Third part in a series of four on Dark Matter
I have already reviewed how it reveals its presence through gravitational effects and the lack of direct evidence of interaction with regular matter. Let’s now look how cosmological evidence also supports the existence of dark matter.
It is now widely accepted that all matter (dark and visible) started out being uniformly distributed just after the Big Bang. To make a long story short, a rapid expansion followed where the Universe cooled down and particles slowed down enough to form nuclei three minutes after the Big Bang. The first atoms appeared 300 000 years later while galaxies formed between a hundred and a thousand million years later.
How did the Universe change from being a gigantic cloud of uniformly distributed matter to containing large structures? Dark matter is probably the one to be blamed.
Dark matter is heavier than regular matter and slowed down earlier. Small quantum fluctuations eventually turned into small lumps of dark matter. These lumps attracted more dark matter under the effect of the gravitational attraction, in a very slow snowball effect. Since dark matter also interacts very weakly, these planted seeds survived well through the stormy moments of the early Universe.
Once matter cooled off as the Universe expanded, it started accumulating on the lumps of dark matter. Hence, dark matter planted the seeds for galaxies. “All this could have happened without dark matter, although it would have taken much more time,” explains Alexandre Arbey, theorist at CERN.
Simulating the formation of the Universe
Not convinced? Nowadays, scientists can reproduce this process using computer simulations. As a starting point, they inject into their models how much matter and dark matter there was right after the Big Bang. The observations of the cosmic microwave background provide these estimates. Then they let it evolve under the attractive effect of gravity and the repulsive effect of the Universe expansion.
All these guesses must converge to reproduce the amount of dark matter leftover today, a quantity called the “relic abundance”. If all is properly tuned, scientists can recreate the whole evolution of the Universe in fast motion from the moment of the Big Bang until today.
The results are striking as can be seen on the three pictures above. These computer-generated images show the distribution of dark matter 470 million years after the Big Bang, then 2.1 and 13.4 billion years later (today). Dark matter first formed small lumps, then long filaments and finally large-scale structures appeared.
Scientists from the French National Centre for Scientific Research (CNRS) just released an amazing video showing how they are now using these mega simulations in the hope to discriminate against different dark matter and dark energy models by comparing these images with current observations.
Cold dark matter
Another way to figure out which theory of dark matter best fits the reality was provided last month by a group of scientists working with the Subaru telescope. They studied the distribution of dark matter in fifty galaxy clusters. Averaging all the data, they found that the dark matter density gradually decreases from the centre of the clusters to their diffuse outskirts.
This new evidence conforms to the predictions of cold dark matter theory (CDM), which states that dark matter is made of slow moving particles. Hot dark matter candidates like neutrinos would be made of particles moving close to the speed of light.
Cold dark matter theory predicts that central regions of galaxy clusters have a lower dark matter density while individual galaxies have a higher concentration parameter.
Unexplained signals from outer space
Astronomers are not just providing clues to the mystery of dark matter but also raising questions. For example, a decade ago, the INTEGRAL-SPI experiment found an intense gamma ray source at 511 keV coming from the galactic centre, exactly where dark matter is most concentrated. This value of 511 keV is precisely the energy corresponding to the electron or positron mass.
This smelled incredibly like dark matter particles annihilating or decaying into pairs of electron and positron, which in turn can annihilate into gamma rays as depicted on the diagrams above. Unfortunately, nowadays the excitement has somewhat wound down since theorists have a hard time reconciling its characteristics with numerous other observations.
Several satellite experiments (HEAT, PAMELA and FERMI) have observed an excess of positrons in cosmic rays. A positron is the antimatter counterpart of the electron. Given matter prevails over antimatter in the Universe (otherwise, we and the galaxies would not be there), astrophysicists have to figure out where these positrons come from.
Many theorists have attempted to explain this in terms of astronomical phenomena but the jury is still out. Could this be the first concrete sign of dark matter? The AMS experiment on-board the International Space Station has already shown that they have high quality data and could provide a definitive answer very soon.
First part in a Dark Matter series: How do we know Dark Matter exists?
Second part in a Dark Matter series: Getting our hands on dark matter
Third part in a Dark Matter series: Cosmology and dark matter
Fourth part in a Dark Matter series: Can the LHC solve the Dark Matter mystery?
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