of undetermined particles, gravitic anomalies with distant spacecraft, polarised galactic spins, superlight neutrinos and many other recent puzzles strike me as pieces of a jigsaw which don’t fit when viewed through the eyes of the nineteenth and twentieth century giants of science. There is a sea-change coming and we need new geniuses to put the pieces together in a different way but more importantly we need to keep spending money on projects like the James Webb telescope to gather more direct and irrefutable evidence.

    • Anonymous says

      What of the tentative idea that most of the matter in the Universe has slipped into another dimension and is therefore detectable by us only by its gravitational signature?

      • Anonymous says

        I can see a flaw in that theory. The infinitely flat denizens of a 2d disc would not be aware their universe was a slice of a 3D sphere but what you are suggesting is that when they measure the mass of objects in their disc they are seeing the gravitic attraction of objects above and below them in the third direction.

        Gravity from mass inside a 2D world can only pull the denizens in the X and Y directions – they are confined there but gravity in our sphere also works in the Z direction so, although mass _directly_ above and below has no effect, any mass above/below but displaced in the X Y directions can have an effect.

        The effect is very weak because the inverse square law is being applied in an extra dimension, and only the XY vectors of that force can be felt in that 2D plane. Imagine a ball bearing under a sheet of glass and a magnet above the glass, it can’t pull the ball up through the glass but it can pull it up/down left/right, the intensity of the pull depending on how far away it is in the Z direction.

        Translating this into four dimensions (let’s leave time out of this), imagine a 4D “qube” there would be a fourth direction in which somehow matter has moved or more likely already exists. You are also assuming gravity can attract in that fourth direction as well. Mass directly (4D)above and (4D)below would have no effect as we are 3D beings and – just like the ball bearing- cannot be moved in the fourth direction. Any matter somehow displaced from us in the fourth direction would therefore be effectively be in another universe but could possibly exert a gravitic force in the fourth direction but only if it is also displaced in one of the XYZ directions as well.

        If that was the case then the 3D universe would behave very weirdly indeed. imagine a supermassive black hole moving in another 4D plane across our path. We wouldn’t feel the pull in the 4th direction but like the ball bearing we would feel a mysterious gravitic attraction which would move across our path as well. OK I know things happen very slowly but I think we would have noticed effects like that by now. It is also strange that extra gravitic attaction just happens to be coming from things in the same vector as mass in our XYZ planes.

        This effect would be very pronounced if MOST of the matter in the universe was displaced from us in this fourth direction and depending on how much it was displaced in the XYZ directions.
        My head hurts trying to think and move in 4D, fortunately better minds than mine are working on this.

      • Anonymous says

        I see. Would this then give any new significance to the mysterious “Great Attractor”?

      • Anonymous says

        I wrote a longer essay on this matter to this blog entry. See above.

        LC

      • Anonymous says

        I wrote a longer essay on this matter to this blog entry. See above.

        LC

  2. Anonymous says

    Dark energy to my mind requires that we think about physics in some radically new way. The “theoretical laboratory” for such is the black hole. Dark energy is conventionally thought of as due to vacuum or zero point energy. This is probably a decent “crutch” for now, but it leaves open a pile of questions about the nature of the vacuum. It also gives no reasoning for how the symmetries of the vacuum might entail the symmetries of spacetime for the universe.

    The problem requires that we abandon what might be called excess baggage. As I see it this requires that we abandon the idea of an event or quantum event occurring at some geometric location in space or spacetime. For a black hole there are two possible ways to observe it. One may remain outside of it and observe physics up to the event horizon as these fields or string appears Lorentz contracted, time dilated and red shifted. Another observer can enter the black hole and observe physics from the interior, where this observe may see the tidal excitation of fields by gravitation up to the singularity. The two observers witness the same quantum physics, but with S matrices on different geometric domains of support. So the elementary quantum event in quantum gravity does not have a fundamental geometric structure.

    The observer which remains outside the black hole can’t really observe the particle on the stretched horizon. That observer can observe the physics closer to the horizon by placing themselves on an accelerated from close to the event horizon. This is actually just as violent as the situation for the infalling observer who encounters the singularity. This accelerated observer which approaches to a distance d to the horizon in a Rindler-like wedge and must accelerate at

    g = c^2/d

    which becomes large in the limit d —> 0. The temperature this observer finds from the horizon is T = 2pig, which becomes enormous as d —> 0. This observer then witnesses this infalling string in this incredible thermal bath. As the observer watches this closer to the horizon the string reaches it Hagedorn temperature and is demolished. In effect this observer is on an accelerated from with their proper time s such that the asymptotic coordinate time t is t ~ cosh(gs). So for g large enough a comparatively short proper time s can be a boosted situation for a coordinate time long enough to where this observer witnesses the Hawking evaporation of the black hole. The demolition of the string by this enormous Unruh radiation is then the transition of string quanta into thermalized quanta which escapes to the exterior world.

