Since the dawn of humanity the cosmos has been studied using visible light; first with the naked eye, and since the early 17th century increasingly with optical telescopes. Telescopes then played a key role in establishing the heliocentric worldview, and allowed a first glimpse at the magnificence and richness of objects in the Universe. In the late 19th century, Maxwell's equations predicted that there is even more than what meets the eye. Driven by a myriad of technological innovations, the 20th century saw the opening of the entire electromagnetic spectrum to study the Universe, from radio waves to gamma-rays. This enabled us to study in great detail many aspects of the Universe, including the early hot phase just after the Big Bang, the formation of galaxies and stars, and extreme objects like neutron stars and black holes.

![Different frequency sky](//images.contentful.com/g94voriuozkd/6ccHhwp2CfrZxLBVNEiJdm/54ce754f03d9991a2b8b78d57267c4e9/image.png)

Caption: The sky at different frequencies (first 4 panels) and neutrino and charged cosmic rays. Source: Planck, ROSAT, Fermi, IceCube

Nowadays, yet another phase in studying the Universe has arrived, with the addition of all kinds of other non-light signals: Highly energetic particles (cosmic rays, discovered in 1912), and the very hard to detect neutrinos (first detection of solar neutrinos in 1968) and gravitational waves. Gravitational waves are the latest addition to these cosmic harbingers, predicted by Einstein already in 1916, and first detected only in 2015. All these different signals provide us with highly specific knowledge about the physical processes that take place in the Universe. The study of objects like black holes, of neutron stars and star forming regions, or explosive/transient events in the Universe, using the full range of these signals, is referred to as “multi-messenger astrophysics”.

Many of these messengers may act at the same time. For example: if two neutron stars spiralling around each other finally merge into a new object, probably a black hole, the in-spiralling leads to the release of gravitational waves, which can be detected. The merging of the two neutron stars itself leads to a powerful nuclear explosion producing much light, but the final compact object also launches jets that give rise to the acceleration of particles such as protons and electrons to very high energies. The energetic electrons will radiate across the entire electromagnetic spectrum, whereas the protons could contribute to the cosmic rays detected on Earth, or they may collide in the neighbourhood of the explosion with other protons and produce highly energetic neutrinos and gamma-rays.

![Artist impression of neutron star merger](//images.contentful.com/g94voriuozkd/4fXscQym0MTJRBKpWJJ00G/d7095776be2b26a2319080de3cde536f/image.png)

Caption: Artist impression of a neutron star merger. Source: University of Warwick/Mark Garlick

Members of GRAPPA play a world-wide leading role in many aspects of multi-messenger astrophysics. The group of Christoph Weniger studies the generation and propagation of cosmic rays in the Milky Way using gamma-ray data.

Multimessenger Astrophysics

Faculty members Colijn, Decowski and Pollmann are looking for experimental signatures of dark matter. Detecting ‘visible’ subatomic particles can already be very challenging, so how does one find an ‘invisible’ kind of particles such as dark matter? This may possible by looking for the signature of a collision between dark matter and known particles. However, the likelihood of such an interaction is very small, which means that dark matter does not collide with other particles often.

The amount of dark matter passing earth is fixed, so to increase the chances of observing a collision, one must provide a large amount of known particles for it to interact with. The known particles used by these researchers come in the form of xenon. Xenon nuclei, at an atomic mass of about 130, provide a large target area. Furthermore, by using xenon in its very dense liquid form, one can fit a lot of xenon into a relatively small space. The latest detector, XENONnT, is filled with 8.5 tonnes of liquid xenon at a temperature of about -100°C.

Xenon is used because when other particles collide with a xenon nucleus, tiny amounts of light and electric charge are released. These are registered with high-precision sensors. A specific signature in the emission of light and charge could demonstrate the presence of dark matter. Due to the small chance of collision between dark matter and xenon, this is not a simple measurement. Normal particles, for example from natural radioactivity and cosmic rays, collide with the xenon atoms much more often. To block these, the detector is in an underground laboratory, underneath a mountain, at the Gran Sasso National Laboratory. This location, with 1400 meters of rock overhead, ensures that atmospheric muons are filtered to a large extent. Other sources of natural radioactivity are controlled by carefully selecting detector materials, and by surrounding the detector with several layers of shielding. 

