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Home Forums General Discussion "Pass me the torch. I think I've found some dark matter."

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This topic contains 26 replies, has 4 voices, and was last updated by  Brian Blake 2 years, 4 months ago.

Viewing 12 posts - 16 through 27 (of 27 total)
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  • #9139

    Tej
    Participant
    Topics: 36
    Replies: 582

    Sorry for late write up as I was very tired after a whirlwind of activities that started with a madman endeavour to putting up a tent in gale force winds previous night, staying up to watch/image the meteor shower, dash back home then out again to attend the lecture, then after the lecture, spending hours discovering the enormous delights of its vicinity, the Museum of London (just wow), and then couple hours of bread and butter work, finally arriving home collapsing on a sofa too exhausted to even go on a computer.

    Ah I was looking around for you, Brian…sorry to hear your ribs are still hampering you, hope it heals up quickly as there are plenty more enticing lectures coming up…many of them by professor Caroline Crawford who did yesterday’s lecture and she is bloody superb. I will make every effort in future (shift work if I have to) to attend her lectures.

    Her lecture yesterday was about “How to search for dark matter”.

    Prof Crawford wasn’t one for preamble or anecdotes and there were no introductions. As soon as she stepped onto the lecture podium, I had split seconds to belt up as she plunged us straight into the composition of the universe as we so far know it..26.8% dark matter, 68.3% dark energy and 4.9% normal matter. Apparently, she did a lecture on dark energy so this is perhaps her “sequel” lecture. She also ensures that we do not confuse dark matter for dark energy and that the focus is solely on dark matter in this lecture.

    Before answering the title question, she spent the first ten minutes explaining the necessary fundamentals of the makeup of our universe.  Ie the four known forces: Gravity, electromagnetism, weak and strong.   

    Gravity is categorised in General Relativity which apply to mass outside the atomic scale.

    Although general relativity and Quantum forces appear incompatible, she explains that physicists still endeavour to find a way to encompass all four forces that neatly describes the universe big and small. The logic being that if current observable theory of the universe begins with the big bang then it must have started with a unified single force which then split into prehistoric early universe forces from which Gravity was the first to split from as general relativity made its distinction from the Quantum world very early on. There have been theories such as supersymmetry (Susy) which grouped all the forces together but her slide had a question mark on it.

    She then focuses on gravity, explaining its nature as it will become the major character player for the lecture’s theme.

    Key characteristics (copied from her slide):

    Drops off with inverse square law
    Depends on mass present
    Always attract
    Acts on ALL matter
    Always additive (not sure what this means)

    So with that 5-10 minute fundamental basics out of the way, she proceeds to talk of why we think there is approximately 26.8% dark matter in the universe.

    It started with an astronomer in the 1930s called Fritz Zwicky, who observed galaxy clusters and realised that these clusters should be flying apart as there is too little mass to bind to each other. This leading to speculation of “missing mass” or mass we cannot see.

    Then decades later, astronomers observed the stars in individual nearby galaxies. They noticed that the outer ring stars should be moving more slowly than the inner stars (by applying the inverse square law of gravity), so slow in fact that they should be flying away from it. But they are not because they appear to be travelling at the same speed as the inner stars.

    So there must be mass we cannot see which contained within and extending much further out than the observable galaxy that exerts enough gravitational forces to allow these outer rim stars to move at the same speed and retain its orbit around the galaxy. She showed a slide of the galaxy and speculated dark matter halo outside the galaxy and it was a huge size compared to the galaxy, I cant recall how much but its a considerable number of factors greater in ratio to the luminous galaxy. (BTW throughout the lecture, she complements her talk with eye catching dynamic and clear-cut pictorial slides, very few text apart from necessary bullet points and labels. Now this is how to do a compelling PP presentation)

    She then proceeds to show the “proof” of the dark matter existence.

    One major evidence is that of gravitational lensing. She briefly explains the curvature of space (again complemented with lovely self explanatory diagrams) and how light bends around mass and indeed how lensing effect appear around the dark matter halos of individual galaxies and whole galaxy clusters.

    So continuing her supremely streamlined investigative lecture of dark matter, she then moves on to investigating the possible candidates of dark matter composition.

    1. Baryonic matter. This is matter that which we can observe. Three candidates are Rocky planets, brown dwarves (failed stars) or stellar remnants. But there are problems with every baryonic candidate. For instance, rocky matter known as MACHOS, are considered as a strong candidate in limited quarters of the scientific community because they do not emit light so could well be the make up of dark matter but rocky planets are made of heavy elements and therefore must originate from star remnants but this cant possibly make up the huge of dark matter out there when doing the calculations. Nevertheless, there are some who still believes that it could well be rocky matter that makes up the missing mass equations so she lets us know that all possibilities are explore no matter how unlikely. If they exist, observations to detect them are in place using Macro lensing (transit of dark matter on a star) but no such thing has been detected so far.

    Just as another aside, one thing that I absolutely appreciate about Prof Crawford’s methodically structured lecture is that she gives all the main perspectives and theories, although, naturally she would give her own inclination and even ruthlessly strikes off a theory but at least she gives the other perspectives for our consideration!