    The infalling observer observes something which is different. Let the infalling observer track the infalling particle closely by letting the observer intersect the light cones of the infalling particle. This observer finds that the string is tidally distorted so it stretches and its transverse directions vanish and modes shift into the longitudinal direction. Further once the tidal force becomes large enough the string can break, where the break point is analogous to a quark-antiquark pair, and further this can continue to where the string rapidly breaks up into a “gas of strings” right before you hit the singularity.

    This leads to an invariance principle for events. Relativity removed the invariance of simultaneity. So we have in the history of physics already an instance of where a “sacred cow” was killed. Quantum mechanics destroyed the idea of a well defined trajectory for a particle and compatible measurements. Now this direction implies there is no invariant meaning to an event. The conservation of information as detected by the two complementary observers tells us that they both observe the same data and how it transforms into the future. The observers witness the same event, but that event is not unique to a region of space (spacetime) in a classical sense. This is because the spacetime itself in a noncommutative system, and further the removal of any geometric structure means it is nonassociative.
    Dark matter is more in line with established physics. The question is whether it is a supersymmetric partner of the Z, photon and Higgs called the neutralino, or maybe it is the axion particle at near absolute zero temperature, or maybe it is some other quantum field not yet considered.

    LC

    • Member
      Steve Nerlich says

      This gets me thinking about Schroedinger’s cat. If one observer can know what the live cat/dead cat outcome is before another observer, then for that other observer there is no wave function to collapse – the outcome is already determined.

      This would be true even though both observers might be seen to make a simultaneous observation (from a privileged frame of reference). This challenges the idea of the co-existence of indeterminant states (which is a daft idea really).

      • Anonymous says

        This is related to that, but where the geometric meaning of a quantum event is removed. The two observers in fact observe the two complementary aspects of the quantum system, but they are forbidden from ever mutually sending classical signals to each other about their observations. This is also related in part to why a galaxy at z > 1 is moving away from us faster than light, but where we are unable to send a signal to it.

        LC

  3. Sam says

    Dark mater is uncountable figure (doubtful 96%). It may exceed 99.9+++++.
    Sam.G
    Space Research Center Of Madras
    India

  4. Sam says

    Dark mater is uncountable figure (doubtful 96%). It may exceed 99.9+++++.
    Sam.G
    Space Research Center Of Madras
    India

  5. uniontera6 says

    Uniontera number 11-1, hope it helps.

    Existence is the time expressed by light itself. http://www.youtube.com/watch?v=AN1jTwHUC7E

    1. Arrow A is the laser work. (Same time work)
    2. Arrow B is the reason of gravity. (also, Higgs boson individually / Different time work)
    3. The rose line is the beginning light. (result from Bible)
    4. The existence of different time. (result from relativity of light)
    5. 2 dimension is not a space. (result from 2-axis, math)
    6. Universe came from nothing. (result from No.5 / cf. Playing with bubbles)
    6-1. A barrier[posititon] exist between the inner and outer bubbles. But universe do not have that barrier. (Cause of Inertia / Reason of Sun / Position of Antimatter / cf. Expansion)
    6-2. Nothing is not a no existence. No existence is a zero dimension. There is two zero dimension. One has a position. The other doesn’t have a position. These different positions were connected by something. That “something” is not a what but a way. (Cause of Light)
    7. Progress / Event / Probability is a time. (This is my trinity.)
    8. Wave-particle duality of light, creation & evolution, uncertainty principle, etc. (result from No.7)
    9. Coalescing of different time. (Cause of Rotation & Spin / Equator has plenty of time.)
    10. Your so-called black hole is a wholly condensed time. (cf. Kerr spacetime)
    11. The reason of “your so-called magnetic field”. (result from step3 and 4)
    11-1. The reason of “your so-called dark matter”. (result from step3 to 4 / Energy-Mass duality / cf. Dark energy “step4 to 3”)
    12. The reason of “your so-called electric field”. (result from the crack of time / Same time’s crack is a different time’s connection. / Cause of Static electricity)
    13. Your so-called “space” is a crack of time. (result from No.12 / Cause of Lightning)
    14. Boundary is a crack of space. (result from No.13 / Same space’s crack is a different time’s contact. / Cause of Superfluid / cf. Fractal)
    15. Boundary is nothing. Your so-called hole is a boundary itself. Also, nothing is K=0 temperature. (result from No.14 / cf. Calabi-Yau, Hilbert space)
    16. This is a wall of your so-called fire. (result from No.15) – copyright ? uniontera

    Our nonsense has come from one time. Uniontera is a key.

    English is not my first language, sorry!

    If it is difficult to understand, just remember & enjoy “Hand touching everything is a time”. This is the only one result of uniontera.