The predecessor of XENONnT, XENON1T, took data between 2016 and 2019, providing world-leading dark matter results. The researchers hope to use the measurement data from XENONnT to prove the existence of dark matter and to determine two properties of the dark matter particle: what mass does it have and what is the likelihood of it interacting with ‘ordinary’ matter? But the experiment is also sensitive to other rare interactions, such as collisions involving neutrinos.

In the future, an even bigger detector using approximately 40 tonnes of xenon will provide an even more precise measurement of the possible interaction of dark matter with xenon atoms. The researchers are currently doing R&D to make this detector, called DARWIN, possible.

Experimental direct dark matter detection: XENON 

Sera Markoff is active in the Event Horizon Telescope (EHT). More information about the EHT can be found [here](https://eventhorizontelescope.org/).

The Event Horizon Telescope

Very-high energy (VHE) gamma rays—covering photon energies from ~100 GeV to >100 TeV— originate from the interaction of highly relativistic particles with gas particles, or with radiation fields. These highly relativistic particles are also detected on Earth and are known as cosmic rays. Since charged cosmic rays are deflected by magnetic fields before being detected, we cannot use them directly to infer from which sources cosmic rays are originating. Gamma rays offer therefore the best tool to identify the sources of cosmic rays in the sky. The gamma rays sources are in most cases energetic sources, such as accreting black holes (in the Milky Way or in the cores of other galaxies) or caused by energetic explosions such as supernovae and gamma-ray bursts. 

A potential other source of gamma rays is dark matter annihilation. According to some theories dark matter consists of ubiquitous, but unidentified,  massive neutral particles, which only weakly interact with each other, or other forms of matter (WIMPs or weakly interacting massive particles). However, WIMPs may self-annihilate, producing energetic known particles, which in the end will produce gamma ray signals. Finding such signals in the gamma rays provide one of the means to learn about the nature of dark matter.

![HESS](//images.ctfassets.net/g94voriuozkd/1X7syqw9psqpLmTMMsiQTr/e4c6df39c49e108a518d7c7823ed03e4/HESS.jpeg)

Fig. 1: The H.E.S.S. IACT array consisting of four 12 meter telescopes and one 25 meter telescope.

At GRAPPA we are involved in two VHE gamma-ray experiments: the High Energy Stereoscopic System (**H.E.S.S.**) located in Namibia (see Fig 1.) and the Cherenkov Telescope Array (**CTA**) to be built in two locations, on La Palma—covering the northern hemisphere—and in Chile—covering the southern hemisphere. Fig. 2 shows an artist impression of CTA.

Both H.E.S.S. and CTA are so-called imaging atmospheric Cherenkov Telescope (IACT) arrays. IACTs detect the faint bluish light that accompany charged particles that move through the atmosphere faster than the local speed of light. These charged particles are produced whenever a VHE photon interacts with particles in the atmosphere, typically producing energetic electron/positrons pairs, which in turn will also produce more gamma-ray light and secondary charged particles. As this happens repeatedly this results in a cascade of particles, which produce a nanosecond flash of Cherenkov light. By imaging Cherenkov light, the brightness and direction of the light bundle can be used to reconstruct the energy and original direction of the original gamma-ray photon. Also cosmic-ray particles produce particle cascades, but these produce light cones that are wider, which allows for separating  cosmic rays from photons entering the atmosphere. 

H.E.S.S. consists of five telescopes, which makes it more likely for the photons to be detected. Moreover, by detecting a Cherenkov event with multiple telescopes, from various angles allows for triangulation of the flashes, which results in a more accurate reconstruction of the direction from which the original gamma-ray photon arrive. In other words, stereoscopy results in sharper gamma-ray images.

