00:00Sous-titrage Société Radio-Canada
00:30At the beginning, the universe was homogeneous.
00:33Matter was distributed in it like a vast fog of undifferentiated particles.
00:39Today, it contains objects with definite shapes, like galaxies in which stars and planets form.
00:45But how and why did the original uniformity give way to today's universe, organized in structures?
00:56At the Meudon Observatory, Jean-Michel Alimi and his team are trying to understand how the universe was formed
01:03and what causes cosmic expansion with the DEUS, Dark Energy Universe Simulation, Full Universe program.
01:11For the first time ever, scientists are going to calculate how matter evolved in the entire observable universe,
01:18from the Big Bang to the present day.
01:24All the observations that have been made of the universe have enabled us to establish the amount of energy and
01:29matter it contains.
01:31Ordinary, visible matter, which we and the planets and galaxies are made of,
01:35only accounts for 4% of the energy in the universe.
01:40By observing the movement of stars at the edges of galaxies, scientists have deduced the existence of other, invisible matter,
01:48only detectable through its gravitational effects, known as dark matter,
01:53which would comprise cosmic structures like galaxies and represent about 20% of the energy in the universe.
02:01But there is something even more mysterious.
02:04We know that the universe is expanding.
02:07If we take two points in space, with time, they move apart,
02:10as is confirmed by observation of receding galaxies.
02:16It's not that the galaxies are actually moving away from one another,
02:20it's the geometric fabric of space itself on which the galaxies are located that's dilating with the passage of time.
02:27In the classic framework of general relativity, without introducing additional parameters,
02:32this expansion cannot be accelerated.
02:35The speed can vary but not increase in the course of time.
02:39Recently, it was discovered that this expansion was speeding up,
02:42and this research earned its authors a Nobel Prize.
02:46How was it possible?
02:47This is where dark energy comes in.
02:50So the theory is that as well as visible matter and dark matter,
02:54there must be another component in the universe that explains this accelerated expansion – dark energy.
03:02Representing more than 75% of the energy in the universe,
03:06it would act as a kind of fluid with negative pressure forcing the universe to dilate.
03:12This dark energy is the mystery of modern cosmology,
03:15going beyond the fundamental laws of physics.
03:18To find out what it is means understanding its role in cosmic structure formation,
03:24but that requires processing far in excess of what has been achieved up to now.
03:29It means creating a numerical simulation not just of a fraction of our universe,
03:34but of the entire observable universe.
03:38At the beginning, just after the Big Bang, matter was distributed evenly in space.
03:44There was no distinguishable structure.
03:46Then, as time passed, the matter in the universe became concentrated into objects,
03:52giving birth to galaxies, galaxy clusters,
03:55and all the cosmic structures that can be observed today.
03:59The challenge that Jean-Michel Alimi's team have taken up
04:03is to monitor this structure formation throughout the history of the universe
04:07so as to reproduce the evolution of all the matter it contains with incomparable accuracy.
04:14The scientists set the initial conditions for their calculation
04:17to concur exactly with the closest image that we possess of the primeval fog
04:22formed by the very first photons emitted just after the Big Bang.
04:27This is the primal glow of cosmic background radiation.
04:32The formation of cosmic structures is then simulated by following the evolution of dark matter particles,
04:38which vastly outnumber particles of ordinary matter.
04:42With 550 billion particles, each of which contains the mass of our galaxy,
04:48the model represents the entire observable universe.
04:52The cosmic structure formation process will take place differently
04:55depending on the nature of the dark energy.
04:57So the researchers decided to repeat their experiment
05:00with three different models of dark energy,
05:03each with its own characteristic manner of structuring the universe.
05:18To achieve the necessary processing power,
05:21they used one of the world's largest computers,
05:24the Curie supercomputer.
05:32Here we are at the CEA's large computing centre
05:35and behind us is the Curie machine acquired by GenC,
05:38which must currently be the fourth most powerful computer in the world
05:41and which can perform several million billion operations per second.
05:45With the arrival of this machine and the vast memory it provides when operating at peak performance,
05:52we were able to envisage this numerical simulation
05:55of how the whole universe's structure was formed
05:58with a high enough resolution
06:00to follow the trajectories and evolution of 550 billion particles.
06:07This computation will require more than 300 hours of processing
06:11on the 80,000 cores of the Curie supercomputer.
06:15That is equivalent to about 3,000 years
06:18if the processing were performed by a single core.
06:21The data generated will represent more than 150 petabytes,
06:25which a rigorous and absolutely necessary selection process
06:29will whittle down to around 2 petabytes,
06:31the equivalent of 400,000 DVDs.
06:35After weeks of processing and interpretation,
06:38the evolution of the entire observable universe
06:41finally appears for the first model.
06:43The others will follow a few months later.
06:47At the beginning of the simulation,
06:49the galaxy particles are homogeneously distributed.
06:52As they are affected by the attractive forces of gravity
06:55and the repulsive forces of dark energy,
06:58structures appear.
06:59Galaxy clusters, superclusters, filaments.
07:07We know that when we look far out into the universe,
07:10we're looking into the past,
07:11so we have reconstructed the distribution of matter
07:15at each point in space,
07:16taking account of this timeline.
07:19We can approach a site
07:20where cosmic structures are formed.
07:23The matter there is broken down along large filaments.
07:27These filaments intersect and form a complex structure,
07:31characteristic of the cosmological model
07:33and so of dark energy.
07:40The most massive object that appears
07:43is more than 10 million billion times the mass of the sun.
07:47We know its structure and we also know its environment.
07:50It is at the intersection of a flow of dark matter,
07:54filaments of dark matter that come and collapse
07:57into this gigantic object
07:59which is only accessible with such huge scale computing.
08:08Comparing this fine structure,
08:11the formation process and the breakdown of matter
08:13will give us a better understanding
08:15of the nature of dark energy
08:16and perhaps even allow us to determine what it is.
08:20The structures obtained in the universe
08:23is simulated for the three cosmological models
08:25can now be compared with one another
08:27and with the structures corresponding to real observations.
08:30This will enable researchers
08:32to refine their model of dark energy
08:34and perhaps succeed in penetrating its mysteries.
08:40The Force
08:41The Force
08:41The Force
08:45The Force
08:48The Force
