The big bang was the origin of everything in the universe. Or so we thought. Most – 95% no less – of the universe is dark matter and dark energy. It has long been assumed that the big bang was the origin of this dark matter and energy in the universe, as well as the matter and energy we can see. But this assumption is wrong. Dark matter and energy have an origin story of their own, writes Katherine Freese and Martin Winkler.
The Dark Universe
The Universe contains a strange and surprising combination of ingredients. All the objects of our daily experience - our bodies, the chairs we sit in, the air we breathe, the Earth, the Sun, all the stars and planets, all made of atoms - all of that adds up to only 5% of the content of our Universe. The other 95% is the dark side: the dark matter makes up 25% of the total, and dark energy the remaining 70%. Galaxies, including our own Milky Way, are made up almost entirely of dark matter. When we look out into the sky, the stars that we identify as being in the Milky Way, are only a tiny piece of the whole Galaxy. The stars lie in a flat plane called the Disk, and are surrounded by a huge spherical object containing mostly Dark Matter. Although the first evidence of Dark Matter was observed 90 years ago in the 1930's, its nature still remains a mystery to this day. New types of elementary particles have been proposed to explain it, and experiments have been searching for these particles for decades, yet the problem still remains unresolved. The nature of dark matter is the longest unsolved problem in all of modern physics.
The other 95% is the dark side: the dark matter makes up 25% of the total, and dark energy the remaining 70%.
The Hot Big Bang
According to the standard paradigm for the Hot Big Bang, all matter and light were created roughly 14 billion years ago in an extremely dense and hot state. All particles were moving extremely rapidly and were packed together extremely tightly. As time went on, the Universe expanded so that the particles moved farther and farther apart and slowed down. Some of them became the matter that makes up the galaxies while others remained relativistic in the form of the Cosmic Microwave Background photons that we still see today and that provide some of the major information that we have about the early Universe as well as its properties today. The basic notion of the Hot Big Bang, that the Universe started out hot and dense and has been cooling off and expanding ever since, is more than a theory - it is correct, although incomplete.
What came before the Big Bang? Particle theorists have been able to provide a well-motivated idea of what led to the Big Bang: the theory of "inflation." In its earliest moments, the Universe was dominated by a vacuum energy, which we can think of as pairs of short-lived particles popping into and out of existence. We know this type of vacuum energy is a real thing; experiments today confirm that conducting plates can be attracted together by the vacuum energy in between them. The early vacuum energy would have been enormous and drove accelerated expansion of the infant universe, stretching out a tiny region of the Universe to the smooth (observable part) of the Universe we see today. But the vacuum had to "reheat," i.e. convert to the matter and light we see today. That reheating process corresponds to the Hot Big Bang.
In the standard paradigm, that reheating led to all the content of today's universe: the light, the ordinary atomic matter, and the dark matter. Early on, all these particles were thought to be interacting with one another very rapidly, leading to thermal equilibrium, i.e. a Universe characterized by a high temperature controlling the behavior of all of the particles.
The Standard Paradigm and Weakly Interacting Massive Particles
According to the standard paradigm the dark matter particles are produced in the Hot Big Bang alongside ordinary matter and light. This assumption roots in the belief that the dark and the visible world are closely connected. The known elementary particles which make up the visible matter interact via one or several of the following fundamental forces: the strong force, the electromagnetic force and the weak force. Gravity is so much weaker than these three forces that it plays no role in the microscopic world of elementary particles (gravity can only be measured on macroscopic objects consisting of zillions of particles). While the inert nature of dark matter immediately rules out that it feels the strong or the electromagnetic forces, it has long been thought that dark matter interacts with ordinary matter via the weak force. Intriguingly, the neutrino -- a particle we know from radioactive decays -- has precisely the property of interacting solely through the weak force. But unfortunately, neutrinos are so light that they cannot easily be bound in astrophysical structures. Neutrino dark matter would, therefore, jeopardize the formation of galaxies in the early Universe. On the other hand, a hypothetical heavier cousin of the neutrino -- dubbed the Weakly Interacting Massive Particle (WIMP) -- would support galaxy formation and is often considered as the leading dark matter candidate.
If dark and visible matter are fundamentally distinct, if they do not share any of the forces shaping the microscopic world, they are subject to their own separate histories.