![air showers](//images.ctfassets.net/g94voriuozkd/6EdOyr6k1qvM7ARNCsjI9O/cccd6e593bea1f9a473cf7e984048dbc/air_showers.png)

Fig. 2: Artist impression of CTA observing light from two Cherenkov events. The narrow cone on the left depicts a shower induced by a VHE gamma-ray photon.

CTA is global gamma-ray observatory, which will improve on the stereoscopic concept by employing many more telescopes spread over a large area. The size of the array will lead to more events being detected, but also to better determine the gamma-ray photon directions. For example, CTA-South will consist of 37 small-sized telescopes (SST, 4m class) telescopes and 23 medium-sized telescopes (12m class) to achieve a sensitivity that is a factor ten better than H.E.S.S.  (Fig. 3). The array is expected to be completed by 2028.

![CTA sensitivity](//images.ctfassets.net/g94voriuozkd/4qvui20JtJBuMQbZCWP6uG/ed14b70cb87293cb3aa72e02f5dbb49d/CTA_sensitivity.png)

Fig. 3: Sensitivity curve for CTA as compared to current gamma-ray telescopes.

GRAPPA member Jacco Vink is a member of the H.E.S.S. collaboration, and is currently functioning as its chair of the astrophysics department of H.E.S.S. GRAPPA members Jacco Vink and Sera Markoff are also members of the CTA consortium and are members of the SST camera consortium, which will build 37 cameras as part of the SSTs consortium. This is an international consortium (Germany, UK, Japan, Netherlands, Australia), which in the Netherlands also involves the Kapteyn Institute and the NOVA submm lab in Groningen.

![CTA camera](//images.ctfassets.net/g94voriuozkd/1PHmAM1G1RhFlAeWQclakw/de1618260d11820f7f3edf58defe4faf/CTA_camera.png)

Fig. 4: Prototype camera for the small-sized telescopes. This camera is relatively compact, as the telescopes will have two mirrors, resulting in a small focal plane. The detector plane will have 32 silicon photomultipliers (SiPM), each having 64 pixels.


Experimental gamma-ray astronomy and high-energy astrophysics

![Gravitational waves Jarnestad](//images.ctfassets.net/g94voriuozkd/1PJ4bH2xTgKJzQu2BN6RVp/a38386b21a0ff33ed881b1c98af83b02/image.png)

The existence of gravitational waves –ripples in the fabric of space-time– were predicted by Albert Einstein in 1916. After decades of painstaking technological improvements these waves were first observed in 2015 by the two LIGO interferometers in the USA and presented in the by now iconic and Nobel Prize winning illustration above. This event was traced back to the merger of two massive black holes into a single black hole. Since 2015 many (>50) events have been observed, with the most spectacular one a signal from the merger of two neutron stars, recorded in 2017 jointly by the two LIGO detectors and the Virgo detector in Italy near Pisa. This merger of two massive blobs of ‘normal matter’ was subsequently observed all across the electro-magnetic spectrum by numerous telescopes and established gravitational waves as a core ingredient in the new field of multi-messenger astronomy and as an entirely new way to ‘listen’ to the Universe.  

![GW detection principles](//images.ctfassets.net/g94voriuozkd/7y1roBQ1BfcxIJu2JO1Apm/24425b2040c1d812056d4389aa21f846/image.png)

The gravitational-waves detection principle is in essence very simple and based on the Michelson interferometer. This device is a perfect match to the signature of a gravitational wave which extends the length of one arm while it contracts the length of the other arm and vice-versa affecting the interference pattern of the two merged beams after they traveled up and down the laser-interferometer arms. This is observed as a brief flicker of light at the output port (photodiode light detector) as detailed in the illustration. In reality a gravitational-waves observatory is an extremely sensitive high-tech infrastructure capable to detect relative length changes between the two 3 (Virgo) or 4 (LIGO) km long arms as small as 0,000 000 0000 000 000 001 meter i.e. ten thousand times smaller than the proton diameter. To do so requires mind boggling technologies. 

