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Dark matter, dark energy & gravity

  1. Mar 3, 2006 #1
    Hi Friends,

    I just want to know how many of you believe that there is dark matter and dark energy in the universe and how many of you believe that there is something wrong with our understanding of gravity.

    :surprised
     
  2. jcsd
  3. Mar 3, 2006 #2

    Chronos

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    Hi Brahma! Welcome to PF. It's not about belief, it's about observational evidence. The conclusions are not speculative. Given what we know, gravity works just fine using Newtonian and Einstein calculations.
     
  4. Mar 3, 2006 #3
    Yes, it is about belief. As I know each and every theory has its own domain of valadity and so general theory of relativity also has it domian of validity: it has not been tested at very small and very large scales. So there is actually no way to know that either GTR is ok at Planck's or Hubble's scale. This simply means that the dark matter and dark energy may be the artifacts of the breakdown of GTR since we do not have any satisfactory candidates for them.
     
  5. Mar 3, 2006 #4

    ZapperZ

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    The issue isn't your explanation. The issue is your liberal use of the word "belief". Such usage implies that the acceptance of GR is not based on any reliable experimental verification. This is false, because your life depends on the validity of GR every time you fly in a commercial airplane, even in the so-called limited range.

    Take note also that you are using speculation to counter an established theory. The argument on "it has not been tested" or "we do not have any" are all speculation on the possiblity that it MAY not work in those range. You can't challenge an established theory using speculaton. It doesn't work that way. I could say that with just about any other physical principle that we have. Using your logic, all we have are nothing more than "beliefs". But unlike you, I have never been able to use a "belief" to make that semiconductor you are using in your modern electronics, or depend on it for my family's well-being.

    Various areas of physics have different degree of certainty. To make a blanket categorization that a theory is just a "belief" just because it hasn't been tested over all the infinite range of validity is absurd.

    Zz.
     
  6. Mar 4, 2006 #5

    Chronos

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    Good point, Zz, but the question is valid and brahma is new here. While we have not explored, and tested current theory at all scales, the scales we have tested to date confirm predictions to a very high degree.
     
  7. Mar 4, 2006 #6

    ZapperZ

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    Certainly. As you can see from my post, I didn't say that this isn't a valid question. I just wish the word "belief" doesn't get used simply because something has been tested over the whole infinite range of validity. The usage of such a word is as faulty as the usage of the word "theory" in the pedestrian manner.

    There are people who "believe" in supernatural things. I want to see them use this belief to run their computer, or fly an airplane. Equating such a belief to be no different with GR is not only wrong, it is insulting.

    Zz.
     
  8. Mar 4, 2006 #7
    Dear folks: Actually I am not very new here. I posted many posts here long time back from some other name but then I was lost in research.

    Let me assure you that I am a faithful researcher: I must believe in homogeneity and isotropy of the universe, I must believe that the universe was originated with a "bang" and GR is "the" theory of gravity and all that...

    However, sometimes I seriously feel that in science there are many occasions when we have more than one explanations for the same phenomena and finally it boils down to our "belief" and particularly on the "believe" of major section of people in the game. I think this is true in this case also.
     
  9. Mar 4, 2006 #8

    ZapperZ

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    But you are confusing between "research front" science versus established science. Dark matter and dark energy are STILL, believe it or not, a highly active research front areas of science. This means that we continue to learn MORE and more about it as more evidence comes in, and a better refinement of our understanding of it. There are no "beliefs" here. The differeing opinions are PART of our methodology.

    I can give you another example. The mechanism of high-Tc superconductors are being seriously worked on, and there are at least 2 schools of thought here. But does that mean that the WHOLE area of superconductivity is based on a "belief" without any emperical evidence? Does that mean that was we have already known so much of conventional superconductors are nothing but "opinions" or personal preferences?

    I still think that it is a mistake for you to use such a word. Many quacks on the 'net would JUMP on what you have just said here and use it to justify their intrusion into such a field. If all of science is nothing more than a belief system, then what's wrong with THEIR beliefs? If you are truly a researcher, then you have done a disservice to your profession by equating such a word with what you do.

    Zz.
     