The prospect of detecting WIMP dark matter has stimulated unprecedented experimental efforts over the last decades. In dedicated underground laboratories, shielded against cosmic radiation by kilometers of rock, an army of physicists has searched for the recoil produced by dark matter scattering off atomic nuclei. If dark matter is subject to the weak force, the rate of such recoils can be predicted and compared against experimental data. Unfortunately, not a single dark matter scattering event has ever been confirmed (one Italian experiment named DAMA has claimed detection but must be checked by other similar detectors) -- although millions of events would have been expected for typical WIMP dark matter. The searches, nevertheless, continue with ever greater precision -- an important effort since there exist variations of WIMP dark matter which give rise to a suppressed recoil signal. A discovery of WIMP or WIMP-like dark matter could, therefore, still be just around the corner.
Beyond the Standard Paradigm: The Dark Big Bang
On the other hand, it might be the time to drastically rethink what the dark matter could be. What if the dark matter is not just a heavier version of a known particle, what if it does not share any interactions with ordinary matter other than gravity? There could even be a whole dark sector of new particles with new "dark forces'' acting upon them. Given the rich structure of our visible world with many particles and forces, why should the dark world be overly simplistic and just consist of a single particle? While such speculations are clearly intriguing, they immediately raise a problem: if gravity is the only force connecting the visible and the dark matter all experimental efforts to directly detect dark matter particles are doomed to fail. Among physicists this is known as the “nightmare scenario” because it would seemingly not give rise to any predictions which can be probed experimentally. And yet, surprisingly, the latest research triggered a dramatic shift of paradigm by revealing a spectacular previously unknown experimental signature of the nightmare scenario -- the sound of the Dark Big Bang.
Gravitational waves from the Dark Big Bang sweeping through the galaxy would stretch and contract the space between the pulsar and Earth and induce tiny perturbations in the arrival time of the pulsar's signals.
As noted, the Hot Big Bang has long been considered as the origin not only of the visible, but also of the dark matter. However, this standard paradigm breaks, if the two forms of matter only interact gravitationally. Gravity is too weak to transform matter and light into dark matter -- even in the hot dense state of the early universe. If dark and visible matter are fundamentally distinct, if they do not share any of the forces shaping the microscopic world, they are subject to their own separate histories. But what could have produced the dark matter if the not the Hot Big Bang? The most plausible possibility is that there has been a second Big Bang, which we call the Dark Big Bang, that took place not long after the Hot Big Bang. Just as the Hot Big Bang converted the energy of empty space - the vacuum energy - into matter and radiation, the Dark Big Bang converted a second type of vacuum energy (dark vacuum) into dark matter. An exciting new idea to search for dark matter is to search for the Dark Big Bang.
The Dark Big Bang occurs through what physicists call a phase transition. Specifically, the first studies of the Dark Big Bang examine the notion that the Dark Big Bang arises in a "first-order" phase transition, which ends in formation of bubbles of the lower energy phase. A familiar example of a first-order phase transition is the boiling of water. During this process gas bubbles form within the liquid phase and water gradually turns into into steam. During the Dark Big Bang the universe evolves from a more energetic vacuum state into a less energetic vacuum state. Similar to the case of boiling water, this occurs through the formation of bubbles which grow and collide until the phase transition is completed and the universe is filled by the new vacuum. The collisions of the bubbles can be viewed as a massive explosion in which the dark matter is created. At the same time, this violent blast causes ripples in the fabric of spacetime which we call gravitational waves.
Gravitational waves are predicted in Einstein's general theory of relativity which postulates that a gravitational wave passing through space makes it shrink and expand periodically. They are often called the sound or the echo of the most violent processes in the universe due to their resemblance to sound waves which also vibrate through a medium like air or water -- only that spacetime itself is the medium in the case of gravitational waves. Dedicated laser experiments at Earth have been constructed to measure the tiniest deformations of spacetime in order to listen in on the cosmic symphony. On September 14, 2015 the LIGO observatory made the first detection of gravitational waves emitted during the collision of two black holes.
Listening to the Sound of the Dark Big Bang
The gravitational waves from the Dark Big Bang, however, exhibit a too low frequency to be discovered by LIGO. Their measurement would require a detector with the incredible size of a quintillion of kilometers. While this may sound preposterous, such a detector actually exists: our Milky Way galaxy. Our home galaxy hosts a population of millisecond pulsars. These fastly rotating, highly magnetized dead stars are cosmic lighthouses which emit beams of radio waves which periodically cross our line of sight. Gravitational waves from the Dark Big Bang sweeping through the galaxy would stretch and contract the space between the pulsar and Earth and induce tiny perturbations in the arrival time of the pulsar's signals. Dedicated pulsar timing array experiments like NANOGrav use radio telescopes to survey such perturbations in a network of milisecond pulsars. Intriguingly on June 29, 2023 the NANOGrav experiments has announced the detection of a gravitational wave signal which is compatible with a cosmic phase transition. Possibly, we heard the sound of the Dark Big Bang for the first time.