Philippa Cole wrote:
We’re still on the hunt for what dark matter
actually is. The most popular candidates are usually some sort of particle, like the WIMP or the axion
. However, an idea that was first considered by Stephen Hawking (among others), is that tiny black holes which formed right after the Big Bang could also do the trick. There have been many searches carried out for these littl’uns, known as primordial black holes
, but there’s been no success just yet. However, whether there’s enough of them floating around to make up some or all of the dark matter or not, we still need to work out how they could have been produced in the first place. For that, we’ll need to go back 13.8 billions years…
Right after the Big Bang, the Universe expanded from a tiny seed by 20 orders of magnitude in approximately one second. That’d be like blowing up a pea to the size of a galaxy in that time. We call this process cosmological inflation
. If everything started off really small and close together before being blown up, then we’d expect everything to look pretty homogeneous
today even in regions which are really far apart, and that’s exactly the case. For example, the temperature of the Cosmic Microwave Background (CMB
) has been measured to be around 3 Kelvin to one part in 100,000 in every direction across extremely large distances. This is why inflation is such a popular idea for how the Universe began.
During the expansion process, any tiny fluctuations that were there initially are able to grow, and it’s these fluctuations that make the Universe not totally smooth and boring everywhere. Once inflation is over, these inhomogeneities can then go on to continue growing and eventually gravitationally collapse
to all of the structure, like stars and galaxies, that we see today. That’s exactly what the 1 part in 100,000 fluctuations that we’ve measured are responsible for. If, however, the initial fluctuations happened to be sufficiently larger in some regions, then they’d be able to seed something really dense – like a black hole.
In order to get large enough fluctuations that can collapse to form primordial black holes immediately after inflation, you need inflation to happen in a very specific way. For the Universe to expand rapidly, you need a lot of energy, and we can model how that energy is spent as a ball rolling down a hill losing potential energy. The dynamics of how this ball rolls down the hill describes the dynamics of inflation. The faster the ball rolls, the quicker the potential energy is lost and the less time fluctuations have to grow. This means that if you want really large densities to be able to grow and be left over at the end of inflation, you need to slow down that ball. Today’s authors propose a cosmological speed bump for doing this. ...