At school I often came into trouble because teachers were irritated and frustrated to continue answering the never-ending ‘why’ questions. ‘Why do we exist?’ and ‘what was there before the Universe?’ It was not appreciated by my teachers. Education did not go well, and let’s just say I wasn’t hanging out with the ‘right crowd’. I have always been curious about the world and universe we live in, my father inspired me to follow that path and convinced me that asking questions is exactly the right way.

And science is getting closer to answer these questions!!

Albert Einstein demonstrated that atoms exist, showed that light comes in discrete packets and proved by elegant arguments that space and time are not absolute. Formerly independent and uniform, space and time became inseparable and variable.

Gravity is one of the four fundamental forces of nature (along with electromagnetic, weak and strong nuclear forces). Einstein’s theory of General Relativity turned Newton’s absolute space and time into a relativistic mash-up — his equations suggested a merged spacetime, a new sort of arena in which the players altered the space of the playing field. It was a physics game changer. No longer did space and time provide a featureless backdrop for matter and energy. Einstein showed in his general theory of relativity, matter and energy warped the spacetime surrounding it. The way in which matter perceives distortions in space-time; gravity is a side-effect of the curvature of spacetime.

An important prediction of Einstein’s theory are gravitational waves: tiny ripples in spacetime caused by violent events in the Universe. Generated by the most extreme events in the cosmos (like the crashing together of neutron stars and black holes), gravitational waves are ripples in the very fabric of the Universe. Gravitational waves are disruptions in space-time, the four-dimensional universe that includes time as well as the three spatial dimensions we are used to), that can only be detected by the most sensitive instruments around the world. The waves are virtually unaffected by matter, which means that they travel through the universe, effectively unchanged, providing incredibly accurate information of the sources billions of lights years away.

Gravitational waves are fundamentally different from, for example, electromagnetic waves. The acceleration of electric charges creates electromagnetic waves, propagating in space and time. However, gravitational waves, created by the acceleration of mass, are waves of the spacetime ‘fabric’ itself. Gravitational waves distort spacetime: they change the distances between large, free objects. A gravitational wave passing through the Solar System creates a time-varying strain in space that periodically changes the distances between all bodies in the Solar System (this strain changes distances perpendicularly to the direction in which the wave moves).

When gravitational waves travel past us, they stretch space-time slightly in one direction and compress it at right angles. Therefore, if we can detect an object being stretched and compressed in this way, we can detect gravitational waves. Due to the tiny size of gravitational waves, the amount of stretch and compression of space is correspondingly tiny.

According to General Relativity matter and energy in the Universe — specifically, the energy density, the pressure, the momentum density, and the shear stress present throughout spacetime — determines the amount and type of spacetime curvature that’s present in all four dimensions: the three spatial dimensions as well as the time dimension. As a result of this spacetime curvature, all entities that exist in this spacetime, including (but not limited to) all massive and massless particles, move not necessarily along straight lines, but rather along geodesics: the shortest paths between any two points defined by the curved space between them, rather than an (incorrectly) assumed flat space. Where spatial curvature is large, the deviations from straight-line paths are large, and the rate at which time passes can dilate significantly as well.

In 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the universe was actually expanding more slowly than it is today. So the expansion of the universe has not been slowing due to gravity, as everyone thought, it has been accelerating. still don’t know what the correct explanation is, but they have given the solution a name. It is called dark energy. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest – everything on Earth, everything ever observed with all of our instruments, all normal matter – adds up to less than 5% of the universe.

One explanation for dark energy is that it is a property of space. As mentioned, Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein’s gravity theory, the version that contains a cosmological constant, makes a second prediction: ‘empty space’ can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe. Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, ‘empty space’ is actually full of temporary (‘virtual’) particles that continually form and then disappear.

Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy.

Then there is dark matter. By fitting a theoretical model of the composition of the universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~68% dark energy, ~27% dark matter, ~5% normal matter.

We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons (a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). This is known because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.

Quantum mechanics describes the machinations of matter and energy on the atomic scale with unerring accuracy. At the atomic scale, quantum mechanics explain some of the most basic facts of our existence: the fact that the atoms we are made of are stable and do not collapse can only be explained by quantum mechanics. On cosmological scales, general relativity explains the existence of black holes, the gravitational bending of light, and gravitational waves from the collisions of distant black holes, and -as aforementioned- the curvature that’s present in all four dimensions: the three spatial dimensions as well as the time dimension. For now, the fact that these two theories are fundamentally incompatible has not caused any practical problems: on atomic scales, gravity is not relevant in experiments, and on cosmological scales, quantum mechanics do not play a noticeable role in what we observe.