Nikhef physicists, engineers and technicians are deeply involved in many aspects of gravitational-waves detection not only for the Virgo observatory (see https://www.virgo-gw.eu/), but also for the    ETpathfinder laser interferometer R&D laboratory:
https://www.einsteintelescope.nl/etpathfinder/ 
being built in Maastricht in preparation for the next generation gravitational-waves observatory Einstein Telescope (ET):
https://www.einsteintelescope.nl/ 
and for the future LISA gravitational-waves mission in space:
https://www.elisascience.org/.
We work, for example, on the characterization and improvement of the Virgo instruments. We design and develop new elaborate vibration attenuations systems (fabulous classical mechanics i.e. coupled pendula etc.), vibration-free cryogenics to allow eventually to cool interferometer mirrors to 10-20 K, and innovative photodetectors for LISA. We design and shape future detectors, with a focus on modern control systems and on optical simulation and design (classic laser optics with a quantum optics twist). Another Nikhef group, the one at Utrecht University, is deeply involved in the so-called ‘pipelines’ to identify gravitational-wave events in the continuous output stream (while taking data) of the Virgo observatory as well as in the analysis of these observations.

Staff members to contact: Frank Linde


Detector R&D for gravitational wave instrumentation at Nikhef

Particle accelerators have allowed us to systematically probe nature at ever smaller distance scales. They have provided unique information from the structure of atoms one hundred years ago to the discovery of new elementary particles, the gauge structure of their interactions, and discovery of the Higgs boson in 2012. The Standard Model (SM) currently describes all known particles and their interactions, but despite its enormous success, many fundamental questions remain unanswered. What is the nature of Dark Matter? What is the origin of the three generations of fermions and of their seemingly random patterns of masses and couplings? Is there a link between fermions and bosons? Why do we observe such an enormous discrepancy between the amount of matter and antimatter present in the Universe? Ideas that solve these problems all predict new phenomena or new particles to appear at smaller distance scales (that is, at very high energies). 

The most powerful accelerator today is the Large Hadron Collider (LHC), located at the CERN laboratory in Geneva, Switzerland. The LHC accelerates and collides protons and by studying in detail the particles produced in these collisions we can fingerprint nature at these distance scales where we hope to find answers to the pressing open questions listed above. Members of the Amsterdam physics community provide a crucial contribution to the LHC efforts by participating in the ATLAS and LHCb experiments as well as with theoretical simulations of collider processes. The ATLAS experiment studies particles at the highest energies. In combination with high collision rates, it allows to study in detail the Higgs boson, gauge bosons, and heavy particles like the top quark. We also look for new particles and phenomena predicted by theoretical extensions of the Standard Model. At the LHCb experiment, we focus on differences between matter and anti-matter. Precision measurements allow to study differences between lepton families and probe details of the  quark mixing matrix. In both experiments we are involved in detector R&D, data-acquisition and computing. Theoretical particle physicists in Amsterdam carry out precise predictions for relevant LHC processes, both with and beyond the SM, develop data interpretation frameworks based on effective field theories, and exploit machine learning tools to optimise the sensitivity to new phenomena. 

![LHC plots](//images.ctfassets.net/g94voriuozkd/6UXEPuxCKs5bLsMTDjrkaL/f79cdc46565ed60090504d7f294d8295/image.png)

Plot caption: left: visualisation of a collision recorded by the Atlas detector of a Higgs boson (decaying into two photons) produced in association with two top quarks, middle: on of the many di-pole magnets in the LHC tunnel and right: the Lagrangian of the Standard Model.

