Learning to See in the Dark

Simon Williams explains how new data from the EDGES all-sky antenna in Australia could shine some light on dark matter.

Dark matter makes up 26.8% of our Universe’s energy and mass, yet we know practically nothing about it. In fact we only really know about luminous matter, such as that we are made from or see in our every day life and this only amounts to about 4% of the Universe’s energy and mass.

So called, dark matter remains to this day an extremely illusive entity, only ever evidenced by cosmologists when looking at the discrepancy between the gravitational pull of a galaxy, and the mass within the galaxy. It seems that the galaxies must have much more matter within them than is visible. So what is this invisible, dark matter? Perhaps we’re about to find out.

New results of the temperature of the early universe from the EDGES all-sky radio antenna experiment could change our view on dark matter entirely. After the big bang occurred, the Universe cooled dramatically, and our current models of the evolution of our Universe suggest that 180 million years after the Big Bang the Universe was about 6 degrees above absolute zero, the coldest possible temperature.

At this time, the first stars started to emerge, and the age of light began. The Universe was still very basic at this point, and comprised mainly of hydrogen, the simplest element with one proton and one electron only. These hydrogen atoms absorbed the ultraviolet light form the newly emerging stars, causing their single electrons to be excited from their ground state energy to a higher energy level: in physics these are called the hyperfine energy levels of hydrogen.

As the electrons lose energy, they drop back down from the higher energy level to the ground state, releasing the energy they originally absorbed. This energy has a very specific frequency of 1420MHz, over time it has been redshifted and is now only 78MHz, making this very easily distinguishable from the background noise of the Universe.

The EDGES experiment measured this frequency, looking at the whole sky and thus an average over the whole visible Universe. The results were astonishing, finding that the Universe was actually only half the temperature 180 million years after the Big Bang than we had predicted.

So why is the Universe so cold, and what is wrong with our models of the early Universe? Rennan Barkana of the Tel Aviv University has come up with a radical idea that could change the face of the field entirely. The only matter that can be colder than hydrogen at this time in the Universe’s history is dark matter.

Barkana thus suggests that the temperature difference arises from dark matter collisions with the hot hydrogen, and consequently removing some of the heat from the hydrogen atoms. If this statement is true, then Barkara admits that this ‘is the first direct observational indication of a non-gravitational interaction’, a truly revolutionary observation in a somewhat impossibly elusive field.

So what does this mean for dark matter? Why do we not see these interactions in the modern Universe? Well in the low temperature and thus low velocity conditions of the early universe, it is quite possible for some reactions between dark and real matter to take place, if the mass of the dark matter particles was less than 4.3GeV, about the mass of a Helium nucleus.

At the moment, physicists are searching for dark matter at masses of greater than 100GeV, hugely larger than the true mass of dark matter if this theory is correct. So are we looking in the wrong place for dark matter?

Before we start tearing down all of the multibillion pound experiments looking for dark matter at masses of 100GeV, lets try looking for the primordial hydrogen signal again, and this could be soon. Two new experiments could be about to shine some light on the dark situation: HERA (Hydrogen Epoch of Reionisation Array), and the Square Kilometre Array both in South Africa.

However, for now we remain in the dark. Physicists must really start to think hard about dark matter, and perhaps forget about our current ideas! In the words of Richard Massey, this could be the first time dark matter is seen ‘doing something rather than nothing’.