Why do the Dark Ages provide a powerful testbed for searches for exotic physics?
The Dark Ages refer to the period after the cosmic microwave background last scattered (400,000 years after Big Bang) and before the first generations of stars formed (about 50 million years after the Big Bang). During this time, the Universe was an extremely simple place - a nearly uniform sea of neutral hydrogen, unaffected by the photons streaming through it. The gas cooled slowly as the Universe expanded, and gravity began to form the cosmic web. But cosmologists can predict the results of these processes with extreme precision. If we can measure the properties of the gas during the Dark Ages, any deviations from these predictions would provide strong evidence for new physics that is not so simple!
Image: A visualization of the cosmic web. Each bright knot is an entire galaxy, while the purple filaments show where material exists between the galaxies.
Credit: NASA/NCSA, UIUC, Frank Summers, STScl, Martin White, Lars Hernquist, Harvard
How might dark matter affect the Dark Ages?
We call the missing matter in the Universe "dark" because it does not produce light. Nor does standard dark matter interact with ordinary matter, except in extremely rare events. However, even in the standard picture dark matter is not entirely dark: dark matter particles that collide with each other may annihilate, for example, producing very high-energy radiation called gamma rays. Today, even if such annihilations occur, the resulting gamma rays are typically swamped by the many other gamma-ray sources in the Universe. But during the Dark Ages, where no other light is produced, even an extremely faint glow from annihilating dark matter would be enough to change the temperature of the hydrogen gas. Alternatively, if even a small fraction of the dark matter has a tiny electric charge, those particles will collide with electrons, allowing the dark and normal matter to exchange their energy. These are just examples - cosmologists have identified several mechanisms that would allow dark matter to affect the properties of the cosmic gas during the Dark Ages. If we can measure precisely the properties of that gas, we can study the mysterious properties of dark matter.
Image: Caption: A diagram of particle annihilation: counterpart particles approach each other, collide, and annihilate into two photons (wavy lines, traveling perpendicularly). Dark matter may annihilate in a similar fashion.
Credit: E. Hoffman
What other exotic processes can affect the Dark Ages?
While dark matter models provide one exciting set of mechanisms that would change the standard picture of the Dark Ages, cosmologists have suggested several others too. For example, in some scenarios, small primordial black holes can form very early in the Universe’s history. Under certain circumstances, these objects can heat the surrounding media, or they can produce radio waves. Even more exotic objects called cosmic strings – vast filament-like structures of very high densities – can have similar effects. While strange processes like these cannot be common in the Universe, the simplicity of the Dark Ages will make even rare anomalies stand out easily.
Image: Snapshot of a cosmic string network during the transition from the radiation epoch to the matter epoch.
Credit: B. Allen & E.P. Shellard cosmic string simulations