People to contact on this:
Atlas experiment: Wouter Verkerke
LHCb experiment: Marcel Merk
Theory: Juan Rojo

Physics at the Large Hadron Collider 

Neutrinos are electrically neutral subatomic particles. They have almost no mass. They also have a very weak interaction with matter. The consequence from this fact is that neutrinos will travel in a straight line from a cosmic source of high-energy neutrinos to Earth, with little absorption. This makes them potentially very good messengers of high-energy cosmic processes. Furthermore, neutrinos are interesting to study by themselves. Neutrinos are able to change their nature (“flavor”) when travelling, a phenomenon called neutrino oscillations. Precise measurements of these oscillations are able to reveal the properties of neutrinos, which is crucial for understanding their role in the history of the Universe.

Whenever neutrinos from the cosmos hit, for example, a water mass on Earth, most of them will simply travel straight through it. Only very occasionally will they collide with water molecules. When this happens, electrically charged muons are produced, which move in nearly the same direction as the neutrinos. These muons produce light as they move through the seawater. This is called Cherenkov radiation.

ANTARES is a neutrino telescope located on the bottom of the sea at a depth of 2500 meter, 40 km from the coast near Toulon, France. There, it searches for Cherenkov radiation as footprints of neutrinos. The seabed location is selected to largely shield the detector from disturbances by particles originating from the atmosphere. ANTARES aims to study neutrinos from the cosmos.

In order to improve the sensitivity for neutrino sources, a new neutrino telescope is needed that is more sensitive than ANTARES. To achieve this, the construction of KM3NeT started several years ago. KM3NeT is a neutrino telescope larger than a cubic kilometre. The building blocks of this detector are so-called Digital Optical Modules (DOMs). These are glass spheres filled with 31 phototubes that detect Cherenkov radiation. KM3NeT consists of hundreds of lines of about a kilometre in length. A single line contains 18 DOMs. Phase 1 of KM3NeT consists of 30 lines; 24 in Italy and 6 in France. Eventually, KM3NeT will consist of some 300 lines in two locations. Researchers hope to use KM3NeT to discover the cosmic sources of high-energy neutrinos, and how particle accelerators in the Universe work. Furthermore, KM3NeT will be able to study neutrino oscillations, and determine the neutrino mass ordering. The first KM3NeT lines have been deployed, and the detector is taking data.

The DUNE experiment operates very differently. A beam of muon neutrinos or muon antineutrinos will be sent from Fermilab near Chicago to a detector in South Dakota in the USA, 1300 kilometers away. Neutrino oscillations turn a fraction of these neutrinos to electron neutrinos or antineutrinos. The DUNE detector is a large volume time-projection-chamber filled with liquid argon, and measures particles from neutrino interactions in the argon with great detail. By studying the rate of electron - and muon neutrinos and antineutrinos, DUNE aims to detect violation of the CP-symmetry in neutrino interactions, and thereby shine light on the role of neutrinos in generating the Universe’ matter-antimatter asymmetry. A prototype detector for DUNE is constructed at CERN.

![KM3NeT detection lines](//images.ctfassets.net/g94voriuozkd/6XoH1YXNszrlQOaukIDVIq/c406cfd6c26a1ae2520e0e1fc3ce9098/image.png)

Caption: artist impression of the KM3NeT detection lines with the Digital Optical Modules in the Mediterranean Sea. Figure: Edward Berbee, Nikhef

In Amsterdam, Nikhef (Bruijn, De Jong) is deeply involved in the construction of the KM3NeT detector DOMs and lines, and in the data analysis. We concentrate on the measurements of neutrino oscillations with the first data: triggering and data selection, detector calibration, neutrino reconstruction, event classification and fitting the data. The Nikhef KM3NeT group is also active in neutrino astronomy, with data from ANTARES, and preparing analyses for a larger KM3NeT detector.

The DUNE group (Decowski, De Jong) is active in the analysis of prototype DUNE data, data acquisition and event classification. 

