Most of the Universe is Missing!

by Nicholas Mee on December 16, 2012

Neptune’s existence was originally deduced from its gravitational influence on the planet Uranus. Only later was it observed through a telescope.

It is a rather embarrassing fact that most of the universe seems to be missing!

There are two ways to determine the amount of material in the universe. One way is to measure its gravitational pull, the other way is to measure the amount of light that is being emitted by luminous objects. All objects have a gravitational attraction, but not all objects emit light, so we would expect the first measure to give an answer that is bigger than the second, and it does.

Dark Matter

Before these calculations were possible astronomers thought that most of the mass of the universe would be in the form of stars, which would emit light, so their expectation was that most of the material in the universe would be visible. It turns out however, that most of the material in the universe does not emit light. Astronomers call this stuff dark matter simply because we cannot see it.

Could There Be Some Mistake?

The Sombrero Galaxy
ESO (European Southern Observatory)

The first question is: could the measurements by wrong. Afterall, it can’t be that easy to measure the properties of galaxies that are millions of light years away. But astronomers have been working on this problem for many decades and the measurements are now quite accurate. For instance, the rate of rotation of a spiral galaxy that we view edge-on can be measured from the Doppler shift of the light from its two edges. The starlight from one edge will be red-shifted as the stars move away from us, while the starlight from the other edge will be blue-shifted as they move towards us. The rate of rotation can then be used to deduce the total gravitational mass of the galaxy. It is clear from these measurements that the rate of rotation of galaxies is so high that they would fly apart if they were solely composed of visible material. This implies that there must be additional material that is invisible to us. So there is no doubt that the universe contains a lot of material that we cannot see.

The Moon does not emit light, but it certainly has a gravitational attraction that raises the tides twice a day.

The Big Mystery

The big mystery is: What is it? And this is where everything becomes much more mirky, because all the obvious answers, such as dark gas clouds that have not yet formed stars or burnt out star remnants cannot be the answer. The Big Bang model of the early universe works very well, I am skipping over the evidence here, but there would be serious problems with it if the universe had formed with much more ordinary matter.

MACHOs and WIMPs

The possible sources of dark matter fall into two categories that are referred to as MaCHOs (Massive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). MaCHOs are the burnt out stars that we have just ruled out, so most physicists believe that the dark matter consists of vast quantities of WIMPs. A WIMP is a stable particle that only interacts very weakly with ordinary matter, which would explain why it forms a separate component of the matter in the universe and also why it has not already been discovered by particle accelerators or observed in cosmic rays.

Cosmic Relics

Most particles that are created in particle accelerators are unstable, which means that they rapidly transform into other particles. Eventually the only particles that are left are members of a small collection of stable particles. These include the protons and electrons from which atoms are formed, photons, which are particles of light and neutrinos. It is fortunate that these particles are stable because otherwise there would be no atoms and no light. Neutrinos were created in vast quantities in the early universe and are still being created by stars and supernovae. It was once thought that this might explain the origin of dark matter. However, we now know that the mass of a neutrino is too small to account for all the dark matter. Most physicists now believe that the dark matter is probably formed of another unknown type of stable particle that was produced in large quantities in the very early universe. Finding such a particle is one of the main targets of the research at the Large Hadron Collider.

Supersymmetry

Many physicists believe that the wimpy relic particle will turn out to be the lightest particle predicted by a theory known as ‘supersymmetry’. If they are correct then the LHC may soon make a discovery that is even more important than its discovery of the Higgs boson. I will be writing a brief article about supersymmetry in the near future.

More Information

For some sensational astronomical images, such as the one of the Sombrero Galaxy above, visit the website of the European Southern Observatory
http://www.eso.org/public/

The relic particle responsible for dark matter might be produced in the proton collisions at the LHC. But it is also being sought by dedicated experiments, such as this Dark Matter Detector in South Dakota:
http://www.ucl.ac.uk/news/news-articles/1211/121115-Dark-matter-detector-installed-and-submerged-in-South-Dakota

{ 15 comments… read them below or add one }

masila December 19, 2012 at 1:04 pm

very fascinating

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Vasant December 19, 2012 at 1:34 pm

Very interesting. The more we try to go deeper the matter is becoming more complex.

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aparna yellapragada December 19, 2012 at 3:06 pm

very informative and interesting 🙂

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Roger Bishop December 19, 2012 at 4:32 pm

Has anyone considered that time may be non-linear. If for example it was exponential, we would not need to worry about time zero, you can never get there ,and there is nothing before it. It would also mean that the red shift would be interpretted differently. It would also affect the need for dark matter, a spinning gallaxy is spinning in at a different rate because time is running slower. This may be complete rubbish , but your comment would be appreciated. Roger.

