For thousands of years, we have looked at the night sky and believed that the illuminated staff was all that made up our universe. Scientists now realize it's not what shines in light but hides in dark that holds the true secret of our sky, There is a mysterious dark matter that binds stars and galaxies together and strange particles like WlMPs, AXlONs and MACHOs might be to blame.
And there is dark repulsive energy that is creating space in the universe and driving galaxies father and further apart to a dismal fate. Combined, dark matter and dark energy make up about 96% of universe, and uncovering their secret is like one-in million shot. If uncovered, the ultimate late of universe might be revealed. Will we crash and burn in horrific collision of gravitational forces or will dark energy tear universe apart ? So, in text, we travel to the dark side of the universe.
Dark Matter
Dark matter is unlike anything we have ever encountered on earth. Billions of these matter pass through everything they encounter each second. They are so massive in weight that they have power to influence the galaxies. Dark matter's invisible presence is everywhere, or so it seems. Science hasn't directly proven that it exists: and observing something that you can't see isn't easy. It neither emits nor absorbs light. In spite of no interaction, there are evidences that it exists. Hunt for dark matter started almost a century ago when scientists developed tools to see deep into night sky. Edwin Hubble found that universe extends beyond Milky Way and there are other 'galaxies'. As astronomers discovered new galaxies, something strange was observed. Fritz Zwicky in 1933 measured the motion of galaxies in 'Coma' cluster and estimated how much mass there was in it. He compared it with the luminous mass (i.e mass of matter he could see) of galaxy. Something didn't add up. The galaxies were moving too fast for notable illuminated stuff. By his calculation, there should be 160 times more illuminated mass to account for the speed. The cluster couldn't be stable unless it contained a large amount of 'dark' matter. According to gravitational concept, further is an object from center of gravity, slower will be its speed, and some applies for galaxies. But it is observed that the angular velocity and thus time period ot revolution around galactic center stores and dust remains constant from central region to the arms. This will unstabilize the galaxy and matter will be thrown out.
Only way to explain the stability of galaxies is to assume a halo of invisible mass surrounding the galaxy, keeping it whole, which is dark matter. First astronomer to figure this out. Ruben, assumed that there is 10 times more dark matter than ordinary illuminated stuffs. This was observed in all galaxies. Thus the hunt for dark matter started. Science realized that dark matter. Although invisible, exposed itself by bending light rays passing through and very close to it. This can be detected by gravitational lensing which is a virtual spotlight that uncovers any invisible stuffs in the universe. Like ordinary matter, dark matter deflects path of light thmugh gravity, and by tracing the distorted light, scientists found that dark matter is concentrated at halo of galaxies Thus, presence of dark matter was suddenly revealed. Scientists could actually generate 3d maps of dark matter in the space. Search began to find out what dark matter is? First guess Was that, it was ordinary matter that doesn't shine. Black holes, MACHOs (Massive Compact Halo objects), failed stars etc. were considered but they were far less to account for the amount of dark matter. Sooner, scientists realized that it wasn't made up of proton-neutron-electron system, so they suspected other particles like neutrino, AXlONs etc, but neutrinos were far light and AXlONs were unstable unlike dark matter. WlMPs (Weakly Interacting Massive particles) are now taken as possible suspect for dark matter but they haven't been detected either. Scientists are trying to capture dark matter many kilometres down the surface of earth avoid of cosmic rays at an underground laboratory named SUDAN lab in USA by trying to collide dark matter with Ge-Si mixture kept at almost absolute zero temperature, however, they have been unsuccessful for a whole decade. Discovery of dark matter is supposed to help the scientists to trace the history of universe upto one-ten thousandth of a second after big bang. However, to realize dark matter precisely, we have to travel to the beginning of universe the Big Bang. It is believed that dark matter was formed at Big Bang and they acted like scaffoldings upon which ordinary matter could embedded itself. Gravity slowly brought dark matter together at places and thus ordinary matter would come together to form galaxies, planets, stars and so on. it acts as frame work for shining galaxies we can see, which matches with the map of dark matter obtaining from gravitational lensing. Had there been no dark energy, there wouldn‘t have been any galaxies and thus no life forms. it is estimated that dark matter makes up 23% of universe while ordinary makes up only 4% but what makes up final 73% ? Scientists were shocked to‘ discover new dominating quantity to universe the dark energy, which drives galaxies apart.
Dark Energy
In 1990, one thing was fairly certain about the expansion of our universe. It might have enough density to stop its expansion and re collapse into singularity, or it might have so littie energy density that it would never stop expanding, but at least gravity was certain to slow down the expansion as time went on. But scientists while measuring the expansion history of universe realized that the expansion of universe wasn't slowing down, it was speeding up..The space between the galaxies were expanding, but not the galaxies or stars. Edwin Hubble first discovered that galaxies were moving away from each other by observing the red shift of lights from two galaxies. He discovered that the distant galaxies move away with greater speed than nearer galaxies. Astronomers also obtained the same results by studying and analyzing the 'level-A supernova‘ explosion in different galaxies. This was a very unusual and shocking discovery. This showed that there is a new type of energy in the universe that creates a repulsive force that drives the universe apart called 'Dark energy’. This invisible energy is what known expected or understood. This energy is present everywhere in the universe and is stronger but directed opposite compared to gravitational force of attraction.
Dark energy is different from dark matter. It doesn't clump in cluster, rather appears to be quite uniform, so we can see same amount of acceleration in every direction. It is the energy of vacuum, or in better words, it is energy of nothing. It was created at Big Bang and has always existed since the beginning of time.
During first 9 billion years after the big bang, the expansion of the universe gradually slowed down. It was because dark matter was highly concentrated at that time while dark energy was very less as there was less space, and hence the effect of dark energy was kept on check by dark matter. But as the universe kept on expanding, the dark matter was dispersed, but dark energy wasn't affected much, and it kept on growing with increase of vacuum. At about 5 billion years ago, a time came when dark energy exceeded dark matter and the universe began to expandmore rapidly with time.
With time dark energy is getting stronger as more space is being created between galaxies. It looks like each galaxy is destined to lonely existence. And that is how everything will end. Einstein's cosmological constant which he himself considered as his biggest blunder, is now called dark energy and is the dominating force in universe which will determine Whether the Universe will end in fire or ice, and the bet till now is that it'll end in ice at almost absolute zero at which no intelligent species will be able to survive. Once a time will come when dark energy will totally dominate over dark matter and universe will enter the stage of 'exponential expansion' i.e for each unit of time, size of universe will be double. One day when we look up at the sky, it will be all vacant when even the galaxy will be broken up. And until dark energy someday changes its sign and becomes gravitationally attractive, the universe will keep on expanding. Knowledge about dark matter and dark energy has led science a step closer to defining the 'Theory of Everything', one equation that will define the working of everything in universe. But for now, dark matter and dark energy continue to be the greatest cosmological question of the 21st century.