The universe itself is full of mysteries. Among all the mysteries, dark matter and dark energy are two of the biggest unsolved mysteries in space science. Scientists only know 5% of the vast universe is composed of dark matter and energy.
“The of all this is that we live in a universe
whose age we can't quite compute,
surrounded by stars whose distances we don't altogether know,
filled with matter we can't identify,
operating conformance with physical laws whose properties we don't truly understand.”
-Bill Bryson
This thought of Bill Bryson beautifully captures the mysteries surrounding our universe, with dark matter and dark energy constituting 95% of its mass and energy.
What are the properties of Dark Energy?
What is Dark Matter?
Scientists consider dark matter as a hypothetical form of matter that has gravity, is invisible, and is non-luminous(does not radiate any form of light). Gravitational effects that cannot be explained by general relativity are taken as evidence that dark matter exists and which, if true, would require that "there is more matter than observed." These effects occur when the formation and evolution of galaxies, gravitational lensing, the current structure of the observable universe, the mass positions in galactic collisions, the motion of galaxies in the intragroup of a cluster, and cosmic microwave background anisotropies are considered.
Evidence of Dark Matter
Now, the question that might arise in your mind is,
“How did we learn about the existence of dark matter if it's invisible?”
Well, while dark matter is invisible, its existence is inferred from its gravitational effects on visible matter. Here's how scientists have come to believe in its existence:
Galaxy rotation curves:
Stars in galaxies should orbit in a way that slows down as they get farther from the center, similar to planets orbiting the sun. However, observations show that stars in the outer regions of galaxies orbit just as fast as those closer to the center. This suggests that there's more mass present than what we can see, exerting additional gravitational pull.
Gravitational lensing:
The phenomenon by which massive objects bend the path of light is gravitational lensing. By observing how light from distant galaxies is distorted, scientists can map the distribution of mass in the universe. In some cases, the amount of mass inferred from lensing is much greater than the amount of visible matter, suggesting the presence of dark matter.
Formation of the diverse structures of the universe:
The huge-scale shape of the universe, with galaxies clustered together in filaments and voids, is an idea to have fashioned due to the gravitational influence of darkish remember. Simulations that encompass dark count numbers can accurately reproduce this found structure, whilst simulations without darkish rely can't.
Our universe roughly consists of 25% of dark matter. The existence of dark matter is surely undeniable.
Types of Dark Matter
There are two main types of dark matter. They are-
1. Baryonic Dark Matter
It is a kind of hypothetical dark matter consisting of baryons, i.e., composed of particles like protons, neutrons, and electrons.
Examples: comets, stars, black holes, etc.
2. Non-Baryonic Dark Matter
They are composed of unknown particles that don't interact with electromagnetic radiation.
Example: primordial black holes
What is Dark Energy?
The kind of energy that acts in opposition to gravity and works as an accelerator in the expansion of the universe is termed Dark Energy. The word "Dark Energy" was first coined by the theoretical cosmologist Michael S. Turner. Around 70% of the universe is made of dark energy.
What are the properties of Dark Energy?
1. Negative pressure:
Unlike ordinary matter and dark matter, which have positive pressure, dark energy has negative pressure. This means it exerts a repulsive force, counteracting gravity's attractive force. This negative pressure is responsible for the accelerated expansion of the universe.
2. Uniform Density:
Dark energy is evenly distributed throughout the universe. Its density remains constant over time, even as the universe expands. This is in contrast to dark matter, whose density decreases as the universe expands.
3. Dominance over time:
Dark energy is the dominant component of the universe's total energy density. Its influence has increased over time, leading to the accelerated expansion of the universe.
4. Weakness:
Dark energy is very weak, but it significantly affects the universe because it is so pervasive.
Who wins the cosmic tug of war?
The ultimate winner in the cosmic tug-of-war between dark matter and dark energy seems to be dark energy. While dark matter initially dominated the universe, holding galaxies and clusters together, dark energy has gradually gained the upper hand. It's now accelerating the universe's expansion, pushing galaxies and clusters apart at an ever-increasing rate. If this trend continues, the universe could eventually become a cold, empty expanse, dominated by the relentless force of dark energy.