  10. Mar 4, 2006 #9

    LeonhardEuler

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    I don't see anything wrong with using the word belief. There is a difference between believing something for experimentally demonstrated reasons and believing it for arbitrary reasons, but belief is the result of either. What is important is that people have reasons thier scientific claims.
    Also, I do think it is somewhat speculative to say that because a theory is verified on some scale that it must be valid on all scales. Classical mechanics was an extremely succesful theory for explaining a wide range of phenomenon from small objects on earth to planets, and yet it turns out not to be true for objects at tiny scales or high speeds. I do not think it is wise to have a rigid belief one way or the other about the validity of a theory on untested scales. There is no real reason to believe a theory should continue working at a scale where there is no experimental evidence to corroborate this.
     
  11. Mar 4, 2006 #10

    ZapperZ

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    But you are forgetting that other eyes are watching things like this. These are the SAME pair of eyes that confuses the use of the word "theory" in science. They think theory is the same as the pedestrian usage of the word in which one is making a GUESS.

    If I were in a purely scientific exchange at a physics conference, I wouldn't have given this a second thought. But many prominent physicists have been quoted out of context (i.e. Einstein and his "imagination is more important than knowlege" and Feynman's "No one understands quantum mechanics). The LAST thing any responsible scientist wants to do is ADD to the pile of library of quotations that someone ignorant of its usage can use.

    I'd like you to point out to me where I actually say that.

    What I did say was that it is a speculation to claim that it MAY not work. I didn't make any claim that it should work everywhere and that this is a open and shut case. That would be a silly thing to do.

    To claim that the acceptance of something is a "belief" simply because it hasn't been tested over all infinite range means that you accept the fact that all of physics is a belief. Now, I challenge you to go find someone on the street and asks someone that if a physics theory is a belief, what does that mean? I will bet you that to that person, it means that is accepted based simply on a matter of FAITH without any convincing empirical evidence. Now how valid is such an impression? Did you think your semiconductor works simply because you believed in it really, really hard? What happens if you have no faith in it? Would it then not work? Really? Have people simply ignored or trivialized reproducibility in experimental physics?

    Zz.
     
  12. Mar 4, 2006 #11
    It may be that dark matter and dark energy are phenomena associated with zero point energy of the vacuum or associated with some energy of spacetime itself. We would then continue to use the GR formulas as before but correct for this phenomena with some appropriate calculation.

    For example, there may be a temperature associated with being in the accelerated reference frame of a gravitational well according to the Unruh effect. This would give rise to an energy density which would add to the original field. When iterated, this might account for DM effects.

    And again there might be a temperature associated with the expansion of space as distant points are seen to be accelerating away from us. This would give rise to added energy density as well. This added energy density would also give rise to even more vacuum energy, etc. And when iterated, this might account for DE effects.

    The first order calculation of these effects contributes a negligible amount. But I do not know what the iterative process would converge to. Perhaps it is enough to account for DM & DE.
     
  13. Mar 4, 2006 #12

    SpaceTiger

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    Zz makes some good points about one's interpretation of the word "belief", but given all of the evidence we've collected so far, I think I could say the following:

    Dark Matter - I'm pretty convinced of its existence. We still don't know exactly what it is, but it would be pretty hard to concoct an alternate theory of gravity that so nicely explained so many different observations.

    Dark Energy - I don't think we really understand what's going on here. It could be one of the more conventional explanations (like a cosmological constant and/or ZPE), but I wouldn't put money on it.


    There's plenty of evidence for that, just look at the CMB.


    Nope. The expanding universe paradigm doesn't rely on a singularity or "bang", as you put it.


    This is, of course, an assumption we make until there is convincing evidence to the contrary. It wouldn't make sense to start by assuming a different theory of gravity.
     
  14. Mar 4, 2006 #13
    Brahma
    Gravity as a force just doesn’t seem to want to fit in very well with what is observed
    and with what experiments that science is capable of now.
    The problem, as I see it, is that the scientific paradigm of our day (even the last 200 years) is based on the idea that gravity is a force that not only holds the universe together but determines it’s configuration (or geometry).
    With this view, then, is the requirement for something unseen and so
    mysterious that it cannot be detected; although we, right here, are
    immersed in it.
    No one wants to change a comfortable paradigm, even when it doesn’t
    work anymore.
    I think what is needed is a new theory of gravity where it is not a force!
    What seems to be ignored is that gravity causes two or more objects to
    accelerate toward each other without overcoming inertia or any
    expenditure of energy.
    The accelerating objects must be, technically, at rest in spacetime!
    Therefore; it is distance between objects that is diminishing that results in
    the effect of gravity.
    But such a new concept would require an entire revamping of the
    paradigm, no big bang, no gravitons and the instant propagation of the
    effect of gravity. Ironically, most of Newton and Einstein’s equations would
    remain.
    Joel
     
    Last edited: Mar 4, 2006
  15. Mar 4, 2006 #14
    something new?

    o:)
    Do you have something in mind?
    I do.
    Joel
     
  16. Mar 4, 2006 #15
    Dear friends: the motivation behind this post is not ask you what in text books, this is about what you yourself feel about the issue on the basis of your experience. I know this type of questions can be asked only in such forums.