![inside protoDUNE](//images.ctfassets.net/g94voriuozkd/1ipRIrfgHXnZ5SMcuSwXVG/533d99e022e47069da30aa9d89f15c0d/image.png)

Caption: inside protoDUNE. Figure: CERN


Experimental neutrino physics: ANTARES, KM3NeT, DUNE

In particle physics experiments, researchers study the nature of elementary particles and fundamental forces by colliding particles. Through the study of these collisions we may find evidence of new, still unknown, fundamental particles that may explain large scale phenomena that we see in the universe, such as the existence of dark matter or the absence of anti-matter in the universe. 
Particle physics experiments make use of multiple layers of particle detectors using different detection techniques that allow us to unravel the physical interactions between the particles created in the collisions. One important detector technology is that of silicon pixel sensors, which allow the reconstruction of the flight trajectories of charged particles. Such silicon pixel sensors have been used in particle physics experiments since a few decades and similar sensors are now also readily used outside particle physics, for example in the camera of your cellular phone. Over the past decades, many technological improvements have been made. The sensors are now thinner, faster and more accurate in recording the spatial hits needed for the reconstruction of the trajectories. 

![Timepix 4](//images.ctfassets.net/g94voriuozkd/4as1lQTTkQkTFoG3P86Xri/b6bd58b483be1ae98875d981c3f73f08/image.png)

Figure: Nikhef contributed to the development of the Timepix 4 chip that has 200 ps time-to-digital converter (TDC) time bins to record hit time of particles with sub-nanosecond precision. There is ongoing development for further improvement of the time resolution.

We now work on what is most likely the most dramatic improvement of the silicon pixel technology since we started using it. We are adding the functionality to, simultaneously to the already very accurate spatial position, also record the time at which the particle has passed our detector to very high accuracy. The current time precision used in the LHC experiments at CERN is 25 ns. We are aiming to improve the precision of this measurement with a factor 1000 to an order of 20 ps. 

![ATLAS t-anti-t](//images.ctfassets.net/g94voriuozkd/7cX2gFvK7qnRS6Ltr3nbQ8/c49ad6956bf1ead7d10708235f8b6ddc/image.png)

ATLAS experiment simulated top-antitop quark event with 200 multiple events originating from the same bunch crossing. To distinguish the different origins of collisions, an excellent time resolution is needed. Figure from cds.cern.ch. 

This technological improvement will allow much better separation of those particle collisions that occur almost, but not exactly, simultaneously. These new fast timing detectors are necessary for the advanced so-called High Luminosity LHC that will boost the discovery potential of the experiments at the large hadron collider at CERN. In the Nikhef detector R&D fast timing group, we study technological solutions for upgrades of the LHCb, ALICE and Atlas experiments. 

![FastTimingDetectors MG 4903](//images.ctfassets.net/g94voriuozkd/2CRxCUy59F1eBIRlyfOWWQ/6ba17b26d9f38e5f2a5f9e8b7f5b232a/FastTimingDetectors_MG_4903.jpg) Image credit: Marco Kraan

![FastTimingDetectors MG 4784(1)](//images.ctfassets.net/g94voriuozkd/F6fzdXfcicjjJlzXnIk96/796d5b1598a0d362412101a335609d62/FastTimingDetectors_MG_4784_1_.jpg) Image credit: Marco Kraan

Laser setups at Nikhef for characterization of silicon sensors. The very new two-photon absorption setup with a very fast, very powerful, and very precise femtosecond laser pulse can be used to characterize sensors for future experiments at the LHC and beyond (right).

To be able to reach this next generation silicon pixel detectors, we study the response of several semiconductor sensor technologies in great detail, and are active in the development and design of new sensors and readout chips. We make use of beam experiment facilities in addition to two laser setups locally in our lab. We make detailed simulation studies of the charge transport and response of various sensor designs, working closely together with the ASIC design team in the Nikhef electronics group.

For possible research projects in our group, see the [Nikhef project wiki](https://wiki.nikhef.nl/education/Master_Projects).


