Intro
Analogue gravity is a field of study in which one devices an experiment to mimic a gravitational scenario/system that is usually inaccessible in our laboratories. Afterall, It would be much easier to study the universe and everything within it, from blackholes to galaxies, if we could create one and have full control of it in our hands.
Except that we do not really understand the universe entirely to create a new one, nor we have the energy and the time to do so. You do not want to wait millions and billions of years to take a single measurement for you next research paper. The processes that occur in our universe happen in a, well, cosmic time. Galaxies need a billion year to form, stars take billions of years to die, etc… . The time scales of cosmic processes are simply too large to study them in real time, meaning to have iterations of the same process over and over again, something you usually want to obtain statistical certainty.
But now imagine you could build a well controllable system, that behaves just like our universe. Even better, the processes that happen in such a “universe” are much faster now so that you can take as many measurements as you like. You can repeat the process, change initial conditions, tune certain parameters just to observe how differently this simulated universe would behave. This is exactly what analogue gravity is about.
How to Simulate a Universe
This “machine” is nothing but water mixed with fluorescent dye in a huge bath tub allowed to rotate through a drain to create a vortex. The scientists then propagate sound waves in the water to study their motion. This rotating fluid/vortex system resembles a rotating blackhole, while the soundwave can be seen as a beam of light. At a certain angular velocity, the scientists show how soundwaves can no longer escape the vortex after crossing a certain horizon, similarly to how light cannot escape a blackhole after crossing its event horizon. Such analogy allows researchers to study unobserved effects that are predicted to occur when light propagates near a rotating blackhole.
It happened that I was recommended this excellent piece of research [1] by my professor that sparked my interest again in analogue gravity and motivated me to write this article.
A group of researchers simulated a universe using a special kind of fluid, a Bose Einstein Condensate (BEC). Such a state of matter occurs after an atomic ensemble crosses a certain phase space density (a ratio of temperature to volume). This BEC is usually in the nanokelvin temperature scale, just above absolute zero. At such a low energy scale, the ensemble can be controlled and studied in great detail. And this BEC cloud behaves like a universe the same way our universe allows light and gravitational waves to propagate thought it. But what kind of waves can propagate through a BEC? Phonons!
Think of soundwaves propagating in air from point A to B. This wave is simply the motion of the air molecules oscillating back and forth, producing a wave that carries sound from a point source A to a receiver at B. The source A is said to have perturbed the system, or created an “excitation” of air molecules which themselves excited more air molecules and so on.. a wave of excited air molecules.
(Strictly speaking, the air molecules are not excited in the quantum mechanical sense, rather acquire additional energy due to the perturbation.)
Phonons are the quantum equivalent of soundwaves. They are excitations in a quantum system like a BEC which propagated through it by causing more excitations, a wave of excited atoms. And similar to how a perturbation in an air medium excite the air molecules producing soundwaves, a perturbation in the BEC produces phonons.
One way of perturbing the BEC is through shining a laser beam on it. The laser beam creates a repulsive force on the atoms in the BEC cloud producing a wave when the laser is pulsed for a short time, an effect similar to dropping a stone in a lake.
(The researchers are using potassium-39 in a harmonic trap as their BEC source and a blue detuning laser at 532nm to perturb the system.)In the image to the right, we see a blob, that’s the BEC cloud. The bottom cloud is just after pulsing it with the laser beam, which leaves a hole in it. As time passes (the middle and top 4ms and 8ms respectively), one can realize a wave like structure propagating through the cloud. That’s the phonon! The right column shows the same images as those on the left column but subtracted from the unperturbed cloud (not shown here).
But how is this close to simulating a universe? Just because a fluid allows wave propagation doesn’t mean it’s a universe. And yes, that is absolutely correct. Analogue gravity aims to understand a specific property of the universe and not the entirety of it in a single system.
Atoms and the Cosmos
The first property the researchers aimed at demonstrating is whether the shape of the BEC clouds affects the trajectory of the phonons. In cosmology, there are three models to the shape of the universe. Flat, negatively curved (hyperbolic), and positively curved (spherical). The shape of the universe dictates the trajectory of light and massive objects in spacetime. If analogous, the shape of the BEC, which depends on its density, should leads to distinct trajectory of the phonons.
For this reason the researchers begin shaping the BEC through different laser trapping techniques, to change its density and see how the phonons will propagate. And to their expectations, when the BEC is hyperbolically curved, the propagation of phonons exhibit concentric waves, while phonons on a spherically curved BEC gets straightened out as you can see below.
The behaviour of waves in spacetime and phonons through a BEC is indeed analogous and obey the same fundamental geometric principles. But do they also obey the same physical principles?
In cosmology and quantum field theory, there’s a process that has been predicted which states that in an rapidly expanding universe, line in the example of the early inflationary stage, particle production occurs due to certain broken symmetries when dealing with quantum fields in curved spacetime [2]. Many suggest that it is this process that gives rise to anisotropies in the cosmic microwave background (CMB) radiation that we observe.
While it has never been experimentally observed by astronomers, it has been observed by the quantum opticians in their BEC lab. The researchers placed their BEC inside a tunable magnetic field, this allows them to change the scattering length of their potassium atoms. Lower scattering means lower interaction. Lower interactions mean that the phonons now propagate slower. And a slower propagation of waves in a static universe is equivalent to a constant propagating speed of a wave in an expanding universe.
Astonishingly, they observed an enhanced density distribution in their cloud due to the “expansion” in a form of random fluctuations. The analogous phenomena in our universe is the Hawking radiation from blackholes, which remains unobserved. Furthermore, the researchers observed how these fluctuations interfere with each other leading to oscillations that were first predicted by Andreĭ Sakharov in 1965. These Sakharov oscillations, as they are known today, might be the reason behind the large-scale pattern of angular fluctuations (ripples) in the temperature of the CMB.
(Watch this nice animation demonstrating how particle production in the early universe could have influenced the CMB)
But to what extent is a BEC analogous to our universe? Can we understand the cosmos from the eye of an atomic cloud trapped in a vacuum glass? While there are indeed several similarities and analogies between the laws they obey, they are different systems. One fits in a room, the other is the room and everything around it. But perhaps one is a window towards phenomena that we are yet to observe through our telescopes and satellites, a companion in the dark.
References:
By Ali Lezeik
PhD student at Leibniz University Hannover, Germany
Article source : lezeik.com
Ali Lezeik is a PhD student, currently researching quantum optics at Leibniz University Hannover. He is part of the Very Large Baseline Atom Interferometry (VLBAI) facility, where he studies the behavior of ultracold quantum gases in a gravitational field.
Passionate about exploring the intersection of gravity and the quantum realm, Ali aims to bridge the gap between theory and experiment.
Glossary
- A phonon is a particle-like unit of vibration in a solid, similar to how photons are particles of light. When atoms in a solid vibrate together in a wave-like pattern, we describe that motion as a phonon. These phonons help in transferring heat and sound through materials.
- A Bose-Einstein Condensate (BEC) forms when certain atoms are cooled near absolute zero, causing them to merge into a single quantum state, behaving like one “super atom.” It’s like people moving in perfect sync as one entity! being!
To create such a BEC, researchers first confine and cool atoms using a Magneto-Optical Trap (MOT), which relies on a combination of laser cooling and a magnetic field gradient (by using anti-Helmholtz coils).
To explore how magnetic field gradients are generated in such systems, visit our Anti-Helmholtz simulator.
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