The core problem in trying to combine these two is the fact that in general relativity, the position of a mass changes the evolution of time. In quantum mechanics, however, masses can be in a superposition: they can be two places at the same time. Combined with general relativity, this would predict that there can also be quantum uncertainty in the definition of time: if heavy thing exist in two places at the same time, there will be two different ‘clocks’, one associated with each of the ‘halves’ of the object in the superposition. However, this is something that quantum mechanics is not able to handle: there is no quantum theory that can account for quantum uncertainty in time!

Attempts to find coherent math that accommodates quantum weirdness with geometric gravity, though, have met formidable technical and conceptual roadblocks. According to quantum mechanics, the forces of nature come in tiny, discrete chunks known as quanta. Gravity is a force, so is space-time itself quantized? According to general relativity that the warping of space and time is what we experience as the force of gravity. According to quantum mechanics that what we experience as the forces of nature really come in discrete, tiny chunks, known as quanta.

Everything in the universe is constantly being stretched and squeezed by gravitational waves, ripples in space-time caused by the movements of massive objects. So, if gravity is the bending of space-time, gravity is a force and all forces are quantized, maybe space-time itself comes in discrete little blocks. Maybe there are fundamental units of space-time at some unfathomably tiny scale? The main question is the role that space-time plays in the physics. For quantum mechanics, space-time is just a background for all the interesting interactions that make up the physics of the universe. Even if its bens and warps, and that bending and warping affect the paths of particles, all physics happens ‘on top’ of that background space-time.

Gravitational wave detectors have opened a new window into the astrophysical universe, enabling the first direct observations of gravitational radiation from collisions of compact objects.

In the quest to unite quantum mechanics with gravity, maybe we should take Einstein’s theory at face value. If gravity simply *is* the mechanics of space-time, then to seek a quantum theory of gravity, we really need to seek a quantum theory of space-time. If we can crack that quantization, then by default, we’ll end up with a quantum theory of gravity, and the problem will be solved.

In the primordial universe, all matter once existed as extremely tiny elementary particles. In the moments immediately following the Big Bang, the very first gravitational waves rang out. The product of quantum fluctuations in the new soup of primordial matter, these earliest ripples through the fabric of space-time were quickly amplified by inflationary processes that drove the universe to explosively expand. Primordial gravitational waves, produced nearly 13.8 billion years ago, still echo through the universe today. The magnitude of these waves is directly determined by the energy scale of the infant universe. But they are drowned out by the crackle of gravitational waves produced by more recent events, such as colliding black holes and neutron stars.

The ability to directly detect gravitational waves opens up a new observational field at the intersection of fundamental physics, astrophysics, and cosmology. Access to spacetime curvatures that are millions of times stronger than in the Solar System, caused by macroscopic objects like black holes that are smashing into each other at a significant fraction of the speed of light. Allowing to probe some of the most fundamental properties of black holes by direct observation, to ‘look’ inside neutron stars, with implications for our understanding of the strong nuclear force that keeps atomic nuclei together. At much larger length scales, gravitational wave sources can be used as cosmic distance markers to map out the large-scale evolution of the Universe. Further into the future we also hope to find primordial gravitational waves that originated a split-second after the Big Bang.

Gravitational waves are being detected on an almost daily basis by LIGO and other gravitational-wave detectors, but primordial gravitational signals are several orders of magnitude fainter than what these detectors can register. It’s expected that the next generation of detectors will be sensitive enough to pick up these earliest ripples.

In the next decade, as more sensitive instruments come online, the new method could be applied to dig up hidden signals of the universe’s first gravitational waves. The pattern and properties of these primordial waves could then reveal clues about the early universe, such as the conditions that drove inflation.

The Einstein Telescope will probe the physics near black-hole horizons (from tests of general relativity to quantum gravity), help understanding the nature of dark matter (such as primordial Black Holes, axion clouds, dark matter accreting on compact objects), and the nature of dark energy and possible modifications of general relativity at cosmological scales. The Einstein Telescope will make it possible to explore the Universe through gravitational waves along its cosmic history up to the cosmological dark ages, shedding light on open questions of fundamental physics and cosmology.

Amazing!!!