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Nicholas Mee December 31, 2012 at 11:14 am

I think the core of your question is: ‘How do we know that time has always passed at the same rate?’

To measure the rate at which time passes we would have to have something to measure it against and it is not obvious what that would be. In other words, if the rate of passage of time changed would we notice the difference? Theoretical physicists do transform the time coordinate in various ways when playing with general relativity and they do employ transformations that make time pass at an exponential rate, when considering black holes and the very early universe for instance, but this is just a change of coordinates and does not have any physical effect.

A closely related question to yours is: ‘Are the constants of nature really constant or do they evolve as the universe ages?’ And in particular: ‘Is the speed of light really constant?’

Physicists have considered models of the universe in which the fundamental constants do change, and there have been recent suggestions based on observations of extremely distant quasars that they might change. (But, personally I am quite sceptical about all this.) It is discussed in ‘The Constants of Nature’ by John Barrow.

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Dick Dastardly December 19, 2012 at 5:47 pm

Maybe there are a lot more black holes and dead stars lurking unseen within galaxies that are responsible for the gravity side of things, i.e. dark matter. Doesn’t explain the dark energy though.

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Nicholas Mee December 21, 2012 at 8:57 am

There cannot be sufficient quantities of black holes, neutron stars and white dwarfs within galaxies to account for all the dark matter, because these dead stars would have formed from ordinary matter. The reasoning behind this statement goes as follows:
In the immediate aftermath of the Big Bang nuclear fusion reactions converted a quarter of the hydrogen into helium. This is well understood theoretically. In fact, the theoretical calculations of nuclear synthesis in the very early universe match observations for the abundance of heavy hydrogen (deuterium), helium-3, helium-4 and all the other very light elements. These calculations are based on the amount of matter observed in the universe. If the universe had formed with much more ordinary matter it would throw these calculations out and they would no longer match the observed cosmic abundances. This is the evidence that I alluded to in the article. It implies that whatever the dark matter is it cannot simply be more ordinary matter.
You are absolutely correct that any explanation of dark matter will not provide an explanation of dark energy. Dark energy is even more mysterious and no-one has any inkling of its origin.

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Dr Mirza December 19, 2012 at 7:28 pm

Very informative now I understand what dark matter is

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Rick H December 19, 2012 at 9:19 pm

Outstanding Writeup Mr. Mee.

I just now happen to be reading , To The Edge of the Universe , Paul Halpren.
and so this stuff is right up my alley. A little over my head of course, but this
Dark Matter, Dark Energy, Dark Flow, the Missing Universe is captivatingly mysterious.
I am hoping to get your new book for xmas. so Thanks Rick

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Ekambaram C.S December 20, 2012 at 4:35 am

I read in the November 20 article of Simons Foundation by Natalie Wolchover that “As supersymmetry fails tests, physicists seek new ideas”. I request you to cover this aspect in your planned article on Supersymmetry. I believe that String theorists have propounded the Supersymmetry theory. Will you please write an article on what is the current theory on the foundational matters.
Thanks
Ekambaram

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Dr, Babu Joseph December 21, 2012 at 6:43 am

Very enlightening info.
Babu Joseph

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Aditya Thakur December 23, 2012 at 7:39 pm

I may be wrong, but why don’t we think about possible extra dimension(s) which we can’t see or even measure? Are we sure that our universe is made up with three dimension (+1 as Time) only? If there is the existence of extra dimension(s) in any particle in the universe then the things will be different, the physicist will have to think in different way. The hidden dimension-ed portion of a particle or a formed body or our universe may add or give us some information(s) which may be measured in our known universe. If that is true then one day we may able to explain dark matter/energy with our known parameters.
The above statement is truly hypothetical, I will eagerly wait for comment/reply.
Aditya Thakur,

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Nicholas Mee December 31, 2012 at 11:23 am

Physicists do consider whether there might be extra spatial dimensions beyond the three that we are aware of. The first suggestion of this kind dates back to 1919 and was made by Theodor Kaluza who was attempting to construct a unified theory of gravity and electromagnetism. These ideas were resurrected in the 1970s and continue to be a critical feature of string theory.

String theory only works in ten dimensions, so there are necessarily six extra dimensions that are assumed to be rolled up very tightly so that we are not normally aware of them. However, if these ideas are correct, the properties of these six extra dimensions would determine the properties of the fundamental particles that we do observe.

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helna January 15, 2013 at 11:01 am

very interesting

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Danis Ponniah April 23, 2015 at 3:14 pm

Dear Nicholas Mee,
Greetings. Thanks for the brilliant article on Dark Matter. I am much benefited by your write-ups. Keep sending them to me.
With kind regards,
Danis Ponniah

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