    So baryonic matter, as a possible candidate has issues in matching with the current model of the universe so she scratches that group off.

    Next candidate:

    2. Non brayonic matter. This was the first time the audience chuckled but Prof Crawford is not one for comic relief breaks and snaps us back into possible candidates of non baryonic matter. And this is where she brings the fundamentals covered earlier in deriving a shopping list of criteria:

    Must experience gravity.
    React to WEAK forces
    Does not react to EM or strong force

    2a. Neutrinos. The only proven non baryonic matter. So could this be the elusive dark matter? It fits the shopping list. But there is a problem. Its too fast and doesn’t have enough mass. So it does not fit with observations. Its a hot dark matter and current observations dictate that dark matter is cold. So she crossed this off and adds further criteria to the shopping list for having enough mass and must be cold. I don’t quite understand why these added criteria but I take her word for it at this point!

    2b. Axions. A theoretical particle for which there is a fascinating possible detection just recently (as per Andy’s original post on this thread). It is one billionth of an electron. Cant recall if she dismisses this or nothing in my notes, sorry.

    Then she moves onto WIMPS. Basically the theory goes that over time, dark matter will decay. This decay remnant may or should become a visible matter or energy.

    And it is this remnant that experiments around the world are hoping to pick up on. The main experiment being the LHC. Given the theory that dark matter was a driving element in the early universe from the big bang, so when particles are collided, if there are any discrepancies of the total energy, then that discrepancy would describe the dark matter. But so far no discrepancies as such has been found. However, with the collide r upgrading to faster speed particle collisions, it is hoped will produce results of such discrepancies of the total detected mass and energy.

    She goes on to describe all sorts of active laboratory searches for dark matter around the world and at large budget scales. There sure are plenty of them. I’m afraid I got a little burnt out with the note takings to describe them but rest assured the astrophysicists and cosmologists are well on the case!

    Hope that gives a taste of what the lecture was like. Prof Crawford is now one of my favourite lecturers, no beating around the bush, superb slide show presentation that’s not filled with text that no one can possibly read and listen to the lecturer at the same time, methodical in the analysis from covering necessary fundamental basics, history of the problem, investigation techniques, different perspectives, different possibilities, absolutely no formulas, not an “=” sign in sight!, yet delivers a riveting and clear cut journey to a conclusion that gives me a greater understanding and direction in find the secrets of the cosmos.

    Her next lecture will about The Transient Universe on 26th November, hopefully more of us will attend that one so we can have better write ups than what I can muster up!

    • This reply was modified 2 years, 10 months ago by  Tej.
    • This reply was modified 2 years, 10 months ago by  Tej.
    #9142

    Brian Blake
    Participant
    Topics: 187
    Replies: 409

    Tej great report.  Yes I was hoping that my ribs would be better by now but the doctor told me it could take another couple of weeks.  Just to let you know Caroline is booked on the current programme of Flamsteed lectures.

    #9175

    Mike Meynell
    Moderator
    Topics: 119
    Replies: 746

    Just catching up with this… great report Tej. Carolin is a great speaker… she last spoke to the Flamsteed back in 2011 about Nebulae.

    Incidentally, I have a transcript of the “Dark Matter Debate” at the Flamsteed if you are interested, a debate which even got a write-up in the New Statesman.

    #9178

    Andy Sawers
    Moderator
    Topics: 113
    Replies: 588

    Yor PhD is in the post, Tej. Great report!!!

    #9183

    Tej
    Participant
    Topics: 36
    Replies: 582

    I think the phD will be taken back from once they click on to all those embarrassing grammatical errors!

    #9184

    Tej
    Participant
    Topics: 36
    Replies: 582

    Thanks for the links, Mike. Caroline covered much of the same in that discussion but interestingly she didn’t mention anything about MOND in her talk.

    #9189

    Mike Meynell
    Moderator
    Topics: 119
    Replies: 746

    …but interestingly she didn’t mention anything about MOND in her talk

    It’s still considered to be a wacky theory by some physicists.

    #9291

    Brian Blake
    Participant
    Topics: 187
    Replies: 409
    #9694

    Brian Blake
    Participant
    Topics: 187
    Replies: 409
    #9695

    Andy Sawers
    Moderator
    Topics: 113
    Replies: 588

    Interesting, indeed – but I’m not sure I’m buying this. It seems slightly odd that they’ve suddenly discovered that the universe’s expansion is slowing down.

    “If the dark energy is growing and dark matter is evaporating we will end up with a big, empty, boring Universe with almost nothing in it…”

    The key word in that sentence is probably ‘If’.

    “This is a hint — not a solid result yet — that should stimulate further observational studies,” Medvedev said. “It may happen to be real, but it may well go away.”

    Won’t argue with that.

    #9696

    Brian Blake
    Participant
    Topics: 187
    Replies: 409

    It will be interesting to see what the Planck data shows.

    #10869

    Brian Blake
    Participant
    Topics: 187
    Replies: 409
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