Dark Matter Still Exists...
January 9, 2017
A few weeks ago there was an announcement by a team of researchers from the University of Amsterdam that claimed they had analyzed the gamma-ray background and found no evidence of dark matter. It was a good result, and a valid claim, but unfortunately some of the online and mainstream media misunderstood exactly what the team was claiming. And while I wrote about this misunderstanding and tried to clarify it at the time, some people I have been speaking with are still confused.
For nearly a century now astronomers and astrophysicists have known that there is unexplained matter in the Universe. It began with the discovery that the stars in galaxies such as our orbit the galactic center as if there were significantly more mass than can be accounted for with stars alone. At first this was believed to be caused by 'dark baryons' and compact halo objects, which are essentially dust and rocks and other forms of ordinary matter that just don't reflect much light. However as telescope technology and techniques improved, this explanation was ruled out.
Then in the 1990s and 2000s measurements of the cosmic microwave background, of supernovae, and of the chemical composition of early stars in the Universe proved that this mysterious dark matter already existed moments after the Big Bang, and continued to exist into modern times. In fact we now believe that more than three quarters of the matter in our Universe is of a form we know nothing about!
What we do know about dark matter is that it has mass, and that it does not emit or reflect light at any significant level. All forms of atomic matter absorb light and re-emit it, or can be heated up and made to emit light. But dark matter is different. As far as we can tell it has no interaction with light at all.
However it is possible that when two particles of dark matter collide and annihilate, it could generate other particles that do then produce high energy photons, known as gamma-rays. At present there are hundreds of candidates for dark matter, each of which contains a number of unknown parameters. Some of these combinations will result in dark matter annihilations, some won't. Some will produce gamma-rays, some won't. Some dark matter candidates will decay, and some of these decays will produce gamma-rays. We just don't know enough about dark matter to know what it will or will not do in the Universe.
If dark matter can annihilate, and if its annihilations produce gamma-rays, then a fraction of those gamma-rays will arrive at the Earth. We already receive a large flux of gamma-rays from supernovae and black holes and various other high energy astrophysics events, so the mere existence of these gamma-rays would not be enough to signal the presence of dark matter.
However there are different signatures for dark matter produced gamma-rays that we could detect. For example most sources of gamma-rays are point-like sources in the sky, whereas dark matter is expected to form larger clouds that would appear to be a distributed source of gamma-rays. We also expect most sources of gamma-rays to produce a range of energies, while dark matter annihilations would likely be limited a smaller range of energies. With dark matter decays it would be a very precise spike in the number of photons at a certain energy. And so with careful study, it would be possible to separate out any large contributions to the gamma-ray background due to dark matter.
And that is what the team from Amsterdam has completed. Using six years of data from the Fermi Large Area Telescope, they analyzed the gamma-ray spectrum searching for anything that could not be explained by known sources. And the result was negative – there are no strong distributed sources of gamma-rays that cannot be explained. There is no signal of dark matter.
However this definitely does not mean that they have ruled out the existence of dark matter. Only a few dark matter candidates could be detected in this manner, and even those can have properties that reduce the gamma-ray production to below measurable levels. This result simply restricts some of the parameters on some of the models of dark matter. It does not disprove the existence of dark matter by any means!
And so the scientific community will go on searching for this most elusive form of matter.
For nearly a century now astronomers and astrophysicists have known that there is unexplained matter in the Universe. It began with the discovery that the stars in galaxies such as our orbit the galactic center as if there were significantly more mass than can be accounted for with stars alone. At first this was believed to be caused by 'dark baryons' and compact halo objects, which are essentially dust and rocks and other forms of ordinary matter that just don't reflect much light. However as telescope technology and techniques improved, this explanation was ruled out.
Then in the 1990s and 2000s measurements of the cosmic microwave background, of supernovae, and of the chemical composition of early stars in the Universe proved that this mysterious dark matter already existed moments after the Big Bang, and continued to exist into modern times. In fact we now believe that more than three quarters of the matter in our Universe is of a form we know nothing about!
What we do know about dark matter is that it has mass, and that it does not emit or reflect light at any significant level. All forms of atomic matter absorb light and re-emit it, or can be heated up and made to emit light. But dark matter is different. As far as we can tell it has no interaction with light at all.
However it is possible that when two particles of dark matter collide and annihilate, it could generate other particles that do then produce high energy photons, known as gamma-rays. At present there are hundreds of candidates for dark matter, each of which contains a number of unknown parameters. Some of these combinations will result in dark matter annihilations, some won't. Some will produce gamma-rays, some won't. Some dark matter candidates will decay, and some of these decays will produce gamma-rays. We just don't know enough about dark matter to know what it will or will not do in the Universe.
If dark matter can annihilate, and if its annihilations produce gamma-rays, then a fraction of those gamma-rays will arrive at the Earth. We already receive a large flux of gamma-rays from supernovae and black holes and various other high energy astrophysics events, so the mere existence of these gamma-rays would not be enough to signal the presence of dark matter.
However there are different signatures for dark matter produced gamma-rays that we could detect. For example most sources of gamma-rays are point-like sources in the sky, whereas dark matter is expected to form larger clouds that would appear to be a distributed source of gamma-rays. We also expect most sources of gamma-rays to produce a range of energies, while dark matter annihilations would likely be limited a smaller range of energies. With dark matter decays it would be a very precise spike in the number of photons at a certain energy. And so with careful study, it would be possible to separate out any large contributions to the gamma-ray background due to dark matter.
And that is what the team from Amsterdam has completed. Using six years of data from the Fermi Large Area Telescope, they analyzed the gamma-ray spectrum searching for anything that could not be explained by known sources. And the result was negative – there are no strong distributed sources of gamma-rays that cannot be explained. There is no signal of dark matter.
However this definitely does not mean that they have ruled out the existence of dark matter. Only a few dark matter candidates could be detected in this manner, and even those can have properties that reduce the gamma-ray production to below measurable levels. This result simply restricts some of the parameters on some of the models of dark matter. It does not disprove the existence of dark matter by any means!
And so the scientific community will go on searching for this most elusive form of matter.
I am a theoretical physicist & mathematician, with training in electronics, programming, robotics, and a number of other related fields.