Fast timing detectors for particle physics experiments

![Early universe](//images.ctfassets.net/g94voriuozkd/6BJtI8ckfQPiNaD3Hq4K1u/fbddbaa62dd86b8c2d245e51bdeec55f/early-universe.jpg)

Figure 1: Temperature fluctuations in the cosmic microwave background as measured by the Planck satellite. The hot and cold spots in this map correspond to density variations in the matter 380 000 years after the Big Bang. A central goal of modern cosmology is to explain the origin of these fluctuations. Credit: Planck collaboration.

The early universe was hot and dense. Interactions between particles were very frequent and energetic. Matter was in the form of free electrons and atomic nuclei with light bouncing between them. As the primordial plasma cooled, the light elements (hydrogen, helium and lithium) formed. About 380 000 years after the Big Bang, the temperature had dropped enough for the first stable atoms to exist. At that moment, photons started to propagate freely through the universe. Today, billions of years later, we observe this afterglow of the Big Bang as a faint microwave radiation. This relic radiation isn’t completely uniform but varies in its intensity across the sky (see Fig. 1). These fluctuations reflect tiny variations in the density of the primordial matter, which over time, and under the influence of gravity, grew. Dense regions were getting denser. Eventually, galaxies, stars and planets formed and the large-scale structure of the universe was created.

But, what created the fluctuations in the cosmic microwave background? What was the fundamental origin of the large-scale structure of the universe? A remarkable fact about our Universe is that the structures we see around us are not distributed randomly but rather display interesting spatial correlations. A central challenge of modern cosmology is to construct a consistent history of the Universe that explains these correlations. Observations suggest that the primordial correlations were created before the hot Big Bang, at energies far exceeding those probed by particle colliders. Members of GRAPPA are playing a leading role in the quest to understand the Universe under these extreme conditions.

One of the leading proposals for the origin of structure in the Universe is “cosmological inflation.” According to this theory, the Universe experienced a burst of very rapid expansion just fractions of a second after the Big Bang. Microscopic quantum fluctuations got stretched to cosmic scales providing the seeds for the large-scale structure of the Universe. The correlations observed in the afterglow of the Big Bang were inherited from correlations in the quantum-mechanical fluctuations during inflation (see Fig. 2). While inflation is a very successful phenomenological model, it is not yet a complete theory. In particular, the physical origin of the inflationary expansion is still a mystery. Members of GRAPPA are investigating the physics of inflation in theories of quantum gravity and from the viewpoint of effective field theory (Baumann, Freivogel). They are also exploring the phenomenological consequences of inflation, including the possibility that inflation gave rise to primordial black holes (Bertone).

![Quantum fluctuations during inflation](//images.ctfassets.net/g94voriuozkd/5xThf9SCx3ZDR6hQcx8ZQV/b4fa2f9b224519e8170127281cc4a222/image.png)

Caption: Quantum fluctuations during inflation produce correlated density fluctuations at the beginning of the hot Big Bang. Researchers at GRAPPA are exploring whether this mechanism was the origin of the large-scale structure of the Universe.

The Early Universe

One of the most important discoveries of the past 50 years in physics and astronomy, is that there is much more to the universe that meets the eye: the universe appears to be permeated by mysterious forms of matter and energy, that shape the universe and determine its structure and evolution.

According to modern cosmology, only 5% of the matter and energy budget of the universe is made of the same form of matter that we see all around us -- which as we know is made of atoms. The remaining 95% is referred to as the ‘dark universe’.

![Dark matter web - Illustris](//images.contentful.com/g94voriuozkd/1NYdziqkYQUSR5TJ9H51d2/4fb5c6f38ea7c8b36e9578b020cf480a/image.png)

Caption: According to modern cosmology, a web of dark matter permeates the universe, and supports all cosmic structures. In this snapshot taken from the Illustris Simulation, we see on the left the dark matter density, and on the right the density of gas in a massive cluster of galaxies.
        Source: Illustris Project

We don’t know much about this dark side of of the universe, but if we look at the matter-energy budget of the universe today, we can confidently say that on very large scales 25% appears to be composed of an unknown form of matter, known as dark matter, and another 70% behaves like a sort of repulsive gravity, that pushes the universe to expand ever faster, and is generically referred to as dark energy.