  6. Anonymous says

    I wrote this to address questions about dark matter as something which has escaped our universe. To address this question it requires that I provide some basic physics which leads to issues of extra dimensions. The issue of whether there are field quanta which escape our spacetime is a matter of extra large dimensions. For those less familiar with this sort of quantum machinery, just try to abstract these ideas.

    A quantum mode with frequency ? ~ n (n = mode number) has a state |N_n>, where the big N corresponds to the number of particles in that mode. There are raising and lowering operators a^†_n, a_n which act on that state as

    a^†_n|N_n> = sqrt{N + 1}|(N+1)_n>

    a_n|N_n> = sqrt{N}|(N-1)_n>

    so as to create or destroy a mode with frequency ? ~ n. The symbol † means “complex conjugate and transpose” for those familiar with matrices and complex variables.

    To use the lore of string theory the x and y moving modes on a closed string are a^†_n and b^†_n. For there to be a real physical state these states must come in even right and left moving terms, eg a^†_na^†_{-n}, b^†_nb^†_{-n} and a^†_nb^†_{-n}. In general we can define “polarizations” as a^†_n + ib^†_n and a^†_n – ib^†_n for left and right polarizations. This is very much like polarization of light, but where here they correspond to right and left moving waves on a closed string. There are the states

    (a^†_n + ib^†_n)(a^†_{-n} + ib^†_{-n}) — > spin m = 2 right moving

    (a^†_n – ib^†_n)(a^†_{-n} – ib^†_{-n}) — > spin m = -2 left moving

    (a^†_n + ib^†_n)a^†_{-n} — > spin m = 1 right moving

    (a^†_n – ib^†_n)a^†_{-n} — > spin m = -1 left moving

    (a^†_n + ib^†_n)b^†_{-n} — > spin m = 1 right moving

    (a^†_n – ib^†_n)b^†_{-n} — > spin m = -1 left moving

    (a^†_n + ib^†_n)(a^†_{-n} – ib^†_{-n}) — > spin m = 0

    There are then 5 possible spin states here (m = -2, -1, 0, 1, 2), but if the field is massless there is only the m = 2 and -2 states. The others correspond to longitudinal modes which are massive. We eliminate those (though there are subtle matters of the m = 0 modes as a dilaton and axion field), in some consistent fashion. These states are sort of compositions of photons, in quantum optics a bit like photon bunching effects, but where this composite state is a graviton. Further, there is a level matching condition in this “pairing” where operators with the n mode are matched with those of a –n mode. This is due to a rather subtle issue involving Noether’s theorem of symmetries and conserved quantities.

    The graviton is a closed string and so is not bound to the space. Open strings with endpoints have those ends anchored to a D-brane. We might think of our universe as a sort of D-brane in three or four dimensions. Closed strings can slip pass or through these in a 10 or 11 dimensional setting. An open string corresponding to particles in our observable universe has its end points anchored in our universe or its corresponding D3-brane, or one endpoint anchored in our D3-brane and on another one — leading to multiverse ideas. So closed strings can slip through our universe, and an open string that manages to attach its endpoints (similar to a quark anti-quark pair in a meson) can annihilate those endpoints and become a closed string. That closed string under any force or field flux in another dimension can lift off of our brane.

    In the string description a graviton is with proper level matching

    G^{??} ~ (a^?)^†_{n} (a^?)^†_{-n}.

    If the spacetime indices ? and ? range over some extra dimension then the graviton has spin 1 gauge bosonic content. Assume we set ? = 5 for an extra dimension and we restrict ? to 0, 1, 2, 3 spacetime. Then

    G^{?5} ~ (a^?)^†_{n} (a^5)^†_{-n},

    And since (a^5)^†_{-n} is restricted to this extra dimension that appears “internal” this is then a vector potential A^?. This is a fairly straight forwards Kaluza-Klein type of theory. So the graviton projected into this other space gives a gauge particle, such as in electromagnetism or the nuclear force. This graviton that is cut somewhere and its endpoints attached to our D-brane (or one point on our D-brane and the other endpoint on another brane) then becomes a gauge particle (photon, gluon etc) that interacts entirely on the brane.

    A particle in our universe can emit a graviton, say from an open string that curls around and pops off a loop. This might be thought of as the transition of a particle p to another state p’ by quantum emitting a graviton. That graviton can be emitted off our D-brane and interact with a particle (string) in another D-brane. The absorption of the closed string by another open string can be thought of where the open and closedstrings touch and merge into a single open string. This is one reason people have been concerned with the prospect that the force of gravity at very small distances might diverge from the Newtonian force law. However, this is likely to be a deviation from our gravity law only on very small scales. It is not likely to be a very large scale effect, such as on the scale of the large attractor.

    LC

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