    I seriously feel that the quality of our observational data is not that good for making the statement about homogeneity and isotropy of the universe at all scales etc.

    In the case of dark matter it is puzzling that why 90 % matter in the universe is unseen or dark. More puzzling is what it is: LSP, neutrinos, axions, MACHOS...

    I know we need dark matter for galaxy formation, being consistent with BBN, explaining rotation curves, explaining increasing mass to light ratio when we go from galaxy to cluster scales etc. Even then, we cannot justify its existence only on the basis of the argument that we need it, without knowing what it is.

    As far as dark energy is concerned everybody knows it is more problematic: fine tunning, coincidence, what it is ? Again we need it for being consistent with the age of the universe, spatial curvature and supernova observations etc.

    It seems to me that people are so much confused about dark matter and dark energy that there is scope for questioning their existence. Particularly, people are coming up with all possible type scenarios some of them are really wired: dark matter and energy are the two different forms of the same thing, they are nothing but the manifestation of higher dimensions, tachyons, new theories of gravity etc.
     
  17. Mar 5, 2006 #16
    Absolutely right !

    These are the facts which we have to look at and learn lessons from our past and keep our eyes open for all options.

    This is what I am pointing towards.
     
  18. Mar 5, 2006 #17
    Most of it is going above my head. I do not know what type of force you are talking about.

    GR which is "the" theory of gravity so far which we have and it clearly syas that "gravity is the deformation of the geometrical structure of the space time by energy".

    So where is the force you are talking about


    :surprised
     
  19. Mar 5, 2006 #18

    Chronos

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    Well, brahma, I was willing to give you the benefit of the doubt, as an inquiring mind. But, apparently you already had your mind made up. It's also apparent you have no idea what you are talking about.
     
  20. Mar 5, 2006 #19

    ZapperZ

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    Dear brahma,

    In all of your replies so far, you have continued to ignore my point of the IMPLICATION of your usage of the word "belief". You still think it is OK to label ALL of physics as a belief since NO PART of physics is verified over all infinite ranges of parameters. You never once addressed the fact that using your logic, Newton's Laws that were used to construct the house you lived in is based on a "belief". You never once addressed how you would explain this to someone who is not familiar with physics. You never once address the case that using your logic, even YOUR statements on here would be considered as nothing more than a "belief". You never once address my point regarding research front science versus established science and how they evolve.

    It appears that you are only interested in a one-conversation or only in someone who agrees with your view.

    I believe that I have given you PLENTY of my "feelings" about such issue, and especially the irresponsibility of the usage of the word in dispute. If this whole thread is not about physics/astronomy, but rather the discussion about people's FEELINGS, then it doesn't belong in this forum but rather should be in the General Discussion forum. So far, it is severely lacking in physics/astronomy content from you.

    Zz.
     
  21. Mar 5, 2006 #20
    Dear friend this is my reply:

    There is no harm in using the term "belief". We have a set of "beliefs" on the basis of our expriences or prejudices. Sometime it is really difficult to find out how much of our "belief" is based on facts and how much of it is based on prejudices. For example in data analysis we have "priors" which may be considered some sort of "beliefs".

    You are true. This is what I have been told also by one of our distinguished professors that "our physical theories or models are true only up to when they do not contradict with our observations or experiments. Once they do; we need to modify them. This simply means that our "physical theories are not sacred". But actually we do not need "sacred" theories or theories which work on all scales for our daily life. However, cosmology is a different subject which sometime needs some such theories.

    I hope my above explanation will satisfy you.

    I think in this forum everybody is familer with physics.

    It will take too much time for me to explain you but, I assure you that everything is just a "belief" or some sort of "interpretaion" of our mind or "subjective", including science. That is why there is always scope for improvement.

    I hope you got my answer.

    Yot are attacking :uhh:

    Bye the way have you read about the explnations which I have given in support of dark matter and energy. I hope thay are quite informative particularly for people who are new in cosmology.

    :blushing:
     
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