Scientists once believed that due to the mass in our universe, the universe's expansion needed to slow down as time passed. We had not observed the universe's expansion slowing down. Theoretically, we had to. right? because our universe consists of much matter and gravity that attracts matter.
In 1998, the Hubble Space Telescope observed very distant supernovae, which showed that the universe was actually expanding more slowly than it is today, meaning it is expanding faster now, and that was unexpected. Now, we don't know what is causing it, but we know that it is happening.
Based on what we know, it can be said that whatever this mysterious energy is, it is working as anti-gravity generating negative pressure that permeates the universe and is stretching the very fabric of space-time. Rather than pulling cosmic things together as gravity does, dark energy drives them apart.
Over time scientists have provided many theories to explain this mysterious energy known as Dark energy. Some say that it is the result of Einsteins' discarded theory of relativity that includes "cosmological constant" Others may say there is some sort of energy fluid filling the space. It is also a possibility that there is something wrong with Einstien's general relativity theory and a new theory could take its place explaining this acceleration. There are different theories but they have given its solution a name and that is Dark Energy.
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| By Szczureq - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25394893 |
How much Dark Energy there is:
About 68% of our universe consists of dark energy, and dark matter is about 27%, and matter we call normal that we see and interact with and all the matter we have observed so far in the universe only makes up less than 5% of the universe.
You may be wondering how can we measure a thing we don't even know what it is? the answer is quite simple because of its effect on the universe.
Explanations for Dark Energy:
Property of Space: Albert Einstein is the first known person who said that empty space is not nothing it has some properties of its own.. the first property Einstein discovered was that more space can come into existence. Einstein's gravity theory which contained the "cosmological constant" also predicts that empty space contains its own energy. this energy would not get diluted as space is expanding because it's a property of space itself. As more space comes more of this energy will come with it as well. This energy as a result would cause the universe to expand faster and faster,
Quantum theory of matter: According to the theory space is full of "virtual" particles popping in and out of existence. The calculation done for the energy it would give to "empty space" came out to be wrong with a large number so, it is highly unlikely the cause. it came 10^120 times bigger. yeah, that's a big difference.
Quintessence: It is a term given by theorists to describe a completely new type of dynamical energy of fluid or field that fills the entire space and its effect on the universe is opposite to that of matter and gravity. you must keep in mind that Quintessence is also just a theoretical concept for now at least. even if it is true we don't know why it even exists and how and what it interacts with. So, it is a complete mystery.
Exploring alternative gravity theory: We may need a completely new theory for gravity that can explain the motion of bodies in the solar system as Einstein's theory is known to do but still gives us new predictions for the universe as we know it today. It is a possibility that Einstein's theory of gravity is not correct and if it is not it would change our understanding of how normal matter and clusters of galaxies behave.
Dark Matter vs Dark Energy:
Do not confuse dark energy with dark matter. Dark matter is a hypothetical form of matter. It does not interact with either light or electromagnetic fields. its existence is inferred because of the gravitational effects in the universe that cannot be explained by general relativity unless we add this matter which cannot be seen.
Dark matter is a mass that cannot be seen but works as normal matter in terms of gravity, unlike dark energy which works as anti-gravity (works as a repulsive force) and causes acceleration in the universe's expansion.
Terms to Know:
Cosmological Constant:
is denoted by the Greek capital letter lambda is a constant coefficient of a term that Einstien added to his general relativity. At that time it was a common belief that the universe is static so, Einstien introduced this constant to account for a constant energy density filling the whole space. After the discovery that our universe is expanding Einstein discarded this constant but now it is being used to describe as a form of dark energy.
Hubble space telescope:
On April 24, 1990, the Hubble Space Telescope was deployed into low Earth orbit which has provided invaluable insights into the universe.
Supernovae:
They are really bright stellar explosions. It marks the end of the massive stars' life. These events unleash an extraordinary amount of energy momentarily outshining entire galaxies. They produce elements that are vital for the birth of new planets and stars.
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| By NASA/ESA, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=407520 |
Sources:
https://www.bbc.com/news/science-environment-13462926
https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/
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