![History of the Universe](//images.contentful.com/g94voriuozkd/3A4lk9TKQtj4eWTOaLgU6H/958ad6ac946ac572e278713e56d406d4/image.png)

Caption: Visualization of the history of the Universe. In order to make sense of the structure and evolution of the universe, we need to postulate the existence of dark matter and dark energy.
        Source: NASA/WMAP Science Team

These are not mere conjectures: the Nobel Prize in Physics was awarded in 2011 to Saul Perlmutter, Brian Schmidt and Adam Riess, for the discovery of the accelerated expansion of the universe, and in 2019 (for one half) to Jim Peebles, for theoretical discoveries in cosmology, including his pioneering work on dark matter.

The existence of dark matter has long been suspected, and at GRAPPA we actually study the fascinating history of this elusive component of the universe (Bertone, van Dongen). But it was only in the 1970s that different lines of evidence merged within the framework of the rising field of physical cosmology, and provided convincing evidence for a form of matter that is fundamentally different from anything we have ever observed in our laboratories, or with our powerful telescopes.

Researchers at GRAPPA try to identify the nature of dark matter with a variety of approaches: we devise strategies to detect high-energy radiation produced by dark matter (Ando, Bertone, Weniger), we set up methods for detecting dark matter ‘clumps’ with gravitational lenses and stellar streams (Ando, Bertone, Weniger), and we explore the new opportunities arising from the booming field of gravitational waves (Baumann, Bertone, Weniger).

Our group is also active in the field of dark energy, with GRAPPA researchers pioneering a new method to determine the expansion rate of the universe, based again on gravitational waves (Nissanke).

Dark Universe

Gravitational waves are ripples in spacetime curvature caused by some of the most violent and energetic events in the Universe. They are an intrinsic prediction of General Relativity, that were first proposed by Albert Einstein in 1916. They are generated by massive asymmetric accelerating masses, such as inspiralling and merging pairs of neutron stars, black holes and white dwarfs (aka. compact objects), and travel at the speed of light, carrying critical information about extreme spacetime, extreme gravity objects and extreme events (such as the Big Bang and Inflation) in the cosmos.

After decades of immense experimental and theoretical work, gravitational wave astrophysics became also observationally-driven field with the discovery of gravitational waves from the first pair of black holes merging as observed by the LIGO detectors. Since 2017, the LIGO detectors have continued to improve in sensitivity and have since been joined by the Virgo detector, located in Pisa, Italy (with NL contributions), as well as KAGRA in Japan. Therefore, we now have a worldwide network of ground-based gravitational wave detectors observing tens of compact of mergers, of different flavours (neutron star-neutron star, neutron star-black hole and black hole-black hole mergers) every year, and uncovering a diversity in the fundamental properties of extreme gravity objects. Such a plethora of new observations have allowed us to ask and now answer many new questions including: how and where do compact object mergers form? How do compact objects form and drive the evolution of the Universe? What is the intrinsic nature of black holes, neutron stars and white dwarfs? In addition, a subset of gravitational wave mergers, notably in systems where there is at least one neutron star, are accompanied by electromagnetic counterparts, for instance radioactively powered kilonova or short gamma ray bursts; such events exemplify multi-messenger astronomy.

At GRAPPA, we are working on many different aspects of gravitational wave astrophysics and physics: gravitational wave and electromagnetic counterpart source modelling (Nissanke and Moesta), gravitational wave parameter inference and machine learning (Nissanke, Weniger), numerical relativity (Moesta), binary and triple stellar evolution (Toonen), primordial black holes and dark matter (Baumann, Bertone and Weniger), gravitational wave cosmology (Nissanke, Baumann) and multi-messenger astrophysics and interpretation (Nissanke, Moesta). Members are active in the Virgo Collaboration and LISA consortium, EU Cost Actions GWVerse, as well as a slew of electromagnetic follow up groups.

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