Saturday, 18 February 2012

15. Dark Matter and Dark Energy

Gravitational effects in large galactic clusters are found to be much higher than what can be expected from the known amount of matter there. Apparently, there is another, unknown, form of matter that is a large fraction of the total, as indicated indirectly by these gravitational effects.  It is called dark matter, because we are unable to observe it, and we infer its existence only through its gravitational effects.

Perhaps neutrinos have something to do with dark matter (if they have a little rest mass). Or perhaps some still undiscovered elementary particles, including some very heavy (but unobserved) ones, are involved.

An idea has been floated that dark matter could be comprised of baryonic matter (protons, neutrons) if it is all tied up in 'brown dwarf stars', or in dense chunks of heavy elements. The technical term for this possibility is 'massive compact halo objects', or MACHOs.

A more likely explanation is that, rather than the normal baryons, their supersymmetric partners ('sbaryons') (cf. Part 14) are involved. This makes sense because it explains why such dark matter does not interact with ordinary mater, except through gravity.

But another possibility is that dark matter is not baryonic at all, and is rather made up of particles like 'axions' or WIMPs ('weakly interacting massive particles'). In the supersymmetry modification of the standard model, there is something called 'R symmetry', which means that the number of superparticles must be conserved in every process. It follows from this that there must exist a lightest supersymmetric particle (LSP), which is stable because there is no lighter superparticle into which it can decay under the R-symmetry constraint. Typically, the LSP is a 'linear combination' of superpartners of: the Higgs boson (Higgsino), the photon (photino), and the B gauge boson (bino).The term 'neutralino' has been coined for such an LSP; it is an example of a cosmological WIMP.

A remarkable thing about modern cosmology is that, when one calculates the number of neutralinos that survived annihilation in the early universe, the agreement with the known amount of dark matter turns out to be extremely good; in fact, too good to be true!

There is not only dark matter, but also dark energy. Till the early 1990s the generally accepted belief in cosmology was that the universe cannot go on expanding at the current rate; the gravitational pull of all matter must at least slow down the rate of expansion. Then something unexpected happened. In 1998 the Hubble Space Telescope observations showed that the expansion of the universe is actually accelerating, rather than slowing down. Three explanations were offered for this:

1. Perhaps the acceleration can be explained in terms of a long-discarded version of Einstein's theory of gravitation, the one that contained a COSMOLOGICAL CONSTANT. In Einstein's theory, it is possible for more space to come into existence. In one version of his theory there was a cosmological constant, 'put by hand' (and later withdrawn). This version makes a prediction that 'empty space' can possess energy of its own. Since this energy is a property of space itself, the energy density would not be diluted as the universe expands. As space expands, more energy comes into existence, and the universe expands faster and faster.

2. Perhaps there is some unknown energy-fluid that fills all space.

3. Perhaps Einstein's theory is wrong, and a new theory is needed that would include a new field that can explain the acceleration.

No matter what the true explanation is, the phrase 'dark energy' was introduced to account for the mystery. It turns out that a whopping ~74% of the universe is dark energy. Dark matter makes up ~22%. The rest (including normal matter) adds up to less than 4% of the universe.

Both dark matter and normal matter pull the universe together. But dark energy does the opposite: It pushes the universe apart. Further observations and their interpretations have conjured up the following scenario: Till ~5 billion years ago, the universe was not having an accelerated rate of expansion. Dark matter dominated the early universe, but dark energy overtook the influence of dark matter ~5 billion years ago. As the universe expands, the domination of dark energy over the effect of dark matter is getting stronger and stronger.

Why should that be so? As stated above, one explanation can come from Einstein's general-relativity theory, with cosmological constant included.

Another explanation for how space possesses energy comes from quantum field theory. In quantum physics when we speak of vacuum, we really mean a space which has a certain minimum-energy state (cf. Part 14).

We may not understand dark matter much, but it is just as well that we have already discovered it. The curvature of spacetime in the universe depends on the overall mass/energy density of the universe. Since the curvature, i.e. the geometry, is observed to be flat or Euclidean, rather than curved (either spherical or hyperboloid), the mass/energy density must have a certain critical value. This is indeed found to be the case, provided we include the contribution from the known amount of dark matter.

Why has the geometry of our universe been flat, almost right after the Big Bang? An answer to this 'flatness problem' comes from the inflation postulate I described in Part 8. The exponentially rapid expansion ('inflation') of our universe from a size much smaller than that of a proton to the size of a tennis ball (or more) smoothed out our spacetime to make it very flat. And dark matter has contributed to this flat geometry.

Here is an interesting URL you might like to spend time on:


  1. Dear Prof. Wadhawan,

    This is Rudra, with whom you exchanged a couple of posts on Nirmukta. I have just gotten back and read your article and I need time to go through it in whole and have some points to make, which I will do later.

    Thank you.

    PS: I hope, I have used the correct salutation.

  2. Dear Prof. Wadhawan,

    Thank you for writing about the dark matter and and dark energy. I feel that dark matter and dark energy are hypothetical entities that have no existence outside the discussions of the astro-physics academia. The galaxy rotation curve, gravitational lensing and the accelerating Universe, are things that at best can be termed as "epicycles", a derogatory term used to describe a non scientific opinion. It is true that it is impossible to calculate the exact mass of the nebula or galaxy from such great distances (for example, Neptune's orbital discrepancies)

    Please refer to this document, by Cooperstock and Tieu, who have theorized that galaxy rotation curves can be explained without needing any type of non Baryonic dark matter.

    They have said that galaxy rotation curves can be explained with some modifications to field equations of the General Relativity (GR).

    Rest later.

    Thank you.

  3. Thanks for the comments, Rudra. The paper you refer to was made public in 2005. I wonder what fraction of cosmologists take it seriously in 2012.

    Even if 'dark matter' and 'dark energy' are little more than just 'labels' or hypotheses at present, they are already a part of a fairly self-consistent model of the universe. A large fraction of cosmologists take them seriously. We can throw them away only if a better model gets accepted by the majority.

    Dark matter and dark energy help in proper book-keeping (in terms of the necessary density of the universe) when it comes to finding an explanation for the experimental fact that our universe has a flat geometry. Evidence for the flat geometry first came in 1997 from the results of the BOOMERANG experiment. The surface of last scatter has a built-in scale regarding gravitational effects. As further confirmed by the satellite probe WMAP launched in June 2001, our universe indeed has flat geometry. You can put in a cosmological constant by hand in Einstein's equations, or you can postulate the presence of dark matter and dark energy, or you can try a third or a fourth alternative. The best theory will win. I have only presented the majority viewpoint in my post.

    In fact, 'dark energy' plays the same role as the cosmological constant in Einstein's equations: Both are 'negative pressure' entities needed for explaining the present accelerated expansion of the universe. The 2011 Nobel Prize for physics has been awarded to Perlmutter, Schmidt and Riess "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".

    The majority view CAN be wrong. We are discussing cutting-edge science here, for which the majority view CAN keep changing till things have stabilized to some kind of a consensus. Dissent is welcome. In fact it is good for the health of science.

  4. Hi,
    Can you please give the citation to the second image.

  5. Yes. It is now included in the blog post.

  6. The Secret of Dark Elements Finally Revealed: