Cosmology (the study of the Universe).

More specifically, the very early universe and how to observe signatures of this period billions of years after the big bang. Recently I have begun working intensively on primordial black holes, which have become a hot topic thanks to LIGO's detection of merging black holes. See below for more details.  

Aimed at non-specialist audience, you can watch my 3 minute presentation in the Famelab competition here


A very slightly more technical "interview" about my work is given here


For the specialists, my research topics include:


Non-Gaussianity of the primordial density perturbation

Primordial black holes - the unique dark matter candidate which is not a new particle


You may find a complete list of my publications by clicking here  

Below is a popular science article about primordial black holes. Comments and feedback are very welcome. 

Many thanks to Elena Sellentin for her patience editing this article and for producing the gif files.



Small scales in a big universe: Searching for golf balls in our galaxy


Wouldn't it be exciting to find an entirely new species of black holes? Of course it would! So called “primordial black holes” have so far been postulated, but not yet been found. Discover in this article, why scientists are so eager to find them, and which new insights into our universe will be unravelled by the detection – or non-detection.


Chris Byrnes – August 2016


What are Primordial Black Holes (PBHs)?

Black holes are objects so strongly bound by gravity that not even light can escape from them. The most famous are black holes which form from a dying star: if the star has exhausted it's fuel, it can no longer produce enough pressure in its core to stop gravitational collapse. Black holes that form in such a collapse are always massive, more massive than the Sun. If many of such solar mass black holes merge, a supermassive black hole is created. Astronomers believe that most centres of galaxies contain supermassive black holes, as massive as about one billion stars.

This article is about another type of black hole, which may have formed long before any stars existed. In fact, these other black holes formed shortly after the big bang. Since they formed so early, they are known as primordial black holes. Unlike astrophysical black holes, they could exist with any mass. In contrast to their cousins that form by the death of a star, primordial black holes haven't been detected, but the search is on and there is a lot to learn from them, whether or not they exist.


Primordial Black Hole formation

The earliest epoch of the universe which we can describe with any confidence is that of inflation, a period very shortly after the Big Bang, when the universe was rapidly expanding. This means that the space between any two neighbouring points in the universe stretched, and drove the two points quickly far way from each other. Inflation therefore made the universe cold and empty, due to the massive expansion. This can be seen in the animation below.


Expansion due to inflation


Imagine you are the blue particle in the middle of the circle. The lightly shaded areas shows how far you can see, and it doesn’t noticeably change during the very short period of time shown in the animation. What does change is the distance between the particles, they are all moving away from each other rapidly, due to the “inflationary” expansion of space. So in this short time, nearly all particles which you could initially see move outside your horizon, and you are left looking at an empty universe. This is inflation.

This dilution of particles is exactly the opposite of black hole formation – after all particles have to be pressed together by gravity, in order to form a black hole. During this time no structures form, but the universe is not exactly smooth, some areas will be slightly more dense than others after inflation ends and the expansion becomes slow. These areas might collapse into a primordial black hole within a fraction of a second of inflation ending.


The speed of light sets the speed of information propagation

Imagine you are lonesome particle in the early universe. Inflation has just driven all other particles far away from you. What do you see? With whom can you exchange information? These questions have the same answer: You can only see that part of your surroundings, from which light rays can reach your eyes. Because light doesn't travel infinitely fast, you will not be able to see all of your surroundings immediately. Instead, there will be a finite sphere around you, with you being at the centre: everything inside that sphere will be visible to you and you can interact with it. The rim of the sphere is the distance from which light rays were just able to reach you in the short time since inflation ended. From outside the sphere, no light has reached you yet - it would have needed to travel faster, but as we all know the speed of light is constant. Beyond this rim, you cannot see. This rim is called “your horizon”, in analogy to the Earths horizon being the greatest distance on land that we can see. The horizon is the maximal distance over which light, and therefore any information can travel. Also the gravitational attraction travels at this finite speed, and therefore you will only feel the gravity of the other matter particles inside your horizon. Likewise, you are protected against the gravity of the particles outside your horizon.

What happens outside the horizon? You don't know and you can't tell. But the longer you wait, the more light rays can reach you from increasingly larger distances. This is because after inflation, the expansion rate of the universe became much lower, and light can propagate between ever larger patches. This means your horizon grows! With time, more and more of your surroundings will become visible and make themselves felt through their gravity. Potentially, gravity could form clumps of matter in the visible sphere around you. But if the matter is hot enough, i.e. if the particles move fast enough and the pressure in your sphere is large enough, then no clumping will take place. Instead the pressure inside your sphere will stabilize all particles against a gravitational collapse, just like in a star. But as the horizon eats its way out into the unknown universe, more and more matter will enter the horizon. As long as the pressure rises accordingly, no collapse will take place. This, however, is not granted.

 Horizon entry and PBH formation


The animation above shows the evolution of the universe after inflation has ended, when the expansion rate becomes small. The horizon scale grows with time because light has had longer to travel. After a dense patch of the universe enters the horizon, it is possible for the expansion in that part of the universe to stop and become a contraction instead, although the horizon keeps growing. The animation does not continue beyond this point, but imagine the frightening possibility that if you are living in a very dense region, the local contraction might be strong enough to collapse the space around you into a primordial black hole. 

You never know what you will find

Waiting and wondering what new amazing territory lies behind your growing horizon, can be dangerous. Imagine just behind your horizon lies a region of relatively cool and dense gas. Once your horizon crosses this region suddenly there is lots more matter that makes itself felt by gravity – but since the gas is cool, it does not bring lots of pressure with it. All of a sudden pressure and gravity inside the horizon are not in balance anymore! A gravitational collapse sets in. If the pressure never rises enough during this collapse, then nothing will stop the collapse – and a primordial black hole will form. 

The density required to make this collapse possible depends on both the size of the overdense region, and on the pressure forces active which try to resist the gravitational collapse. In the early universe, pressure forces were in general very weak. Once light has had enough time to travel and pass information over a given region, it can in principle collapse. However it will only do so if the gravitational attraction is strong enough and works quickly enough to counter the expansion of the universe, i.e. if a patch of the universe stops expanding and then starts shrinking to become a primordial black hole.

The delicate interplay between causality, expansion and the gravitational attraction caused by an overdensity is a complex problem, and the exact criteria for a region to collapse to form a PBH is under active investigation. What all researchers do agree on is that the overdensity needs to be at least of order ten percent more dense than the average density of the universe. By the standards of the early universe this is a large overdensity, in comparison the density perturbations, which we observe in the cosmic microwave background, are less than 0.01%. However, the density perturbations observed on the cosmic microwave background are at much larger scales, and so the fact that we haven’t detected any PBHs constrains the allowed density perturbations to be less than a few percent on the much smaller relevant scales. This allows us to rule out models of inflation in which the amplitude of perturbations grows on small scales, they would produce too many PBHs, in contradiction with observations.


The difference in scales between the cosmic microwave background and those where primordial black holes could be detected, if they formed in sufficient numbers, is enormous. It is between ten and thirty orders of magnitude, while the cosmic microwave background only probes about 3 orders of magnitude in length scales. Hence there is a huge range of scales about which we know rather little, with a huge amount of information left for us to unlock.  


Smallness: an observational nightmare and why PBHs can help

Satellites and telescopes have a limited resolution. Therefore, they can observe only a very limited range of scales, at best just one billion billionth of the range of scales between the horizon at the end of inflation, which is the smallest the horizon ever became (remember that it starts growing after inflation ends) and our horizon scale today.

The smallest scales of our universe are impossible to resolve with a satellite. So what about all of those unobserved scales? How does one probe them? It turns out that for a huge range of scales spanning about 20 orders of magnitude, our best window onto the early universe is provided by the fruitful search for primordial black holes. Today the horizon is about 50 billion light years away from us, and the smallest scales which a telescope can accurately probe for cosmological purposes correspond to about 10 million light years (corresponding to 1020 kilometers). In comparison, the search for PBHs has taught us about what the early universe looked like on scales of only one metre.

So how about the smaller scales, can we measure them? The naïve answer is yes, of course we can measure the details of our galaxy and solar system. The problem is that they are the result of messy astrophysics, turbulent gas flows and chaotic systems. No memory is preserved in the final form of our solar system to tell us about the physics of such scales at the time inflation ended 14 billion years ago. In contrast, the dynamics between inflation and the cosmic microwave background is fairly simple and well understood; meaning what we see on the cosmic microwave background can easily be translated into the physics of inflation. To learn about the smaller scales, we need to look for something which remains stable for billions of years, the best example of which are black holes. Searches have been made for PBHs over a large range of mass scales.

However black holes are not expected to be completely stable. Hawking radiation, the light rays believed to be emitted as black holes evaporate becomes more and more important as the black holes becomes lighter. Therefore massive PBHs are stable and could be found through searches for gravitational lensing , while lighter ones would be decaying today through Hawking evaporation and satellites are searching for the decay products, such as gamma rays. One might expect that searching for PBHs which have decayed long ago is impossible, since they now longer exist, but the radiation they cause would lead to changes in the universe thermal history, which could for example spoil the success of big bang nucleosynthesis <>.

The bottom line is that these observations severely constrain the fraction of the energy density which is allowed to have collapsed into PBHs. The constraints depend on the assumed mass of the PBH at its time of formation, and vary from less than one part in a million to less than one part in a billion billion, measured at the time when the PBH formed. These constraints tell us that regions in the universe with over densities large enough to form PBHs were extremely rare, and hence the universe must have been quite smooth on all scales, not only on the CMB scales which are known to be extremely smooth. This non-detection of PBHs is not a problem for our theory of the early universe, but it rules out theories all cosmological theories which generate significant density perturbations on any scale, not only on the limited range of scales which telescopes can directly probe.

Models of inflation predict that the amplitude of the density perturbations depends on scale. On the largest observed scales they have to predict an amplitude of about 0.001%, otherwise observations of the cosmic microwave background rue out the theory. But if the perturbations grow slightly as one moves to smaller scales, then over the huge range of scales which PBHs probe the amplitude will probably become too large, and predict a larger number of PBHs than observations allow. Hence searches for small black holes are a window onto very small scale physics of the early universe, giving us information which we cannot learn in any other way.


Inflation leaves the universe empty, as described by the top animation. Today we live in a universe filled with particles and light. The energy of the field which caused inflation must have been converted into particles and photons. This process, called reheating, must have happened but is very poorly understood. We don’t even know the properties of the field that caused inflation, and the energy scales of reheating are probably much higher than those we can ever reach in a laboratory on Earth. An additional difficulty is that the relevant length scales for studying reheating are those similar to the horizon scale at the end of inflation when reheating began, and remember these scales are the smallest the horizon has ever been, far too small to measure with the normal cosmological probes. But one thing most simulations of reheating find is that it was a violent process, involving large transfers of energy, which generated large density perturbations. If these perturbations were large enough, PBHs will have formed, and the fact that we haven’t detected any already puts a limit on how “violent” the reheating process could have been.



The observational search for PBHs continues in many ways, as new experiments search for them in complementary ways. For example the Fermi Gamma-ray space telescope is observing all forms of gamma rays, and during the final stages of evaporation a PBH is expected to emit many gamma rays. A detection remains elusive but the reward for success would surely include a Nobel prize, and learning a lot more about the very beginning of our universe.

Looking and not finding, is not the same as not looking, so was not a fruitless search. We have already learnt about the big bang from our search for PBHs. So far, already the non-detection of PBH has taught us lots about the early universe. Imagine what we will learn, if we finally detect them!